TGSL Tuning & troubleshooting

[dfbowers]

That is an interesting point, depending on how much high-ferrite ground could affect the TX frequency. However, I assume ferrite will always reduce the TX frequency, which might reduce the gain/sensitivity of the TGSL by moving away from the RX resonant freq, but wouldn't cause a problem with phase shifts.



That may be desired for certain TGSL constructed a particular way. I personally suspect however it is not necessary in general, and some MDs will naturally need a different phase shift. I'm working on finding a way to explain why and how to predict the correct phase shift for each TGLS.



I would be interested in seeing an actual comparison test with real chips and oscilloscope pics.



Don't forget -- the null signal phase (20 deg or whatever) has nothing to do with the target signal phase. The phase between the drain and gate of TR4, TR5 that you observe is only for the null signal, not the target signal. The null signal only makes a DC voltage at the source of TR4, TR5 -- which is ignored by the next op amp filter!

So the null signal phase is not really important in the way you think (I think).

What matters is the relationship between the sync pulse phase (gate of TR4, TR5) and the target signal, which you cannot see normally, it is so small. You can change the null signal phase all you want and it does not change the phase of target signals. So don't worry too much about the null signal phase as a way to null your coils -- use other criteria. Except this warning -- make sure the DC voltage on the source of TR4, TR5 does not go too far less than ground, or you will forward bias the JFET and make a mess. In other words, shift your coils to pick a different null signal phase.

I have not completely made my mind up on the effect of the null signal phase on ground balancing. Qiaozhi and I had a long confused discussion on this and I accepted the idea that ground/ferrite would amplitude-modulate the null signal without changing its phase. Since then I have been considering that this is only partially true -- that the null signal may be composed of a "magnetic" component and also a "non-magnetic" component, in which case ground/ferrite would only modulate the "magnetic" component. I also have been thinking that ferrite truly may be a "zero phase" target, which really is different from simply modulating any null phase.

Sorry, getting too complicated. I need to do much more thinking about it. But my final advice is don't get too concerned about your null signal phase for the purpose of nulling your coils. And don't worry that the wrong null signal phase will hurt your DISC control -- it should not. Do make sure your JFet gate-source voltage is not forward biasing the JFet junction.

-SB

Simon, can you elaborate on the conditions required to forward bias the JFet junction?
I have read your post in the past regarding this but have largely ignored it because I have not had any issues.. Are we speaking of badly nulled coils to cause this?

Don

[johnsmith77]

The important point is that the target phase is not affected by your null signal phase -- the two can be considered separately.


Regards,

-SB

I agree with this: the target phase is not affected by your null signal phase.
Lets say null phase= ph0
Target phase= phtarget
Total phase = ph0 +- phtarget (+ or - depends on target.)
I think ph0 is important as ph0 affects rx/geb sampling phase and rx/disc sampling phase. Geb phase can be trimmed, but disc not.
Maybe i am not correct, it is important understand how it works for future designs.

[johnsmith77]

I am interested in seeing what it will do for a TGSL that is already getting good depth.

Winter weather is about here so I hope to be back to experimenting with this soon.

Jerry

This message is for me?
Tgsl works good with separate power supply and not so good when only some capacitors are connected. I wonder if 7660 could work with less spikes and rails than chopper stage power supply.

[Jerry]

I am interested in seeing what it will do for a TGSL that is already getting good depth.

Winter weather is about here so I hope to be back to experimenting with this soon.

Jerry

[simonbaker]

That is a good test and it showed some very promising results!

When you talk about the tone being faint and short, do you mean that it is that way at all depths or it gets fainter and shorter as the depth increases?

Ivconic did experiment with a threshold control on U107 and he adopted that in the IGSL for U8a & b. I have not done this one yet but I think it could help the short tones in particular. Right now it may be just the peaks getting above the threshold and so they are shorter.

My ideal is to have the get fainter as it goes deeper but not shorter, so you can get a sense for the depth by listening to the target.

Thanks for posting your test results.

Jerry

When I first built the TGSL on a breadboard I noticed terrible voltage spikes on the supply rails at the TX frequency, due to the negative supply circuitry. I was very concerned and thought it must be fixed.

But other people's TGSL seemed to work very well. So I began to think maybe the voltage spikes just don't matter. First, because they are at the TX frequency, they will not make a problem at the Synchronous Detector, because they would just make a constant voltage output. And the rest of the circuit is a very low-bandpass filter that probably ignores them. Well, I'm not sure, I recall I thought I saw the spikes in the op amp outputs. If they could chop up the inputs to the final comparators, that might act like noise and kill weak signals. But those spikes are so fast, I don't think they do.

However, if we are seeing new evidence that these spikes are a problem, this is a good reason to take measures to reduce the spikes. However, be careful in case they are not the real problem.

-SB

[simonbaker]

I think a little different. I try give you an example and explain how i imagine this when no target applied.

On Tr gates comes signal 14.5khz from Tx coil.
On Tr drain comes signal 14.5khz received by Rx coil, shifted and modulated by Tx signal from Tr gate. Because of it we don't see nice sinus on Tr drains as wee see on LF353 pin 7 - it is modulated here. Depending on phase shift between Rx/Tx on Tr drain signal will be positive or negative, also it influence Tr source voltages.

When disc pot are turned, shift also changes, this changes voltage on Tr 4 source.
If we have wrong phase shift at min disc pot position, then we will no be able achieve correct disc at all pot positions.
As you know disc pot changes cut off point for target metal.

Example again (if we have wrong phase shift tx/rx) then imagine:

disc pot min (phase shift between Rx/Tx disc sampling pulse -40) then voltage on Tr s = -V.
disc pot middle (phase shift between Rx/tx disc sampling pulse 0) then voltage on Tr s = --V (more negative).
disc pot middle (phase shift between Rx/tx disc sampling pulse 40) then voltage on Tr s = -V.

Tr s voltage when disc pot min and max will be equall so disc at min and max will act the same. Only from Min to middle and from middle to max different.
Disc can't act the same on min and max. Also this applies to geb.

There is how i think

Also depending on this, some TGSL made projects has iron rejection at min pot, some not.
Mine has iron rejection when disc pot turned at 1.
Depending on phase shift i can make that iron will be rejected when disc pot turned at 0 or at 3.

We may be talking about different things, I'm not sure.

The phase of the TR4 gate pulse (square wave) is important and is controlled by the disc pot. If you have a mistake in your circuit there you will have trouble.

The phase of target signals is determined by the type of metal mainly and also by the RX coil resonant frequency compared to the TX oscillator frequency. But it is not affected by the "null" signal phase, which is the signal you see when you null your coils. That null signal phase is easily changed as you shift your coils back and forth near the minimum. But the null signal phase does not affect the target signal. The two signals are added together linearly. Because TR4 is essentially a linear operation on the two signals, the output is the sum of the two outputs as if they weren't combined.

Once you glue your coils, the null signal does not change, and it creates a constant voltage on capacitor C12/C15. A target signal arises as you sweep your coil over the target. If the phase of the target signal is less than the "discrimination phase" set by your DISC pot, the output will be negative. Otherwise, the output will be positive and maybe make a beep. The important point is that the target phase is not affected by your null signal phase -- the two can be considered separately.

It is not easy to explain. You need to google "Synchronous Detector" and "lock-in amplifier" and look at the mathematics.

I believe I am correct in this, but I'm always open to discussion in case I made an error somewhere. However, my experiments seem to confirm that the null signal phase does not affect how the DISC control affects target signals.

Regards,

-SB

[Qiaozhi]

hello 2 any 1
I want to understand the meaning of this shortcut ( V.C.O )
I found V.C.O in another metal detector
Greetings

It means Voltage Controlled Oscillator.
In other words, it is an oscillator whose output frequency is controlled by an applied voltage.

[johnsmith77]

It's long and loud up to 15cm, after that it gets gradually shorter and weaker. It is normal, right? I need to check some videos in the Youtube to compare.

It might be good idea to change all bypass electrolytic capacitors to low ESR ones to lower the ripple.

The rescue blanket I got has plastic on both sidesso I can't make a connection to the aluminium inside.

You can try emery paper to reach aluminium on rescue blanket.
My tgsl act the same, only sound lowers about 20cm, can't say very accurate.
I read that Salih's tgsl act the same.

There can be used IC chip 7660 for -5V. I remember it was discused on tgsl thread, need find some more information about it. Not shure if it is good solution here.

Also i attach White's Classic ID power supply circuit, it very likely to tgsl's, but some differences.
Whites don't use chopper stage on this Classic id circuit. But voltage is 8V only instead of 14V in TGSL (8+6 (not 5 when measured) )

[Mullihaka]

When you talk about the tone being faint and short, do you mean that it is that way at all depths or it gets fainter and shorter as the depth increases?

It's long and loud up to 15cm, after that it gets gradually shorter and weaker. It is normal, right? I need to check some videos in the Youtube to compare.

It might be good idea to change all bypass electrolytic capacitors to low ESR ones to lower the ripple.

So much things to try!

Johnsmith77: I have foil. The rescue blanket I got has plastic on both sidesso I can't make a connection to the aluminium inside. R37 is connected to - voltage.

[salih]

your Experience

hello 2 any 1
I want to understand the meaning of this shortcut ( V.C.O )
I found V.C.O in another metal detector
Greetings

[Jerry]

I disconnected the emitter of TR7 from the + rail. Then I soldered 6V battery pack between anode of D6 and ground (battery minus on anode).

The detection distance is now about 33cm, but the tone is still faint and short.

All voltage rails are now quite clean: the 14.5 kHz ripple is below 2mVpp while it was 40mV before modifications.

That is a good test and it showed some very promising results!

When you talk about the tone being faint and short, do you mean that it is that way at all depths or it gets fainter and shorter as the depth increases?

Ivconic did experiment with a threshold control on U107 and he adopted that in the IGSL for U8a & b. I have not done this one yet but I think it could help the short tones in particular. Right now it may be just the peaks getting above the threshold and so they are shorter.

My ideal is to have the get fainter as it goes deeper but not shorter, so you can get a sense for the depth by listening to the target.

Thanks for posting your test results.

Jerry

[johnsmith77]

The detection distance is now about 33cm, but the tone is still faint and short.

You achieved distance from 15cm to 33cm with separate power supply. Great!

What is your coil shielding? (mylar, foil)
I managed achieve a little better distance when shielded coil with less overallaping shielding.
It could help when making another coil.

About dfbowers mod: do you have only shorten r37 or connected to -5v also?

Sound is weak in all tgsl, but Ivconic used time delay circuit with 555 to make it constant in all distance.

[Mullihaka]

Do we have test results?

I disconnected the emitter of TR7 from the + rail. Then I soldered 6V battery pack between anode of D6 and ground (battery minus on anode).

The detection distance is now about 33cm, but the tone is still faint and short.

All voltage rails are now quite clean: the 14.5 kHz ripple is below 2mVpp while it was 40mV before modifications.

There's total of 10 pieces of 330nF capacitors, soldered between the + rail and ground, where they are close enough for a SMD capacitor to reach them. I think these capacitors are not needed if the minus voltage is taken from a separate battery.

I will next try to get the tone loud and long, as it is up to 20cm distance for a 2 euro coin. If successfull, then I conenct the TR7 back and install some bypass caps more. It is not convenient to have two batteries.

I think that even if the spikes are synchronous with the sampling FET, they still mask the detection, as the coin movement across the coil is not synchronous and the voaltge it creates, is much lower than the 40mV forest of spikes. I'll take some pictures and add later.

[johnsmith77]

I guess I will next disconnect TR7 and TR8 and supply the negative voltage from an additional battery to see, if this makes the detection better.

Do we have test results?

[Jerry]

The interference is definitely internal; it's the TR7/TR8, which "modulate" both positive and negative voltage rails with the TX frequency. The rise time of the pulse is short, approximately 150ns. I'm wondering, if TR7/TR8 could be changed to ones with lower frequency range, just to lower the ripple.

I remember some conversation about this a long time ago. As I recall the scheme is set up so the transmit frequency is divided down to perform other functions so that any transients that are generated would be in sync with the transmit freq and not cause problems. Do not remember much in the way of details but that the gist of it.

I am sure others can fill in the blanks.

Jerry