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

[Aziraphel]

The solution is to put a HP filter on the input and disable the input during transients. Similar circuit solutions existed in the early days of pi metal detectors. But I don't like this solution, partly because turning the input on and off itself causes transients.
I wonder if you have found a simple and elegant solution to this problem?
P.S. and in the picture, I think, high-frequency noise is depicted against the background of a voltage drop in the coil. I don’t know how this relates to low-frequency noise filtering.

[moodz]

... But I don't like this solution, partly because turning the input on and off itself causes transients.
I wonder if you have found a simple and elegant solution to this problem?
....

You have made the key point ... if you consider the transmit and recieve waveforms from a discrete sinewave ( fourier ) analysis ...every waveform in the system no matter what amplitude or period can be decomposed to an additive series of sinewaves. (note : the keyword here is additive ).

If a non linear event occurs ( eg diode clamping, blocking, amplifier overload, switching ) which causes distortion of the waveforms then due to the non linearity a convolution ( mulitplication ) will occur and you end up with frequency products you proabably dont want..... note : here the unwanted event is multiplication not addition as previously mentioned.

So imagine we have a PI fundamental tx frequency of 1 kHZ pulses ... then the Fo of this pulse train is a 1 Khz sine wave ... imagine also that we have a 50 or 60 hertz mains interference signal of 1 millivolt in the recieve coil.
If a non linear event occurs ( eg preamp overload, clamping diode distortion etc ) then this will cause the 50/60 hertz mains frequence to be modulated onto the 1 Khz Fo signal. ( multiplication in the time domain cause frequency addition / subtraction AKA "difference" in the frequency domain. )
So now our 1 Khz Fo will have 3 components .... 1000 - 50, 1000 and 1000 + 50. ie 950 hertz, 1000 hertz and 1050 hertz ( for the 50 hertz case ).

We could call this "intermodulation".

So when you take a sample from the output of the preamp at 1 khz rate ... you will in fact then demodulate the difference signals from 1 Khz ( 950 and 1050 hertz ) .... and your sampled signal will have 50 hertz interference on it.

Because it would be very difficult to construct a filter that can separate 1050 and 950 hertz from 1000 hertz signals ... some PIs use sampling rates that are multiples of the mains rate to cancel the effect .. however this is not optimal and does not cancel other noises that may occur at the rx coil.

This same scenario applies to earth field ( modulated by swing speed ) for example or any other interfering signal at the RX coil.

This effect is the same in mono or DD PIs.

A solution I found was to demodulate the preamp output signal by mixing - mulitplying ( ie demodulating ) it with Fo ( eg 1 Khz ) and then LP filter the demodulated ouput and subtracting this from the input of the preamp thus cancelling noise below Fo ( 1 Khz ) by negative feedback.

In summary it could be called a demodulating negative feedback band pass amplifier.

In this way any convolution products produced by non - linearity in the frontend of the RX ( clamping / amp overload etc ) are substantially blocked and prevent modulation of the desired frequency components ( 1 Khz upwards ) from being distorted by earth field or mains etc.

You can see this preamp in the MAGPI 3 circuit. Even though this circuit uses active damping the preamp in that circuit will work on any PI including regular resistor type damping.

The solution has been out there for some time... unfortunately lots of folk have missile lock on expensive low noise amplifiers as the solution to their problem.

moodz.