Threshold Pot doesn't work

[green]

I assume the preamp is the same as before, which had an output noise of 5.7uv/rtHz. The NBW of the demod is 7.6Hz and its effective gain is 1 so the total integrated preamp noise seen at the output of the demod is 15.7uv rms. This does not include noise generated in the demod... that is the so-called kT/C noise which is equal to v = sqrt(kT/C) = 354nv rms. This is about 1/40th of the preamp noise so we consider it "in the mud" and ignore it.

This seems to imply that the total output noise of the demod is just 12.5uv rms but that only includes one Nyquist zone. The PI is a wideband system and all that wideband noise gets folded into the demod output. How many Nyquist zones get folded in depend on the BW of the preamp; the wider you make it, the more noise gets folded in. Ferinstance, if the preamp has a BW = 3.2MHz (NBW = 5MHz) and the sample rate is 1kHz (Nyquist BW = 500Hz) then you are folding in 10,000 Nyquist zones. One of your lower BWs -- 94kHz -- only folds in 188 Nyquist zones.

This may seem depressing but Nyquist zones don't fold equally. The first Nyquist zone has a relative weighting of 1, but after that the other Nyquist zones are sinc-weighted and diminish pretty quickly. You'll need an Excel spreadsheet to figure it all out, you can't simulate this stuff without a periodic steady-state simulator which tends to run $50,000. I don't remember the numbers but something like 50 Nyquist zones doubles the noise, 200 zone triples the noise, etc. So you take that 15.7uv rms above and multiply it by a Nyquist gain factor that you have to figure out.

One last thing... all this was for a single sample window. If you also have an EFE sample window then the noise goes up by another sqrt(2).

I don't expect anyone to understand all this, it begs for some graphics and equations to make sense of it.

Thanks for the reply, some things to think about. Any thoughts on the test? Maybe include a 100us GB sample with a 10us target sample.

[Carl-NC]

The only test is to put the output of the demod on an oscope and see what it looks like. You can change the preamp BW, or change the demod sample width, and see how the noise changes.

[h9361]

Hello
Dear Carl
I remember that you mentioned in your article "If a 1kΩ resistor is used forR42a, the minimum pulse delay can betaken down to 8?s".
My question is that do you think i can reach less delay, for exp, 5us or etc?
Thanks

[Carl-NC]

Getting the 555 timer to do that would be quite easy. Getting the coil & preamp to do that would be quite difficult.

[green]

Don't know how to calculate noise at integrator out. Thinking of doing a test with my circuit to try and learn something. Coil not connected, noise caused be amplifier and the two 1k input resistors. Record p-p volts at amplifier out and integrator out at different amplifier cutoff frequencies with two different sample times. Any suggestions for improving the test?

amplifier cutoff set by the capacitors I have

Finally got around to trying a test. Thinking I should add target effect plus noise at different cutoff frequencies. Any thoughts what I might add or do different? Would like to not repeat changing cutoff frequencies to many times.

I set first delay time(adjustable), second delay for 4us and GEB sample time for about 100us. Then adjust target sample time for minimum ground signal when GEB on.

I see one of the target samples is seconds instead of us

[h9361]

Finally got around to trying a test. Thinking I should add target effect plus noise at different cutoff frequencies. Any thoughts what I might add or do different? Would like to not repeat changing cutoff frequencies to many times.

I set first delay time(adjustable), second delay for 4us and GEB sample time for about 100us. Then adjust target sample time for minimum ground signal when GEB on.

I see one of the target samples is seconds instead of us

Thanks dear green.
I used a new PCB rev. D, now i have no noise.
Now i'm working over 4delay ground balancing (suggested by Mr. Carl.). My result is very very nice and i can work in 10us-25us with high stability over mineralization.

[h9361]

Hello
Dear Carl.
I have a question about GPX-5000 coils. As you know, Minelab is using litz wire for its detector. Do you know its litz details (number of strands and size awg)?!
Download file, you can see Litz wires with its details.
https://file.io/8l7gJ5VzdcSP

[Carl-NC]

No, I don't know.

[h9361]

No, I don't know.

Hello
Dear Carl
I hope you are passing good days.
I have question about SW2 (AutoTrack, Rev. D). I tested my HH1 with opened SW2 for testing pinpointing mode.
After SW2 opened, my threshold pot don't work anymore! It works only in SW2 is closed.
Is there any way that be able to work in pinpointing mode with THD pot, like other detectors?
Thanks.

[Carl-NC]

Probably your IC8 has too much input bias current and is drifting. What are you using for IC8?

[h9361]

I'm using TL072.

[Carl-NC]

Where did you buy your opamps?

[green]

Some more testing. Reply #95 has the nickel tested at 139mm (should be 108mm, I had three spacers on the desk. looks like I measured all three but used only two) integrator test_1

1 tau, 3 tau and quarter were spaced at 54mm. The nickel spaced at 108mm. ground spaced 12 mm.

Reducing amplifier response reduced noise at amplifier out, not so much at integrator out.

[h9361]

Where did you buy your opamps?

Sorry for late replying.
My TL072 is Chinese.

[h9361]

Some more testing. Reply #95 has the nickel tested at 139mm (should be 108mm, I had three spacers on the desk. looks like I measured all three but used only two) integrator test_1

1 tau, 3 tau and quarter were spaced at 54mm. The nickel spaced at 108mm. ground spaced 12 mm.

Reducing amplifier response reduced noise at amplifier out, not so much at integrator out.

Thanks for your explanation.