Moodz' Awesome Gold Pulse Induction Version 3 - MAGPI V3 Project

[moodz]

Thanks Moodz, can you explain the reason for the higher TX frequency compared to the lower found in other PI designs (typically < 2kHz)?

The MAGPI uses a IRF9640 which has a 200 volt avalanche spec. For a 5 volt supply the TX timing is set so that the flyback sits a little below the avalanche voltage ( approx 220 - 240 volts on my devices ).

The flyback voltage is proportional to how fast the coil is damped. Using a conventional damping resistor is much slower than active damping. So because the damping time is much faster on the MAGPI the TX on time has to be shorter than conventional PIs.

I did notice some time ago someone has patented this ( not me ).

[Gunghouk]

Thanks for that. I thought it was something to do with ground penetration but maybe that was VLF.

[moodz]

Thanks for that. I thought it was something to do with ground penetration but maybe that was VLF.

A slower PI can penetrate deeper into the ground but the coil current peak will be higher on a slower PI so is the penetration a function of frequency or coil current or both ? ( not sure myself )

[Gunghouk]

I thought it was the TX pulsewidth that changed coil current

From FFT tests I've undertaken, reducing pulsewidth causes the transmitted amplitude of each harmonic to approach similar levels but at a much reduced overall power output of course.

I think I went down to 20uS pulse width at 1kHz at which point the power was probably unusable though each harmonic upto about 15th were of similar amplitude.

Wouldn't reducing frequency allow greater gain from your high spec front end amp?

[moodz]

I thought it was the TX pulsewidth that changed coil current

From FFT tests I've undertaken, reducing pulsewidth causes the transmitted amplitude of each harmonic to be approximately the same but at a much reduced overall power output of course.

That is right of course but I was referring a symmetrical pulse ( equal time on / off ) so the coil current ( unless it saturates ) is proportional to TX frequency.

For interest I just dialled up a TX on time of 100us and off time of 900 us ( ie 1khz repetition ) and the MAGPI handled it OK however I did not adust the sample timings so sensitivity to sub microsecond targets disappeared.

[Gunghouk]

I would love to bury a sensor and see what amplitude of signal it actually picks up for varying freq & pulse width. The results would make a great waterfall diagram and FFT the results also.

[moodz]

That would be great ... I noticed that the detector seems to have more lag at the lower TX frequency even though the TX current has tripled ... the sensitivity has not increased.
But it is still detecting the 1 microsecond target.

[Gunghouk]

Target saturation ? You could try reducing the pulsewidth until the sensitivity noticably drops then crank back to where the sensitivity is 'normal' then alter the freq for that PW. Maybe there's a sweetspot for PW and/or freq for that target size/material in air/ground.

Of course it's going to be different for every coil !?

BTW whats the lower freq breakpoint for your RX amp, higher than the 1kHz TX freq ?

[moodz]

Target saturation ? You could try reducing the pulsewidth until the sensitivity noticably drops then crank back to where the sensitivity is 'normal' then alter the freq for that PW. Maybe there's a sweetspot for PW and/or freq for that target size/material in air/ground.

Of course it's going to be different for every coil !?

BTW whats the lower freq breakpoint for your RX amp, higher than the 1kHz TX freq ?

No I meant coil saturation as in the LM7805 current limiting kicking in. But then again maybe not I just checked it and the limit is around 2.4 amps.

What is interesting is that despite the coil current change there was not much change in sensitivity ... just response time. Not enough testing though.
The lag in response time would be due to less samples per second moving from 14 Khz to 1 Khz. - filters take longer to charge.

The breakpoint is 10 Khz so about 20 db down at 1 Khz ... but all the harmonics have the same target info modulated onto them and the 1 Khz signal has more harmonics as they are spaced at 1 Khz instead of 14 Khz in the passband.


spectrum of 14 khz square wave vs 100u / 1000u tx

[Gunghouk]

Yeah once you get to 50% duty cycle it's the fundamental and only the odd harmonics by amplitude ratio 1/3 + 1/5 etc. Once again is there an optimum duty cycle where the power & harmonic distribution is evenly spread ?

[moodz]

Like this ?

[Gunghouk]

Yes that would seem to be the ideal. What are the conditions here ?

[Gunghouk]

Reduce F to 500Hz and the harmonic 'nit' comb gets finer

[moodz]

Reduce F to 500Hz and the harmonic 'nit' comb gets finer

Its just the mathematics of time domain pulses to frequency domain.

For instance here is a spectrum from DC to 1 GHz. (In the time domain its a single pulse of 1 million volts / amps that is 1 nanosecond wide )
The individual spectral peaks cant be seen as they occur every 1 nanohertz so they merge to form a continuous line.

However no matter how "exciting" these spectrums look for metal detecting there is no time domain in FFT and nuggets are not frequency selective or they would be used in radios

[moodz]

In the meantime JLKing has made the following suggested change to improve the preamp low frequency rolloff.
A second order filter in the feedback element. Requires minimal components.
Thanks JLKing !

Picture says it all.