Popular Electronics, Feb. 1969

[WM6]

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:

I discuss here two RX coil. Maybe this is acceptable in miniature coil, but in such case deep will be miniature too. If our goal is deep sensitivity over existing VLF and PI solutions, two RX coil become a very awkward thing.

[mikebg]

I discuss here two RX coil. Maybe this is acceptable in miniature coil, but in such case deep will be miniature too. If our goal is deep sensitivity over existing VLF and PI solutions, two RX coil become a very awkward thing.

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:

[WM6]

The TX coil must be oriented so as to provide intensive and vertically oriented down to earth magnetic flux.

You mean physical oriented, like this?:

[mikebg]

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]

You mean physical oriented, like this?:

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.

[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:

Hi mikebg
CCO Metal Detector – New Seekers
Written by Barnaby Brown August 07, 2009
TO the best of the author’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 centimetres’ movement this way or that being permissible – 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.

[technos]

Hi mikebg
CCO Metal Detector – New Seekers
Written by Barnaby Brown August 07, 2009
TO the best of the author’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

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 for posting that one.

[mikebg]

Maikl,
CCO Metal Detector – New Seekers
"Written by Barnaby Brown August 07, 2009
TO the best of the author’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.

[WM6]

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 for posting that one.

Thanks technos, for all clarifications.

Can you explain what does you mean by terms "wide scanning detector". Wideband regarding search frequencies, covered search field or some others mean?

[WM6]

What about this bridge MD solution?

[technos]

Can you explain what does you mean by terms "wide scanning detector". Wideband regarding search frequencies, covered search field or some others mean?

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]

What about this bridge MD solution?

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."

[WM6]

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."

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?

[maikl]

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 for posting that one.

Hi technos. Coil Coupled Operation (CCO) Metal Detector:Search oscillator IC1a oscillates at around 500kHz, depending on the positioning of the coils on the search head. The presence of metal induces changes both in the inductance and coupling of the two coils, thereby inducing a shift in oscillator IC1a’s frequency.A second oscillator, IC1b, together with buffer IC1c, serves as a beat frequency oscillator, oscillating at around 2·7MHz. This higher frequency avoids frequency lock, and also increases sensitivity. An LC oscillator was chosen here for its stability, to avoid problems with drift. The CCO MD in postings # 39 and # 48.

[mikebg]

BB Metal Detector by Scarborough

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?