Threshold Pot doesn't work

[GeoMax]

Hi H9361,

what is the benefit to choose between single and double sampling?

[h9361]

Ideally high inductance gives you more "transformer gain" but practically there is not much difference.

Please see, we have a limitation for adding switch between single/double sampling on pcb.
One a method on pcb is, remove one pine of R27, c18 and R25. Then, connect them ( the opened pins) together in air.
See attached, do you have better solution?

[h9361]

Ideally high inductance gives you more "transformer gain" but practically there is not much difference.

I study Lorenz z2 user manual, i understand that he put both single and double samplings.
The option DEL is simple one sampling and the option GND is two samplings (Target + EFE).
Dear Carl, i read your HHD articles, you mentioned that we can reach to one single sampling via removing R25 and C18 and R27, then connect pin2 of IC8 to ground.
I want to add a simple switch for single/double sampling on my HHD PCB.
Do you think i can reach via following switch without removing R25 ( because i can not more edit on PCB)?

[h9361]

Ideally high inductance gives you more "transformer gain" but practically there is not much difference.

Thank you.
Dear carl i have a guestion about you HHD.
I adjusted follwing settings:
Delay 10-12 us
EFE delay at its maximum, near 300us
Sample width 20us
Frequency 1000pps
TX 100us
Threshold is its lower pit ..pit

I also made a big coil 125cm x 125cm using SFTP Cat 5, 500uH.
I tested this coil as fixed on ground with Iron shovel, my depth was 200 to 230cm, not more.

Again i made another bigger coil using cat 5, 150x150cm.
My depth is increased 10cm, not more.

Similarly, i bring a lorenz z2 with 3m frame but i fold it so that its size converted to 150x150 ( this has been said in its user manual).
I tested it with Iron shovel at fixed coil, depth 370 cm!

There is a big question, why i can not reach to this test!
As you know, lorenz delay will be increased more than 70us if we fold frame to 150cm. But lorenz is detecting the shovel from higher depth with the higher delay, while i can not reach this depth with lower delay!

Do you have any suggestion for changing my hhd?
I thought my audio part had lower sensitivity.
So I increased gain of audio part R29 from 100k to 220k, but no result.
I think problem may be in threshold process.

[Carl-NC]

Ideally high inductance gives you more "transformer gain" but practically there is not much difference.

[h9361]

10k. But why is your RX coil only 2uH?

I wrote wrong, 200uH is correct.
Before, i had experienced only RX (200 uH) without TX , i could detect old metals.
Generally, i have a question.
please consider following:
500uH+5n = 100Khz
5mH+500p= 100Khz

Which of them is better? higher or lower inductance?!

[Carl-NC]

10k. But why is your RX coil only 2uH?

[h9361]

Practically, they don't affect the signal. Some detectors leave out R1, I prefer to include it to dampen potential oscillations.

thanks.
Do you have a suggestion about R1 for 10khz to 120khz?
My RX is 2uH.

[Carl-NC]

Practically, they don't affect the signal. Some detectors leave out R1, I prefer to include it to dampen potential oscillations.

[h9361]

I don't understand your schematic (bandpassmain.jpg) unless you are trying to build an energy-theft detector, or maybe PI. It is not what you would use for VLF. That's one of the problems with this thread, you have jumped around a lot of different topics so I often don't know what you are trying to do. It would have been better to use separate threads.

Anyway, this is normally how I would make a VLF bandpass preamp:


C1,R2 form the high-pass filter and C2,R3 are the low-pass filter. It would be a good exercise for you to put this in Spice and see how it works, and how to change the values to get the frequency range you want.

Please forgive me.
About RX and R1, are they considered as high pass filter (LR)?

[Carl-NC]

Is C1 and R8 as High pass filter and C3 and R6 as low pass filter?

Yes.

[h9361]

I'll disagree, your results don't look right. Here is what I get:

Your peak results are -29dB and it should be close to +40dB, and your highpass cutoff is too low at ~200Hz. Make sure you are using a wideband opamp (I use the NE5534) or an ideal opamp, and make sure it is properly powered.

Thanks for information.
Please see my schematic,
Is C1 and R8 as High pass filter and C3 and R6 as low pass filter?

I simulated it:

[Carl-NC]

I'll disagree, your results don't look right. Here is what I get:

Your peak results are -29dB and it should be close to +40dB, and your highpass cutoff is too low at ~200Hz. Make sure you are using a wideband opamp (I use the NE5534) or an ideal opamp, and make sure it is properly powered.

[h9361]

A simple hand calculation tells me that the circuit has a break frequency (Fb) of 3.39kHz, and a cutoff frequency (Fc) of 33.86kHz, so your simulation now looks correct.

Thanks dear Qiaozhi.
As you said, range is between 3.39K to 33.86K.
See attached, i added a white box for this range.
But i can not understand this. for exp, the 1.5K is out of the range, but i see it at the highest gain.
Or, see 33.86 that is in the range, while it has low gain.
I thought that we have high gain only for 3.3 to 33.8 Khz.

[Qiaozhi]

A simple hand calculation tells me that the circuit has a break frequency (Fb) of 3.39kHz, and a cutoff frequency (Fc) of 33.86kHz, so your simulation now looks correct.