TGSL Tuning & troubleshooting

[Qiaozhi]

This is video from last field test where I tested my pcb with dfbowers coil. My mods include "dfbowers mod" for increased sensitivity range, box enclosure to keep light out, trimming capacitor for RX coil tuning, jumper from audio ground to battery ground to help reduce supply rail pulses.

http://www.youtube.com/watch?v=Z8mVtXH4lKY

Sorry, the video is annoying and not good. I'm just showing it for the record. Traffic and wind noise bad. It's difficult to see where I'm waving the target -- I need a good target holder like dfbowers "merry-go-round" in his basement. The field test is proving difficult to do well. Need better location.

My conclusion is: well, my pcb occasionally can get detection around 30 cm I suppose you can say -- but quite delicate setting of pots. I feel dfbowers gets more consistent, fatter tone in range from 24 to 30 cm. Also, dfbowers has some GB pot setting, my GB pot is at minimum.

It is worth pointing out some differences in my PCB compared to dfbowers PCB, and then pondering which, if any, of these differences matter.

1. dfbowers uses etched PCB as per Ivconic layout. My pcb is hand-wired with 26 gauge buss wire and laid out on a 4.5 by 5.5 board. Possible disadvantages of my board -- higher noise due to spread out; higher buss resistance and coupling of signals such as large audio pulse; more chance of bad solder joints.

Future experiments -- thicker buss wire; change layout to make audio ground next to battery ground; etched board.

2. pot wires - dfbowers has shorter pot/switch wires, particularly disc pot. Possible advantage -- less noise pickup.

3. My pcb has test loops sticking up for probes -- possible disadvantage -- pickup noise.

4. dfbowers has the .1uF bypass capacitors that Ivconic added. I left those off my pcb for simplicity and spacing. Possible disadvantage of my PCB - signal coupling from buss rails to ICs. My feeling is that for the big audio pulse, the small bypass capacitors don't make much difference, but maybe important and maybe reduce motorboating effect.

Future experiments -- add bypass capacitors and assess the difference.

5. dfbowers uses 4.7k resistors, I use 5.1k resistors at front end of LF353. dfbowers may have about 8% higher gain at LF353. Significant? Don't know.

6. dfbowers uses back to back 10uf capacitors for the LM358 filter stage -- I use single non-polar 4.7uF cap. dfbowers has perhaps slightly slower response because of it -- signicant? Hardly seems possible, but...

7. dfbowers uses two transistors for darlington stage of audio amplifier -- I use a single darlington transistor. I don't see a significant difference there, but Ivconic has found the audio stage is very critical to the performance.

8. dfbowers uses two 9 volt batteries in series (18 volts), inside enclosure, for power supply. I use 12 volt AA battery pack external to enclosure, attached by about 9 inch cord. Perhaps my power cord picks up noise? Perhaps resistance of longer cord causes buss coupling? I would think not significant.

9. I have a 22k resistor in the oscillator where dfbowers has a 24k resistor. I could try changing that, but don't expect significant effect.

10. dfbowers I think has resistors to stabilize the unused LM358 op amp as per Ivconic's layout. I chose to stabilize it with a simple wire from the output to inverting input and grounding the non-inverting input, as per some examples I found in a reference. I hope that difference is not significant.

11. I have the "sb mod" (might as well call it that since I'm the only fool who uses it), which is a voltage divider from the oscillator output that feeds a reduced voltage to the discrimination and GB phase shift circuits. The purpose is to get rid of the "jaggie oscillations" in the DISC phase shifter. However, it is possible it introduces a small phase shift of its own (less than 5% probably), and also may reduce the GB phase shift range a little. It also slightly may couple the GB and phase shift pots a little due to the loading effect.

Future experiments -- I can jumper over this voltage divider to assess it's effect.

So some more fool for thought -- I mean food for thought -- to try to find those small critical things that make the TGSL circuit work as well as possible.

Again, as Qiaozhi says, the extra air depth may be of no value. I still want to use it as a basis of comparing PCB construction. Also, it may indicate higher quality of signal at the lower depths also.

-SB

One suggestion - have you tried comparing the noise level between PCBs at the inputs to the comparators (U106a pin 3 and U106b pin 5)?

[dfbowers]

This is video from last field test where I tested my pcb with dfbowers coil. My mods include "dfbowers mod" for increased sensitivity range, box enclosure to keep light out, trimming capacitor for RX coil tuning, jumper from audio ground to battery ground to help reduce supply rail pulses.

http://www.youtube.com/watch?v=Z8mVtXH4lKY

Sorry, the video is annoying and not good. I'm just showing it for the record. Traffic and wind noise bad. It's difficult to see where I'm waving the target -- I need a good target holder like dfbowers "merry-go-round" in his basement. The field test is proving difficult to do well. Need better location.

My conclusion is: well, my pcb occasionally can get detection around 30 cm I suppose you can say -- but quite delicate setting of pots. I feel dfbowers gets more consistent, fatter tone in range from 24 to 30 cm. Also, dfbowers has some GB pot setting, my GB pot is at minimum.

It is worth pointing out some differences in my PCB compared to dfbowers PCB, and then pondering which, if any, of these differences matter.

1. dfbowers uses etched PCB as per Ivconic layout. My pcb is hand-wired with 26 gauge buss wire and laid out on a 4.5 by 5.5 board. Possible disadvantages of my board -- higher noise due to spread out; higher buss resistance and coupling of signals such as large audio pulse; more chance of bad solder joints.

Future experiments -- thicker buss wire; change layout to make audio ground next to battery ground; etched board.

2. pot wires - dfbowers has shorter pot/switch wires, particularly disc pot. Possible advantage -- less noise pickup.

3. My pcb has test loops sticking up for probes -- possible disadvantage -- pickup noise.

4. dfbowers has the .1uF bypass capacitors that Ivconic added. I left those off my pcb for simplicity and spacing. Possible disadvantage of my PCB - signal coupling from buss rails to ICs. My feeling is that for the big audio pulse, the small bypass capacitors don't make much difference, but maybe important and maybe reduce motorboating effect.

Future experiments -- add bypass capacitors and assess the difference.

5. dfbowers uses 4.7k resistors, I use 5.1k resistors at front end of LF353. dfbowers may have about 8% higher gain at LF353. Significant? Don't know.

6. dfbowers uses back to back 10uf capacitors for the LM358 filter stage -- I use single non-polar 4.7uF cap. dfbowers has perhaps slightly slower response because of it -- signicant? Hardly seems possible, but...

7. dfbowers uses two transistors for darlington stage of audio amplifier -- I use a single darlington transistor. I don't see a significant difference there, but Ivconic has found the audio stage is very critical to the performance.

8. dfbowers uses two 9 volt batteries in series (18 volts), inside enclosure, for power supply. I use 12 volt AA battery pack external to enclosure, attached by about 9 inch cord. Perhaps my power cord picks up noise? Perhaps resistance of longer cord causes buss coupling? I would think not significant.

9. I have a 22k resistor in the oscillator where dfbowers has a 24k resistor. I could try changing that, but don't expect significant effect.

10. dfbowers I think has resistors to stabilize the unused LM358 op amp as per Ivconic's layout. I chose to stabilize it with a simple wire from the output to inverting input and grounding the non-inverting input, as per some examples I found in a reference. I hope that difference is not significant.

11. I have the "sb mod" (might as well call it that since I'm the only fool who uses it), which is a voltage divider from the oscillator output that feeds a reduced voltage to the discrimination and GB phase shift circuits. The purpose is to get rid of the "jaggie oscillations" in the DISC phase shifter. However, it is possible it introduces a small phase shift of its own (less than 5% probably), and also may reduce the GB phase shift range a little. It also slightly may couple the GB and phase shift pots a little due to the loading effect.

Future experiments -- I can jumper over this voltage divider to assess it's effect.

So some more fool for thought -- I mean food for thought -- to try to find those small critical things that make the TGSL circuit work as well as possible.

Again, as Qiaozhi says, the extra air depth may be of no value. I still want to use it as a basis of comparing PCB construction. Also, it may indicate higher quality of signal at the lower depths also.

-SB

Simon,

I might suggest trying the .1uF bypass caps, particularly for the LM308s. If TL071s or TL081s are used, they have internal bypass caps.

[simonbaker]

One suggestion - have you tried comparing the noise level between PCBs at the inputs to the comparators (U106a pin 3 and U106b pin 5)?

I think you have hit the nail on the head -- I haven't found a good way to do it because it is so low frequency, it needs a good averager.

But here's what I find, on my workbench, and it seems suspicious/auspicious...

I'm comparing my PCB to your PCB, both using your coil. No headphones plugged in, since didn't seem to influence readings.

Note: I've got two voltmeters and they don't give exactly the same results, so there is some confusion. Also, the noise seems very "non-stationary" and varies from minute to minute -- by the time I move the probes from my PCB to yours, it can be very different! So I simply have to watch for minutes at a time and get a sense of it.

But these seem like typical results:

My PCB:

1. Power off -- about 1.3 mV +- .1 mV (may depend on if caps discharged)

2. Power on -- about 30 mV +- 7 mV -- varies from time to time.

3. Power on, RX coil shorted 4 inch jumper -- about .3 mV +- .3 mV

Because of test 3, I generally felt that the PCB noise was not the limiting factor. But I should redo this test in the field.

Now, I have not found a nice way to jumper the RX coil in your PCB to do test 3 yet.

Here's typical for your PCB.

1. Power off -- less than 1 mV.

2. Power on -- 5 mV +- 5 mV. Sometimes there will be periods where it is maybe 13 mV +- 8 mV.

Sheesh -- now as I write it is around 44 mV +- 8 mV. It just won't stay put.


Well -- I get the feeling your PCB maybe gets 10 to 20 millivolts less noise than mine on my workbench with the coil connected, but it could be just illusion.

Observation -- hold on. I just observed that if I turn the DISC pot up with your PCB, the noise level seems to go from about 25 mV to around 50 mV (I think).

With my PCB, the noise seems higher but more constant across the DISC pot range.

I don't know if I'm getting anywhere, probably need to test in noise-free environment.

I'm wondering if possibly there is some oscillator noise, particularly amplitude modulation, being a factor here. Your PCB maybe has less modulation. Or perhaps your null point ends up different than mine so you get less null signal noise at low DISC pot settings, but as the sync pulse is moved, the AM noise shows up.

But why does it come and go? That sounds more like EMI, and EMI would be phase-random I would think, and not affected by DISC pot.

Wow, this is tough to get to bottom of. I'll keep trying. I may have to spend $$$ for one of those USB oscilloscopes that can really do some data averaging. I wish I could run our PCBs simulataneously to sample same noise, but not feasible for many reasons.

-SB

[dfbowers]

I think you have hit the nail on the head -- I haven't found a good way to do it because it is so low frequency, it needs a good averager.

But here's what I find, on my workbench, and it seems suspicious/auspicious...

I'm comparing my PCB to your PCB, both using your coil. No headphones plugged in, since didn't seem to influence readings.

Note: I've got two voltmeters and they don't give exactly the same results, so there is some confusion. Also, the noise seems very "non-stationary" and varies from minute to minute -- by the time I move the probes from my PCB to yours, it can be very different! So I simply have to watch for minutes at a time and get a sense of it.

But these seem like typical results:

My PCB:

1. Power off -- about 1.3 mV +- .1 mV (may depend on if caps discharged)

2. Power on -- about 30 mV +- 7 mV -- varies from time to time.

3. Power on, RX coil shorted 4 inch jumper -- about .3 mV +- .3 mV

Because of test 3, I generally felt that the PCB noise was not the limiting factor. But I should redo this test in the field.

Now, I have not found a nice way to jumper the RX coil in your PCB to do test 3 yet.

Here's typical for your PCB.

1. Power off -- less than 1 mV.

2. Power on -- 5 mV +- 5 mV. Sometimes there will be periods where it is maybe 13 mV +- 8 mV.

Sheesh -- now as I write it is around 44 mV +- 8 mV. It just won't stay put.


Well -- I get the feeling your PCB maybe gets 10 to 20 millivolts less noise than mine on my workbench with the coil connected, but it could be just illusion.

Observation -- hold on. I just observed that if I turn the DISC pot up with your PCB, the noise level seems to go from about 25 mV to around 50 mV (I think).

With my PCB, the noise seems higher but more constant across the DISC pot range.

I don't know if I'm getting anywhere, probably need to test in noise-free environment.

I'm wondering if possibly there is some oscillator noise, particularly amplitude modulation, being a factor here. Your PCB maybe has less modulation. Or perhaps your null point ends up different than mine so you get less null signal noise at low DISC pot settings, but as the sync pulse is moved, the AM noise shows up.

But why does it come and go? That sounds more like EMI, and EMI would be phase-random I would think, and not affected by DISC pot.

Wow, this is tough to get to bottom of. I'll keep trying. I may have to spend $$$ for one of those USB oscilloscopes that can really do some data averaging. I wish I could run our PCBs simulataneously to sample same noise, but not feasible for many reasons.

-SB

I have noticed noise at random times as well.. I have no idea where it comes from but can make a 5 - 6 cm difference in bench testing easy... NOT just with the TGSL.

Now that I have a revolving testor at a set distance to the coil, I can eliminate other variables in testing. I just turn everything on at different times and check.

[simonbaker]

followup

Don't trust cheap AC voltmeters too much.

I'm watching the noise on oscope, comparing dfbowers PCB and mine, switching his coil back and forth.

Still difficult because noise changes, but some observations:

1. dfbowers oscillator 13.70 kHz. My oscillator 13.82 kHz. Pretty close.

2. Looking at noise at U106 pin 3 at very slow sweep, it seems that dfbowers noise is more sinusoidal and lower frequency, mine seems higher frequency. When move dfbowers DISC pot up 70%, seems noise becomes higher frequency, maybe a little smaller. Noise seems to look sine-like in the 10 to 25 Hz range.

Both PCBs seemed to have similar noise amplitude. But hard to eyeball it, could still be 30 % different. But frequency difference seemed more noticeable.

Maybe AC voltmeter fooled by frequency -- measures higher frequencies more because of low frequency roll-off.

Could frequency diff be simply due to different oscillator frequencies, beating against 60 cycle power?

Maybe I learned nothing applicable to field tests.

-SB

[Qiaozhi]

Don't trust cheap AC voltmeters too much.

I'm watching the noise on oscope, comparing dfbowers PCB and mine, switching his coil back and forth.

Still difficult because noise changes, but some observations:

1. dfbowers oscillator 13.70 kHz. My oscillator 13.82 kHz. Pretty close.

2. Looking at noise at U106 pin 3 at very slow sweep, it seems that dfbowers noise is more sinusoidal and lower frequency, mine seems higher frequency. When move dfbowers DISC pot up 70%, seems noise becomes higher frequency, maybe a little smaller. Noise seems to look sine-like in the 10 to 25 Hz range.

Both PCBs seemed to have similar noise amplitude. But hard to eyeball it, could still be 30 % different. But frequency difference seemed more noticeable.

Maybe AC voltmeter fooled by frequency -- measures higher frequencies more because of low frequency roll-off.

Could frequency diff be simply due to different oscillator frequencies, beating against 60 cycle power?

Maybe I learned nothing applicable to field tests.

-SB

Can you also check the outputs of the comparators to see if they are oscillating?

As you can see from the TGSL circuit, the comparators are not using hysteresis to tame any instability (motor-boating) when the input hovers near the reference voltage.

[simonbaker]

Can you also check the outputs of the comparators to see if they are oscillating?

As you can see from the TGSL circuit, the comparators are not using hysteresis to tame any instability (motor-boating) when the input hovers near the reference voltage.

For these noise tests I used min-sensitivity and so the comparators were not motorboating -- however, they were being triggered by the noise. But the speaker does not sound because the pulses are too short to get through the next low pass filter.

My PCB, on my noisy workbench, does not fully motorboat with the RX coil active because the noise effectively breaks up the motorboating. If I short out the RX coil, then I can get a pretty clear motorboating at the max sensitivity.

I can't explain why, but in my field tests, I was able to get motorboating below the max sensitivity, which turned into steady tone at max sens. It's as if when I'm in the mountains, there is a bias on the comparator inputs. I previously determined it was due to the sunlight on the diodes, but this time I had it covered quite well -- just a small hole where the cable came out. Next time I'll plug that completely.

Oscillator Trimming
------------------

I tried trimming the oscillator frequency to see the effect. Nominal frequency is 13.82 kHz.

At 13.55 kHz I get a strong tendency to motorboat, more than at 13.82 kHz, but I'm not sure it's from feedback, or rather the noise signal -- or maybe a confluence of both. The noise signal looks like a lower frequency sine wave.

At 13.32 kHz, the circuit will not motorboat at any sensitivity setting. The noise chatter seems suppressed.

At 13.10 kHz, the circuit seems less noise, I can turn the sensitivity up much higher without chatter. Detection of a Euro seems cleaner and clearer also.

However, I didn't retrim the RX coil for each oscillator setting, so probably the gain was just dropping. To test that, I added 1 nF and retested and indeed noise signal was higher (probably due to higher gain). So not sure that proved much, although strong motorboating at 13.55 kHz seemed to indicate some interaction of noise spectrum with oscillator frequency.

-SB

P.S. The more I test, the more I find that slight changes in oscillator frequency make big changes in the noise characteristic, suggesting interaction with some signal source such as 60 Hz power.

But overall dfbowers seems to have better noise characteristics at LM308 output, smoother, smaller.

Also, dfbowers DC offsets are LM308 output are 1, 0 mV for DISC, GB. Mine are about 5.5, 4.5 mV, which could partly be reason I can get into motorboat region.

[porkluvr]

Can you also check the outputs of the comparators to see if they are oscillating?

As you can see from the TGSL circuit, the comparators are not using hysteresis to tame any instability (motor-boating) when the input hovers near the reference voltage.

Qiaozhi, wouldn't this 100k resistor be hysteresis feedback? It's only applied to the GEB channel but since this is a wired-And configuration it should apply to both channels. (?)

Simon, maybe you have a different configuration there, or an open resistor or diode?

[Wiltran]

TGS Transmit coil Frequency adjust

I am building TGS, and need to know how to fine tune freq. on TGS transmit coil. I tried shortening coil and can get to 14.359 khz.(freq. counter) The inductance meaured on a LCR meter is 5.9 MH. Is there anyway I can get it to 14.5khz? T he amount of coil turns turned out to be quit a bit less about 90t with 30AWG when tied and constrained.

[Qiaozhi]

Qiaozhi, wouldn't this 100k resistor be hysteresis feedback? It's only applied to the GEB channel but since this is a wired-And configuration it should apply to both channels. (?)

Simon, maybe you have a different configuration there, or an open resistor or diode?

I suspect that it's intended purpose, but it will not provide any hysteresis.

For a comparator used in the non-inverting configuration, the hysteresis voltage can be calculated with the following equation.
HYST = R1 * (VOH - VOL) / R2
where R2 is the positive feedback resistor.
In that case the hysteresis works out as zero volts, because R1 equals zero ohms.

Even if the PCB track connecting U105 pin6 to U106b pin 5 had a resistance of 0.1 ohms, this would only provide a hysteresis of 10uV, which would be completely lost in the noise.

[simonbaker]

apples, oranges

Qiaozhi, wouldn't this 100k resistor be hysteresis feedback? It's only applied to the GEB channel but since this is a wired-And configuration it should apply to both channels. (?)

Simon, maybe you have a different configuration there, or an open resistor or diode?

I have always believed that the 100k resistor is a "feedforward" path whose purpose was simply to make the TGSL audio louder for stronger targets. It effectively sets the pull-up voltage of the comparators to the output of the GB LM308, which is then passed through to the audio signal by the next stage. Note also that in the original TGS, the GB LM308 does not have the limiting feedback diode, so it's output range is potentially higher -- this corroborates the purpose of the feedforward resistor to modulate the audio volume. Also as you mentioned, it is only on one side of the "AND" logic and wouldn't be effective, so doesn't make sense for hysteresis.

Also, as Qiaozhi points out, the 100k resistor won't make a hysteresis feedback, primarily because the LM308 output is a low impedance "AC ground" that sucks away any feedback signal.

I haven't come to a conclusion about motorboating yet or if it is a problem, because generally noise is the limiting factor that keeps me from turning the sensitivity pot to a region where it motorboats. And it is perhaps true that all TGSLs will motorboat if operated at a certain threshold. But no doubt you can build the circuit to have less tendency to motorboat. Hysteresis could help that, but I'm not sure you want it for other reasons. On the other hand, I have often considered adding hysteresis just to lengthen the audio beep. But I think it might make the chatter more disruptive.

-------- noise personality ----------

My last fooling around, I mean tests, comparing the LM308 output noise between my PCB and dfbowers PCB revealed that the noise "personality" (how it looks on an oscilloscope) is extremely sensitive to the oscillator frequency.

I found by changing my oscillator frequency by merely 200 Hz caused the noise to look completely different. It went from about 20 Hz wave to about 2 Hz wave. It may not change the basic noise power, but it really looks different.

So to really compare noise between PCBs, I think I need to trim my oscillator to be as close to dfbowers frequency as possible. Otherwise it's just craziness.

So I need to rig up a more permanent trimmer for my PCB.

I'm also trimming my R6 capacitor in the RX coil "tank" to match closely also.

I'm going to look at the null signal also -- it should be similar phase and amplitude -- if not, something to dig into.

-SB

[simonbaker]

I am building TGS, and need to know how to fine tune freq. on TGS transmit coil. I tried shortening coil and can get to 14.359 khz.(freq. counter) The inductance meaured on a LCR meter is 5.9 MH. Is there anyway I can get it to 14.5khz? T he amount of coil turns turned out to be quit a bit less about 90t with 30AWG when tied and constrained.

First, you may not need to adjust the frequency -- you're probably close enough. But if you want to, you can try trimming capacitor C2 (.022uF) to a slightly lower value, or even C1 (.22 uF) to a slightly lower value. You'd have to rig up a custom capacitor to do that.

If your coil is not glued, of course you can keep taking off turns. But after gluing, shielding, etc., the frequency could still change a little.

Or, theoretically, you could leave the frequency where it is and slightly increase the capacitor C6 (parallel with RX coil) by a tiny bit (.3 nF?). Theoretically, this adjusts the RX coil resonance slightly lower to compensate for the lower oscillator frequency. But you probably won't notice the difference, and I'm not sure anyone knows the optimal RX resonant frequency relative to the oscillator frequency anyway.

dfbowers made an excellent TGSL at 13.70 kHz frequency. So the frequency is not magic by itself apparently. But it is always comforting to get it as close as possible.

-SB

[simonbaker]

trimming

Re-measured dfbowers oscillator freq with his coil = 13.72 kHz.

Trimmed mine to 13.72 kHz with kluged capacitor.

Meter only accurate to 10 Hz, so still not exact, but will try comparing now.

-SB

[porkluvr]

For a comparator used in the non-inverting configuration, the hysteresis voltage can be calculated with the following equation.
HYST = R1 * (VOH - VOL) / R2
where R2 is the positive feedback resistor.
In that case the hysteresis works out as zero volts, because R1 equals zero ohms.

I'll go along with that. I overlooked that there is a formula for hysteresis and that an input resistor is needed. I should glue the formula to my forehead so I won't forget it.

I have always believed that the 100k resistor is a "feedforward" path whose purpose was simply to make the TGSL audio louder for stronger targets. It effectively sets the pull-up voltage of the comparators to the output of the GB LM308, which is then passed through to the audio signal by the next stage. Note also that in the original TGS, the GB LM308 does not have the limiting.

-SB

Feed-forward makes sense. I've read that before. When it's late at night, my wishful thinking can sometimes lead to confabulation having no basis in reality.
That also holds true in the daytime, come to think of it.

Porkluvr, excuse me for joke
<snip>
Experiment showed, that TX circuit diagram of Eagle Spectrum has worst properties than

all other circuits we have tested with salty water. When operates at low power mode as

marked in posting #529, by lowering search head even to not enough conductive earth, the amplitude of oscillation so diminishes that change can be measured by a conventional AC voltmeter. This is because LC tank is pumped by constant current portions as in your circuit with 555 (posting #585). My circuit in posting #595 simply increases or decreases the current portion stabilysing amplitude across TX coil.

I wasn't mad. I was laughing too hard to be mad.

What you're suggesting is that the Lobo style system will lose power at the beach. You might be right but my "Plan A" PCB is already pretty far along in the design stage. I could hunt gold near my neighborhood without going to the beach.

I do have a plan B. I chose the Lobo drive circuit over that found in the Minelab Relic Hawk because it is simpler and uses less power.
But the RH circuit looks like a PID as you describe, and my experiments show that it is readily adaptable to the 14kHz range. It actually seems to work better above the "stock" 5kHz frequency because it NEEDS a high Q coil which is easy to devise at 14kHz. I do not know the math behind the PID controller but trial and error is a viable tool if I let LTspice do the heavy lifting.

[simonbaker]

similar noise personality

Adjusting my oscillator as close as possible to dfbowers made our two noise signals look much more similar.

So at least in my environment, you have to be very careful in trying to compare apples to apples. Oscillator frequency very important.

I still have the impression that dfbowers noise may be slightly less amplitude, but it is very difficult to tell -- in the time it takes to move the probes over and reconnect the coil, the noise character easily changes quite a bit. My oscillator has drifted down a little, maybe between 13.71 and 13.72 kHz.

However, the two PCBs now are more similar in measurements, including air depth test -- both fairly lousy due to noisy environment, so not useful really to compare here.

But let's assume dfbowers has lower noise signal at output of LM308. The noise clearly comes from the RX coil, because shorting the RX coil makes the noise go away. Why would his PCB detect a lower noise signal from the same coil? I don't know. A gain difference would explain it. I tried to match the RX resonant tank, but a difference there would make a different gain. We do have different resistors in the LF353 preamp (4.7k dfbowers, 5.1k SB), but his gain should be higher, not lower. His slightly lower capacitor at C14, C15 could boost gain a tad. Otherwise, I don't know -- and it may just be an illusion.

The best I can think of to truly measure this noise would be to hook up to a digital sampler and take averages, switching the probes exactly every 30 seconds or so for several hours. Not about to happen soon.

-SB