Replies to Minipulse Plus

[KingJL]

Thanks, but I think this integrator is different. It acts different in spice. Maybe I'm missing something. What is the gain and frequency response formula?

Gain is R23/R25, LP cutoff is 1/( 2×3.1416×R20×C13 ), band stop is 1/( 2×3.1416×R24×C13 ).

[KingJL]

I tried to twist various coils I made into a figure eight and the noise went down but the sensitivity went down with it by a lot.

In the figure 8, only the rx coil is formed into the 8
( or a facimile thereof ), the tx coil circumscribes the entire rx figure 8.

[Tinkerer]

KingJL

Probably the next best thing after the Litz wire, is fine strand tinned insulated wire. It seems that the eddy currents are much less and the insulation reduces the inter-wire capacitance. Twisted, fine stranded, tinned insulated tween leads in parallel give even better results.

[mickstv]

Thanks, but I think this integrator is different. It acts different in spice. Maybe I'm missing something. What is the gain and frequency response formula?

Here's another link. Eric posted the thread, one of the circuits is the same as the MPP, you might find some useful info.

http://www.geotech1.com/forums/showthread.php?20470-Integrator-Circuits-for-PI

[green]

Hi Green, check the link below.


http://www.geotech1.com/forums/showt...ntegrator-gain

Thanks, but I think this integrator is different. It acts different in spice. Maybe I'm missing something. What is the gain and frequency response formula?

[josewashere]

Today I tried a MAX412 (as suggested by a member) in the preamp in place of the Ne5532 . Its better than the ne5532 and about the same as the LM4562 that works best in my MPP. I think that's it for me trying op amps. LM4562 stays.

[josewashere]

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.

I think its ok for me were it is at the moment but yes if you swing too fast you don't get a response. Its definitely worth some experimentation to increase the delay if the sensitivity stays the same. I tried to twist various coils I made into a figure eight and the noise went down but the sensitivity went down with it by a lot. A double D design is interesting to me as it should be quieter on bad ground.
Only one of my home made coils is as good as the commercial coiltek coil. Its smaller 9 inch vs 11 inch but when you take the size into account its as good as the coiltek. All others are worse in some way.
I think I am going to buy some proper litz wire one day and give that a good try. I just want to eliminate eddy currents within the coil. Also want to have a go at a basket weave coil one day.

[mickstv]

Been getting a brain cramp trying to figure frequency response and gain for the integrator. Tried building it in spice. Changing sample time changed frequency response and gain. Don't know if I did it right. Could someone give the formula for gain and frequency response for the integrator.

Hi Green, check the link below.

http://www.geotech1.com/forums/showthread.php?19079-Calculating-integrator-gain

[green]

Been getting a brain cramp trying to figure frequency response and gain for the integrator. Tried building it in spice. Changing sample time changed frequency response and gain. Don't know if I did it right. Could someone give the formula for gain and frequency response for the integrator.

[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.

[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.

[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.

[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.

[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.

[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.