TGSL Tuning & troubleshooting

[simonbaker]

waveforms

This is for dfbowers and anyone with good working TGSL:

I am interested in seeing the waveforms on a scope for:

1. TX oscillator
2. Pin 1 LF353 (disc driver output)
3. Pin 1 of U102 LM393 (output to JFet gate) when disc pot is turned all the way up.

4. Also measurement of DC voltage at source of JFet TR4 (disc channel) when disc pot is turned all the way up.

Regards,

-SB

[Stefano]

Rx output was taken at preamplifier output (pin 7 IC 101a)
Thaks for hints sb, I'll try a.s.a.p.

[simonbaker]

Hi SB. I tried this afternoon: with 1 euro unfortunately not much more: 18-20 cm. And I've built a 10" DD, not a 8" !!!
I also noticed that: If the Rx resonance freq. is higher than Tx (as it is supposed to be in the TGSL design) the phase at null is always negative (so not +20° but instead -20/-40°). If the Rx freq. is lower, the phase at null point is positive.
With negative phases I was able to reject ferrite and disc worked fine, with positive I had more noise and disc and geb were sort of "out of tuning" and didn't work correctly, but got more depth.
Pay attention that 1KHz less as Rx resonance (13.5 Kz) can be obtained only with 20 and more nF, so I think too much far from the given 15nF value!!!
And also: shifting from a higher to a lower rx freq than tx one should reverse coil leads.

Apart of understand how coil must be set I might have another issue in my circuit around U106, because I noticed that turning the sensibility trimmer had no effect, but I will investigate on it in future. I hope to better improve the depth there...

Usually null is taken slightly to side of minimum point, not exactly on it. I think Qiaohzi said that you choose the side with more coil overlap. Is that what you do?

- Is there any metal near your coil (including oscilloscope, table legs, etc.) when you null?

- Where do you measure Rx voltage and phase at?

I have also played with rx resonant freq lower than tx by raising tx freq higher (17 - 20 kHz). I noticed similar need to reverse leads. And detector worked, even though a bit different! I don't know what it means though at this point, if there is a strong reason that Rx resonance should be greater than Tx freq or not. Maybe both designs possible.

Sensitivity pot at U106 is tricky. Sometimes you just notice more noise beeps when you turn it. Also, it requires precision op amps for U104 because their quiescent output should be very near zero volts. You can check by disconnecting RX coil. Outputs of U104 should have DC voltage very near zero (hopefully within a millivolt).

In my opinion, a good sensitivity circuit should be able to make noise at some point when turning pot. If you have no noise ever, then perhaps you can improve the gain somewhere.

Note: the sensitivity pot circuit has minimum threshold of about 5 millivolts (I think) at input of LM393, so you cannot turn it more sensitive than that (unless you jumper the 1k resistor R37).

-SB

[Stefano]

Hi SB. I tried this afternoon: with 1 euro unfortunately not much more: 18-20 cm. And I've built a 10" DD, not a 8" !!!
I also noticed that: If the Rx resonance freq. is higher than Tx (as it is supposed to be in the TGSL design) the phase at null is always negative (so not +20° but instead -20/-40°). If the Rx freq. is lower, the phase at null point is positive.
With negative phases I was able to reject ferrite and disc worked fine, with positive I had more noise and disc and geb were sort of "out of tuning" and didn't work correctly, but got more depth.
Pay attention that 1KHz less as Rx resonance (13.5 Kz) can be obtained only with 20 and more nF, so I think too much far from the given 15nF value!!!
And also: shifting from a higher to a lower rx freq than tx one should reverse coil leads.

Apart of understand how coil must be set I might have another issue in my circuit around U106, because I noticed that turning the sensibility trimmer had no effect, but I will investigate on it in future. I hope to better improve the depth there...

[simonbaker]

I had little time for testing my TGSL last night and I also noticed the same thing.
My homebuilt coils are 5.7 and 6.3 unshielded at the moment. I played with the Rx tuning cap in a wide range trying to find the value wich gave me the +20° offset at minimal nulling. I found that what I wanted was just the minimum residual voltage, not the correct phase. In my case I was able to get a good and stable working even with -20°/-40° if coils adjusted for right nulling!
In neither case I was able to get more than 16-18cm for a 10 euro cent coin (it's a small coin), but I could reject ferrite in all metal with the trimmer set to about mid-range and disc mode fully rejected iron.
By changing the cap to have a closer tx-Rx tuning freq I could get some 2-3 cm more but it became extremely noisy and couldn't reject ferrite anymore and got iron always detected in disc mode, so I suppose that this means that the phase behaviour of targets changes with tx-rx tuning frequency.

That makes sense to me what you say.

I think many things can affect the null signal phase, such as nearby metal while you are nulling, the type of shielding, various capacitance of cable, coils, etc. So I suspect the exact phase of null signal is not critical, better to null by other criteria as you found.

For TGSL, the null phase does has some importance because if completely wrong, it will cause a negative voltage at the capacitors C15, C12 at the source side of the JFet detectors. This makes it hard for the JFets to work properly because they might be forward biased part of the time.

I hope to experiment some more with Tx, Rx frequency closeness, but I think the specs for TGSL are as good as possible without changing the design.

16 -18 cm seems pretty good for small coin. What do you get for 1 Euro?

-SB

[Stefano]

If you get near RX resonance, the phase shifts dramatically with small frequency changes. The discrim. circuit would need to be redesigned because any target signal near resonance is offset quite a bit

I had little time for testing my TGSL last night and I also noticed the same thing.
My homebuilt coils are 5.7 and 6.3 unshielded at the moment. I played with the Rx tuning cap in a wide range trying to find the value wich gave me the +20° offset at minimal nulling. I found that what I wanted was just the minimum residual voltage, not the correct phase. In my case I was able to get a good and stable working even with -20°/-40° if coils adjusted for right nulling!
In neither case I was able to get more than 16-18cm for a 10 euro cent coin (it's a small coin), but I could reject ferrite in all metal with the trimmer set to about mid-range and disc mode fully rejected iron.
By changing the cap to have a closer tx-Rx tuning freq I could get some 2-3 cm more but it became extremely noisy and couldn't reject ferrite anymore and got iron always detected in disc mode, so I suppose that this means that the phase behaviour of targets changes with tx-rx tuning frequency.

[erolunall]

Hi İvconic

I think everybody miss you & your excellent works over MD.

Geotech forums are going very slowly without you.

Best Wishes
Erol

[nakky]

Substantially; what is speaker? Coil! Coil driven by low frequency power output. So you have search head on one side and speaker ("tx-ed" coil) on the other side of pcb.... 3 divisions (by 4024) from main TX frequency are passed to audio out and "TX-ed" through "coil" (speaker)....fine..Only problem is in fact that those 3 divisions are in direct (harmonical) resonance with main frequency..... See what is the problem? I faced that long time ago...maybe at my second or third tgsl. Eversince than i tried to overcome it. TGS//TGSL is such concept and nothing we can do to overcome that issue....unless apply some major changes and possible change its charactrer too. Compromise is to pack it in metal enclosure and assure 90 degreees oriented relation between speaker and the rest of "hot" electronics. Couple ferrite perles will not harm to; tied on speaker leads...etc...etc...

I am still alive and i am still following this subject....but will not take a part in discussions ...unless if those were started by members i do respect for real (like this was the case)...
Cheers!

Hi Ivconic,happy to see you.

Grt Nakky

[simonbaker]

Oh yea, I do remember the post by Ivconic on the phone jack thing.. I have been using headphones with TGSL #1 so I forgot the symptoms I had while testing it until I build TGSL #2 and started experimenting with a hardwired speaker again. Now, is the reason that Tesoro uses off resonance coils to avoid funny things like this as well? Maybe when their final products are assembled and put into enclosures the synchronous audio is not a problem but testing on a bench can be? I guess by solving one problem and creating clean audio by driving the audio from the oscillator they can cause the potential for another.. Anyway, this probably doesn't matter if we all finish our projects.

Don

If a circuit doesn't work until it is on a minimal PCB and in an enclosure, I always wonder how the first designs were developed (you really got to trust your theory) - being a putzer, I like stuff that works on the breadboard, but MD circuits seem too sensitive to go too far that way, although I do have a mediocre version of the TGSL working on a solderless breadboard -- still very useful for some tests.

Yes driving audio from oscillator turns out to be really smart because most noise, spikes, etc from audio/speaker gets turned into a constant voltage by synchronous detector. Yet there is obviously some kind of feedback that causes those beeps - I'm thinking the "constant voltage" I referred to (from emi when speaker sounds) appears initially as a pulse that travels nicely through the very narrow low-pass amplifier section (approx 15 Hz low pass filter), and causes hiccuping if the gain is high enough. Sort of a whole separate sub-band feedback path.

By "off-resonance" coils, if you mean RX coil resonance being quite higher than TX oscillator -- I think reason for that is phase stability. If you get near RX resonance, the phase shifts dramatically with small frequency changes. But boy do you get a lot more gain! Ultimately I'm trying to work out some way to take advantage of that. It potentially could kick @$$. The discrim circuit would need to be redesigned because any target signal near resonance is offset quite a bit phase-wise from where we see it now, I believe.

Your observations as usual are quite helpful to anyone trying to get a good build of one of these beasts. Thanks.

-SB

[dfbowers]

Substantially; what is speaker? Coil! Coil driven by low frequency power output. So you have search head on one side and speaker ("tx-ed" coil) on the other side of pcb.... 3 divisions (by 4024) from main TX frequency are passed to audio out and "TX-ed" through "coil" (speaker)....fine..Only problem is in fact that those 3 divisions are in direct (harmonical) resonance with main frequency..... See what is the problem? I faced that long time ago...maybe at my second or third tgsl. Eversince than i tried to overcome it. TGS//TGSL is such concept and nothing we can do to overcome that issue....unless apply some major changes and possible change its charactrer too. Compromise is to pack it in metal enclosure and assure 90 degreees oriented relation between speaker and the rest of "hot" electronics. Couple ferrite perles will not harm to; tied on speaker leads...etc...etc...

I am still alive and i am still following this subject....but will not take a part in discussions ...unless if those were started by members i do respect for real (like this was the case)...
Cheers!

I thought maybe you were listening..

Thanks!

Don

[ivconic]

Substantially; what is speaker? Coil! Coil driven by low frequency power output. So you have search head on one side and speaker ("tx-ed" coil) on the other side of pcb.... 3 divisions (by 4024) from main TX frequency are passed to audio out and "TX-ed" through "coil" (speaker)....fine..Only problem is in fact that those 3 divisions are in direct (harmonical) resonance with main frequency..... See what is the problem? I faced that long time ago...maybe at my second or third tgsl. Eversince than i tried to overcome it. TGS//TGSL is such concept and nothing we can do to overcome that issue....unless apply some major changes and possible change its charactrer too. Compromise is to pack it in metal enclosure and assure 90 degreees oriented relation between speaker and the rest of "hot" electronics. Couple ferrite perles will not harm to; tied on speaker leads...etc...etc...

I am still alive and i am still following this subject....but will not take a part in discussions ...unless if those were started by members i do respect for real (like this was the case)...
Cheers!

[dfbowers]

Yes, Ivconic and others spent quite a time trying to understand why audio was flaky, unpredictable, and different with headphones, speaker, jack, etc. You may have hit on something. The crazy thing is that the feedback that can cause beeps may actually improve sensitivity and audio when just below the threshold of beeping. But it is a wildcard factor. If there is a way to improve sensitivity/response, it would be better to do it in a controlled way. But I like what you have discovered, especially orientation.

Have you tried headphones and compared to the performance with a speaker on your TGSLs?

-SB

Oh yea, I do remember the post by Ivconic on the phone jack thing.. I have been using headphones with TGSL #1 so I forgot the symptoms I had while testing it until I build TGSL #2 and started experimenting with a hardwired speaker again. Now, is the reason that Tesoro uses off resonance coils to avoid funny things like this as well? Maybe when their final products are assembled and put into enclosures the synchronous audio is not a problem but testing on a bench can be? I guess by solving one problem and creating clean audio by driving the audio from the oscillator they can cause the potential for another.. Anyway, this probably doesn't matter if we all finish our projects.

Don

[simonbaker]

Ivconic posted something a while back and it's starting to make a little more sense now. While experimenting with my latest TGSL circuit on my work bench, I noticed that the proximity of the speaker to the coil has much to do with the audio. If the speaker is within about two feet of the coil, the circuit could go into a slow oscillation after passing metal over the coil. Likewise, the orientation of the speaker in relation to the coil has some bearing on the quality of the audio - short vs long, broken vs solid sound! Audio can be improved by making sure that the speaker coil stays perpendicular to the search coil! (Not in the same geometric plane) In my case, it's a small 2", 8 ohm magnetic speaker. Just another variable to be aware of when testing an troubleshooting.. There HAS to be some magnetic coupling between the two and hence some feedback. It's best to keep the two separated as much as possible..

I can't find the exact post, but Ivconic mentioned that once he put his circuit in an metal enclosure, the audio improved. It seems to support my observation.

I will attempt to put together a short video demonstrating.

Don

Yes, Ivconic and others spent quite a time trying to understand why audio was flaky, unpredictable, and different with headphones, speaker, jack, etc. You may have hit on something. The crazy thing is that the feedback that can cause beeps may actually improve sensitivity and audio when just below the threshold of beeping. But it is a wildcard factor. If there is a way to improve sensitivity/response, it would be better to do it in a controlled way. But I like what you have discovered, especially orientation.

Have you tried headphones and compared to the performance with a speaker on your TGSLs?

-SB

[lunamay]

i noticed same thing..only on 5W 8 ohm speaker..like you said it's not good to keep the speaker near pcb and coil ..and metal encloser realy helps..me too
best regards

[dfbowers]

Ivconic posted something a while back and it's starting to make a little more sense now. While experimenting with my latest TGSL circuit on my work bench, I noticed that the proximity of the speaker to the coil has much to do with the audio. If the speaker is within about two feet of the coil, the circuit could go into a slow oscillation after passing metal over the coil. Likewise, the orientation of the speaker in relation to the coil has some bearing on the quality of the audio - short vs long, broken vs solid sound! Audio can be improved by making sure that the speaker coil stays perpendicular to the search coil! (Not in the same geometric plane) In my case, it's a small 2", 8 ohm magnetic speaker. Just another variable to be aware of when testing an troubleshooting.. There HAS to be some magnetic coupling between the two and hence some feedback. It's best to keep the two separated as much as possible..

I can't find the exact post, but Ivconic mentioned that once he put his circuit in an metal enclosure, the audio improved. It seems to support my observation.

I will attempt to put together a short video demonstrating.

Don