Pulse Induction Hz difference

[BILL96]

I have a pretty good understanding of how VLF frequencies work and what may work best for different applications. My primary interest is gold detecting and I notice that PI detectors come with different operating frequencies, some 1200hz , some 1500hz, etc. Can someone give me a brief guide as to to why the different frequencies and use's.
Thanks, Bill

 

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[giuhalftrack]

the PI detector works in different way to VLF.. you don't have to think at frequency as a continuous wave but at the frequency of pulses (number of pulses x second).

the detector gives an impulse to the coil, this impulse interact with the object in the ground.
after the pulse, the detector "listen" to the coil to get a "response" by the target, the decay time of the "mirrored" pulse given by the object gives you some form of discrimination, you have to choose the delay (after the pulse) after you listen to the eventually returned signal by the object, metal with high conductivity gives you a longer decay time so we can say that the returned pulse is more delayed.
This way of works gives you a better understand on the fact they have a mono coil with one winding, the coil in fact it's alternatively transmitting and than receving.

This is a very simplified explanation.

In the surf PI clone that I build I remember that there was a trimmer to set this delay time, and i set the unit to the delay more suitable for detecting gold.

 

[BILL96]

When shopping for PI detector is the operating HZ a factor to consider ?

Thanks

 

[giuhalftrack]

When shopping for PI detector is the operating HZ a factor to consider ?

Thanks

the frequency it is not of great importance, what you want is a "fast coil" in which microvolt changes in the the impulse decay (influenced by the target response) can be appreciated and detected.

Remember that in any other design you are measuring a frequency variation like amplitude (less common), difference of frequency (BFO), changing of phase (VLF), so the frequency itself is what you are "measuring" in your circuitry.

The PI detectors acts emitting a steady magnetic field (the pulse) so it is the pulse width that you can control in some design, because a longer pulse mean more depth, and the moment in time in which you look at the decay tail measuring the changes (sampling).

The book "inside metal detector" by Overtone and Moreland states:

"..the important operating parameters of the PI detector center on timing of the coil and sampling pulses...
the pulse frequency is the rate at which the coil is pulsed, and for basic design, this parameter is not critical..."

I will take this book as absolute truth

 

[BILL96]

Very interesting, thanks for your help.

Bill

 

[Jim in Idaho]

Another thing to consider is non-ferrous metals decay faster. That's why a lower delay time is important. Otherwise the signal from those would be gone before the detectors started "listening" for the signal. So, really short delays are important for gold detectors.
Jim

 

[BILL96]

Good information, have more research t do.

Thanks

 

[pulltaboo]

I'll try to describe it in less geeky terms…
You have a camera with flashlight attached, and you use it to look for things hiding in a dense mist.
VLF frequency is the color of that flashlight. Different colors penetrate the mist differently and let you see smaller or more distant objects more clearly.
PI frequency is the frame rate of your camera (or strobe frequency of the flashlight). If it's too low for your sweep speed, or you have a lot of visual clutter, you'll see blur… or nothing at all.

 

[BILL96]

Good explanation, things are becoming clearer
Thanks