Coil Saturation in a PI?

[ivconic]

Many years ago I held one of these at a treasure show, I recall it was 70+ pounds. It belonged to Carl Fismer and was for sale. I wish I had the money back then to buy it.

You are not that naive as you appear!
Such old silver usually is not purified good and also consist of 5-15% of gold too!

[green]

I would say additional Gain in the pre and post amps, then 1500pps into 100nF/100k Ohm integrator. Plus the use of the Three sample method with a Long second sample (~110us) to detect/disc high conductors.

Forgot about second sample. Greater gains at longer delay times for the 200us Tx. Still not double the signal, maybe missing something else. Does depend on quarter simulation being correct.

[waltr]

Forgot about second sample. Greater gains at longer delay times for the 200us Tx. Still not double the signal, maybe missing something else. Does depend on quarter simulation being correct.

Note in your sim that at 100us after TX off the signal is double for a 200us pulse than for a 100us pulse. Remember that long TC targets do not have a high signal early in sampling compared to short TC targets. The signal increase is later. This is fairly easy to see is your spice sim and also doing the math for target TC's in excel.

Yes, and the longer integration time into the 100nF cap as well as the pps (& sampling) rate.
Much of the sensitivity in a PI detector is in the integrator stage.
Carl has posted this on how the integrator works. I did an Excel spreadsheet to simulate an integrator and play with values & timing. This is a worth while exercise to learn how this works and to optimize values.

by Carl
link:

The purpose of the integrator is to constructively add successive positive samples (resulting from a target) while
averaging out everything else. That is, random noise (which is both positive and negative) will average to zero. So the
integrator needs to have a charge rate that is fast enough (compared to the sample rate) to build up a signal, and a
discharge rate that will get rid of the signal fairly quickly once the target is gone, so you don't get an audio response
long past the target.

So let's make up some numbers. Sample rate is 1kHz, so sample period is 1ms. Sample width is 10us or 1%. Let's say a
weak target signal from the preamp is 10mV. A single sweep of the coil takes 1 second.

We get 1000 pulses per swing, so as we go over a target we might get 10-20 positive samples. Let's say exactly 10.
Without an integrator, this would result in 10 samples that are 10mV high with a 1% duty cycle.
This is a time-averaged signal of 100uV and probably will never trigger the audio.

Let's say we use a single-ended integrator shown below; R1=1k, R2=100k, C1=0.1uF. When the switch is closed vin (=10mV)
is converted to a 10uA current (i1) by R1 and charges the cap. dv/dt = i/C1 which gives 10uA/0.1uF = 100V/s, so in 10us the
cap charges up by a whopping 1mV. Wow, a whole millivolt? Yup.

When the switch opens the integrator is left with R2 discharging C1, at a time constant t = R2*C1 = 10ms. We have 990us
of discharge time (sample switch open) so the signal drops to e^(-990us/10ms) = 90% of whatever the sampled value is. So
when the switch is closed, the cap charges by 1mV, and when the switch is open the cap discharges by 10%.

We assumed a 10mV signal for 10 samples, so at the end of each of the 10 samples we have:

1: 1.00mV
2: 1.90mV
3: 2.71mV
4: 3.44mV
5: 4.10mV
6: 4.68mV
7: 5.22mV
8: 5.69mV
9: 6.12mV
10: 6.51mV

6.5mV doesn't sound like much, but it's 65 times better than the average 100uV without the integrator. And the integrator
is usually followed by a gain stage so if it has a gain of 100 (e.g., HH) then the final voltage applied to the audio is 650mV.
That's much better.

Finally, when there is no more target signal, the integrator cap will discharge in about 3t (95%) so a time constant of
10ms means the integrator output dies off in about 30ms.

- Carl


If anyone wants to learn more about pulse induction RX signal integration, look up the following phrase "lock in amplifier theory".
You should see links to many PDF files from equipment manufactures and universities explaining how lock in amplifiers extract
signals buried in noise. What Carl has explained above gets into the theory about how a lock-in amplifier works and how similar
it is to what happens when you can integrate many RX signals when searching for very small or very deep targets.

The key number is how many samples are being integrated while the target in within the coil area. This translates into coil size,
sweep speed and the pulse rate of the TX pulse. Eric Foster would integrate many signals in his 3000PPS PI machines and integrate
from 500 to 1500 RX signals to improve the RX sensitivity rather than putting more power into the TX signal.

Short TX pulses are optimum for small low TC targets and allows earlier sampling to detect at 10 us or earlier on small gold
targets. By integrating many samples target signals can be extracted from the noise.

For or those interested, you can easily see low lock in amplifier theory can be applied to pulse induction integration but pay
attention to:
1. Full stimulation of your target based on its time constant.
2. Lowest delay you can achieve.
3. Optimum coil size for target.
4. Sweep speed to optimize the number of RX samples being integrate.
5. Maximum PPS rate you can operate at for your desired targets while fully stimulating them.

I hope this adds a new perspective on integration.

Joseph J. Rogowski

[Ferric Toes]

If you vary the length of the TX pulse from short to long while observing the decay of a target signal after preamplification, you will see that the decay curve 'fills out' as the pulse is lengthened. This is best seen with a more conductive target e.g. a silver US quarter or even a clad. The TX pulse should be flat topped all the way so that the coil resistance is limiting the current rather than the inductance. The range of TX widths should be, say, 50uS to 1mS and adjusted by a potentiometer. The TX width that corresponds to the point that there is no further change in the shape of the receiver decay curve is what I call target saturation. Any increase in the length of the TX pulse after this is just wasting energy as the magnetic field within the object is invariant. Prior to that point the internal field is still limited by the opposing eddy currents from when the TX pulse switched on.

Eric.

[ivconic]

If you vary the length of the TX pulse from short to long while observing the decay of a target signal after preamplification, you will see that the decay curve 'fills out' as the pulse is lengthened. This is best seen with a more conductive target e.g. a silver US quarter or even a clad. The TX pulse should be flat topped all the way so that the coil resistance is limiting the current rather than the inductance. The range of TX widths should be, say, 50uS to 1mS and adjusted by a potentiometer. The TX width that corresponds to the point that there is no further change in the shape of the receiver decay curve is what I call target saturation. Any increase in the length of the TX pulse after this is just wasting energy as the magnetic field within the object is invariant. Prior to that point the internal field is still limited by the opposing eddy currents from when the TX pulse switched on.
Eric.

One and only Eric!
Thanks!
...

"...Target saturation is an unclear term to me..."

[6666]

Hi Waltr, thanks for your reply in post #22

[6666]

I would say additional Gain in the pre and post amps, then 1500pps into 100nF/100k Ohm integrator. Plus the use of the Three sample method with a Long second sample (~110us) to detect/disc high conductors.

Hi Waltr, did your values for the intergrator come from your spread sheet ?, with your three sample method would you have a working timing sequence , what do you vary to compensate for ground variations ?, thanks

[bklein]

Is there a way to see the target response other than using and being influenced by the TX waveform? Some kind of local sensing just sensing the target?

[waltr]

Hi Waltr, did your values for the intergrator come from your spread sheet ?, with your three sample method would you have a working timing sequence , what do you vary to compensate for ground variations ?, thanks

The three sample method is used First for GEB. It's secondary effect is high/low TC discrimination, much like on the TDI.

Starting values for the integrator are from Carl's Hammer Head but I did play with other values in the spread sheet and found Carl's values were good. Changes were adding gain stage after the sampling switch and lowering the input series R (one the charges the integrator cap) from 1k to 220 Ohm.
Also played with the values in the TDI schematics and could see why the integrator Mod to the TDI was a big improvement.
All details are in my build thread:
https://www.geotech1.com/forums/show...ake-on-the-HH2

[6666]

The three sample method is used First for GEB. It's secondary effect is high/low TC discrimination, much like on the TDI.

Starting values for the integrator are from Carl's Hammer Head but I did play with other values in the spread sheet and found Carl's values were good. Changes were adding gain stage after the sampling switch and lowering the input series R (one the charges the integrator cap) from 1k to 220 Ohm.
Also played with the values in the TDI schematics and could see why the integrator Mod to the TDI was a big improvement.
All details are in my build thread:
https://www.geotech1.com/forums/show...ake-on-the-HH2

Thanks Waltr, had a look through your build thread and found some references, will have a read after dinner.

[green]

Charted a silver quarter, constant current flat tops after 20us. Tx on time, 50, 100, 200, 300, 400 and 500us. Charted some California ground also.
Not much increase in signal after 300us with either target. Both targets have a greater increase in signal at 50us vs 10us after Tx off. Appears greater increase in signal with the quarter at longer of times gets canceled if detector is adjusted to cancel ground signal.

[Ferric Toes]

Many years ago I held one of these at a treasure show, I recall it was 70+ pounds. It belonged to Carl Fismer and was for sale. I wish I had the money back then to buy it.

Ahh---- the Atocha big pile; that brings back memories of the early 1980's. The ultimate in large conductive objects where you are dealing in milliseconds, not microseconds. Nowhere near saturation even with a 1.5mS Tx pulses. Unfortunately I only have two bars left now which make great doorstops.

Eric.

[ivconic]

Ahh---- the Atocha big pile; that brings back memories of the early 1980's. The ultimate in large conductive objects where you are dealing in milliseconds, not microseconds. Nowhere near saturation even with a 1.5mS Tx pulses. Unfortunately I only have two bars left now which make great doorstops.

Eric.[ATTACH=CONFIG]47780[/ATTACH]

I am all green in face now... problem is i can't find green, angry and envy smiley!


P.S.

[Carl-NC]

Ahh---- the Atocha big pile; that brings back memories of the early 1980's. The ultimate in large conductive objects where you are dealing in milliseconds, not microseconds. Nowhere near saturation even with a 1.5mS Tx pulses. Unfortunately I only have two bars left now which make great doorstops.

Eric.

Every time I see one of those I start drooling.

[Ferric Toes]

I am all green in face now... problem is i can't find green, angry and envy smiley!

Every time I see one of those I start drooling.

I'd better come clean now, as I can't have members getting worked up. Even worse, I might get an extradition warrant to face charges. Those bars are just aluminium replicas of the same size and shape as the real ones. Understandably, Treasure Salvors were not keen to even loan one for tests, although I did have access to one of them in the US when working with TS. A few were found prior to locating the 'big pile', which was Mel Fisher's main priority when he found Spanish archive records showing nearly 1000 seventy pound bars were on board. The top bar in the picture has a hole bored from one end, several inches into the bar so that I could prove by inserting a ferrite cored coil, that an external pulse field would take some relatively considerable time to penetrate. Even with the less conductive aluminium it was a few milliseconds. Decay was also as long. Mint marks on the bar were done yesterday with marker pen. The bottom bar was sprayed with gold paint a few years ago. They make a good talking point with visitors; useful as door stops, and the top one for ironing the carpet when it gets rippled - hence the shine.

Eric.