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

[moodz]

So you are using a digital based brick wall type filters as to completely stop any signal below 10 Khz?
Interesting.
An AD822 would work well.

Woody after all these years of modifying Minelab detectors you dont understand what the spectrum of a PI is ? The spectrum in the post above is the TX coil current spectrum off a plain damped PI with no mods / filters or any modifcations.

Here is a real spectrum of 1 Khz below.

There is a DC component because that is the average DC current of the monopolar pulses however targets dont respond to constant mag fields. ( so we dont need this for target infos )

This is the spectrum the target sees and responds to ( ie 1khz 3 Khz 5Khz etc) .... not 10 Hz or whatever.

[detectormods]

Oh, thank you for explaining what you are trying post, I thought you were transmitting at 10 KHz and observing higher mixing products. So thanks for explaining what it is. But I do find it strange that do not realise that P.I detectors are wide bandwidth in nature and sub10 hertz noise figures are very important.

[Detectorist#1]

Hi moodz,
"Good points .. but ... the PI signal stimulating the target caused eddy currents to flow during the collapse of the TX current waveform and the target response has no signal information below the TX primary modulation frequency ?

So if we say have a TX frequency of 1kHz then we are looking at demodulating target signals at 1 Khz upwards.

But the typical opamp noise performance looks likes this ... so why would we amplify anything below the primary TX frequency ?"
Sorry if I not understand your idea!
It is very interesting that "no signal information below the TX primary modulation frequency" - this conclusion is after simulation.
Very strange conclusion for me.
It is interesting - if some other PI MD hobbyist will agree with this conclusion.

[JoyJo]

And what can be said about the ad8091 operational amplifier. The recovery time is relatively low, and they are available at an acceptable price.

[Carl-NC]

It is interesting - if some other PI MD hobbyist will agree with this conclusion.

moodz is correct, at the preamp the spectrum of interest starts at the TX frequency. There is no need to for the preamp to have a flat response to DC, you can add high pass filtering to knock down EMI. Except in the ITMD projects, I've always used a DC preamp just because "that's the way it's always been done." Whether it is beneficial depends on the TX frequency. If you have a large coil cache hunter running at 100Hz, then it won't help much. But a nugget detector running at several kHz should see EMI improvement.

[lucifer]

Here is a real spectrum of 1 Khz below.

There is a DC component because that is the average DC current of the monopolar pulses however targets dont respond to constant mag fields. ( so we dont need this for target infos )

This is the spectrum the target sees and responds to ( ie 1khz 3 Khz 5Khz etc) .... not 10 Hz or whatever.

moodz,
Can you please post the images again? Seems there's some problem and they don't open for me.

[Carl-NC]

The squares are what I also see.

[Detectorist#1]

Hi Carl,
Thank you for your additional information for spectrum of signals in PI preamp!

[moodz]

moodz,
Can you please post the images again? Seems there's some problem and they don't open for me.

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

[moodz]

Hi moodz,
"Good points .. but ... the PI signal stimulating the target caused eddy currents to flow during the collapse of the TX current waveform and the target response has no signal information below the TX primary modulation frequency ?

So if we say have a TX frequency of 1kHz then we are looking at demodulating target signals at 1 Khz upwards.

But the typical opamp noise performance looks likes this ... so why would we amplify anything below the primary TX frequency ?"
Sorry if I not understand your idea!
It is very interesting that "no signal information below the TX primary modulation frequency" - this conclusion is after simulation.
Very strange conclusion for me.
It is interesting - if some other PI MD hobbyist will agree with this conclusion.

Detectorist#1 you seem to think I simulate everything ... that is not the case. I usually breadboard a circuit for real , then simulate it to see what the mathematics behind it is , then proceed to PCB.

My background and training is in RF .. I have always known that the PI TX spectrum did not extend below the primary TX frequency. However as Carl said the preamps have always extended down to DC and this is probably becuase there early PI machines were low frequency.

However several years ago my designs in the field ( not the simulator ) had alot of trouble with EMI ( mains hum ) and magnetic field ( earth field and some types of rocks with residual magnetics ) . I tried a high pass preamp and have never looked back since then. If your TX frequency is high enough and you have good rejection of LF in your preamp then you dont have to sample for Earth field correction and mains hum disappears.

[moodz]

Oh, thank you for explaining what you are trying post, I thought you were transmitting at 10 KHz and observing higher mixing products. So thanks for explaining what it is. But I do find it strange that do not realise that P.I detectors are wide bandwidth in nature and sub10 hertz noise figures are very important.

Any waveform ( including all PI waveforms ) can be decomposed to a series of sine waves however the series ( or spectrum ) starts with the fundamental frequency. So if you have a 1 Khz waveform of some shape ( square / triangle / ramp , weird minelab transmit waveform , whatever ) the spectrum STARTS at 1 Khz and is broadband ( ie extending from 1 Khz upwards but NOT downwards in frequency ) and will not contain sub 10 Hz components.

if you think about it .. how stimulating would a sub 10 Hz waveform be to a 0.1 gram gold nugget. ?

[detectormods]

Any waveform ( including all PI waveforms ) can be decomposed to a series of sine waves however the series ( or spectrum ) starts with the fundamental frequency. So if you have a 1 Khz waveform of some shape ( square / triangle / ramp , weird minelab transmit waveform , whatever ) the spectrum STARTS at 1 Khz and is broadband ( ie extending from 1 Khz upwards but NOT downwards in frequency ) and will not contain sub 10 Hz components.

if you think about it .. how stimulating would a sub 10 Hz waveform be to a 0.1 gram gold nugget. ?

True, but I posted from experience, by testing, as an example but let’s look at hard facts, yes , lower noise in the 1Khz -100 KHz range is much more important than noise at 10Hz , more important is the slew rate, any laggard behaviour will not capture the dynamics of a target response. But, this is my observance from experimenting, too much bandwidth add noise, but I observed that high noise specification in the lower frequency domain translates into additional noise into the higher frequency spectrum, is it via some mixing component of sub harmonics? I do not know, I have built input stages with the opa1612 ltc2054 that should be exceptional but I get a rise in noise floor, I have even tried building impedance converters but for some reason I do not end up with the performance of the low noise low frequency specified part. I myself would be over moon if someone can explain to me why this happens.

[moodz]

True, but I posted from experience, by testing, as an example but let’s look at hard facts, yes , lower noise in the 1Khz -100 KHz range is much more important than noise at 10Hz , more important is the slew rate, any laggard behaviour will not capture the dynamics of a target response. But, this is my observance from experimenting, too much bandwidth add noise, but I observed that high noise specification in the lower frequency domain translates into additional noise into the higher frequency spectrum, is it via some mixing component of sub harmonics? I do not know, I have built input stages with the opa1612 ltc2054 that should be exceptional but I get a rise in noise floor, I have even tried building impedance converters but for some reason I do not end up with the performance of the low noise low frequency specified part. I myself would be over moon if someone can explain to me why this happens.

I ran across the same problem ... you cannot just sub in a highpass ( actually its a band pass ) filter and hope all will be good.

There are three parts or time periods to the waveform coming into the preamp.
Considering a "traditional" PI

The first part is the time during TX ON when the current is ramping up in the TX coil.
The second part is the flyback period ( very short ) .. this is when the front end protection clamping and damping or switching is active.
The third part is the actual RX period starting directly after the fly back.

Basically because the RX period is not continous ( interrupted by TX and FLYBACK ) and non linear response of preamp then the RX is considered to be modulated.

Any form of modulation results in convolution or frequency mixing of the input waveforms.

The RX waveform consists of UNWANTED LOW frequency components ( dc offsets, earth field, mains hum etc ) AND WANTED HIGH frequency target responses ( and ground response .. deal with later )

The preamp non linear response to high level signals causes the LOW frequency unwanted components to be modulated onto the HIGH frequency wanted components.

So you need a preamp that does not cross modulate the LOW with the HIGH and problem solved ( earth field and mains hum etc anyway )

Traditionally using high spec opamps partly solves this as the low distortion / low noise / high speed / recovery of the these amps minimises cross modulation and overload.


... are you following where this is leading to ?

[detectormods]

I ran across the same problem ... you cannot just sub in a highpass ( actually its a band pass ) filter and hope all will be good.

There are three parts or time periods to the waveform coming into the preamp.
Considering a "traditional" PI

The first part is the time during TX ON when the current is ramping up in the TX coil.
The second part is the flyback period ( very short ) .. this is when the front end protection clamping and damping or switching is active.
The third part is the actual RX period starting directly after the fly back.

Basically because the RX period is not continous ( interrupted by TX and FLYBACK ) and non linear response of preamp then the RX is considered to be modulated.

Any form of modulation results in convolution or frequency mixing of the input waveforms.

The RX waveform consists of UNWANTED LOW frequency components ( dc offsets, earth field, mains hum etc ) AND WANTED HIGH frequency target responses ( and ground response .. deal with later )

The preamp non linear response to high level signals causes the LOW frequency unwanted components to be modulated onto the HIGH frequency wanted components.

So you need a preamp that does not cross modulate the LOW with the HIGH and problem solved ( earth field and mains hum etc anyway )

Traditionally using high spec opamps partly solves this as the low distortion / low noise / high speed / recovery of the these amps minimises cross modulation and overload.


... are you following where this is leading to ?

A new design.

[Carl-NC]

... are you following where this is leading to ?

A new design.

LMAO, good one Woody.

[moodz]

Well you are getting to the point.

The plot below shows output spectrum from a "traditional" preamp vs output of "new design" preamp. There is a 10 millivolt 50 hertz mains signal hitting the coil in both cases.
The gain of both preamps is 100.

The "new design" clearly attenuates the 50 hertz mains by 40 db ( ie 100 ) AND the noise floor is 20dB lower.