Replies to Minipulse Plus

[liudengyuan]

I've built a mini. The effect is not very good, increasing the power supply to the I 24V, the effect is not ideal

[CAS]

Just a quick question - what is the best way of calculating the optimum value of damping resistor (R1)?

[joop]

Calculating is not an option... Watch the output of the NE5532 (TP3) as you change the damping resistor. When the damping resistor is to low, the decay curve will have spikes. I have a 360 uH coil and use around 300 Ohms. The original minipulse coil uses 500 uH with 470 Ohms. So you will need to know the inductance of your coil.

[josewashere]

Also with the revised timings on this unit a 500uH inductance is probably too much for the mpp. 350uH probably a much better choice but yes if the damping resistor value is too high you will see ringing on the preamp output waveform and even on the coil connections too with a scope. You aim for the highest value before it starts to ring. Too low a value and you loose sensitivity. Every different coil will use a different value but the standard 470ohm resistor will work with most coils that I tried.

[Qiaozhi]

I've built a mini. The effect is not very good, increasing the power supply to the I 24V, the effect is not ideal

You cannot run the MPP from 24V. The LT1054 has an operating supply voltage range from 3.5V to 15V.

[liudengyuan]

I did not use LT1054, I am using other modules, Tucker converted into a plus or minus 24V 5V, and with isolation

[josewashere]

Yes I agree there is terrible marketing hype surrounding detectors. Have you noticed that even the buyers of this hype largley help propagate the hype too lo.
Many years ago I owned a garret ads deepseeker vlf. With it I could ping a coin at about 35cm in air and the reality is that there is not much improvement since then. All the current vlf detectors that I see today would be no better than that, most are probably worse. In air there wouldn't be many PI's that can beat that either.
The laws of physics can't be overcome.

Good GB and as you say lower noise front end is were improvements can be found.

I certainly hope so. The other PI projects can benefit from this approach as well, but I'll have to see where to put what to make them tick.
In essence there is no difference between any of the simple projects against the much more expensive commercial ones. Except for the preamp noise and the ways of reducing it. MPP is limited with the coil damping resistance to 4nV/sqrt(Hz) or so. A select few of the commercial products take care of input noise in a meaningful way. QED for example. Everything else is just a clever play of marketing wits and front panel presentation.

[Qiaozhi]

Yes I agree there is terrible marketing hype surrounding detectors. Have you noticed that even the buyers of this hype largley help propagate the hype too lo.
Many years ago I owned a garret ads deepseeker vlf. With it I could ping a coin at about 35cm in air and the reality is that there is not much improvement since then. All the current vlf detectors that I see today would be no better than that, most are probably worse. In air there wouldn't be many PI's that can beat that either.
The laws of physics can't be overcome.

Good GB and as you say lower noise front end is were improvements can be found.

Unfortunately air tests do not equate to depth in the ground. Modern motion detectors have an advantage over the older designs (such as the Garrett ADS) with better target separation and recovery, plus target ID.
Also, it's worth noting that PI detectors generally will not do so well in an air test when compared to VLFs, but in real soil the situation is different. VLFs can lose a third of their air test depth (and often much worse), whereas the PI detector's loss is small. Today's designs seem to be optimized more towards detection of targets in contaminated soil, so comparisons with older designs (using air or non-mineralized/homogeneous soil tests) will show little improvement in depth. Plus, as you say, the laws of physics haven't changed.

[liudengyuan]

George's view of the same,

[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,

[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?

[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.

[Davor]

I'm curious how will NJM2068 do.

[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.

[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.