What is the best frequency response for the preamp in a PI ?

[moodz]

..and not only that ... but you dont get a set of free steak knives BUT the intermod distortion caused by the mains signal on the TX signal is also resolved. ( this PI is operating at 10 KHZ ).
Green is the new preamp.

With apologies to Detectorist#1 who does not like sims - thought I dont know any engineers who dont use sims nowadays.

[detectormods]

..and not only that ... but you dont get a set of free steak knives BUT the intermod distortion caused by the mains signal on the TX signal is also resolved. ( this PI is operating at 10 KHZ ).
Green is the new preamp.

With apologies to Detectorist#1 who does not like sims - thought I dont know any engineers who dont use sims nowadays.

As an RF guy, you removed the sidebands, but we all know of a couple of manufactures that would benefit greatly from attenuating 50/60 Hz getting into the domain of the golden goose.

[Detectorist#1]

Hi moodz,

Please, use sims and show us the phase/frequency plot. This plot is very important for PI MD. Thank you in advance!

[moodz]

Hi moodz,

Please, use sims and show us the phase/frequency plot. This plot is very important for PI MD. Thank you in advance!

Here you go ... this was made for a 14 KHz PI.

[Gunghouk]

So where would you put the filter breakpoints, f/2 and 2f perhaps?

[moodz]

So where would you put the filter breakpoints, f/2 and 2f perhaps?

The breakpoints should be at f and as high as the bandwith of your opamp will go but less than the SRF ( self resonant frequency ) of your coil

In my previous post above I said 14 Khz for the bode plot ... but it should have been 1.4 Khz.

You want to pass the primary f and odd harmonics ideally up to f9 ( f f3 f5 f7 f9 ).

If you go much below f ( fundamental ) then you are liffing the skirt of the LF rolloff and degrading the LF rejection.

Heres a qeustion ... why is the phase response flat in the LF rolloff shown above in post #49 ?

[Gunghouk]

Aren't you losing 6db of gain at the fundamental with that as your low breakpoint hence my suggestion of f/2?
My theory is very rusty but isn't there -90 deg shift at the low and +90 deg at the high breakpoints and somewhere between both until zero at the geometric mean frequency point for a 2nd order bandpass filter?
Would a bandpass filter in DSP solve phase shifting?

[moodz]

Aren't you losing 6db of gain at the fundamental with that as your low breakpoint hence my suggestion of f/2?
My theory is very rusty but isn't there -90 deg shift at the low and +90 deg at the high breakpoints and somewhere between both until zero at the geometric mean frequency point for a 2nd order bandpass filter?
Would a bandpass filter in DSP solve phase shifting?

[moodz]

Technically yes you would be losing at the breakpoints but over the whole bandpass the loss is not so critical. So if we are trying to squeeze every db then yes f/2 would be a good start point and 11f could be the endpoint.

I have seen many try to do DSP on a PI ( without demodulation ) and it never worked as far as I know. All the interesting stuff happens in a few microseconds out of a hundred or so and DSP does not deal with this well. Also must remember we need gain as well as filtering. The MAGPI now has a 60 db preamp, 75 db diff amp after demod and 12 bits of ADC.

[Gunghouk]

So is this with passive filter components around the preamp to give 2nd order filtering and single stage 60dB gain ? (f/10 and f10 would give bang on 1kHz geometric freq and zero phase change @ 1kHz Tx )

A couple of other questions, how critical, if at all, is the magnitude of phase shift introduced in the passband ?

Is a gyrator BP design with low Q too severe to be considered in this application?

Presumably target composition/descrimination relies on as little phase change during amplification as possible. Is this assumption correct ?

What are the real sources of noise outside of the home/workshop that require filtering by a BP filter in the field?

Sorry for all the questions, I'm a noob see

[Gunghouk]

Of course f/11 and f11 fit your previously stated upper minimum frequency at least

Fmean = SQRT (Flow x Fhigh)

[moodz]

So is this with passive filter components around the preamp to give 2nd order filtering and single stage 60dB gain ? (f/10 and f10 would give bang on 1kHz geometric freq and zero phase change @ 1kHz Tx )

A couple of other questions, how critical, if at all, is the magnitude of phase shift introduced in the passband ?

I am not sure about that ... no-one that I know of has modelled a PI RX chain to figure if phase shifts are critical or not ... However I suspect they are and someone needs to investigate it.

Is a gyrator BP design with low Q too severe to be considered in this application?
I have not tried one ... you could .. one important factor is that the input impedance to the ( preamp / filter ) should not impact the damping.

Presumably target composition/descrimination relies on as little phase change during amplification as possible. Is this assumption correct ?
Yes and no ... So far I can separate X and R and indicate ferrous / non ferrous. However it can be shown for some targets no change in the X signal as the targets are conductive but dont identify as ferrous or non- ferrous but still give an R signal.

What are the real sources of noise outside of the home/workshop that require filtering by a BP filter in the field?

In the field there are Earth Magnetic Field, mains EMI 50/60 hertz depending on your location, rocks that have a net magnetic field, earth currents from SWER lines mains ( single wire earth return ), DC offsets in your frontend / dissimiliar metals and or , thermal noise, 1/f noise from the opamp itself, LF noise from resistors in your front end.

Sorry for all the questions, I'm a noob see

Dont worry I have not met any experts in the field yet .. were all noobs IMHO

[Gunghouk]

Sorry about that ... this board has trouble with PNG images ... I forgot to convert to JPG.

Actually this harmonic content is for equal duty cycle/pure square wave. For a typical PI with, say 40uS TX on in 1000uS @1khz the even harmonics are as pronounced as the odd and are similar in amplitude to the fundamental well past 11f.

See here

https://www.analog.com/media/en/tech..._book_Ch13.pdf

Page 257

[moodz]

Actually this harmonic content is for equal duty cycle/pure square wave. For a typical PI with, say 40uS TX on in 1000uS @1khz the even harmonics are as pronounced as the odd and are similar in amplitude to the fundamental well past 11f.

See here

https://www.analog.com/media/en/tech..._book_Ch13.pdf

Page 257

I think you might be missing the point of the example ... it does not matter what duty cycle or shape of the TX signal is ... it was merely demonstrating the fact is that there is no useful spectral information below F (the TX fundamental ).
So the "accepted practice" of designing low noise preamps for PIs that extended down to DC is wrong.
Understanding this allows you build a better PI detector than state of the art detectors as they exist today.

[Gunghouk]

I really do get it but I've been dragged down a rabbit hole, which probably should be in another thread, wondering if the amplitude of the TX harmonics should at least be all the same amplitude as far as possible and do particular harmonics 'excite' particular target types.
This is leading me to wonder about modulating pulse width, frequency sweeping and multi-pulse TX signal harmonic content.
Sorry for derailing your thread.

Are the teeth of this nit comb fine enough to catch those pesky sub gramme critters? (rhetorical)