TX for metal detectors

[mikebg]

TX for metal detectors

1. Optimal power of TX

Despite homebrew is not popular recently for hams, my hobby is to design TXs, RXs and antennas for QRP amateur radio.
QRP means low TX power, maximal efficiency of TX circuit and exactly matched antenna. The RX for ham radio extracts faint signals masked by larger noise and interferences.

The RX in a sensitive MD (metal detector) also should extract masked signals and sensitivity also should be limited by noise. However at conventional MDs this is not so (because they are not designed by radio amateurs:-).
The TGT (target) signals at conventional MD are not so faint as in amateur radio, because they are not masked by noise and interference, but by AIR & GND - a signal combination generated by own TX.
"Generated by own TX" means if we increase the power of TX twice, the TGT signal will increase twice, and the combination AIR & GND also will increase twice, ie the modulation index remains the same. This is very bad for incompetent designers, but ideal for my QRP concept. I can can reduce the power of TX until MD starts to loose sensitivity because there are interference and noise in input. In my concept, the TX should be low power and efficient, because it drains current from battery. The weight of metal detector depends on weight of battery. In a low power and efficient TX we can use more lightweight battery.
So arises the question:
"How to determine the optimal minimum power of TX for MD?"
My answer is: "Maximal TX power is possible when there is ABC (automatic balance control) and AGC (automatic gain control). The ABC slowly try to compensate all signals in input of RX which have TX frequency. The AGC slowly try to increase the gain of RFA (radio frequency amplifier) to maximal allowed level. With correct operating ABC and AGC, we can reduce TX power until MD starts to loose sensitivity."
Since this is said too summary and not so easy to understand, with following representation I will try to explain the concept comprehensively and more visually.
The RFA should amplify received signal before demodulation with maximal possible gain. The possible gain of RFA is limited by signals in its imput without target. We can increase the gain of RFA until its output starts to saturate by undesired input signal. In the left side of attached image is shown the output when starts saturation by undesired AIR & GND signal in input. We should slight reduce this gain to avoid distortion.
There are mine detectors and some honebrew projects of MDs which use two buttons for manual compensation (see BALANCE below) of AIR & GND in input of RFA. If ideal compensation is possible, the gain of RFA can be increased until its output starts to saturate by undesired EMI (interference) and noise in input. In the right side of attached image is shown visual effect at wideband RFA when output starts to saturate only by noise in input. Despite EMI is zero, this is not the ideal case. The ideal solution is a narrow band RFA to reduce the input noise, then we can increase the gain.
Since depth of detecting depends on gain of RFA, every conventional metal detector should be improved with three buttons: two for BALANCE and one for GAIN as shown in other threads. However for best improvement should be used ABC and AGC instead manual controls.

If we upgrade with manual controls, what is the procedure for adjusting TX power?
We should add a control TX POWER, imagine a potentiometer with switch to disable TX as shown below. What is the correct algorithm for adjusting control TX POWER?

[mikebg]

The first step of adjustment

FIRST STEP: Connect an oscope to output of RFA (or design a saturation alert circuit:-).

The first thing we need for optimal adjustment of the TX power, is to have an alert or indicator for saturation of RFA output. This is necessary because we have a manual GAIN. (At AGC we have no saturation in output because the automatics so reduces the gain of RFA, that output voltage is always below saturation thresholds). At adjustments in the laboratory will connect an oscope to output of RFA to watch if there is distortion. However we search in the field with metal detector having manual GAIN control. We need to design visual and audible RFA saturation indicator.
I have read several instructions for use of metal detectors, which have manual GAIN control of RFA. However I not found design, which has indicator for RFA saturation. Experiments are recommended in manuals how operator to understand that the GAIN is too high and that he must turn back the GAIN control, however, so he can not achieve an optimal setting.
I think the sound alarm for saturation of the RFA is more useful than visual alert. So I designed a circuit diagram that triggered existing audio modulator in my detector. It creates level H at saturation of RFA output. If your audio modulator is triggering by level L of the control signal, you must change the type of transistor as shown below on the righthand circuit diagram.

[mikebg]

2. Disable TX and watch EMI

2. Set TX POWER to zero.
When TX is disabled, disappears AIR & GND signal, but remains EMI (interference) and noise. Now you can increase the gain of RFA to watch with oscope if EMI contains mains frequency and its harmonics.
The GAIN control should be able to increase amplification until output starts to saturate with EMI. The attached circuit diagram is of an incompetent designed RFA, which amplifies all frequencies starting with DC. If the circuit of your RFA seems so, you should add a capacitor in series to R1. Remove R5 because it generates noise, and remove R3 because is useless.

[mikebg]

For example Eagle Spectrum has manual GAIN control in 16 steps (realised with 4 electronic switches of U3 as shown below).
It has also manual TX POWER control but in 2 steps only (realised with transistor Q11 used as switch). For precise power control you should connect a potentiometer 1kohm in series to R151 and disconnet R152. To disable TX, disconnect wire leading from pin 4 of J1 to rail TX P1.
The oscope and SATURATION ALERT circuit should be connected to C4 (rail OUT 5).

[mikebg]

3. Decrease EMI, decrease generation of thermal noise, increase RFA gain

3. Decrease EMI, decrease generation of thermal noise, increase RFA gain.

After you have TX disabled and connected an oscope to output of RFA, you see amplified EM interference containing mains frequency and its harmonics. However, surely you not see noise because EMI is more large.

A. Decrease amplification of EMI.

If your RFA has an incompetent designed circuit diagram wich amplifies even DC (as shown in above), you should add a capacitor C2 (see the circuit attached here) to cut low frequencies. This also decreases thermal noise because narrows the amplified frequency band.
How to calculate capacitance of C2?
C2 forms cutoff frequency fc in spectral characteristic of gain, shown below as Bode plot. For calculation choose fc = (0.6 ... 0.7) f_tx. Then you can use the formula in attached image to calculate C2 at given R1. However in most projects R1 is not chosen correctly and it generates more noise than the resistance of RX coil. Often resistance of RX coil is choosen too low, which is useless but increases the weight of search head.

B. Decrease receiving of EMI.

To suppress interference in front end, you should use two RX coils connected in series but in opposite direction. This is known as TWIN LOOP configuration (difference sensor or planar gradiometer).

C. Decrease generation of thermal noise.

After decreasing of EMI is made, you should increase the gain to see on oscope thermal noise.
Let calculate the maximal gain of RFA in Eagle (see above posting). The first stage with U1 has gain for noninverting input
A1 = 1+(R2+R3)/R1 = 1+(12E3+43E3)/825 = 67.7 times or in decibels
Av =36.6dB.
The gain of second stage with U2A is maximal when are connected to COM all resistors R12||R13||R14||R15. This makes an equivalent resistance Re=533 ohm. Then the max gain of second stage is
A2 = 1+R8/Re = 1+4020/533 = 8.54 times or in decibels 18.6 dB.
Both stages amplify A1*A2 = 67.7*8.54 = 578 times or in decibels
Av = 36.6+18.6 = 55.2dB.
At suppressed EM interference you will see thermal noise because this is too high gain and you are not in the field (mountain:-) but in a room with oscope. For comparison , the RFA of a vintage "professional" metal detector in 80-ies C-Scope VLF.TR 1220B has gain only 9 times. This is preferable because its high TX frequency generates large GND signal.
We can increase the gain of every preamp builded with opamp by decreasing resistance of R1. However the change of R1 leads to a phase shift. In Eagle Spectrum, the timeconstant T= R1*C2 = 825*47E-9 = 39us forms cutoff frequency fc = 0.16/T= 0.16/39E-6 = 4100Hz which is near to TX frequency 6500Hz . To avoid phase shift we should increase C2. If for example we decrease R1 twice, we should increase C2 twice. For Eagle this is R1=410ohm instead 825 ohm, and C2=100nF instead 47nF.
Decreasing resistance of R1 decreases generated by it thermal noise.
In ideal case when we have a noiseless opamp, for minimal noise we should choose R1< coil r. For Eagle the r of RX coil is 100 ohm and should choose R1<100ohm. However we have a noisy IC NE5534 which generates noise density 4nV/rtHz as a 1000 ohm resistor.
We can choose R1 near to 1/5 of virtual "noise resistance" NE5534, ie 200 ohm without significant incresing thermal noise in input. Then no need of RX coil having so low resistance. If we design a RX coil suitable by noise for preamp with 5534, we can reduce the weight of winding by calculating so wire diameter that to achieve coil resistance 200 ohm. Then we can use R1=200 ohm and both resistances should not contribute significant to total thermal noise.
Here is the algorithm for front end design giving minimal weight of RX winding and optimal low noise.
1. Transform noise density from datasheet of used IC in resistance of a noise generating resistor Rn. I have posted in the forum a table for this purpose. Table shows that NE5534 is noisy as 1000 ohm resistor.
2. Choose RX coil resistance Rn/8 < r < Rn/2. For NE5534 this means 125 < r < 500. Since I use TWIN LOOP, each RX coil is 180 ohm which means total r coil = 360 ohm.
3. Choose R1 < coil r. Since NE5534 is too noisy, this rule is not so important. I choosed R1= 390 ohm, ie my R1 > coil and despite this is not optimal, it is used in every conventional metal detector (ecause there is AIR & GND signal).
In next step we will reduce the AIR & GND signal.

[simonbaker]

Another way to summarize some of these ideas is to say that "signal-to-noise" is the real factor we want to maximize. This means you need to do a good noise analysis to understand thoroughly the noise you are dealing with, and it also follows that adding power is not useful unless it improves the signal-to-noise ratio, and it makes sense to reduce power as long as the signal-to-noise does not suffer.

Also, if you find you can take advantage of more gain in the RFA (I wish), but the "null" signal is limiting the gain because of saturating the input, do you consider canceling the null electronically? I think you call the null signal the "air signal".


Just chiming in while waiting for coil varnish to dry... always find your ideas interesting.

-SB

[mikebg]

Also, if you find you can take advantage of more gain in the RFA (I wish), but the "null" signal is limiting the gain because of saturating the input, do you consider canceling the null electronically? I think you call the null signal the "air signal".

Just chiming in while waiting for coil varnish to dry... always find your ideas interesting.

-SB

Simon,
All signals existing in input of a conventional RX for MD are described in other thread:
http://www.geotech1.com/forums/showt...eferrerid=2910
In that thread is given the idea for improvements.

This thread describes how to make a conventional metal detector to operate with QRP TX because is useless to increase TX power.

Posting #1 explains the general concept: instead ABC and AGC we have 4 manual controls to adjust minimal TX power. Shown is how seems output of RFA when sensitivity is limited only by noise.

Posting #2 explains the first step of adjustment for minimal TX power. We need saturation alert circuit (if we have no AGC). Every metal detector with manual GAIN control must have a saturation alert.

Posting #3 explains the second step of adjustment when we watch the EMI signal. Shown is a bad circuit of an incompetent designed RFA. It amplifies all interferences and useless resistors increase noise in input .

Posting #4 shows as example a better designed front end which has manual controls GAIN and TX POWER mentioned in posting #1, but has no mentioned balance controls.

Posting #5 describes how to decrease:
- amplification of EMI,
- receiving of EMI and
- generation of thermal noise by correct design method.

Posting # 6 shows that you need an additional visual explanation. For this purpose, I made the following image. I'm waiting your questions for this image.

[Qiaozhi]

Simon,
All signals existing in input of a conventional RX for MD are described in other thread:
http://www.geotech1.com/forums/showt...eferrerid=2910
In that thread is given the idea for improvements.

This thread describes how to make a conventional metal detector to operate with QRP TX because is useless to increase TX power.

Posting #1 explains the general concept: instead ABC and AGC we have 4 manual controls to adjust minimal TX power. Shown is how seems output of RFA when sensitivity is limited only by noise.

Posting #2 explains the first step of adjustment for minimal TX power. We need saturation alert circuit (if we have no AGC). Every metal detector with manual GAIN control must have a saturation alert.

Posting #3 explains the second step of adjustment when we watch the EMI signal. Shown is a bad circuit of an incompetent designed RFA. It amplifies all interferences and useless resistors increase noise in input .

Posting #4 shows as example a better designed front end which has manual controls GAIN and TX POWER mentioned in posting #1, but has no mentioned balance controls.

Posting #5 describes how to decrease:
- amplification of EMI,
- receiving of EMI and
- generation of thermal noise by correct design method.

Posting # 6 shows that you need an additional visual explanation. For this purpose, I made the following image. I'm waiting your questions for this image.

Have you used any of these principles to build your own metal detector? If so, how about some photos and test results in the real world?

[maikl]

I have the same question as Qiaozhi. And if you have not done anything practical (but only in theory), put here the complete scheme. Maybe someone will have time to make practical and tell us how it works (good or bad).

[mikebg]

I have the same question as Qiaozhi. And if you have not done anything practical (but only in theory), put here the complete scheme. Maybe someone will have time to make practical and tell us how it works (good or bad).

Here is the complete circuit of TX:
http://www.geotech1.com/forums/showt...eferrerid=2910
See posting #6 in that thread.
For manual adjustable power of TX, replace R9 with potentiometer named "TX POWER".
Here is the circuit of RFA:
http://www.geotech1.com/forums/showt...eferrerid=2910
See postings #5 and 6 in that thread. Instead potentiometers BALANCE X, BALANCE Y, and GAIN are used automatic controls AGC and ABC.

People, I know that most of participants of this forum are builders, not designers. Builders need excellent designed circuit diagrams and pcb. However this is created by designers.
Some participants in this forum are creative designers. There are designers, who copy blocks and stages from existing well operating designs, but they try to modify and improve them. Designers need to know what is the ideal operating block diagram and formulas to calculate its parameters and components.
Note that my postings for designers are in section "General Electronics", not in section "Projects". The postings, which contains solutions of theoretical problems, relates to all existing and future projects. If someone understand described principles, he can improve every model metal detector.
Every hobbist in this forum may be assigned to a category:
- Exploiter, buying metal detectors to use them. Under the law of Murphy, he is not satisfied with their search capabilities so seeks for more new models.
- Experimenter who prefers to make tests. Although he does not like to think and analyze in advance, he was forced to do so because he wish to understand why his experiment is not successful.
- Builder seeking a design to begin its realisation. However, he has not enough knowledge to understand whether the project is well designed, what results can be obtained and whether it is worth the labor and materiel costs,
- Modifier, which is trying to improve something in his machine, but usually does not know what to modify and how in general should be improved, what are the theoretical maximum possible results,
- Collectoner who has so many models in his colection that he needs a building to make an exhibition showing the development of this technique over the years .
- Designer who has the necessary knowledge to design block and schematic diagrams. It is able to help other categories of hobbies, but they hardly understand him because they are accustomed to think as designers.
There are hobbists which belong to several of mentioned categories. They have serious family problems because they are always busy, do not pay enough attention to their children and their families, spend a lot of money on his hobby, which reflects badly on the family budget.
More than two years ago, ham radio designers have created a worldwide group for theoretical knowledge - (R)EMI group, to fill a serious vacuum in our hobby. I will quote how Carl Moreland describes this vacuum, but this will happen after I find his article.

[mikebg]

I will quote how Carl Moreland describes this vacuum, but this will happen after I find his article.

The paper is "Induction Basics" by Carl Moreland:

"Some hobbies, such as collecting stamps or coins, are educational, while other hobbies, like building models,tend to exercise creativity. Metal detecting is clearly on the side of educational, providing you take the time to learn about the items you find, and the context in which they are found. Metal detecting can also have a creative
side to it. Some folks enjoy building their own accessories, such as digging tools. A few even modify their
detectors, most often for ergonomics."

"But only a small handful get down and dirty into the electronics of the metal detector. Mostly because metal detector technology is not well-understood by the majority of users, and there is darn little information available on the technical operation of detectors. Add to this a reluctance by manufacturers to share technical information—even a tendency to provide misleading information, particularly in advertising—and you wind up with a majority of detector users having a wrong impression of how it all works."

"Compare this to another electronics-based hobby, amateur radio, in which many enthusiasts not only have a good working knowledge of the electronics, but often build and modify their own equipment, and with amateur radio
magazines routinely covering technical issues."

To fill this vacuum over the two years I have performed in this forum about the role of the spokesman for (R)EMI * group. For me was very difficult because I am not familiar with English terminology in the field of metal detektors, nobody helped me, I have no editor and postings had to be coordinated and approved in advance by several experts who don't know English. Now all that is past. I learned many things, but when a plan drawn up by the group end its operation last July Morning, reported was that it remained a lot of information that should be sent to the forum. I promised to finish quickly initiated topics, if two people helping me. But there was no wanting, so I still long time will post the forum theoretical information about EMI (Electromagnetic induction) and EMI (lectromagnetic interference :-)*.

The group had intended not to publish projects for builders, but I broke that rule, posting designed by me circuit diagrams for different stages and sections specific to MD projects. The group was very angry because of this imperfection found and principal shortcomings of my projects. They asked me not to publish my circuits for builders until my term is over. Now I am free from this rule and may participate as a designer for each project. I am not accustomed to working alone. When radio amateurs design, a tradition is the collective work because the transceiver is too complicated device. Most designers participate in the first stage - the analysis of block diagram. When the most suitable block diagram is established, we form groups, each of which designs own circuit diagram, but all designs have equal block diagram.
___
* The abbreviation (R)EMI means "Reinventing, rename, revision, redesign and rediscovering Electromagnetic
induction ".

[Qiaozhi]

Here is the complete circuit of TX:
http://www.geotech1.com/forums/showt...eferrerid=2910
See posting #6 in that thread.
For manual adjustable power of TX, replace R9 with potentiometer named "TX POWER".
Here is the circuit of RFA:
http://www.geotech1.com/forums/showt...eferrerid=2910
See postings #5 and 6 in that thread. Instead potentiometers BALANCE X, BALANCE Y, and GAIN are used automatic controls AGC and ABC.

People, I know that most of participants of this forum are builders, not designers. Builders need excellent designed circuit diagrams and pcb. However this is created by designers.
Some participants in this forum are creative designers. There are designers, who copy blocks and stages from existing well operating designs, but they try to modify and improve them. Designers need to know what is the ideal operating block diagram and formulas to calculate its parameters and components.
Note that my postings for designers are in section "General Electronics", not in section "Projects". The postings, which contains solutions of theoretical problems, relates to all existing and future projects. If someone understand described principles, he can improve every model metal detector.
Every hobbist in this forum may be assigned to a category:
- Exploiter, buying metal detectors to use them. Under the law of Murphy, he is not satisfied with their search capabilities so seeks for more new models.
- Experimenter who prefers to make tests. Although he does not like to think and analyze in advance, he was forced to do so because he wish to understand why his experiment is not successful.
- Builder seeking a design to begin its realisation. However, he has not enough knowledge to understand whether the project is well designed, what results can be obtained and whether it is worth the labor and materiel costs,
- Modifier, which is trying to improve something in his machine, but usually does not know what to modify and how in general should be improved, what are the theoretical maximum possible results,
- Collectoner who has so many models in his colection that he needs a building to make an exhibition showing the development of this technique over the years .
- Designer who has the necessary knowledge to design block and schematic diagrams. It is able to help other categories of hobbies, but they hardly understand him because they are accustomed to think as designers.
There are hobbists which belong to several of mentioned categories. They have serious family problems because they are always busy, do not pay enough attention to their children and their families, spend a lot of money on his hobby, which reflects badly on the family budget.
More than two years ago, ham radio designers have created a worldwide group for theoretical knowledge - (R)EMI group, to fill a serious vacuum in our hobby. I will quote how Carl Moreland describes this vacuum, but this will happen after I find his article.

In other words, you're saying that you haven't actually built any of these circuits and it's all theory?

You cannot simply lump people into one of your defined categories. Many members here are designers, builders, experimenters, modifiers and hobbyists.

[WM6]

"Compare this to another electronics-based hobby, amateur radio, in which many enthusiasts not only have a good working knowledge of the electronics, but often build and modify their own equipment, and with amateur radio
magazines routinely covering technical issues."

The group had intended not to publish projects for builders, but I broke that rule, posting designed by me circuit diagrams for different stages and sections specific to MD projects. .

Hi mikebg

you are doing a good work here, even it remain on someway disabled. Thank you.

Must say that I do not know any knowledgeable amateur radio hobbyst, that would confine itself only to design block diagrams and circuit diagrams for different stages.

This simply would not work, so all knowledgeable amateur radio hobbyst are at the same time, designers, builders, experimenters etc.

As long as ideas are tested in practice and then corrected according to the practical feedback, remain at the level of theoretical fantasies only.

This not mean that your theorethical hints are not very usefull, but if you can go an step further and merge your many individual schematic parts into one comprehensive scheme of MD for practical testing, then you become five star designer.

[mikebg]

WM6, I was elected for spokesman of the group for several reasons. What can make one person in a group of designers who have less knowledge of each of its members, who hates to make DX radio comunications because is nervous and difficult hears, does not know English, and as a designer prefers to copy foreign designs instead to think and make its own. The group decided that a such man may not be useful as nothing but as a spokesman. "He will use a machine translator and will learn something".
I really learned electricity. Note that to understand text in the following posting I learned what is difference between mutual inductance and self-inductance:
The term "inductance" is a shorted incorrect term for "self-inductance".
Mutual inductance has sign (plus or minus).
Self-inductance is specific case when mutual inductance is always positive.
Coupling coefficient at mutual inductance is less than unity and can be set to zero (induction balance).
Coupling coefficient at self-inductance is always unity. Self-inductance (monocoil) is specific case of mutual inductance, when AIR signal is maximal. From this follows that monocoil is the worst type sensor for metal detecting, because we need to set AIR signal to zero.
But let me continue with a description of setting for minimum TX power.

[WM6]

.
But let me continue with a description of setting for minimum TX power.

You are welcome.