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Date Posted: 14:33:15 08/28/02 Wed
Author: Reg
Subject: Re: Missed a couple of questions
In reply to: Steve Herschbach 's message, "Re: PI's and small gold nuggets" on 11:16:59 08/28/02 Wed

Hi Steve,

I thought I covered all the ground but left out a couple of things that might be of interest.

One, The duration of the pulse has an effect on depth capability of certain objects only. Normally, a shorter pulse width is sufficient for hunting for gold. A simple rule of thumb is if the signal decays in 50 usec, then a 50 usec pulse is all that is needed.

However, larger gold may have a very long time constant so a longer pulse length would be a benefit to add some depth. Just how much depth, I couldn't say, but there should be an increase.

Actually, a PI could be set up as several different PI detectors if a person wanted to do that. The limiting factor would be pc board space and the circuitry used.

One simple way to do this would be with a microprocessor. Then a quick switch could be made by simply flipping something as simple as a toggle switch.

A PI basically pulses the coil for a finite length of time, then takes a sample of the received signal at some point later in time.

The signal after the pulse shuts off will drop very rapidly to near 0V and then decay slowly to 0. When an detectable object enters the field of the coil, the decay rate changes, thus causing a minute voltage difference between a sample taken with no object present and one with a target in the field of the coil.

If this difference is amplified and processed, a response can be generated indicating an object is present.

Sampling of the receive signal can be done most any time. However, because the next stage of ampification is usually large, the sample is usually taken when the voltage is quite small.

One thing I didn't mention is actually there are 2 samples taken even if a form of ground balance isn't used. The second sample is normally taken much later after most of the targt signals have decayed away.

This later sample is needed to eliminate the effects of the earth's magnetic field. When a coil of wire is moved in the earth's field, a very small voltage is produced. The later sample is taken and basically subtracted from the first to eliminate this earth field effect. If this second sample isn't taken and then subtracted from the first then rapid sweeping of the coil in the air would produce a very loud response.

Ideally, this second sample and the subtraction should be such that there is a perfect elimination of the field effect signal. Because of slight variations in component values, component drift, and other factors including a very high sensitivity, it is possible the earth field effect isn't perfectly eliminated.

It may also cause a slight response from certain hotrocks to be heard.

A slight difference can lead to some other slight quirks. On extremely weak targets it may show up as a slight increase in signal from a target when sweeping from one direction than from another.

So, most PI's actually take 2 samples but only one is used for target detection. Normally discussions ignore this second sample since it really doesn't affect the detector's response to a desired target, except to subtract the field effect signal.

Now, in the case of the Minelab, actually several samples are really taken. It is just some of the samples are to eliminate the earth field effects.

Now a quick brief bit of history. George Payne noticed that the ground residual response response was a function of the pulse in 1972. His technique to eliminate the ground was to pulse the coil with a negative pulse twice and right after the second pulse, pulse the coil with a smaller positive pulse. The residual ground response from the immediately preceeding negative pulse caused the ground response in the positive direction to be of a different amplitude. This basic detector design also had a form of iron discrimination. Unfortunately, the detector was never built.

Just thought I would toss that one in.

Reg

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