TESORO GOLDEN SABRE

[pito]

If you have unsymmetrical positive and negative voltages the limitation will be oslo unsymmetrical, for small signal where is no limitation sin will be without flats.

[ArchibaldSTM]

pito...It's not a problem. TGSL has power supply +8 volts / -5 volts

[kt315]

but something is wrong with the preamp.

where he bought the opamp? is it chinese special for aviation and space usage?

[pito]

Thanks, another question, what could be the reason of not having a perfect sine shape in the RX preamp output? My RX waves have always some flat bottom.

Positive voltage: +7.97v, negative voltage: -6.37v. = This is your problem; the voltages should be the same

[Carl-NC]

After looking at the video again, I think the null is OK, but something is wrong with the preamp. I would check the negative power supply, see if it is connected correctly and maintaining regulation. Or try a different opamp.

[pito]

the problem never goes away = it is go away very easy with this phase shifter + potentiometer for amplitude regulation.


There is no need to waste time for perfect coil nulling, just make it less than 1V ........

[pito]

Proper bias/offset make sine symmetric, to high gain make amplitude limit.

@cristiano.sar

Go from this (right drawing )

to this


Then reduce gain and get this

[Carl-NC]

The fact that the problem never goes away no matter where the RX coil is placed suggests that the main problem might be a bad null. Another possibility is that there is too much preamp gain.

[pito]

not very well nulled. = flats on top and bottom, he got it only on one side

[Carl-NC]

Thanks, another question, what could be the reason of not having a perfect sine shape in the RX preamp output? My RX waves have always some flat bottom.

You either have too much offset in the preamp or the coil is not very well nulled.

[pito]

some flat bottom. = incorrect bias

[cristiano.sar]

Thanks, another question, what could be the reason of not having a perfect sine shape in the RX preamp output? My RX waves have always some flat bottom.

[pito]

In VLF direction finders the shield is used to have sharp null , the same is in metal detectors.....

[cristiano.sar]

I'm experimenting coil nulling with my TGSL.
My coil is shielded with graphite (about 1.2k resistance) and connected to the board via usb 2.0 cable, using the shield connected to pin TX-2.
While observing the waveforms in oscilloscope I noticed that when I have the shield disconnected and move the RX the coil around I have more room to find the desired phase shift while with the shield connected it seems more difficult if not impossible to get it.
Is this a normal behaviour?

Shield disconnected: https://youtube.com/shorts/tSZ8m-IoOu4
Shield connected: https://youtube.com/shorts/KqA1BemDo3s

[cristiano.sar]

I'm having a similar problem.
Positive voltage: +7.97v, negative voltage: -6.37v.
My TGSL TX amplitude is around 10v, checked the resistance of J1-1 to ground with the TX coil disconnected and it measures about 0.7 Meg, should I worry about?
Thanks.

Well, I just found that the problem is my scope, the button at 5v/Div does not work very well, just moving it some millimeter clockwise or using the 2v/Div I can get around 14 and 15v of TX amplitude.