Problem RX Form (TGSL)

[h9361]

Hello
Dear All
I have built TGSL and i tried to null RX/TX.
I connected TX and RX (pin7 from LF353) to Scope.
But my RX is Square waveform, not sine form!
I am using 0.2mm wire, my TX=6mH and RX=6.5
I also reversed TX wires, but no result.
Do you have any suggestion for check?
Thank you.

[Carl-NC]

Is this a DD or concentric coil, and did you build it yourself?

[h9361]

Is this a DD or concentric coil, and did you build it yourself?

Thank you.
It is DD. I tried more and my form is OK ( attached).
But my sensitivity is very very low. Do you have any idea for check?
Thank you.

[Carl-NC]

If the RX waveform is measured at the output of the preamp then it appears to be clipping on the low side, which means your coil null is bad. The idle signal at the preamp output should be 100mV - 1V pp max. Obviously, make sure there is no metal near the coil.

[h9361]

If the RX waveform is measured at the output of the preamp then it appears to be clipping on the low side, which means your coil null is bad. The idle signal at the preamp output should be 100mV - 1V pp max. Obviously, make sure there is no metal near the coil.

Thank you.
My sensitivity is solved, my problem was in GB part.

[h9361]

If the RX waveform is measured at the output of the preamp then it appears to be clipping on the low side, which means your coil null is bad. The idle signal at the preamp output should be 100mV - 1V pp max. Obviously, make sure there is no metal near the coil.

Hello
Dear Carl
I have a problem about TGSL.
All of my TGSL is OK but i can not touch any pots or switch ( Disc/All or mono/double) or battery because it detects them as metal!
Do you have any suggestion for check?
I read other post about this, but no one has provided a solution.

[h9361]

Thank you.
My sensitivity is solved, my problem was in GB part.

I played with my TGSL, it is stable and my coil form is OK.
I have built a 22cm coil and tuned its GB so that not to detect ferrite.
You can see its O-scope form in attached.
I think my sensitivity is low and i think that there is a problem.
My coin diameter 3cm and my depth is 17cm! I saw several videos from TGSL that detect coin from 25cm!
I change sensitivity pot, but it has a very little effects!
Do you have suggestion for test?

[h9361]

If the RX waveform is measured at the output of the preamp then it appears to be clipping on the low side, which means your coil null is bad. The idle signal at the preamp output should be 100mV - 1V pp max. Obviously, make sure there is no metal near the coil.

Hello
Dear Carl
I have a technical question.
As you know, TGSL is using BF245C for switch.
Do you think its sensitivity will be lower if i use BF245B?
Thank you for your information.

[Carl-NC]

I would think that the BF245B would be a better choice. I have no idea why your sensitivity is low. First thing is to double-check resistor and capacitor values to make sure they are all installed correctly.

[h9361]

I would think that the BF245B would be a better choice. I have no idea why your sensitivity is low. First thing is to double-check resistor and capacitor values to make sure they are all installed correctly.

Thank you dear Carl.
I will playing with Gain resistor.

[h9361]

I would think that the BF245B would be a better choice. I have no idea why your sensitivity is low. First thing is to double-check resistor and capacitor values to make sure they are all installed correctly.

Finally, i solved the sensitivity problem, thank you for your kindness.

[h9361]

I would think that the BF245B would be a better choice. I have no idea why your sensitivity is low. First thing is to double-check resistor and capacitor values to make sure they are all installed correctly.

Hello
Dear Carl
I have a technical question.
I tested my TGSL over 25cm coil and i can detect a coin 28-29cm.
I also made a 60cm coil and tested its depth. Unfortunately its depth is like 25cm coil, not more.
I checked 60cm coil on Scope, its nulling is correct. Do you think it is normal in VLF small and big coil?!
Thank you

[Carl-NC]

Yes, a big coil will go deeper on big targets but may not go as deep on a small target like a single coin. Try a soft drink can.

[h9361]

Yes, a big coil will go deeper on big targets but may not go as deep on a small target like a single coin. Try a soft drink can.

Thank you dear Carl for your valuable information.
I tested a Steel round tray with a diameter of 50 with my 60cm coil.
My TGSL detects it from 130cm, i think the depth is low.
Do you think its sensitivity will be increased if i decrease inductance of TX ( adjust it the same frequency) in uH, instead of mH?

[Carl-NC]

Maybe, usually a lower TX inductance produces a stronger TX field.