TGSL Experiments

[Qiaozhi]

Yes, Tx and Rx frequencies are different.

Returns from an energised target get through the Rx chain as an I.F. signal after the mixer of

= 1.76kHz

Bit like a superhet Radio.

The bit I dont get is ... I assume Rx listens to signals from an energised ringing target - which re-radiates a component different to what it was energised with.

IE we energise taget with 14kHz, we listen on 16kHz

Any takers for this last bit?

Steve

A lot of people get confused by this.

One thing you must understand is that, even though the RX coil is tuned to 16kHz, the signal received is still 14kHz. Think of the RX coil / capacitor as a filter. You put 14kHz in, and you get 14kHz out, albeit shifted in phase and a lower amplitude.

Since the sample pulses for the synchronous demodulators (analog switches - JETs, in this case) are triggered from the TX oscillator, the RX signal needs to be phase-shifted to allow correct sampling of the in-phase and quadrature components. This is the reason for the different frequencies. It has nothing to do with creating an intermediate frequency, as per superhet designs.

[simonbaker]

A lot of people get confused by this.

One thing you must understand is that, even though the RX coil is tuned to 16kHz, the signal received is still 14kHz. Think of the RX coil / capacitor as a filter. You put 14kHz in, and you get 14kHz out, albeit shifted in phase and a lower amplitude.

Since the sample pulses for the synchronous demodulators (analog switches - JETs, in this case) are triggered from the TX oscillator, the RX signal needs to be phase-shifted to allow correct sampling of the in-phase and quadrature components. This is the reason for the different frequencies. It has nothing to do with creating an intermediate frequency, as per superhet designs.

Right on.

In fact, the Rx signal has very little phase shift due to the Rx tank circuit since it is so far off resonance -- which is good for TGSL design.

An "on-resonance" design might be possible (on my list of experiments), but the sync pulses would have to be radically shifted in phase, and the stability of the phase would be a huge challenge -- but certainly an interesting design problem with who-knows-what advantages/disadvantages.

-SB

[Qiaozhi]

Right on.

In fact, the Rx signal has very little phase shift due to the Rx tank circuit since it is so far off resonance -- which is good for TGSL design.

An "on-resonance" design might be possible (on my list of experiments), but the sync pulses would have to be radically shifted in phase, and the stability of the phase would be a huge challenge -- but certainly an interesting design problem with who-knows-what advantages/disadvantages.

-SB

Yes, instability will be your biggest challenge. But remember that Nexus have managed to achieve it; so it is possible.

[golfnut]

Quadrature

So we Tx and Rx on 14.6kHz.

The reason the Rx is off - is to help with the quadrature phase demod of the return???



Im still a bit blinded by the terminology here... Im thinking that if we want to extract the phase shifts from a quadrature system - there are a lot of cheap reciever chips that do this stuff.

One advantage I could potentially see is that you could then have the Rx ON CHANNEL and this would boost the sens

Steve

[golfnut]

Hi qiahzi

Do you mean the slope of the off centre rx front end contributes to an amplitude change from a phase change - due to the slope of its response?
#

[golfnut]

I was going to use a source at 16.3kHz - to tune my Rx for biggest signal .

I guess tis is not a good idea now ?


Confused again

[simonbaker]

I was going to use a source at 16.3kHz - to tune my Rx for biggest signal .

I guess tis is not a good idea now ?


Confused again

I think your idea is still good -- you want to achieve RX resonance at 16.3 kHz, and your idea is just a way to figure out the best capacitors.

But if you have a good LRC meter, much easier to just measure.

-SB

[Jerry]

I was going to use a source at 16.3kHz - to tune my Rx for biggest signal .

I guess tis is not a good idea now ?


Confused again

I think that is still a good idea. That is the way I tuned up my rx front end to start with.
Gives a good starting point that you can tweak from.

Jerry

[simonbaker]

So we Tx and Rx on 14.6kHz.

The reason the Rx is off - is to help with the quadrature phase demod of the return???



Im still a bit blinded by the terminology here... Im thinking that if we want to extract the phase shifts from a quadrature system - there are a lot of cheap reciever chips that do this stuff.

One advantage I could potentially see is that you could then have the Rx ON CHANNEL and this would boost the sens

Steve

The TGSL synchronous detector is not a full quadrature system such as a synchronous amplifier where phase and amplitude is extracted.

It is really a two-channel "phase detector", where each channel has a phase threshold (set by the DISC and GB pots). Phases greater than the threshold create a positive pulse, phases equal to the threshold create no pulse, and phases less than the threshold create a negative pulse. The pulse is your sweep of the search head over the target.

When both channels create a positive pulse, the audio beep is triggered, indicating a target that satisfies your discrimination setting and is not a ground signal.

The Rx is off-resonance mainly to make it easier to set correct phase thresholds without worrying about component tolerances, aging, etc., I think. When you are off resonance, variations in the parts don't change the phase much.


-SB

[Qiaozhi]

The Rx is off-resonance mainly to make it easier to set correct phase thresholds without worrying about component tolerances, aging, etc., I think. When you are off resonance, variations in the parts don't change the phase much.
-SB

This is correct. The design is optimised to improve its manufacturability. If the coils were resonant, the manufacturing costs would increase, as would the final product price.

[dfbowers]

The TGSL synchronous detector is not a full quadrature system such as a synchronous amplifier where phase and amplitude is extracted.

It is really a two-channel "phase detector", where each channel has a phase threshold (set by the DISC and GB pots). Phases greater than the threshold create a positive pulse, phases equal to the threshold create no pulse, and phases less than the threshold create a negative pulse. The pulse is your sweep of the search head over the target.

When both channels create a positive pulse, the audio beep is triggered, indicating a target that satisfies your discrimination setting and is not a ground signal.

The Rx is off-resonance mainly to make it easier to set correct phase thresholds without worrying about component tolerances, aging, etc., I think. When you are off resonance, variations in the parts don't change the phase much.


-SB

You have a gift of explaining things in a way that I cannot Simon.. Too bad you are not a teacher, or are you??

BTW, have you seen any examples in schematics posted here that utilize a full quadrature system?

I seem to remember playing around with resonance on the TGSL late one night and the closer I got to resonance, the more sensitive it was. I quit when I could not adjust phasing correctly anymore and didn't want to tear my project apart to try to get phasing correct again.

Also, I got discouraged when reading some of the older threads:
http://www.geotech1.com/forums/archi...p/t-15365.html

[golfnut]

thx all

I did some work last night.


I got my 16.3kHz source working. Detail available if required.

- Lightly coupled it across my Rx tuned cct with 0.1nF caps

-I stopped my Tx oscillator - so no confusion / interaction..

Pulled the 2 tuning caps off the pcb

- made a switched cap bank with 22, 10, 4n7, 3n3, 2n2, 1
on a DIP switch (as per other post)

Put ac voltmeter on pin 1 of Rx preamp.

got range of 0.3Vac to Tuned Peak of 2.7Vac.

I neede a 15.5nF to center my coil at 16.3kHz.

Enjoyed the experience.
Quality.

S

[simonbaker]

You have a gift of explaining things in a way that I cannot Simon.. Too bad you are not a teacher, or are you??

BTW, have you seen any examples in schematics posted here that utilize a full quadrature system?

I seem to remember playing around with resonance on the TGSL late one night and the closer I got to resonance, the more sensitive it was. I quit when I could not adjust phasing correctly anymore and didn't want to tear my project apart to try to get phasing correct again.

Also, I got discouraged when reading some of the older threads:
http://www.geotech1.com/forums/archi...p/t-15365.html

Not a teacher, haven't given up trying to do things yet (those who can't do, ...). But if they ever offer degrees in metal detectors, I'll try to get one... BTW, your explanations set the standard, your docs have changed the landscape.

I'm thinking that an on-resonance design may need an "active tuning" system to keep it calibrated. This probably would be easiest with a mostly digital design, where changing phase shifts is simply adding a number. Aziz will probably beat me to it. I have not yet identified any schematics that are on resonance, although Q indicated some designs exist.

-SB

[simonbaker]

I did some work last night.


I got my 16.3kHz source working. Detail available if required.

- Lightly coupled it across my Rx tuned cct with 0.1nF caps

-I stopped my Tx oscillator - so no confusion / interaction..

Pulled the 2 tuning caps off the pcb

- made a switched cap bank with 22, 10, 4n7, 3n3, 2n2, 1
on a DIP switch (as per other post)

Put ac voltmeter on pin 1 of Rx preamp.

got range of 0.3Vac to Tuned Peak of 2.7Vac.

I neede a 15.5nF to center my coil at 16.3kHz.

Enjoyed the experience.
Quality.

S

Nice execution! I love it when a plan comes together...

[Dennis the Mennis]

I built Iron box for TGSL and Connected with (-)
When I'm close to the Iron box my tgsl Becomes a weak sensitivity
what I do?
help
Greetings

Hi Salih ,

What happens if you disconnect the ground from your pcb to the metal housing ?

I don't know if the old TGS housing was also connected to GND ?

kind regards ,

Dennis the Mennis