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**Melbeta** · 01-10-2025, 07:06 PM

Now above, you have all of my parts and details photos that I have on the PRG metal detector........Now guys post this, and post that, all are concerned with their own discoveries, and I got 11 pages of those postings about their own
PRG and what they learned, etc., etc.,,,,,,,,,,Here it is, labeled Mike, etc............What is the below??? It is 11 pages of technical material postings.....................Enjoy reading them!
MELBETA

A typical metal detector
This detector is based on the use of balanced - mitted inductive bridge and is working with
low frequency signal frequency. Induktion bridge consists of two systems coils, which are mutually perpendicular and form input and output audio-frequency circuit amplifiers with high amplification. When
coils so designed that in post - bed very close to the ideal perpendicularity is the them sufficient inductive link, which would to introduce feedback to an extent suffi - tangential to oscillate amplifier. When
However, the group gets close coils metal, balance is disrupted, there is a backward binding, an amplifier is vibrated and arises hear - Net realizable signal. Diagram of apparatus is shown in Fig 14th
Basically, it is unstable low-frequency amplifier with high amplification. From scheme the obvious way system involving coils. Because the intensity of magnetic field distances are shrinking rapidly and the effect of metal - tion course in this field is rapidly disappearing with shrinkage of its size, is very ob -
Gravity to develop equipment that could zjiÅ¡ - indeed face small objects at a distance.
For detectors that are used for localization coil ends, when the number of turns and da -
It current coil array at a distance axis coil depends on its diameter. The larger the diameter, thus making the field. The However, the larger diameter coil, the greater must be metal object from the field could not affect - thread. For this type of detector is always necessary use a compromise between the size of objects,
to be detected, and the distances in to be detected.
Described detector detects aluminum ring concluded beverage cans, or seven - centimeter nail in depth of 5 cm. Larger objects such as the lid of the bin, it is possible determine the depth of 50 cm. Detector is more sensitive for ferrous materials, because they are more influence on the magnetic field.
When designing the electrical parts must be taken to deploy components to the amplifier stable, that has not been implemented parasitic neck - between the input and output amplifiers.
Very important is the construction of sets of coils. The main material for its mechanical construction is wood, drawing, figure 15th In the In addition to making non-coated wire uses no metal. Note that both
horizontal coil with adjustable position order to set the minimum bond.
Horizontal coils have 470 turns, vertical coil has 870 turns. Coils are the cabinet an electronic circuit connected coaxial cable, as shown in wiring diagram.
Popular Electronics February 1969

WM6, thank you. You did a great job and showed how we can do translations from pdf.
Yesterday the (R) EMI group draw a block diagram, which differs from that described in the article. Now, I expect they to write a description of their block diagram, which will probably be quite different in principle. The attached diagram represents an oscillator. It will oscillate at a frequency for which phase shift in feedback loop is 0 or 360 deg. The reinvention continues. MikeBG

Hi Qiaozhi, what is your L-calculator saying about inductance of coils? In my calculations I get:
Rx=18mH (x2) and Tx=36mH WM6…

OK - I've looked at this very quickly. According to your translation, the horizontal coil has 470 turns and the vertical coil has 870 turns. The diagram in the article shows: horiz = 3.8125" (96.8mm diameter) and vert = 1.5" (38.1mm diameter). If we assume 0.2mm thickness for the wire, then:
Horizontal = 46.9mH and Vertical = 46.3mH. Seems quite different to your results. Qiaozhi

Please check it again. You use diameter as circle (e.g. Rx diameter = 96.8mm) but it is not in round form. Rx coil outer measure of (horizontal coil) are: 3 3/4" x 2 1/4" this can not give us (circle) diameter of 3.8125". Can you check this again? WM6

If RX coil is 3.75" x 2.25", then circumference is 12" (304.8mm). If this was a circle it would have a radius of 97mm. The inductance is then 47mH. Qiaozhi

I don't think that the inductance of a non-circular coil is the same as a circular coil with the same circumference. Isn't the inductance a function of the area it encloses? Squeezing a circular coil until the sides touch would dramatically reduce its inductance. Gwil

Block diagram of IBR (Induction Balanced Regenerator)

Block diagram (shown above in posting # 16) is similar to a blocking generator - a wideband amplifier with regenerative feedback via mutual inductance. Without tuned circuits, it will generate pulses as multivibrator. To generate as a sine wave Meissner (Armstrong) oscillator, needs a tuned circuit, ie to one of the coils to be connected capacitor to obtain the required phase shift. The mutual inductance M between coils L1 and L2 shoud be minimal to increase the modulation index of target signal. This means induction balance or minimal K (coefficient of coupling).
The feedback loop is closed in two ways:
- The upper signal path is through mutual parameters M, L, R, C between L1 and L2. The change of TX current in L1 induces voltage in RX coil L2 by mutual inductance M. The phase of mutual induced signal is independent on frequency. It remains always in quadrature, ie in phase lead 90 deg to excitating TX current. This voltage is shown in complex plane below as a signal AIR. Each frequency represented by a point in streight line 1. When frequency increases, the point moves upward.
NOTE: We can close feedback loop by adding an additional network or mutual parameters L, R, C. Thus the phase of AIR signal can be made frequency dependent. This is done by C7 and R5 in circuit diagram (shown above in posting #5). The additional closure is not shown in the above block diagram.
- The lower signal path is through block TARGET. The phase characteristic of TGT signal can be extracted from curved line 3 for ferrous metal. When TX frequency changes from zero to infinity, his point on the curve moves in shown direction. Thus, at ferrous target, the signal can have any phase in the range from +90 deg in 1st quadrant to minus 90 deg in 4th quadrant. At nonferrous target (shown with curved line 4), the phase may be in 4th quadrant only. The signal can have any phase in the range from 0 deg to minus 90 deg. Note that ferrous and nonferrous conductors have a cuttoff frequency at which phase lag is 90 deg, but only ferrous target has a resonance frequency, at which visual phase lag is zero. This is quasi-resonance because phase lag is 360 deg.
The system may oscillate if it has 2 conditions are met:
1. Phase delay must be 360 deg or a multiple of 360 deg for some frequency . For a given target, this condition can be set in block PHASE SHIFT, which contains a PHASE control.
2. The gain of closed feedback loop must be greater than one at the some frequency. For this purpose anywhere in the feedback loop must be connected a potentiometer (attenuator) GAIN control. This is R9 in above circuit diagram. If the gain of closed loop is greater than one for more frequencies, the circuit will generate as multivibrator.
The sensing network in above block diagram can not serve for visual analysis because is too simplified. We should connect a comprehensive block diagram of sensing network containing ground with target buried in it and two RX coils. The TGT signal undergoes changes due to double pass the energy through the ground.
MikeBG

WM6,
The gain of closed feedback loop is a frequency or time dependent function. In order for a circuit to start oscillate, it must satisfy the Barkhausen criteria:
1. The loop gain must be >1 for one frequency or for more frequencies and
2. The loop phase must be a multiple of 360 degrees.
For example, if we have microphone and loudspeaker set up in an auditorium, the feedback is provided by sound waves traveling from the loudspeaker back to the microphone. At what frequency will oscillate this system depends on all stages in the system, giving phase delays. If you do experiment with such a system, would establish that the frequency varies when changing the distance between microphone and loudspeaker. This shows at what frequency Barkhausen requirements are met. Search WEB for "Barkhausen criteria", "Barkhausen criterion", "Barkhausen stability criterion", "Nyquist criterion", "Oscillator theory".
In our case, the frequency of oscillation will depend on the frequency properties of target and ground. All four possible properties are shown in the upper polar plot. Question arises: "How to make the frequency of oscillation depends only on the metal of the target?" We must combine the ground line with the curved line of target because the target is buried in ground. MikeBG

WM&, The Barkhausen's criterium is written simple in block diagram as Z x Y>1, but Z and Y are complex functions with Re and Im parts (or magnitude and phase) depending on frequency. Written with magnitude and phase, the Barkhausen's condition for oscillations means:
1. There is one or more frequencies at which
A1 x A2 x A3 x A4 x A5 x A6 > 1,
where Ai is amplification (gain) of i-th block. We need GAIN control in block 6 to adjust threshold of oscillation.
2. There is one or more frequencies at which
ф1 + ф2 + ф3 + ф4 + ф5 + ф6 = 360 deg,
where фi is phase lag of i-th block. To discriminate targets, we need PHASE control in block 5 to select frequency of oscillation in region where targets differs in transfer impedance Z2=Re+jIm. The phase of target signal is
ф2=artan(Im/Re).
We need DISPLAY PROCESSOR to transform frequency of TX oscillation in a comfortable audio frequency and/or to demodulate it producing DC signal for visual indicator.
Since in the 1969 project "Different" is missing a block 7, the operator must hear the frequency of oscillation. This is very annoying for discrimination and as sound if it is above 2KHz.
Leslie Huggard published "Houndog" metal detector 10 years after the publication of "Different" metal locator . The principle of operation is the same: sensing network connected in regenerative loop. The author had 10 years to improve the block diagram and introduce missing in project "Different" blocks AUDIO DISPLAY and PHASE SHIFTER. This is indeed done. For audio display using Amplitude demodulator with Q1 and piezoelectric buzzer. For phase shift and balance using potentiometer R7 and capacitor C7. However, these components change phase in steps. Potentiometer R7 regulates fine balance of input voltage induced by mutual inductance between TX coil L3 and RX coil (L1+L2), but in balancing point, the phase reverses in 180 deg. Phase should be adjusted gradually in region where targets vary in phase.
MikeBG

The DISPLAY PROCESSOR In this circuit diagram, IC1b, IC1c, IC2 and X1 are used for block 7 - DISPLAY PROCESSOR. They operates as BFO for audio discrimination. MikeBG

I too have been looking at simple detectors. The only question I have is whether this circuit oscillates until in the presence of metal? That is, in order for the feedback circuit to oscillate it needs to have a change of inductance in the bridge circuit. Which is quite different from detecting an already transmitting signal as a VLF/TR does, and more like a BFO actually. In turn this would be limiting to depth by quite a bit.
Technos

Which solution by your mean "would be limiting to depth": oscillating or non-oscillating in "0" position? WM6

Inductors L1/L2 along with potentiometers R6/R7 form a bridge circuit that are adjusted to be in balance. The bridge circuit feeds the LM386 acting as a differential amplifier. Since the bridge is in balance, there is no voltage on the output of U1 LM386. When a metal target (a coin) enters within range of coils L1/L2 their inductance will change, producing a voltage on the output of U1. This feedbacks through coil L3 C6/C7 and causes U1 to oscillate at that tank frequency. Simultaneously this current turns on Q1 and the buzzer.

Until there is a metal object within range of L1/L2, there is no oscillation and hence there is no transmitted signal like a VLF/TR. Such a signal has photons that can go a certain distance, whose strength follows the inverse square law. http://hyperphysics.phy-astr.gsu.edu...orces/isq.html

Inductive coupling, on the other hand, does not follow the inverse square law, but rather falls off much more rapidly. So greatly generalizing;

Field Intensity at distance d away from coil I [vlf/tr] 1/(d^2) > 1/(d^d) [intensity of flux aka change of inductance]

This is not exact formulas, and is an apples and oranges comparison. But think of a magnet only attracting something a few inches away as compared to a beam of flashlight that can go yards away.

BFOs because they rely upon the change of inductance (and hence creates a difference of frequency that "beats") in their coils fall under this as well.

Under this light, this "hounddog" circuit is kind of a "dysfunctional" circuit in this respect, as the circuit is "held up" by L1/L2 bridge circuit rather than detecting the oscillating signal thrown off by L3 (and then the target).

To have a true TR circuit you would need to rearrange the circuit to something like the link (presumably Russian) Qaiozhi posted above, where an oscillator is continuously operating. Printed in 1972 way before it's time, and only four transistors. Does anybody know where it first appeared? Technos

Thank you technos for your kind explanations.

Yes, circuit is from russian source. Unfortunately, it is easy only regarding the number of semiconductors, but also requires the winding of trafo, which is very annoying work. I am interesting in such simple (Tx oscillating) version of T/R schematic, only not trafos inclusive, to do some test. WM6

The block diagram of IBR MD in posting # 36 is valid also for conventional BFO metal detectors. They have many drawbacks because are "Specific case" of that block diagram:
1. The blocks 1 and 3 are directly connected. Sensing network of BFO uses the worst type sensor "Monocoil" instead induction balanced sensor with 3 or more coils.
At Monocoil, the signal AIR is maximal because the mutual inductance between TX coil and RX coils is degenerated in a self-inductance (coefficient of linkage K = 1). The AIR signal makes the target signal with extremely low modulation index. At induction balanced sensor we can increase modulation index making AIR signal weak (linkage K = 0), however, this is not sufficient because the single RX coil receives a strong GND signal.
The GND signal not only decreases modulation index, but it is modulated with frequencies below 6Hz as target signal.
Therefore we must have second RX coil connected as TWIN LOOP to supress the GND signal.
2. The lack of inductive balanced sensor does not allow conventional BFO MD to work without oscillation as IB MD. It is just one oscillator, a special case of "induction balanced regenerator", which is not "induction balanced", nor "regenerator".
We can construct IBR MD, which is superior to conventional options BFO MD, but this is not enough. It will have inherent weaknesses of each "regen radio" - amplifyes thermal noises generated in input and preamp. Must design low noise preamp for block 4. MikeBG

Goldenscull, you can build one good deep penetrating metal detector, but dont call it BFO MD. It can be IBR MD and the BFO can be block #7 of block diagram.
Disadvantages of CCO MD in posting # 39 (see also the attached below file):
1. Too large supply voltage. If using HCMOS inverters instead 4069, the circuit can be powered by 2 x AA cells (even by 2.4Volts!).
2. Negative feedback via R is too weak, so the input resistance of inverting amplifier is not zero. This decreases Q factor of tuned circuit L2-C1 and increases bandwidth, ie the preamp generates more noise.
3. The 500KHz operating frequency is too high.
4. The capacitance of the capacitor C2 is too large. You can increase the number of turns on L2 and this the sensitivity, if using smaller capacitance for C2.
5. IC1a is loaded unduly with DC current in operating point. Instead of R2 must be connected a capacitor.
6. Is not intended a control knob to adjust the sound frequency.
7. Is not intended to suppress the GND signal using a second RX coil, connected in series to R2.
8. Remains not used 3 inverters of IC1. They can replace the action of IC2. MikeBG

Similar to the 3 coils MD published in Popular Electronics is the PRG Gradiometer. Seems very serious scientific MD instrument. Not all the schematic I show... See pics and patent here: ESTEBAN

Esteban, In 60 years there has been no digital voltmeters in Bulgaria. Note that operating point voltages indicated in the circuit diagram (posting #84) are irreal because are measured by a primitive voltmeter, having low resistance. A modern voltmeter would show operating voltage near to Vbe = - 0,2 V, because transistors are made of germanium. MikeBG

GoldenScull, 50 years ago Vaino RONKA invented and introduced good MDs for detection in 10 meters depth under ground. Search for this name in WEB and for his patents in USPTO. MikeBG

Maikl, the CCO MD in postings # 39 and # 48 is a bad circuit with 8 drawbacks. You can calculate L and C for the lower frequency, such as 15KHz, but the operation will not improve substantially because the remaining drawbacks continue to hinder.
The task of a constructor, even if amateur, is primarily to find the best block diagram of the project according to which must operate the circuit diagram. Once you know what should be block diagram, you can copy foreign designs and circuit diagrams if they have such block diagram.
There is, however incompetently designed circuit diagrams as shown below in right. MikeBG

Circuit to the left serves to explain how operates a conventional narrow band inverting amplifier with normal opamp. The work of the opamp needs some intermediate voltage, such as 6V, to which are connected signal source and load resistance. In the case, our signal source is RX coil L1. We can connect L1 directly to the pin 2 without using a capacitor C1 to get wide band amplifier, but it is impossible for the circuit to the right. Will understand why later. Now let's see how opamp operates:
It has a very high gain, so the feedback through R2 is very strong. It supports the potential of pin2 same with that of pin3, though a short circuit between them. This is tantamount to connect the right-hand lead of capacitor C1 with lower lead of coil L1. Thus the Q-factor of LC tank is high because depends on coil resistance r1 only. For DC operating point, output voltage at pin 1 follows the voltage of pin 3. When we connect a load resistor between output and intermediate voltage, through it will not flow DC component becauce both leads are connected to the same potential.
How operates the circuit with CMOS transistors in right:
It produces self the intermediate voltage. If the complementary transistors have the same parameters, we will have at 2 pin output 6V DC. If you conect a DC voltmeter to be sure that it does not show the half of the supply voltage. Current, which consumes one inverting amplifier depends very much on supply voltage. Amplification is much less than that of the operational amplifier shown to the left and is influenced by supply voltage. This makes weak the feedback, so input impedance in pin 1 is not zero. Appears an input resistance "r in", which deteriorates the Q-factor of LC tank because it is connected in series to the coil resistance "r1". Most importantly, we can not connect a signal source and load resistor withot capacitor because it will amend the operating point and will proceed DC component, in the case of 3mA, which is too high load for 4069. The attachment AN88 contains information about analog applications of digital CMOC IC. MikeBG

I wouldn't call them bad circuits. It depends entirely upon the application. It must be remembered that the 4069 is not really intended for linear circuit operation, but in certain instances can be made to act that way. The reason that a 4069 might be used in this manner is for a cost-sensitive application. What would make it a bad circuit is unreliability. For example, if the circuit only worked with half of the 4069s out there, then it would be a bad circuit. If the 4069 was stressed in a manner that led to its failure, then that would be a bad circuit. But if the circuit worked with all 4069s within its specification, then that would be an acceptable (good) circuit. Technos
Thank you mikebg for your kind explanations.I am very interested in building this circuit to see how it does with ground effects and depth performance. Maikl
Maikl, read this again:
"The task of a constructor, even if amateur, is primarily to find the best block diagram of the project according to which must operate the circuit diagram. Once you know what should be block diagram, you can copy foreign designs and circuit diagrams if they have such block diagram." [by (R)EMI group]. If you are "very interested in building this circuit to see how it does with ground effects and depth performance", you should know the correct block diagram. Now you know that CCO in postings # 39 and # 48 is a bad circuit with 8 drawbacks.
The correct block diagram uses two RX coils to suppress ground effects. This is configuration TWIN LOOP. The correct block diagram of IBR regenerates weak signals but not oscillates. The block diagram in posting #16 should show the connection of both RX coils so: MikeBG
Hi mikebg
CCO Metal Detector â€“ New Seekers Written by Barnaby Brown August 07, 2009
TO the best of the authorâ€™s knowledge, the metal detector circuit shown in Fig.1. represents
another new genre. In principle, it is based on a transformer coupled oscillator (TCO), a well
known oscillator type. This essentially consists of an amplifier which, by means of a transformer,
feeds the output back to the input, thus sustaining oscillation.
The circuit is presented merely as an experimental idea. With some care, an old Victorian
penny should induce a clear shift in tone at 160mm in a crystal earpiece, and a perceptible shift in
tone at up to 250mm.
In Concept: The concept differs from beat frequency operation (BFO) in that its performance far
outstrips that of BFO. Also, unlike BFO, it is dependent on the balance of two coils to boost
sensitivity. However, it differs from IB (induction balance) in that its Rx (receiver) section is active rather than passive, being an integral part of a transformer (or coil) coupled oscillator. Also, unlike IB, the circuit employs a beat frequency oscillator to produce an audible tone.
Furthermore, the CCO detector does not require the critical placement of coils as IB does,
with a few centimetresâ€™ movement this way or that being permissible â€“ on condition that a suitable audible heterodyne is tuned in. Also, unlike IB, its sensitivity is not localised around the
intersection of the two coils, but covers the full area of both. This means that, while not ideally
suited to pinpointing finds, it lends itself well to sweeping an area. As with BFO and IB, it also
offers discrimination between ferrous and non-ferrous metals. GURU
Actually, the hounddog circuit posted above is a variant of a "bridge circuit," though a somewhat dysfunctional one. Estaban's schematic would be another example of a bridge circuit i.e. induction balance type. The russian miniature detector 1972 (I posted a link above - does anybody have the original Russian source for this??) is a very good example of a transformer coupled oscillator (it looks like a bridge circuit, but it's not) and is a simple TR detector that's close (but not quite) to offering VLF discriminating. "Different 1969" is most likely another example of the dysfunctional bridge/hounddog. I call it "dysfunctional" because it's not always oscillating and more like a BFO in performance instead of a TR. The CCO detectors are BFO circuits that use a transformer coupled oscillator and operate at 500 KHz (the one transisotor AM radio metal detectors are an example) like many BFOs (frequency shifts) do.

The patent 3002262 is *not* a patent for a wide scanning detector (though it shows a search head for one), but rather a very informative patent for making one. Thanks mikebg. Technos
Maikl, CCO Metal Detector â€“ New Seekers "Written by Barnaby Brown August 07, 2009
TO the best of the authorâ€™s knowledge, the metal detector circuit shown in Fig.1. represents
another new genre......."The artcle is written by Barnaby Brown, but the text, and the CCO is "genre" of Rev Thomas Scarborough.Search WEB and this forum for "CCO" and "Thomas Scarborough" and you should find such masterpieces in electronics, that only a rev can imagine. MikeBG
What about this bridge MD solution? WM6
Nothing with frequencies, but yes it means a large search area under coil. Like what the bigfoot coil does also - you cover a larger area with the detector than with a smaller coil. Technos
Interesting circuit, where did you find it? Without looking too closely, it appears the bridge circuit there is L1, L2 VR1, VR2 (actually the resistances that VR1,VR2 control).

You need to have four "arms" to have a bridge circuit. For example, L1 would be one "arm";
http://en.wikipedia.org/wiki/Bridge_circuit

It can be tricky telling sometimes as there are many different types of "bridge" circuits, and you may need to redraw the schematic to find the "bridge." Technos
From here: It seems to me as double (symetrical VR1+VR2 & VR1a+VR2a with common reference L1 + L2) bridge: Or phase shift separate two second arms from bridge? WM6
Thomas Scarborough wrote: "Various embodiments of the BB metal detector have been published, and it has been widely described in the press as a new genre.
Instead of using a search and a reference oscillator as with BFO, or Tx and Rx coils as with IB, it uses two transmitters or search oscillators with IB-style coil overlap. The frequencies of the two oscillators are then mixed in similar fashion to BFO, to produce an audible heterodyne. On the surface of it, this design would seem to represent little more than a twinned BFO metal detector. However, what makes it different above all else, and significantly increases its range, is that each coil modifies the frequency of the adjacent oscillator through mutual coupling. This introduces the "balance" that is present in an IB metal detector, and boosts sensitivity well beyond that of BFO. Since the concept borrows from both BFO and IB, I have given a nod to each of these by naming it a Beat Balance Metal Detector, or BB for short. Happy hunting!"
Here is the "new genre" Beat Balance Metal Detector made from discrete components.. Where are the discrete components? How is determined operating point of opamps? How will work a opamp with 100% positive feedback? MikeBG
in 1974....my DAD decides to upgrade from the Garrett BFO and gets the PRG.....PHASE READOUT GRADIOMETER....BASICALLY IT WAS A TR WITH A TID METER.....at this time only the very basic discriminators were being introduced to the Metal Detecting Community.....so this was a major improvement to anything else being used at the time....it would be 1980 or 1982 before anyone else would come out with the VLF TID detectors....I filled a metal pot (that was also dug up)...full of silver coins mainly silver quarters and silver half dollars......found a lot of gold rings...silver rings....and many silver and brass medals and insignias .......it was weatherproof which was another major innovation at the time....it would detect to maybe 10 inches on a silver quarter but the TID meter was only accurate to 6 inches or so....I had a few techniques where I could tell if the target was deep ....found some historic items also......(...I sold most of the above years ago..)..this detector was years ahead of its time...and today it is very rare.....the Manufacturer has stated that only a few hundred were made....BUT THE DEALER WHO WE BOUGHT IT FROM SAID THAT ONLY 50 WERE MADE..........in almost 7 years of searching I have Never seen one on FeeBay...although I have tracked one down using the old investigation methods the current owner is refusing to sell....I hope one day to be able to get it back JOE RANGER
The Phase Readout Gradiometer, or PRG, which was a TR-IB designed by a group of NASA engineers and marketed by Technos Inc.of Maitland, Florida (still in business) for hunting the wet salt, non black sand/low mineral Florida beaches. It was a straight shaft with a meter on it that measured Target I.D.; this was around 1973, and did so in a completely different method than today's TID units. It did not have ground balance capability, or autotune either, and this was several years before George Payne would patent the synchronous phase circuit we know as motion (George also submitted a patent in '78 for a discriminating time domain detector). The PRG cost around $900, this at a time when a top of the line detector from someone else cost about 1/3 of that, and BFO's were still being used by a great many people and it was the only detector you saw prospectors with. The search coil looked like two bowls used for feeding a dog, one upside down and the other atop it; at its widest in the middle it was around 10" and, from top to bottom was about 8" tall, and it would hit a quarter around 12" deep. It had an unusual tuning procedure it recommended, and worked by tone. You first get it to give a low tone on iron and everything above would give a high tone; you then repeated the procedure with a pull tab giving the low tone, and you then disregarded the low tone and dug only the high tone after looking at the meter to see the if it was a target you wanted; it read from 100 down to 0. Now it still continued to identify everything but you had to monitor the meter for all of the low tones to see where they read out if you were interested in nickles/rings. It was also heavy and difficult to read the meter, and depth decreased rapidly with an increase in ground minerals.Teknetics led to its demise with first the 9000, and then the 8500, and White's was right behind them with the 6000Di.
But we will never know where we would be today if this design had continued to evolve.
Maybe one day it will get a second look. TECHNOS

The PGR runs at 7.2 khz
Two tone audio = 800 Hz for iron
1100 hz for non ferrous Esteban

WM6,
Please search section "Modifications" in this forum for Sharky.
Sharky was reported in November 2007 for excellent results: TwinLoop detects coke can at 180cm in air, and a 1 cent coin at 55 cm with modified "Twin Loop Treasure Seeker". It uses 3 coils sensor in a PI machine.
Search also in section "Projects" the thread "Gary's Twin Coil PI". It also can work with TWIN LOOP sensor providing excellent results.
But let's continue with the block diagram of IBR Metal Detector.
When you know the right block diagram, you can find errors and weaknesses of the various circuit diagrams and designs using this principle and to correct them.
Consider if the circuit diagram of "Different 1969" adhere to the block diagram according to our requirements.
1. Coil configuration. The RX coils must be connected as TWIN LOOP, ie in series and in opposite directions to have their signals subtracted. Thereby suppressing the signal from the ground and the interference from distant sources and even by working nearby other metal detectors.
This is not done. In the original, each RX coil shunt any signal generated by the other RX coil. The signal from the remote interference source and from working nearby metal detector is not suppressed because it induces synphase voltages in each RX coil. Proper connection is shown in the block diagram and below in an example design.
The TX coil must be oriented so as to provide intensive and vertically oriented down to earth magnetic flux. However, in the original sensing head, a horizontally oriented magnetic field runs through the TX coil, which gives rise to two weak magnetic fluxes in different directions on both ends of the sensor.
Proper configuration has been long known. Here's an patented example: MikeBG
You mean physical oriented, like this?: WM6
Yes, this is the principle of loop orientation used in "Different 1969". This is the sensor with orthogonal coil configuration given as example in article "Coil Basics" by Carl Moreland. The RX coils in this configuration can suppress the ground signal if are connected in series as shown in posting #16 , however this sensor can not suppress signals from remote sources and nearby metal detectors. MikeBG (2 Horiz & 1 Vertical)
2. Block 4 PREAMP is a wide band amplifier because the RX coils are not connected in tuned circuit. That means more thermal noise generated in input. It is preferable instead capacitor C7 connected to TX coil, to connect a capacitor to RX coils.
3. Block 5 PHASE SHIFTER in the "Different 1969" is formed by TX tank circuit formed by C7. The tank provides voltage signal without phase shift at resonance frequency, but its phase diagram is very steep in this point; in a narrow band the phase varies from +90 deg to minus 90 deg. That means the regenerative loop has trend to oscillate in a frequency determined preferably by phase characteristic of TX tank, instead by those of target.
The phase shift control by R7 is insignificant because the cutoff frequency of this branch varies from 10Hz to 28Hz. The capacitance of C5 should provide for an audible cutoff frequency in order different targets to sound different.
In fact, both potentiometers R7 and R9 control the gain of regenerative loop without phase shift.
4. Block 7 for DISPLAY OF OSCILLATION is an amplifier without volume control. It amplifies the TX frequency, which is an unpleasant high tone. MikeBG

[IMG]file:///C:/Users/JOHNDO~1/AppData/Local/Temp/msoclip1/01/clip_image001.gif[/IMG]

Is my idea of modernization of MT-66, shown in posting # 92 could be part of HOUNDOG modification?
REMI group was disappointed with me because I have no sense of reinventor.
I was very close to reinvent of HOUNDOG, publishing idea for its modification in posting # 92. I never thought to connect TX coil in above circuit diagram to output of LM386.
For me, sensors of MT-60, 62 and 66 are like the configuration shown in postings # 1 and # 8. Really RX coil of HOUNDOG has center tap and it can be split as TWIN LOOP.
My idea was to build for MT-66 a narrow band amplifier with LM386 working at 2500Hz. For that purpose I use a tank circuit L1-C2 having high Q-factor. However the modified HOUNDOG should work in a broad band at higher central frequency, for example 8KHz.
How should seems a modernized HOUNDOG? MikeBG

Metal detector with BLACK HOLE ANTENNA
The (R)EMI group forced me to start the thread "Popular Electronics, Feb. 1969" when the members reinvented the BFO (frequency shift) metal detector. The BFO MD is specific case of "Induction Balanced Regenerator" or IBR MD. At conventional BFO MD, the Barkhausen's criterium is satisfyed without target, thats why it allways oscillates in the absence of targets.
The normal IBR MD starts to oscillate only if there is a target, because the target closes the feedback path.
Experts of (R)EMI group say, the IBR MD has a lot of advantages:
Can operate in three modes. Can be designed as a narrow band metal detector with bandwidth maximum 12Hz (preferably 4Hz). Can be designed as broadband metal detector covering the whole audio frequency range (and even more when it should search for gold nuggets).
In narrow band mode, thermal noise is minimal, so there will be sensitivity superior to conventional metal detectors. It can be configured so that the Barkhausen's criteria can be fulfilled only for one species targets, thus discriminate against any other targets.
In a wide band mode, the IBR MD can be designed so that it starts to oscillate at frequency, which depends mainly on the target spectral characteristic. So you can use a frequency meter for visual (or spectrum shift with BFO for audio) TID (target identification). In continous oscillating BFO mode, it delivers more frequency shift than conventional BFO with monocoil.
The IBR MD is the most energy economical type detector because it not oscillates and no audio in the absence of targets. According Leslie Huggard, it operates with a 9V battery for almost one year. Due to negligible power consumption, it can be powered with photovoltaic battery.
Because of regeneration, the IBR MD operates as BLACK HOLE ANTENNA.
If you ask: "What means BLACK HOLE ANTENNA?", the answer is: "Search in WEB!"
You will find several documents written by John Sutton, Craig Spaniol and other NASA collaborants, which explain the term "Black hole antenna" or "Energy sucking antenna". Because of regeneration (positive feedback) the active antenna absorbs from environment more energy than expects from its dimension. That means when an IBR metal detector uses a small coil, it operates as if uses a large coil. This is illustrated with following drawing:

Here's how looks the nearfield without regeneration (no BLACK HOLE EFFECT).

And here is what happens with regeneration (there is BLACK HOLE EFFECT). MikeBG
Revised block diagram of metal detectors in this thread. The previous name of block diagram INDUCTION BALANCED REGENERATOR is changed in REGEN MD to satisfy 3 operational modes:
Mode A. The circuit allways oscillates. A specific case is the conventional BFO metal detector with monocoil. In this case the mutual inductance between TX and RX coils is transformed in self-inductance of the monocoil. Because of large AIR signal, the frequency shift at monocoil is minimal. Mode B. The circuit operates as IB MD with "Black Hole Antenna". It starts to oscillate when there is target. If the loop gain is wideband, oscillating frequency depends more of targets phase characteristic. A specific case is when the loop gain is maintained to unity by slow acting AGC . Then the gain of amplifier should be so large, that TX coil excitates target with rail-to-rail noise. Mode C. The circuit oscillates without target. The target diminishes amplitude or stops oscillation.
This US patents of John F. Sutton
5,311,198 Active antenna
5,296,866 Active antenna
5,015,963 Synchronous demodulatoris an absorbing metal detector. MikeBG
2,747,152 Greene (see bridge portion of system) WM6
Pinpointer with BLACK HOLE ANTENNA. NASA
The block diagram of this design should be specific case of block diagrams in postings ##116, 60 and 16. WM6
Yes, the Gold Guns AL707 and AL718 are passive metal detectors. For operation they need large signal from VLF station in searched region. They are primitive variants of genuine passive detector invented by Vaino Ronka and described in patent US3,500,175. Ronka uses second ferrite antenna for receiving reference signal and measures amplitude and phase of target signal.
Search WEB for Geonics EM16. The company produces sparate TX for using EM16 in regions where no VLF stations. MikeBG

**Melbeta** · 01-10-2025, 07:11 PM

Now there will be other drawings, shown from some of the technical material, and one will have to ask me for this and for that, I will dig it out, stick it in, and you will learn more. But if you work with STANS reverse engineered unit, it will be easier to build it........But if you want more, send me some of the drawings you do..........I got a lot, you are smart, and you and I we can all work together on the PRG unit....[email protected]
MELBETA

**Melbeta** · 01-11-2025, 03:22 PM

Now I am going to present to the reader, an Copy Abstract, identifying other Companies Copyrights, contained within this particular Copyright Abstract,
which is an pleasure to read of the other companies DESIGNS, and other companies ABSTRACT CLAIMS... I maintain this as an LEARNING Episode
for the reader of this ABSTRACT COPY...........
MELBETA

US 3826973 A
Abstract
An electromagnetic gradiometer includes a pair of coaxial pickup coils arranged on opposite sides of a transmitter coil for deriving a magnetic field in response to a fixed frequency a.c. source. The pickup coils are connected to null circuitry which derives a substantially zero a.c. voltage in response to neither a ferrous nor a conductive body being in the field. In response to ferrous and conducting bodies being in the field, non-zero a.c. voltages having phases displaced relative to each other are produced. Detection circuitry responds to the amplitude and phase of the voltage derived from the pickup coils to generate aural outputs at two different frequencies which enable the ferrous and conductive bodies to be easily distinguished even though the device scans rapidly over an area. Quantitative phase information, derived by a phase comparator, is provided by a meter to provide a characteristic signature of the electrical and physical characteristics of a detected object, so the object can be identified. The nulling circuit includes a two-tap resistive voltage divider connected in shunt across output terminals of the two pickup coils. First and second of the taps are respectively connected via a resistor and a capacitor to a common terminal for the windings.

Description (OCR text may contain errors)
United States Patent [1 1 Pflaum July 30, 1974 ELECTROMAGNETIC GRADIOMETER [75] Inventor: Norman C. Pflaum, Miami, Fla.
[73] Assignee: Richard Benson, Miami, Fla. a part interest 22 Filed: Jan.10, 1973 21 App1.No.:322,400
[56] References Cited I UNITED STATES PATENTS 5/1945 Millington 324/5 6/1951 Ostlund 324/3 2,744,232 5/1956 Shawhan et al.... 324/3 2,858.505 10/1958 Shawhan 324/41 3,015,060 12/1961 McLaughlin ct a1 324/4 3,020,470 2/1962 Shawhan et al 324/3 3,471,772 10/1969 Smith 324/3 Primary Examiner-Gerard R. Strecker [57]

ABSTRACT An electromagnetic gradiometer includes a pair of coaxial pickup coils arranged on opposite sides of a transmitter coil for deriving a magnetic field in response to a fixed frequency a.c. source. The pickup coils are connected to null circuitry which derives a substantially zero a.c. voltage in response to neither a ferrous nor a conductive body being in the field. in response to ferrous and conducting bodies being in the field, non-zero a.c. voltages having phases displaced relative to each other are produced. Detection circuitry responds to the amplitude and phase of the voltage derived from the pickup coils to generate aural outputs at two different frequencies which enable the ferrous and conductive bodies to be easily distinguished even though the device scans rapidly over an area. Quantitative phase information, derived by a phase comparator, is provided by a meter to provide a characteristic signature of the electrical and physical characteristics of a detected object, so the object can be identified. The nulling circuit includes a two-tap resistive voltage divider connected in shunt across output terminals of the two pickup coils. First and second of the taps are respectively connected via a resistor and a capacitor to a common terminal for the windmgs.

15 Claims, 4 Drawing Figures r PRlMARY l OUTPUT l REcEwEiz \7 (mom 13 I cuzcun 1. Y l 1% l \8 l i VNUABLE e c e g c I RE- emu AMP. TUNED I RECHHER TRANSDUCER AMPUHER l 1 aizmiomareiz seusoiz I I PHASE ADJUSTABLE I UNUUNCTlON ME mo I 14 COMPARATOR 1155mm OSULLATOR G AMPUHER I l l l J 3' l 5\NE wave e c o r i mY OSULLMOR' METER uANTiTATwE POWER PHASE) AMPuHett PAIENTEBIIII30 I974 SHEEI 2 [IF 4 F/a Z// I I I I I I I l I I I I I I I I I I I I I I I I I l I I I Mai I3 ULULW 1 ELECTROMAGNETIC GRADIOMETER FIELD OF INVENTION The present invention relates generally to electromagnetic gradiometers and more particularly to an electromagnetic gradiometer for detecting and distinguishing between ferrous and conductive bodies.

BACKGROUND OF THE INVENTION Electromagnetic systems and gradiometers are known in the art, as exemplified by US. Pat. Nos. 2,744,232, 2,915,699, 3,020,470 and 3,471,772. Typically, systems of the type shown by these patents include an excitation coil driven from a fixed, constant frequency a.c. source and a pair of pickup coils connected in a differential relationship. In response to the two pickup coils being loaded in the same manner relative to the excitation coil, i.e., a balanced condition, as exists in response to neither a ferrous nor a conductive body being in the area being monitored by the gradiometer, a virtually null output is derived from the differential coils. To compensate for coil variations which occur, for example, as a function of temperature, relatively complex nulling circuitry may be provided to assure the derivation of a zero output for the balanced condition. In response to differential loading of the two pickup coils, i.e., an unbalanced condition, as occurs in response to a ferrous or conductive body being in the area monitored, a finite, non-zero voltage is derived between the two pickup coils. In response to a ferrous body being in the field, the non-zero differential voltage has a first phase relative to the excitation voltage phase. In response to highly conductive body being in the monitored area,.the magnetic field causes eddy currents to be induced in the conductive body to cause a significant change in phase relative to the voltage derived for a ferrous body. For bodies having intermediate conductivity and ferrous properties, the phase shift is between the two extreme values.

Generally prior art systems of this type have failed to provide a facile arrangement for distinguishing between ferrous and conductive bodies and for enabling such bodies to be easily located. In general, the prior art has relied solely upon phase displacement of a differential signal to activate an output device which may be either of the visual or aural type. Further, some of the prior art devices have required relative manipulation between the excitation and pickup coils in order to distinguish between ferrous and conductive bodies and locate same.

BRIEF DESCRIPTION OF THE INVENTION Frequency is derived to indicate detection of a ferrous body. The amplitude of the first aural frequency is varied in response to the amplitude of the voltage derived from the pickup coil to provide an indication of the magnitude of the magnetic unbalance. which in turn is a function of the geometry, orientation and particularly distance of the ferrous body from the coil assembly. In response to a conductive body being in the magnetic field between the excitation and pickup coils, the resulting phase displacement causes the frequency, i.e., tone, of the aural signal to shift from the frequency associated with detection of a ferrous body, whereby an operator is provided with a definite indication of a conductive body being detected. The aural signal associated with the second frequency is always derived at constant amplitude, but a measure of the conductivity, orientation, geometry and location of the conductive target is derived by providing a meter driven by a phase comparator which causes the aural tone frequency shift. The aural tone amplitude is controlled so that, in response to a balanced condition being sensed by the pickup coil, no aural signal is derived.

The presence or absence of aural tones at two different frequencies enables the operator to scan a particular area under investigation at a very rapid rate. In response to an aural tone being derived during the rapid scan operation, the operator repeats the scan at a relatively slow rate, while observing the sound intensity and frequency, as well as the meter deflection to enable the ferrous or conductive body to be located and identified with relatively great precision.

A further feature of the invention resides in an extremely simple and easily adjusted nulling circuit for compensating impedance variations of a pair of differentially connected pickup coils, which variations result from changes in ambient conditions or inconsistencies in manufacture. The nulling circuit comprises two resistive voltage dividers connected in shunt across output leads of the two pickup coils. The resistive voltage divider means includes a pair of taps, one of which is connected through a resistor to a common, shielded lead for the two pickup windings, while the other is connected through a capacitor to the common lead. The resistance and capacitor connected to the two taps provide zero and phase shifts for voltages developed at the taps so that compensation for resistive and reactive changes of the pickup coil assembly can be provided merely by changing the positions of the taps. The taps, resistive voltage dividers and impedances connected between the taps and the common terminal are arranged in a bridge-like circuit to provide the desired null characteristic.
A further feature of the invention is that sensitivity of the system to distinguish between ferrous and nonferrous, conductive metals can be varied as desired. Variable sensitivity is important in monitored host regions having relatively high conductivity, such as salt water. To this end, a threshold detector having a variable input setting responds to the output of the phase detector to control the frequency of the aural tone. Only in response to the phase shift exceeding a predetermined level, controlled by the operator as desired and as a function host medium conductivity, is the aural frequency shifted to indicate the presence of a conductive body, rather than a ferrous body.

A further feature of the invention relates to the ability of the system to detect certain ferrous and conductive non-ferrous bodies in a medium having a relatively high magnetic permeability, as occurs in certain mineralized soils. In a highly sensitive system. the high magnetic permeability of the host medium may result in a sufficient unbalance between the two pickup coils to It is, accordingly, an object of the present invention to provide a new and improved electromagnetic gradiometer device for detecting and distinguishing between conductive and ferrous bodies and for locating same.
An additional object of the invention is to provide a new and improved device for detecting and distinguishing between ferrous and conductive bodies and for locating same, wherein different distinguishable signals responsive to the phase and amplitude of a magnetic field passing through the body are employed.

An additional object is to provide new and improved circuitry for nulling a pair of differential coils having impedance characteristics that are susceptible to change and not consistent from one unit to the next during manufacture.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one embodiment thereof, especially when taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram of a preferred embodiment of the invention;
FIGS. 2A and 2B are a detailed circuit diagram of the system illustrated in FIG. 1', and
FIG. 3 is a perspective and partially phantom view of a portable device embodying the principles of the present invention and particularly adapted for use in de tecting and distinguishing between sub-surface ferrous and conductive bodies.

DETAILED DESCRIPTION OF THE DRAWING The detailed description of the invention is concerned with a portable device particularly adapted to locate sub-surface conductive and ferrous bodies, such as pipelines. cables. lost articles, treasures and archeological finds, as well as mineralize deposits or contaminants. It is to be understood, however, that the principles of the invention are equally applicable to systems wherein the requirement for portability does not exist, such as in industrial applications or personnel surveillance for, e.g., detecting concealed ferrous and nonferrous, conductive weapons.

Reference is now made to the block diagram of FIG. 1 wherein there is illustrated a centrally located transmitting coil 11 and pickup coils l2 and 13, which are coaxial with each other and transmitter coil 11. Pickup coils I2 and 13 are fixedly and permanently mounted on opposite sides of similarly mounted transmitting coil 11; the pickup coils are wound in opposite directions relative to each other, with opposite ends being connected in a differential relationship. The other ends of pickup coils 12 and 13 have a common ground connection that extends to each of the other circuit elements illustrated in FIG. 1.

Transmitter coil 11 is connected to be excited by a fixed, constant frequency sine wave source. which preferably derives a frequency in the audio range. Sine wave source 14 comprises a stable oscillator which drives the excitation winding through a power amplifier tuned to the frequency of the oscillator. Pickup coils l2 and 13 are positioned relative to transmitting coil 11 in such a manner that in response to a balanced impedance subsisting between the transmitter coil and each of the pickup coils a zero, null voltage is derived between the opposite ends of the pickup coils. In response to either a ferrous or conductive, non-ferrous body being in the region scanned by the assembly including coils 1113, a finite, non-zero voltage is derived between the opposite ends of pickup coils 12 and 13. The nonzero voltage is derived because of an unbalanced magnetic coupling between the two pickup coils 12 and 13; the unbalance occurs due to the electromagnetic gradient resulting from the closer proximity between the conductive or ferrous body and one of the pickup coils relative to the other pickup coil.

Coils 11-13 are preferably mounted on a dielectric, non-magnetic temperature stable cylinder (not shown), which may be fabricated from certain glasses, so that there is an air core coupling the magnetic fields of transmitter coil 11 to each of pickup coils l2 and 13. Pickup coils l2 and 13 are arranged vertically on the cylinder with respect to transmitter coil 11, so that all of coil 12 is above the transmitter coil and all of coil 13 is below the transmitter coil. It is to be understood, however, that in certain instances it may be desired to change the orientation of the pickup and transmitter coils 11-13 so that they are coplanar or otherwise disposed. In response to a ferrous body causing the magnetic unbalance, the finite voltage developed between the opposite ends of coils 12 and 13 has a first, substantially predetermined phase relative to the phase of the sine wave derived from oscillator 14. In response to a conductive body causing the unbalance, the finite voltage developed between the opposite ends of the pickup coils is variable over a relatively wide range, which is distinguishable from the predetermined phase resulting from a ferrous body causing the unbalance.

Because pickup coils 12 and 13 have a tendency to change impedance with respect to each other and transmitter coil 11 as a function of ambient conditions (particularly temperature) and difficulties in manufacturing all of the assemblies of coil 11-13 do not have precisely the same characteristics, the opposite ends of the pickup coils are connected to nulling circuit 15. Nulling circuit 15 includes a network whereby a relatively exact zero voltage is derived from it in response to neither a conductive nor a ferrous body being in the magnetic field coupling transmitter coil 11 to pickup coils 12 and 13. Null circuit 15 includes adjustable impedance elements for compensating impedance changes of the coil assembly including windings or coils 11-13.

The ac. output of the pickup coils 12 and'13, after being zeroed by the nulling circuit 15, is coupled to a pair of cascaded a.c. amplifiers l6 and 17. Amplifier 16 is a preamplifier having a fixed gain and relatively wide bandwidth, while amplifier I7 is a variable gain device with a narrow band pass at a center frequency approximately equal to the frequency of the sine wave applied by oscillator 14 to transmitter coil 11. The ac. output of variable gain amplifier 17 is a sine or clipped wave indicative of the amplitude and phase derived between the opposite ends of coils l2 and 13. If the coil assembly is traversing a medium having a contaminated high magnetic permeability, such as certain mineralized soils, the gain of amplifier 17 is adjusted to a relatively low level, to enable certain conductive and ferrous targets to be distinguished from the contaminated medium being traversed. However, if it is desired to provide optimum sensitivity and enable small, conductive and ferrous bodies to be detected in an uncontaminated medium, the gain of amplifier 17 is adjusted to a high level.
The variable amplitude signal derived from amplifier 17 is fed to rectifier 18. Rectifier 18 derives a d.c. signal having an amplitude approximately proportional to the amplitude of the a.c. signal derived from variable gain, tuned amplifier 17. In response to the phase of the signal derived from amplifier 17 indicating that a ferrous object is in the magnetic field of 'the coil assembly, the amplitude of the d.c. signal derived from rectifier 18 controls the amplitude of an audio frequency, F derived from a free running relaxation oscillator 19. The amplitude of the output of variable gain, tuned amplifier 17 controls the amplitude of the F component derived from oscillator 19 so that the F, component is fed with increased amplitude through variable gain gate 21 as the ferrous signal increases. For a null output of amplifier 17, oscillator 19 derives the F, component which is blocked by gate 21. To these ends, the gain of gate 21 is controlled by the output voltage of rectifier 18 to increase from zero as the rectifier output increases from zero. The output of variable gain gate 21 is fed to audio amplifier 22 which drives electroacoustic transducer 23, which derives an aural output having a frequency and amplitude that is an approximate replica of the audio frequency signal supplied to amplifier 22.
In response to a conductive object causing an unbalance between coils 12 and 13, speaker 23 derives an aural tone having a constant amplitude and frequency,

F which is easily distinguishable by the ear from the frequency F To this end, the a.c. output of coils 12 and 13 is applied without phase shift via amplifiers 16 and 17, as one input to phase comparator 24, the other input of which is responsive to an output of oscillator 14. The output of oscillator 14 applied to phase comparator 24 has the same frequency as the sine wave applied by the oscillator to transmitting coil 11, but in phase shifted relative to the wave applied to the transmitting coil. The phase shifted wave applied to phase comparator 24 in response to the output of oscillator 14 functions as a switching voltage to open and close electronic switches in the phase comparator. Phase comparator 24 derives a d.c. voltage proportional to the phase angle of the a.c. output of variable gain, tuned amplifier 17 relative to the predetermined phase indicative of a ferrous object causing the unbalance of coils 12 and 13. Phase comparator 24 is arranged so that zero and maximum d.c. levels are derived by it in response to bodies respectively having a high magnetic permeability and high conductivity causing the unbalance.

The d.c. voltage derived by phase comparator 24 is applied to a variable threshold circuit 25, which derives a bilevel signal indicative of whether the threshold thereof has or has not been exceeded. The threshold of circuit 25 is adjusted by the operator as a function of the conductivity of the medium through which the magnetic field of the coil assembly is passing. 1f the medium has a relatively high conductivity, such as exists for a salt water background, the threshold level is set at a relatively high level because the medium causes a significant phase shift in the a.c. output of tuned amplifier 17. y

In response to the threshold of circuit 25 being exceeded, the circuit derives a binary one signal which is applied in parallel to a frequency control input terminal of relaxation oscillator 19 and gate 21. The binary one signal applied to oscillator 19 activates the oscillator so that the oscillator derives the frequency F with a binary zero signal fed by threshold circuit 25 to the oscillator, the oscillator derives the F frequency. The binary one signal fed to gate 21 drives the gate to a full open condition, whereby the F frequency of oscillator 19 is passed through the gate at constant amplitude to audio amplifier 22 and speaker 23. The d.c. variable gain control signal derived from rectifier 18 has no effect on the signal passed through gate 21 when the binary one signal is applied to the gate by threshold circuit 25.

To provide a visual indication of the phase shift resulting from a conductive body being in the monitored region, the d.c. output of phase comparator 24 is fed to d.c. voltmeter 26. Voltmeter 26 provides a quantitative signature of the electrical and physical characteristics of the detected object, thereby aiding in the identification thereof. When the signal level is above a certain threshold the phase information is independent of distance. in response to a pure ferrous target, there is a null reading of phase indicating meter 26, but the inten sity of an aural tone is varied in accordance with the imbalance caused by the target. To locate and identify an object the coil assembly is scanned using conven tional scan techniques and an operator observes the frequency and amplitude of aural tones, as well as the meter output.

Reference is now made to FIG. 2 of the drawing wherein there is illustrated a detailed circuit diagram of the system illustrated in FIG. 1. The circuit of FIG. 2 is powered exclusively by a 6 volt, d.c. battery source 31 connected between plus terminal 32 and ground terminal 33. The 6 volt source between terminal 32 and 33 supplies collector emitter potential for the transistors included in the various circuits, each of which transistors is of the NPN type unless otherwise indicated.

Oscillator and power amplifier 14 comprise an oscillating stage comprising transistor 34 connected in a modified Colpitts configuration including collector tuned circuit 35, whereby audio frequency oscillations are derived and a.c. coupled via capacitor 36 to emitter follower transistor stage 37. A.C. voltage developed across load resistor 38 of emitter follower 37 is a.c. coupled via capacitor 39 to primary 40 of transformer 41, the secondary 42 of which drives a push-pull power amplifier 43 comprising transistor 44 and 45 and output transformer 46. Shunting primary winding 47 of transformer 46 is capacitor 48, whereby a sine wave at the oscillation frequency (in one embodiment 7 KHz) of transistor 34 is developed across transformer secondary winding 49 which is connected via a shielded cable 51 to transmitting coil 11. The shield of cable 51 is connected to ground tenninal 33.

Pickup coils 12 and 13 are connected to nulling circuit'*15 via a shielded cable 53 having a shield connected to ground terminal 33 and a common, center point for the two pickup coils. Opposite ends of the pickup coils 12 and 13 are connected to opposite ends of primary winding 54 of transformer 55 via leads 56 and 57 included in shielded cable 53.

Nulling circuit includes a resistive voltage divider means comprising slide wires 58 and 59 having variably positioned taps 61 and 62. Opposite ends of slide wires 58 and 59 are connected to shunt leads 56 and 57. Taps 61 and 62 are connected to the shield of cable 53 via resistor 64 and capacitor 65, respectively. Resistor 64 and capacitor 65 respectively introduce zero and 90 phase shifts for the a.c. voltages at taps 61 and 62, as coupled to the shield.

The circuitry associated with each of slide wires 58 and 59, in effect, comprises two bridge circuits to provide nulling for the resistive and reactive impedance unbalance between pickup coils 12 and 13, which may occur because of temperature variations or the inability to initially fabricate the pickup coils to have exactly identical impedances. Slider 61 is positioned so that it, in effect, provides a resistive balance for the real impedance component of the unbalance between pickup coils 12 and 13. Tap 62 is positioned to compensate for the phase error introduced by the reactive or imaginary impedance component unbalance between pickup coils l2 and 13. The phase unbalance is compensated by a 190 phase lag introduced by capacitor 65 and divider 59. Capacitor 65 provides a 90 phase shift, but since the capacitor is connected across both of coils 12 and 13 via potentiometer 59, the movement of tap 62 from one side of the other yields an effective i90 phase shift, enabling the nulling of both inductive and capacitive offsets.

Sliders 61 and 62 are adjusted so that if neither a ferrous nor a conductive body is in the magnetic field coupling transmitting coil 11 with receiving coils 12 and 13, a zero voltage is derived between leads 56 and 57. In response to an unbalance in the magnetic field due to eddy currents being induced in a conductive body, a finite voltage is developed between leads 56 and 57 and coupled to primary winding 54 with a phase which varies as a function of the conductivity, geometry, orientation, and possibly separation of the conductive body from the coil assembly. The phase is a function of separation only if the amplitude exceeds the threshold established by circuit for amplitude within the threshold limit, phase is essentially independent of distance between the coil assembly and target. In response to a ferrous body causing an unbalance, a finite voltage is developed across leads 56 and 57 at a predetermined phase; the amplitude of the voltage is dependent upon the geometry, target orientation and separation of the ferrous body from the coil assembly.

The nulled signal developed across primary winding 54 is a.c. coupled via transformer 55 and its secondary winding 62 to fixed gain preamplifier 16 that comprises common emitter transistor 61. The collector of preamplifier transistor 61 is connected via a.c. coupling capacitor 64 to the base of variable gain, tuned amplifier 17, which comprises common emitter transistor 63, 75, and 76. The gain of transistor stage 63 is determined by the setting of potentiometer 65 which is connected between one electrode of capacitor 64 and ground. Potentiometer 65 includes a variably positioned slide wire 66 that is a.c. coupled to the base of transistor 63 via coupling capacitor 67.

The position of slider 66 is adjusted as a function of the magnetic permeability of a host medium being scanned by the coil assembly. In a host medium having a high magnetic permeability, slider 66 is adjusted so that a small portion of the voltage developed across potentiometer 65 is fed to the base of transistor 63, whereby the possibility of driving the transistor into saturation because of the permeability of the medium is prevented, while enabling detection of certain ferrous and conductive bodies in the medium. If the host medium has very low magnetic permeability and it is desired to have the greatest possible sensitivity, the position of slider 66 is adjusted so that the entire voltage developed across potentiometer 65 is coupled to the base of transistor 63.

Tuning is provided for the filter of the variable gain cascade amplifier 17 comprising transistors 63, 75, and 76 by a tank circuit including inductance 71 and capacitor 72. The voltage developed across the tank circuit is shaped by back-to-back parallel clipping diodes 73 and 74 and a.c. coupled to the base of a further transistor stage comprising transistor 75. The output voltage developed across the collector of transistor stage 75 is a.c. coupled to an additional transistor stage comprising transistor 76; the circuitry associated with transistor stage 76 is identical with that associated with stage 75. If the essentially sine wave variations at the collector of transistor 63 exceed a predetermined level, the clipping diodes of stages 75 and 76 shape the variations into square waves that enable phase comparator 24 to function more accurately and independently of voltage magnitude.

The a.c. voltage developed at the collector of transistor 76 is coupled to half wave rectifier 18 via coupling capacitor 77. Rectifier 18 includes shunt diode 78 and series diode 79 which are poled to shunt each negative half cycle to ground terminal 33 and pass each positive half cycle to a low pass filter. The low pass filter comprises the parallel combination of resistor 81 and capacitor 82, across which is developed a dc. voltage proportional to the amplitude of the a.c. voltage derived from the collector of transistor 76.

The a.c. voltage derived at the collector of transistor 76 is also fed through capacitor 77 to one input of phase comparator 24, the other input of which is responsive to a replica of the a.c. voltage fed to transmitting coil 11, as derived from the collector of transistor 45. Phase comparator 24 includes a 90 phase shifter 83 that is d.c. coupled to the collector of transistor 45 and comprises a pair of cascaded phase lag circuits comprising series resistors 84 and 85 and shunt capacitors 86 and 87. The 90 phase shifted voltage derived from phase shifter 83 is d.c. coupled to the base of common emitter, PNP transistor 88, the collector of which is do coupled to the emitter of PNP transistor 89.

During alternate half cycles of finite voltages coupled to transformer winding 54 by pickup coils 12 and 13, the emitter collector path of transistor 89 is driven into a conducting state. To this end, the voltage coupled through capacitor 77 is a.c. coupled to the base of transistor 91, the collector of which is d.c. coupled to the base of transistor 89. During the half cycle when diode 79 is activated into a conducting state the collector voltage of transistor 91 is negative to enable the emitter collector path of transistor 89 to be forward biased. depending upon the phase of the voltage applied to the emitter thereof. Hence, there is developed at the collector of transistor 89 a dc. voltage having a magnitude indicative of the phase displacement between the two ac. signals applied to the phase comparator. Sufficient gain is provided by the circuitry to enable the amplitude of the dc. voltage developed at the collector of transistor 89 to be virtually independent of the amplitude of the finite voltage developed between pickup coils 12 and 13. The voltage at the collector of transistor 89 is do coupled to do. voltmeter 26 via resistor 90.

The dc. voltage developed at the collector of transistor 89 is coupled to adjustable threshold circuit 25 that includes a slide wire 82 which is connected between the collector of transistor 89 and ground. Slide wire 92 includes a variably positioned tap 93 across which is connected smoothing capacitor 94, so that the voltage developed across capacitor 94 is indicative of and proportional to the average voltage developed at collector 89. Tap 93 is set on slide wire 92 at a position determined by the conductivity of the medium being monitored by the coil assembly. For a highly conductive host medium, which has a tendency to introduce a significant relatively constant phase shift, slider 93 is positioned so that a relatively small portion of the voltage developed across potentiometer 92 is coupled to capacitor 94; if the host medium has a relatively low conductivity and great sensitivity is desired, slider 93 is positioned so that the maximum voltage developed across slide wire 92 is coupled tov capacitor 94.

The voltage developed across capacitor 94 is fed to a two stage, high gain d.c. amplifier comprising transistor 95 and PNP transistor 96, the base of which is connected to the collector of transistor 95. The base of transistor 95 is connected to be responsive to the voltage developed across capacitor 94, whereby transistor 95 is driven into a conducting state in response to the voltage across capacitor 94 exceeding a predetermined level, determined by the phase difference between the inputs to phase comparator 24 and the position of slider 93. The base of transistor 96 is biased and connected to the collector of transistor 95 so that the former transistor is driven into saturation and cutoff in response to conduction and cutoff of the latter transistor. In response to transistor 96 being respectively saturated and cutoff its collector swings from approximately the plus dc. voltage at terminal 32 to ground.

The bilevel signal voltages developed at the collector of transistor 96 control the oscillation frequency of relaxation oscillator 19. Oscillator 19 includes unijunction transistor 101, having an emitter 102 connected to charging capacitor 103 and bases 104 and 105 which are connected in series between plus 6 volt terminal 32 and ground 33 via load resistor 106. Constant bias for emitter 102 is provided by connecting the electrodes of capacitor 103 to the plus 6 volt source at terminal 32 via resistor 107 and 108. Variable bias is provided by connecting the electrodes of capacitor 103 to the collector of transistor 96 via resistors 109 and 110. In response to the collector voltage of transistor 96 being at ground potential, capacitor 103 is charged at a relatively slow rate so that a relatively low audio frequency (typically 1 R112) square wave is-developed across load resistor 106. In response to the collector of transistor 96 being at a relatively high level, additional charging current is provided for capacitor 103 to increase the oscillation frequency of transistor 101, whereby a second, higher audio frequency voltage (typically 1.5 KHz) is developed across load 106. Audio tones at l and 1.5 KHz are considerably easier to hear than 7 X112 tones coupled to the coil assembly.

The bi-frequency voltage developed across load resistor 106 of oscillator 19 is fed to the base of transistor 111 which is included in gate 21. The collector of transistor 111 is do coupled via resistor 112 to the collector of transistor 96 so that it can be forward biased in response to transistor 96 being driven into a conducting state. The forward bias applied to transistor 111 from the collector of transistor 96 causes the gain of transistor 111 to be relatively high and constant, whereby the 1.5 KHz signal is derived with constant amplitude across resistor 112.

The collector of transistor 111 can also be variably biased to a level dependent upon the amplitude of the d.c. voltage derived by rectifier 18. To this end, the collector of transistor 111 is do coupled via resistor 113 to the variable voltage developed across resistor 81 and capacitor 82 of rectifier 18. If transistor 96 is in a conducting state, the variable d.c. level fed to the collector of transistor 111 via resistor 113 has no effect on the gain of transistor 111 because the voltage developed across resistor 81 and capacitor 82 is lower than the voltage at the collector of transistor 96 and the low impedance of the transistor emitter collector path to the plus 6 volt source. However, in response to transistor 96 being cut off, whereby ground voltage is applied to one end of resistor 112, the gain of transistor 111 is controlled by thevoltage developed across resistor 81 and capacitor 82 such that the transistor gain increases as the voltage across resistor 81 and capacitor 82 increases, whereby the transistor output voltage is a direct function of the ac. voltage derived from coils l2 and 13.

The voltage developed at the collector of transistor 111 is ac. coupled via capacitor 112 to the base of transistor 113 which comprises audio amplifier 22. Connected in the collector circuit of transistor 113 is the primary winding of audio transformer 114, the sec ondary of which is connected to an electrical to aural transducer 23, which may be a loudspeaker and/or a set of head phones.

Reference is now made to FIG. 3 of the drawing wherein there is illustrated a perspective and partially phantom view of a portable instrument embodying the principles of the present invention and particularly adapted for use in detecting and distinguishing between sub-surface ferrous and conductive bodies. The device illustrated includes a cylindrical head portion 201, manufactured of a dielectric, non-ferrous material, such as plastic. Fixedly mounted inside of head portion 201 is dielectric, non-ferrous, temperature stable cylinder 202 which is preferably fabricated of certain glasses. Cylinder 202 is held in situ by suitable retaining ribs 203 (or foam insulation) which are fixedly mounted between it and the interior of head 201. Rigid polyurethane foam provides thermal insulation from the surface of housing 21 to cylinder 202 and the components thereon, as well as cushioning for mechanical isolation to prevent cylinder deformation or impact breakage. Cylinder 202 comprises a carrier for coaxial, vertically oriented transmitting and receiving coils 11-13.

Head 201 is fixedly secured to a hollow cylindrical wand 205 that is fabricated of a non-metallic material so that it does not distort the magnetic field of the assembly comprising coils 11-13. Wand 204 is divided into a lower, plastic segment 205 which is fixedly connected to an upper, non-magnetic, metallic segment 206, which may be fabricated from aluminum. The handle 217, consists of a segment of 206. Meter 26, which provides an indication of the phase angle amplitude derived from phase comparator 24, is mounted on the upper portion 206 of the wand to enable the operator to readily obtain'a visual indication of the phase shift caused by a conductive body.

Mounted below the upper portion of the metallic portion 206 of wand 204 is housing 209 for the electronic circuitry illustrated in FIG. 2. Housing 209 is fixedly mounted on wand segment 206 by clamps 208. Circuitry within housing 209 is connected to the assembly including coils ll-l3 and meter 26 via leads that run through the interior of hollow wand 204. Positioned on the upper surface of container 209 and beneath the lower surface of wand 204 are knobs 211 which control the settings of the sliders 58, 59, 66 and 93 which control nulling, amplitude sensitivity and phase sensitivity of the instrument. On-off switch 212 is also mounted on the upper surface of container 209 beneath wand 204. By positioning the control knobs 211 and on-off switch 212 between the upper surface of housing 209and the lower surface of wand 204, the possibility of the controls being inadvertently adjusted is materially obviated. Housing 209 also includes an electro-acoustic transducer, in the form of a loudspeaker (not illustrated) or a jack (not illustrated) particularly adapted for enabling a set of headphones to monitor the generated audio tone. Power for the on circuitry within housing 209 is provided by batteries located inside of wand 204.

In use, the operator grasps handle 217 with one hand and'with the other hand is able to adjust controls 21]. The instrument is swung at a relatively rapid rate by the operator as he walks over an area of interest until an aural tone is detected. In response to the aural tone being detected, the operator stops walking and slowly swings the instrument until the amplitude of the aural tone reaches a maximum or the deflection of mter 26 reaches a maximum. The operator is thereby apprised of the location and character (ferrous or conductive) of the body.

While there has been described and illustrated one specific embodiment of the invention, it will be clear that variations in the details of the embodiment specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims. For example, the sensing head can be altered with regard to geometry, configuration and size. Variations can be provided for the frequencies of oscillators l4 and 19. For certain types of targets, it may be desirable to use multiple coil excitation frequencies that may be manually selected or derived by frequency sweeping techniques. Multiple excitation frequencies may be simultaneously fed to the transmitting coil, in which case plural detection channels are provided, one for each of the excitation frequencys. It is also possible to reverse the variable amplitude relationship for the ferrous and non-ferrous indications, whereby the non-ferrous indication is derived as a tone of variable intensity and the ferrous indication is visually read from a meter.

1 claim:
1. Apparatus for detecting and distinguishing between ferrous and electrically conductive bodies comprising coil means, a constant frequency a.c. source connected to said coil means to excite said coil means so that the coil means generates an a.c. magnetic field, said coil means being arranged so that in response to: (a) neither a ferrous body nor a conductive body being in the field there is generated by the coil means a substantially zero a.c. voltage, (b) a ferrous body being in the field there is generated by the coil means a nonzero a.c. voltage having a predetermined phase relative to the phase of the a.c. excitation voltage for the coil means, and (c) a conductive body being in the field there is generated by the coil means a non-zero a.c. voltage having a phase displaced from said predetermined phase relative to the phase of the a.c. excitation voltage; detector means responsive to the magnitude and phase of the voltage generated by the coil means for generating a first, variable amplitude aural signal in response to the coil means generating non-zero a.c. voltage having the predetermined phase with respect to the a.c. excitation voltage and for generating a second aural signal in response to the coil means generating a non-zero a.c. voltage having a phase displaced from said predetermined phase, said first and second aural signals being at differing frequencies to produce tones aurally distinguishable from each other, said variable amplitude being responsive to the amplitude of the a.c. voltage.

2. Apparatus for detecting and distinguishing between ferrous and electrically conductive bodies comprising coil means, a constant frequency a.c. source connected to said coil means to excite said coil means so that .the coil means generates an a.c. magnetic field, said coil means being arranged so that in response to: (a) neither a ferrous body nor a conductive body being in the field there is generated by the coil means a substantially zero a.c. voltage, (b) a ferrous body being in the field there is generated by the coil means a nonzero a.c. voltage having a predetermined phase relative to the phase of the a.c. excitation voltage for the coil means, and (c) a conductive body being in the field there is generated by the coil means a non-zero a.c. voltage having a phase displaced from said pre-determined phase relative to the phase of the a.c. excitation voltage; means for selectively generating an output signal selectively having first and second different distinguishable frequency components, phase detector means connected to be responsive to the coil means generating a nonzero a.c. voltage having a phase differing by at least a pre-selected value from the predetermined phase for activating the means for generating the signal output so only the first component is derived thereby, said means for generating the signal output generating only the second component while the coil means derives an a.c. voltage having the predetermined phase, means for varying the amplitude of the second component in response to the amplitude of the voltage generated by the coil means, and output means responsive to the first component and the second component with the variable amplitude.

3. The apparatus of claim 2 wherein said frequencies are audio frequencies and said output means comprises means for generating distinguishable aural signals having tonal frequencies respectively corresponding to said first and second audio frequency components.

4. The apparatus of claim 3 wherein the phase detector means includes means for generating a voltage amplitude indicative of the phase difference of the voltage generated by the coil means relative to the predetermined phase, and further output means for visually indicating the phase indicating voltage amplitude.

5. The apparatus of claim 2 further including variable gain means connected between the winding means and the phase detector means, as well as the means for varying the amplitude.

6. The apparatus of claim 2 wherein the coil means includes an excitation coil means and first and second differential windings respectively disposed on opposite sides of said first coil means, a common terminal for one end of each of said differential windings; a nulling circuit including first and second terminals respectively connected to second ends of said differential windings, voltage divider means connected between said first and second terminals, said voltage divider means including a pair of taps, means for connecting one of said taps to said common terminal without introducing any substantial phase shift, and means for connecting the other of said taps to said common terminal with the introduction of approximately 90 phase shift.

7. Apparatus for detecting and distinguishing between ferrous and electrically conductive bodies comprising coil means, a constant frequency a.c. source connected to said coil means to excite said coil means so that the coil means generates an a.c. magnetic field, said coil means being arranged so that in response to: (a) a ferrous body-being in the field there is generated by the coil means a non-zero a.c. voltage having a predetermined phase relative to the phase of the a.c. excitation voltage for the coil means, and (b) a conductive body being in the field there is generated by the coil means a non-zero a.c. voltage having a phase of the a.c. excitation voltage; phase detector means responsive to the phase of the a.c. voltage generated by the coil means, a voltage controlled oscillator for generating a signal having a frequency controlled by the phase indication generated by the phase detector, and means for controlling the amplitude of the signal frequency generated by the oscillator in response to the amplitude of the a.c. voltage generated by the coil means.

8. The apparatus of claim 7 wherein the phase detector means includes means for generating a bilevel signal voltage indicative of the phase of the a.c. voltage deviating by at least a predetermined amount from the predetermined phase, said voltage controlled oscillator being responsive to the bilevel signal voltage.

9. The apparatus of claim 8 further including output means, and wherein the coil means is arranged so that in response to neither a ferrous body nor a conductive body being in the field there is generated by the coil means a substantially zero voltage, the amplitude control means including means responsive to the bilevel signal for feeding the frequency generated by the oscillator to the output means with substantially constant amplitude while one of the levels is derived, and means for feeding the frequency generated by the oscillator to the output means at an amplitude that tends to follow the amplitude of the a.c. voltage generated by the coil means while the other level is derived.

10. The apparatus of claim 8 further including means for at will controlling the pre-detennined phase.

11. The apparatus of claim 7 wherein the coil means is arranged so that in response to neither a ferrous body nor a conductive body being in the field there is generated by the coil means a substantially zero voltage, and means for feeding the frequency generated by the oscillator to the output means at an amplitude that tends to follow the amplitude of the a.c. voltage generated by the coil means while the other level is derived.

12. Apparatus for detecting and distinguishing between ferrous and electrically conductive bodies comprising coil means, a constant frequency a.c. source connected to said coil means to excite said coil means so that the coil means generates an a.c. magnetic field, said coil means being arranged so that in response to: (a) a ferrous body being in the field there is generated by the coil means a non-zero a.c. voltage having a predetermined phase relative to the phase of the a.c. excitation voltage for the coil means, and (b) a conductive body being in the field there is generated by the coil means a non-zero a.c. voltage having a phase displaced from said predetermined phase relative to the phase of the a.c. excitation voltage; phase detector means responsive to the phase of the a.c. voltage derived from the coil means for generating a bilevel signal voltage indicative of the phase of the a.c. voltage deviating by at least a predetermined amount from the predetermined phase, a voltage controlled oscillator for generating a signal having audio frequencies F, and F 2 in response to the phase indication generated by the phase detector, said frequencies F, and F being respectively generated in response to the first and second levels of the bilevel signal voltage, means for controlling the amplitude of the frequency F, signal generated by the oscillator in response to the amplitude of the a.c. voltage generated by the coil means while the first level is generated and for maintaining the frequency F signal amplitude constant while the second level is generated, and aural output means responsive to the variable and constant amplitude signals of frequencies F, and F said F, and F 2 frequencies being aurally distinguishable from each other.

13. The apparatus of claim 12 wherein the phase detector means includes means for generating a dc. output voltage approximately proportional to the phase of the a.c. voltage generated by the coil means, and a dc. voltmeter connected to be responsive to the dc. voltage for visually displaying an indication of the dc. voltage.

14. The apparatus of claim 12 further including means for varying the phase deviation required to change the level of the bilevel signal voltage.

15. The apparatus of claim 12 further including variable gain means connected between the coil means and the phase detector and the means for controlling the amplitude of the F, frequency signal.

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**Melbeta** · 01-11-2025, 03:52 PM

Make sure you read the PRG POSTS, they are hidden in the text....... I pulled them out, and stuck them below, so no one misses them.......
They are hidden inside the SQUARE beneath the name of Melbeta.........
MELBETA

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Phase Read Out Gradiometer...........

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