TGSL Tuning & troubleshooting

[simonbaker]

Hi Q coil

Here's a comparison of a 24 gauge "high-Q" coil vs. dfbowers coil -- and the 24 gauge coil isn't shielded! Don't forget to add some balast or she'll list to port...

I originally was working on both TX and RX high-Q coils. Then I felt that using an off-resonance high-q coil for the RX coil could make a worse noise profile -- it amplifies noise at the resonant frequency. I think maybe also the increased gain gave me problems trying to get rid of motorboating and chatter. But it could be interesting in "on-resonance" designs -- however, makes phase shift even more difficult to stabilize.

So for now, main interest would be in just using it for TX coil. Let's see if anything interesting.

-SB

[mikebg]

You have my curiosity up on using a higher Q for the Tx coil.. I'm going to wind one tonight and test tomorrow using 26 gauge wire instead of 30 gauge. Stay tuned!

I have been reading a lot on the subject the past few days and it would seem like a common conclusion is that by increasing power, we gain more distance in bench testing and lightly mineralized soil, but we also increase noise and ground saturation as well.. and doubling the power gets us little return on the investment. Guess that explains my obervation with the Nautilus.

Instead of investing in more power, another thing to do is improve the S/N ratio. One way to do that is to run at full resonance and a high Q coil. I guess when winter rolls around and I'm cooped up, that's what I'm going to experiment with next.

In conventional metal detectors should not be used TX or RX tank having high Q-factor. We need complicated automatic controls to avoid unwanted problems created by the high Q.
Below is shown equivalent circuit diagram of a LC tank near to conductive ground. Soil conductivity imports in tank circuit a virtual impedance consisting of positive resistance r_gnd and negative inductance noted as -L_gnd. Both imported parameters are variable because depend on frequency and on distance coil – soil as shown in the circuit symbolicaly with arrows.
At high Q-factor of tank circuit, the coil resistance r_coil appears relatively small compared to the imported large r_gnd resistance, therefore we obtain a greater modulation index. At high Q of TX tank, side effects are suppressed more effectively with P-I controller as in TX circuit of Garrett. At high Q of RX tank, we need AGC and modifying the whole block diagram of detector. The reference voltage for synchronous demodulation should be extracted from received signal (not from voltage across TX coil as in conventional detectors).

[dfbowers]

In conventional metal detectors should not be used TX or RX tank having high Q-factor. We need complicated automatic controls to avoid unwanted problems created by the high Q.
Below is shown equivalent circuit diagram of a LC tank near to conductive ground. Soil conductivity imports in tank circuit a virtual impedance consisting of positive resistance r_gnd and negative inductance noted as -L_gnd. Both imported parameters are variable because depend on frequency and on distance coil – soil as shown in the circuit symbolicaly with arrows.
At high Q-factor of tank circuit, the coil resistance r_coil appears relatively small compared to the imported large r_gnd resistance, therefore we obtain a greater modulation index. At high Q of TX tank, side effects are suppressed more effectively with P-I controller as in TX circuit of Garrett. At high Q of RX tank, we need AGC and modifying the whole block diagram of detector. The reference voltage for synchronous demodulation should be extracted from received signal (not from voltage across TX coil as in conventional detectors).

Yes, good point. In fact I was reading about several different coils offered for Nautilus metal detectors and there were both high and low Q versions made.. not sure if one of them might have been a special order item or not.. but Low Q was offered as a solution for very bad ground. High Q can offer better peformance in mild ground.


http://www.findmall.com/read.php?39,1039662

Don

[simonbaker]

In conventional metal detectors should not be used TX or RX tank having high Q-factor. We need complicated automatic controls to avoid unwanted problems created by the high Q.
Below is shown equivalent circuit diagram of a LC tank near to conductive ground. Soil conductivity imports in tank circuit a virtual impedance consisting of positive resistance r_gnd and negative inductance noted as -L_gnd. Both imported parameters are variable because depend on frequency and on distance coil – soil as shown in the circuit symbolicaly with arrows.
At high Q-factor of tank circuit, the coil resistance r_coil appears relatively small compared to the imported large r_gnd resistance, therefore we obtain a greater modulation index. At high Q of TX tank, side effects are suppressed more effectively with P-I controller as in TX circuit of Garrett. At high Q of RX tank, we need AGC and modifying the whole block diagram of detector. The reference voltage for synchronous demodulation should be extracted from received signal (not from voltage across TX coil as in conventional detectors).

Yes good point that high-Q tx oscillator is affected more by ground and targets. Makes design more complicated no doubt -- potentially could even be useful. Perhaps there is a trade-off point, and maybe the spec we follow now is near optimal. Worth an experiment or two to check it out.

If you really trust your ground-balance circuitry, maybe the hi-Q coil not so big a problem?

-SB

[simonbaker]

GB, DISC effect on air tests

To get back to troubleshooting our anemic TGSLs, some hard data would be useful to understand how our DISC and GB pot settings affect depth. This would have to be collected by someone with an excellent working TGSL and, perhaps, a very low noise basement.

Some graphs would be useful:

1. Air depth vs. GB pot position. Assume DISC pot is at minimum.

2. Air depth vs. DISC pot position. Assume GB pot is at minimum.

3. Air depth vs. DISC pot position with GB pot position ground-balanced for ferrite.

Pot positions can be displayed in per cent.

To make this most useful, the following additional data should be taken:

1. Phase range of DISC sync pulse, relative to TX oscillator -- min and max phase values.

2. Phase range of GB sync pulse, relative to TX oscillator -- min and max phase values.

(Diagram needed to explain how phase measured, such as zero crossings, etc.)

3. Oscillator frequency.

4. RX coil resonant frequency (or RX inductance, resistance, and capacitor C6 values).

5. Null phase measurement (make sure to account for slope of zero crossing).

The point of this is that it seems with my TGSL circuits, the GB pot particularly can greatly reduce the air depth as it is turned toward max. Perhaps one problem is having a null signal phase that requires GB pot to be at very high end. So by arranging a different null phase, we can use a better GB pot setting, and maybe get better depth.

Of course this always means: find a low-noise area for testing! Otherwise, impossible to evaluate.

-SB

[mikebg]

Don,
I can not agree with opinions in cited postings that Hot Soil needs low Q coil. If the term "Hot Soil" means nonconductive ferromagnetic earth, then in practice we have no imported resistance. As shown in the equivalent diagram below, there is no significant imported resistance despite mineralisation in earth is in principle wrth lossy ferrites. The imported parameter is only a positive inductance, independent on frequency. You can test this with ferrite rod for AM antenna and then use for comparison a "Hot Rock". The problem is only soil conductivity and mainly in salty beach.

Simon,
the bad thing of TX modulation is that AIR signal becomes modulated as if it is an additional GND signal. However it has different phase and GND balance circuit can't eliminate both signals simultaneously. The effect in practice is that you can not adjust enough good ground balance.

[simonbaker]

Don,
Simon,
the bad thing of TX modulation is that AIR signal becomes modulated as if it is an additional GND signal. However it has different phase and GND balance circuit can't eliminate both signals simultaneously. The effect in practice is that you can not adjust enough good ground balance.

I'm not totally clear on that, can you describe more? What do you mean by air signal is modulated? What kind of TX modulation are you referring to?

Regards,

-SB

[dfbowers]

Don,
I can not agree with opinions in cited postings that Hot Soil needs low Q coil. If the term "Hot Soil" means nonconductive ferromagnetic earth, then in practice we have no imported resistance. As shown in the equivalent diagram below, there is no significant imported resistance despite mineralisation in earth is in principle wrth lossy ferrites. The imported parameter is only a positive inductance, independent on frequency. You can test this with ferrite rod for AM antenna and then use for comparison a "Hot Rock". The problem is only soil conductivity and mainly in salty beach.

Simon,
the bad thing of TX modulation is that AIR signal becomes modulated as if it is an additional GND signal. However it has different phase and GND balance circuit can't eliminate both signals simultaneously. The effect in practice is that you can not adjust enough good ground balance.

I will admit.. your concepts of import resistance are new to me.. I approach this this from the hobbiest perspective, although I have an EE degree I am severely out of practice! I'm on a learning curve so bear with me.

Other interesting reading on Q factor as it pertains to MD coils:
http://www.nexusdetectors.com/Scienc...detectors.html

[mikebg]

I'm not totally clear on that, can you describe more? What do you mean by air signal is modulated? What kind of TX modulation are you referring to?

When TX coil moves above conductive water or earth, there is amplitude modulation of TX current caused by r_gnd (see posting #572). When TX coil is connected in self-oscillating circuit (as in your case), equivalent LRC parameters of tank circuit determine oscillating frequency. Then the self-oscillator obtains an angle modulation caused by -L_gnd in addition to AM.

When TX coil moves above magnetic nonconductive soil, there is only modulation of imported inductance +L_gnd (shown with an arrow in posting #576). In this case the self-oscillating TX obtains only angle modulation.
The P controller (with JFET transistor in your circuit) can suppress only amplitude modulation (because uses a diode as amplitude detector).
The angle modulation of AIR signal is not important because synchronous demodulator can't see it. It is blind since uses reference voltage derived from TX. However the demodulator sees a remained AM of AIR signal because P controller can not whole suppress it (as this can make a P-I controller).
At ideal balance in RX input, the AIR signal is zero. Old mine detectors have two knobs to minimize AIR signal and this improves significant the properties. However for best results should be used ABC (Automatic Balance Control).
The modulation of TX signal is transfered in the GND signal as addition of its own modulation caused by change of distance coil - earth.

What we can make to solve these problems?
1. To use separate oscillator driving an amplifier loaded with TX coil.
2. To use P-I or P-I-D controller for amplitude stabilisation of TX.
3. To eliminate AIR signal usin manual balance with two knobs or ABC.
4. To suppress GND signal using RX coils placed in TWIN LOOP configuration.
5. To increase distance TX coil-earth placing it in almost orthogonal coil configuration.

[simonbaker]

I will admit.. your concepts of import resistance are new to me.. I approach this this from the hobbiest perspective, although I have an EE degree I am severely out of practice! I'm on a learning curve so bear with me.

Other interesting reading on Q factor as it pertains to MD coils:
http://www.nexusdetectors.com/Scienc...detectors.html

I'll try a quick paraphrase of what I think maybe mikebg is saying, in terms I'm more familiar with. Then he can correct and add ideas that may be new to me.

It sounds like he's saying the ground affects the TX coil in two main ways: 1) like ideal ferrite, and 2) a "dissipative" effect that saps energy, like a resistor.

My understanding is that anything that saps energy can be modeled as a resistor (including radiative energy loss) for our purposes here. It might be more intuitive to model the ground by a transformer where the TX coil is one coil, and the other coil is an imaginary coil connected to a circuit with some resistance (and possibly reactive components to be more complete).

Ideal ferrite, on the other hand, acts like a slug in a coil and simply modulates the inductance of the coil. In the real world we know that ferrite also dissipates some energy, and therefore really should be modeled by adding a small resistor in the circuit also.

So I think mikebg is saying that when the coil passes over just ferrite, the inductance of the coil effectively changes, causing a frequency change (he says angle) in the oscillator without a significant change in the amplitude of the signal? But are we talking current or voltage here?

Anyway, when the coil passes over conductive ground, it's like a resistor being added in series with the coil, which effectively drops the Q of the circuit and changes the amplitude of the signal (voltage or current?) without changing the resonant frequency and therefore not changing the oscillator frequency. I'm not sure it is quite that simple -- as mikebg is aware, an oscillator frequency is determined by the frequency at which the feedback phase shift is exactly 360 deg (or multiple) -- I'm not sure if the resistance is part of that phase shift equation even though it is not part of the resonance equation.

I'd better stop there.

-SB

[Qiaozhi]

I'll try a quick paraphrase of what I think maybe mikebg is saying, in terms I'm more familiar with. Then he can correct and add ideas that may be new to me.

It sounds like he's saying the ground affects the TX coil in two main ways: 1) like ideal ferrite, and 2) a "dissipative" effect that saps energy, like a resistor.

My understanding is that anything that saps energy can be modeled as a resistor (including radiative energy loss) for our purposes here. It might be more intuitive to model the ground by a transformer where the TX coil is one coil, and the other coil is an imaginary coil connected to a circuit with some resistance (and possibly reactive components to be more complete).

Ideal ferrite, on the other hand, acts like a slug in a coil and simply modulates the inductance of the coil. In the real world we know that ferrite also dissipates some energy, and therefore really should be modeled by adding a small resistor in the circuit also.

So I think mikebg is saying that when the coil passes over just ferrite, the inductance of the coil effectively changes, causing a frequency change (he says angle) in the oscillator without a significant change in the amplitude of the signal? But are we talking current or voltage here?

Anyway, when the coil passes over conductive ground, it's like a resistor being added in series with the coil, which effectively drops the Q of the circuit and changes the amplitude of the signal (voltage or current?) without changing the resonant frequency and therefore not changing the oscillator frequency. I'm not sure it is quite that simple -- as mikebg is aware, an oscillator frequency is determined by the frequency at which the feedback phase shift is exactly 360 deg (or multiple) -- I'm not sure if the resistance is part of that phase shift equation even though it is not part of the resonance equation.

I'd better stop there.

-SB

I think what mikebg is trying to say is simply this:

If you use a self-oscillating transmit circuit (like the TGSL) then the frequency of the oscillator can change as the coil is moved over the ground, even when there is no target present.
The solution is to use a forced transmitter, as any inductance change in the coil will not result in phase jitter of the TX signal. Any change in inductance will result in an amplitude change, but no phase-shift.

[mikebg]

Any change in inductance will result in an amplitude change, but no phase-shift.

Qiaozhi, the reverse is true:
Any change in equivalent inductance of LC tank circuit causes angle modulation when it is part of self-oscillating circuit. Any change in energy loss of LC tank circuit causes AM if there no a controller for amplitude stabilisation.
"Angle modulation" is a composite term in the field of signal processing, which means both frequency modulation and phase modulation, because one is derivative of the other.

[dfbowers]

I always wondered about the draw backs of using a self oscillating transmit circuit, and optimizing matching coils. Nevertheless, with the TGSL design, it works sufficiently.

Much appreciated is the model on how the ground can negatively influence the oscillator.

I have been contemplating an idea of substituting the Tx section from the Volksturm Sm circuit for some time since it's driven, for the TGSL colpitts oscillator. I would just have to clock it at 16 kHz instead if 8kHz. Any opinion on the benefits?

Don

[Qiaozhi]

Qiaozhi, the reverse is true:
Any change in equivalent inductance of LC tank circuit causes angle modulation when it is part of self-oscillating circuit. Any change in energy loss of LC tank circuit causes AM if there no a controller for amplitude stabilisation.
"Angle modulation" is a composite term in the field of signal processing, which means both frequency modulation and phase modulation, because one is derivative of the other.

You have misread my post. I was referring to the forced oscillator approach, not the TGSL circuit.

[porkluvr]

I think what mikebg is trying to say is simply this:

If you use a self-oscillating transmit circuit (like the TGSL) then the frequency of the oscillator can change as the coil is moved over the ground, even when there is no target present.
The solution is to use a forced transmitter, as any inductance change in the coil will not result in phase jitter of the TX signal. Any change in inductance will result in an amplitude change, but no phase-shift.

Here is one method for having a driven coil. The circuit is adapted from the Tesoro Lobo.

Main disadvantage being that there are two somewhat critical potentiometer adjustments to be made back and forth before you get best performance.

The RC components shown in this diagram are based on the LM555 spice model, but a LMC555 or TLC555 etc. should be used. I have not a good model for
either of those. Optimum RC values may be different for CMOS 555 versions.

If your tank coil-capacitor values put their natural resonant frequency outside of the LMC555 RC oscillator's range with the components shown, then R4
or C2 would need adjusted.

You'll have the same problem with instability in the discrimination phase opamp as before. I would deal with that by placing a 5pf or 10pf capacitor
across pins 1 and 2 of the opamp.

My experiments show me that by using something like 1.76mH (and 68nF for the capacitor) an increase in field strength can result. That might seem
unlikely considering that tank reactances are lower, but based on having relative field strength being proportional to the product
of coil turns TIMES the coil current, that's what I saw after simulating with those components,
looking at coil turns and coil currents, and doing some quick math. Maybe should consider the reduction in coil resistance from using
a lesser coil allowing higher current. Other than that I have no explanation. I used the coil calculator in MiscEl to determine how much wire
would be needed in various coils for comparison.

My statement of having higher possible field strength with lesser coil comes with the condition that a reasonable amount of distortion in the
Colpitts waveform be allowed or else no comparison can be made.
I need to say that before somebody reduces their emitter resistor to 500 ohms, has 25% distortion on the transmitter reference signal,
and then tells me I am full of baloney because they got more power.

edit: Doh. Now I see dfbowers post, so he has beat me to it. Also, look at the Fisher Goldbug. Its 19.2kHz is a little closer to the original TGSL frequency.
The goldbug uses about 1mH to resonate with 68nF.
I believe that the Tesoro Lobo (at 20kHz) uses about a 1.3mH tx coil and 47nF, but tank components values were omitted from the RE'd schematic that I have
seen (not intending to throw stones at Mr. Lahr, or whoever it was kind enough to make the drawing).

The "Coil Parameters" chart list the Lobo transmit coil as being 10mH and the receive coil as 1.3mH which I think is backwards, but I have no proof and hardly
anyone listens to me anyway. http://www.geotech1.com/cgi-bin/page...&file=info.dat

Change my first sentence to read "Here is ANOTHER method...".