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

[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]

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.

[Detectorist#1]

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

[Carl-NC]

The squares are what I also see.

[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]

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.

[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.

[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.

[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.

[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]

Woody and Dectroist#1 you are both missing the point of this thread ... its not about the design of preamps ... its about the required frequency response of the preamp.

There is no 10 Hz or other frequencies of interest below the primary TX frequency in a PI at the coil preamp level. ( unless your TX frequency was 10 Hz )

The target signals only become low frequency when they are demodulated from the TX carrier.

Below is the spectrum of a PI operating at 10 Khz ... figure it out.

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.

[moodz]

Old but works very well, if you want to aim at 0.2 Db NF at 10Hz there are only a few cheap ways to do it.

Woody and Dectroist#1 you are both missing the point of this thread ... its not about the design of preamps ... its about the required frequency response of the preamp.

There is no 10 Hz or other frequencies of interest below the primary TX frequency in a PI at the coil preamp level. ( unless your TX frequency was 10 Hz )

The target signals only become low frequency when they are demodulated from the TX carrier.

Below is the spectrum of a PI operating at 10 Khz ... figure it out.

[detectormods]

Hi detectormods,

This idea to use midle power transistors before Opamp is very old. The problem is that this very low level of the noise is possible only when you use very low values of the resistors as the source of the signal. The PI first stage is not those case. MD SD2000 from Minelab use that idea, but the result is moderate - not fantastiq.

Old but works very well, if you want to aim at 0.2 Db NF at 10Hz there are only a few cheap ways to do it.

[Detectorist#1]

Hi detectormods,

This idea to use midle power transistors before Opamp is very old. The problem is that this very low level of the noise is possible only when you use very low values of the resistors as the source of the signal. The PI first stage is not those case. MD SD2000 from Minelab use that idea, but the result is moderate - not fantastiq.

[Detectorist#1]

Hi moodz,
I not see the answer of my question - how to bild optimal design with NE5534 to avoid using of AD797 in the preamp stage in PI design. Using of OPA2188 after demodulation is not nessesery - the level of the signal is far more than noise at this point.