Replies to Minipulse Plus

[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]

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.

[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]

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

[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.

[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.

[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.

[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!

[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.

[Qiaozhi]

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).

Reducing the sample pulse width was disappointing, as there was no noticeable change. Personally I've always found that increasing the sample pulse width tends to increase the depth, but at the expense of a degradation in signal-to-noise ratio. So in this case I was expecting a decrease in noise, and a corresponding decrease in depth.

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 samplinding g (the sample/hold circuits will tend to integrate to the signal level at the end of the sample period) can be realized.

The GS4 has a 100nF cap fitted in that position, and I've been meaning to try this mod. However, now I've made the change, I couldn't honestly say that there was any noticeable improvement. Without the cap, the cutoff frequency is limited by the GBW of the opamp, but with the 100nF cap Fc = 28.4Hz. However, there was a small reduction in "chattiness" by using a 1uF cap, which reduces Fc to 2.8Hz. The mod did not affect the recovery speed of the detector.

As I've mentioned before, the original Minipulse had a pot for adjusting the audio sensitivity. This sensitivity pot was simply acting as an attenuator on the output of U9a, and the adjustment was so slight that I made the decision to remove it along with the battery check circuit. If you monitor TP9 with an oscilloscope, you can readily see what's causing the audio instability when indoors. Since it seems somewhat pointless to boost the signal and then attenuate it after the opamp, a better solution would be to make R33 adjustable. At the moment, U9a has a gain of 47x. I tried replacing R33 with a 2k resistor (which reduces the gain to 23.5x), and although the beast was calmed somewhat, it was still the same result as reducing the sensitivity to minimum on the original Minipulse. The best result obtained was with R33 set to 4k7 (which reduces the gain even further to 10x), with minimal to no loss of depth. So it would seem to be worth fitting an external 5k sensitivity pot in place of R33.

[Qiaozhi]

Ivconic - I've had a thought about the problem you experienced with the MPP, where the audio amplitude went lower as the target got closer to the coil.
Please check the values of R10 (10k) and C10 (10n), and//or monitor the secondary pulse at TP6 compared to the TX pulse at TP1. This pulse should start at around 100us after TX switch-off. If R10 or C10 are a much lower value, then it's possible that the secondary sample is too close the main sample. In which case you would get the amplitude reduction problem.

[ivconic]

Ivconic - I've had a thought about the problem you experienced with the MPP, where the audio amplitude went lower as the target got closer to the coil.
Please check the values of R10 (10k) and C10 (10n), and//or monitor the secondary pulse at TP6 compared to the TX pulse at TP1. This pulse should start at around 100us after TX switch-off. If R10 or C10 are a much lower value, then it's possible that the secondary sample is too close the main sample. In which case you would get the amplitude reduction problem.

Thanks!
I will inspect it further.

[josewashere]

[QUOTE=Qiaozhi;195570]Reducing the sample pulse width was disappointing, as there was no noticeable change. Personally I've always found that increasing the sample pulse width tends to increase the depth, but at the expense of a degradation in signal-to-noise ratio. So in this case I was expecting a decrease in noise, and a corresponding decrease in depth.

Qiaozhi I just re tested my configuration using the sample width modification (R11, C11) by KingJL and my detector has definitely increased 1.5cm detection distance in air. I'm using a LM4562NA in the preamp. The LM4562 gave me about 1cm extra when I swapped it out 2 weeks ago. All up from my tests tonight 33cm for a men's gold band ring using a coiltek 11 inch mono coil. All test was done in my workshop so it could be a bit better again outdoors.
All in all I really like this detector.
Next a GB mod and I will be very happy!
I did try the 47nf cap across R22 and it didn't do anything for my detectors chattiness. 1 uF does settle it a bit but for now I just left it out. Still considering the 5k pot in place of R33.

[Qiaozhi]

The LM4562 gave me about 1cm extra when I swapped it out 2 weeks ago. All up from my tests tonight 33cm for a men's gold band ring using a coiltek 11 inch mono coil.

Maybe that's the reason for my [non] results, as I'm still using an NE5534 and the original Minipulse coil.

[KingJL]

Qiaozhi I just re tested my configuration using the sample width modification (R11, C11) by KingJL and my detector has definitely increased 1.5cm detection distance in air. I'm using a LM4562NA in the preamp.

If you are still interested in experimenting, try the R11/C11 configuration that results in 15 usec (6.8k, 2n2) and see if the response is a little better for a normal detector swing rate. The combination that give 10 usec may require a little slower swing rate to achieve adequate integration time.

As far as coils... my favorite 2 coils, both of which I made 7 yrs ago and keep using on all my PI's, are a 10.5" DD and a 9" × 4" eliptical shaped figure 8. In both, the tx is ~275 uH and rx is ~380 (I was shooting for 300 and 400 respectively but missed by about 1 turn in each). I love the dd... it gives me the added advantage of ferrous metal discrimination. The figure 8 I use in noisy envirionments and for bench testing as it is extremely immune to environment and ground (salty damp soil) noise. It's only drawback is the small detection profile (the front 4.5" of the figure requireing at least double the scans to cover the same area as the dd.