Replies to Minipulse Plus

[Davor]

I think "destroys" may be a little strong. But yes it does change the response of the integrator. I noticed this impulse response while testing/learning GS4 circuitry (Eric Foster incorprated this into the GS4). Although the impulse response is somewhat changed, it tends to improve the bandpass charactaristics... at least I like the result. Everything in the real world is a trade-off... there is no achieving the ideal... and every idea of the desired is relative to the one that desires.

Well, I checked what can be done with this as I abandoned the idea after seeing what it does with 100n and I simply did not try tuning the capacitor. But apparently yes, it can perform, only not with 100n. With 47n it behaves OK, but with 100n it seriously overshoots. See for yourself, the pink trace is 100n, while the bluish one is 47n.

[josewashere]

Yes, R11 would be correct... I sometimes forget which version of schematic I last referenced. In the original minipulse schematic posted, the reference was R16, but in the minipulse-plus the reference is R11.

I just did the changes to r11 and c11. It increased the detection distance by about 1cm and overall chattiness stayed the same!

[KingJL]

100n in parallel with R22 destroys otherwise perfect impulse response to the targets, so it is not the kind of mod we want. What happens is that integrator response overshoots, so it results in masking of small targets in vicinity of a large one.

I think "destroys" may be a little strong. But yes it does change the response of the integrator. I noticed this impulse response while testing/learning GS4 circuitry (Eric Foster incorprated this into the GS4). Although the impulse response is somewhat changed, it tends to improve the bandpass charactaristics... at least I like the result. Everything in the real world is a trade-off... there is no achieving the ideal... and every idea of the desired is relative to the one that desires.

[KingJL]

... Did you mean R11? R22 is 1k and the only 22k resistor is R11

Yes, R11 would be correct... I sometimes forget which version of schematic I last referenced. In the original minipulse schematic posted, the reference was R16, but in the minipulse-plus the reference is R11.

[Davor]

100n in parallel with R22 destroys otherwise perfect impulse response to the targets, so it is not the kind of mod we want. What happens is that integrator response overshoots, so it results in masking of small targets in vicinity of a large one.

[josewashere]

For those that wish for increased sensitivity, I would suggest changing the value of R16 from 22K down to 6.8K. This will reduce the sample pulse width from ~48 usec down to ~15 usec (changing C11 from 2n2 to 1n5 in conjunction withnthe change of R16 will bring the sample pulse width down to ~10 usec). To slightly reduce the "chattiness" and injection noise from the switches, try putting a 100n in parallel with R22. The longer sample widths made sense with the original higher inductance coil. But with the faster coils in the 300 uh range, earlier effective sampling (the sample/hold circuits will tend to integrate to the signal level at the end of the sample period) can be realized.

Thanks for the modification. Did you mean R11? R22 is 1k and the only 22k resistor is R11

[KingJL]

For those that wish for increased sensitivity, I would suggest changing the value of R16 from 22K down to 6.8K. This will reduce the sample pulse width from ~48 usec down to ~15 usec (changing C11 from 2n2 to 1n5 in conjunction withnthe change of R16 will bring the sample pulse width down to ~10 usec). To slightly reduce the "chattiness" and injection noise from the switches, try putting a 100n in parallel with R22. The longer sample widths made sense with the original higher inductance coil. But with the faster coils in the 300 uh range, earlier effective sampling (the sample/hold circuits will tend to integrate to the signal level at the end of the sample period) can be realized.

[josewashere]

If you trigger the scope from TP1, and monitor TP3, you can see the reason why the TL072 gives a smoother threshold.
Using the original Minipulse coil, the NE5532 comes out of saturation at 21us, whereas the TL072 takes 31us. Also, the TL072 provides a lower gain and hence a lower noise level. You could probably get the same smooth threshold by simply lowering the overall gain of the preamp.
Adjusting PIs indoors can sometimes be problematic due to the external EMI. Once you have it outside in a shielded case, then most of the twitchiness goes away.

I was way off track then.. I appreciate that you took the time to check it out.
Mine is in a metal box and using shielded cable and a shielded coil its still pretty jumpy with the ne5532 even in the back yard but I do live in a RF noisy location. I guess I should take it for an outing into the country side (low mineralised ground) too see how it behaves.

[Qiaozhi]

Sounds like a great plan!
I'm starting to think that the chatty threshold is caused by the EFE sampling somehow and as the tl072 is not seeing longer delays its also not seeing the EFE sample giving a smoother threshold.
The threshold is so smooth that it really feels like the preamp stage could probably use more gain with a corresponding increase in sensitivity. Some posts ago Davor did mention that the efe circuit could be introducing more noise into the system.
Then again I could just be way off track here.

If you trigger the scope from TP1, and monitor TP3, you can see the reason why the TL072 gives a smoother threshold.
Using the original Minipulse coil, the NE5532 comes out of saturation at 21us, whereas the TL072 takes 31us. Also, the TL072 provides a lower gain and hence a lower noise level. You could probably get the same smooth threshold by simply lowering the overall gain of the preamp.
Adjusting PIs indoors can sometimes be problematic due to the external EMI. Once you have it outside in a shielded case, then most of the twitchiness goes away.

[josewashere]

The NE5532 has a unity gain bandwidth of 10MHz, but the TL072 only has 3MHz.
Tonight I'll physically try swapping out the NE5532 and examine what's happening on the scope.

Sounds like a great plan!
I'm starting to think that the chatty threshold is caused by the EFE sampling somehow and as the tl072 is not seeing longer delays its also not seeing the EFE sample giving a smoother threshold.
The threshold is so smooth that it really feels like the preamp stage could probably use more gain with a corresponding increase in sensitivity. Some posts ago Davor did mention that the efe circuit could be introducing more noise into the system.
Then again I could just be way off track here.

[josewashere]

I'm curious how will NJM2068 do.

I have actually tried the NJM2068 and didn't notice any difference. So far a LM4562NA was a bit more sensitive than the ne5532 but with an increase in chatter.

[Davor]

I'm curious how will NJM2068 do.

[Qiaozhi]

On your mpp when using the tl072 chip do you find that it stops detecting at the longer delays? Mine is now detecting at the same distance as with the ne5532 with a much smother threshold but it stops responding to any metal at the longer delays. Do you think some component changes around the preamp chip will help? or is it just the tl072 not being suitable for this design?

The NE5532 has a unity gain bandwidth of 10MHz, but the TL072 only has 3MHz.
Tonight I'll physically try swapping out the NE5532 and examine what's happening on the scope.

[josewashere]

Sure I realise that a vlf is going to loose a lot of detection distance when going through the ground and a PI is not going to loose nearly as much.

On your mpp when using the tl072 chip do you find that it stops detecting at the longer delays? Mine is now detecting at the same distance as with the ne5532 with a much smother threshold but it stops responding to any metal at the longer delays. Do you think some component changes around the preamp chip will help? or is it just the tl072 not being suitable for this design?

[liudengyuan]

Unfortunately, so many years have passed. Delta Forum is not changed, we need serious research, experiment, experience, and better electronics and software technology, in order to make a better machine,