PI SAMPLE DELAY QUESTION

[baum7154]

i remember a electronics engineer who makes detectors many years ago saying that you can sample in the ringing to get more sensitivity.....im not sure if this is correct cause no one so far has made any comment on this....my detectors use a 390 ohm resistor to get a clean back emf curve with no ringing or over shoot...maybe my resistor value is too low !!!! maybe some ringing and over shoot is good to sample early ???? but im out of my depth here and im not really sure !!!

I'm not sure where the 390 ohm value came from but it seems to be prolific in the designs of many detectors and it is far too low for any fast/high performance mono coils I have wound. Quantum leaps in shorter sample delays are possible with a well designed fast coil and a proper critical damping resistor. An adjustable damping network is now built into my CHANCE PI detector with quality high power components as I like to try a variety of coil designs and the adjustment feature makes it possible to optimize the coil performance quickly. In my opinion, getting rid of that compromise 390 ohm resistor is one of the most significant things to improve detector performance.

Regards,


Dan

[daverave]

i think your correct about the 390 ohm resistor...its far too low for a fast coil...what sort of proformance you getting with the chance ??? i have never got around to making my chance detector...can it discriminate against iron using a mono coil ????

[daverave]

With the resistor network trick it's easy to test this theory and determine how much ringing is ok.
In the past I've found that a small amount of ringing does increase sensitivity, but too much has the reverse effect. Setting up a PI correctly is all about compromise.

i have always used a 1k 2W potentiometer across the coil to test for the damping...without any other resistors !!! is that incorrect doing it like that ????

[baum7154]

i have always used a 1k 2W potentiometer across the coil to test for the damping...without any other resistors !!! is that incorrect doing it like that ????

The danger is that with just the pot you are able to lower the damping resistor to '0' or very low ohms and that will eventually burn the wiper contact in the pot... and worse. That is why a network with a series resistor like Qiaozhi describes that sets the absolute lower resistance limit is important. Also I am finding that 1K is often not enough resistance to critically damp a very fast coil. If you have not already done so you should read BB SAILOR's paper on building a fast PI mono coil. In it are tests and methods to measure your coils and determine Self Resonant Frequency. It would be very good to determine the Self Resonant Frequency and Inductance of your coils because once that is known you can easily determine the 'out of circuit' damping resistance required for your specific coil(pi X SRF X uh = Rd). This in turn will at least let you know if the 390 ohm resistor is even close to the proper value. However critical damping must be performed while the coil is connected to the detector because of additional capacitances, resistance and inductance added by the electronics.

As far as the CHANCE PI detector goes yes, it does discriminate between colored metals and iron…but with a caveat. Initial detection at low signal levels defaults to the left or GOLD/ALUMINUM end of the VDI scale accompanied by a low audio tone that also indicates GOLD/ALUMINUM. As the signal strength rises above 3 units the discrimination begins to kick in but jumps around from left end to right end of the VDI accompanied by a combination of low and a few higher pitched melodic tones if iron is present. As the signal level rises above 6 units the VDI generally stabilizes at the right end accompanied by melodic high pitched tones if iron is present. (It is also interesting to note that Minelabs has mentioned in a paper exactly this same phenomenon of VDI instability at lower receive signal levels.) These iron associated high melodic tones sometimes described as 'Organ Grinder Tones' confuse many people as to their meaning. Gold and aluminum are indicated by low tones, silver by a very pure, very high pitched tone, and copper by a pure high but slightly lower pitched tone. Both Silver and Copper are indicated at about 85% to 95% to the right of on VDI scale…Iron is on the far right end of the scale. To someone who has spent time with a CHANCE PI these silver and copper tones are unmistakable from the low gold tone and the melodic iron tone, and the VDI indication follows the tones. Does this mean that the discrimination is 100% accurate at signal levels above 6 units? No, and I don't think any PI discrimination is 100% accurate on iron discrimination because a rusty razor blade will still indicate as gold or aluminum or a flake of rust from an old steam boiler will still indicate as gold or aluminum even at higher signal levels. Sometimes we just have to dig to know the truth.

I hope this helps,

Dan

[daverave]

i will have to use resistor network in future to test the damping...i have to do things by trial and error cause im not a mathmatical person !!!! i really wish i could do calulations to work out parameters but maths was always my weakest subject at school...i just love all this detector science cause it makes my old brain think !!!! its just getting it all into my head and trying to understand !!!!

[Qiaozhi]

In another words , it looks like that some portion of energy is really "stored" in the target , being released and received just after the transmitter current stopping ( and transmitter field collapsing ) . And it really looks like we need a TIME to pump this energy into the target during the ON state of the transmitter circuit .... so the whole case is very similar to a well-familiar process of the capacitor recharging - it needs time to charge and needs time to discharge , and so on . And this is why the idea of "target charging" isn't a nonsense anyhow , as I think .

I can follow your argument, but your conclusions are incorrect.
Although making the TX-on pulse longer gives the result of providing a larger RX signal, this is not caused by any target charging. The first reason why the RX signal is stronger is because any eddy currents that were generated during the initial on-time will be reduced or will have completely dissipated. These initial eddy currents have a negative impact on the target response. The second reason is because the magnetic field will have reached the maximum possible for that coil/circuit configuration.
If you use a shorter TX on-time, the coil current may be lower when the TX pulse is turned off (reducing the strength of the resultant magnetic field), and any eddy currents in the target will not have decayed away. Result: a lower RX signal, and nothing to do with any fictitious target charging.

Obviously it is always good to discuss these things. But ... if target charging is a reality, then what physical mechanism is responsible?

[daverave]

just a couple of questions....do i measure the damping resistance with all three resistors connected or just the pot with the series resistor ???? also i was looking at the bb sailor self resonance test...do we use a sine wave and do i adjust to get max peak and thus this shows resonance ????

[Qiaozhi]

just a couple of questions....do i measure the damping resistance with all three resistors connected or just the pot with the series resistor ????

You need to measure the whole resistor network. (i.e. [the pot + series resistor] + parallel resistor).
Just to be clear: the original damping resistor must also be disconnected during the test.

[baum7154]

Yes all three resistors in the network should be connected during measurement. The network replaces the 390 ohm damping resistor so the 390 must be removed. Yes we use a sine wave and adjust frequency to get the max peak.

Good Luck,

Dan

[mschmahl]

Yes we use a sine wave and adjust frequency to get the max peak.

Good Luck,

Dan

Is there an other methode than using a signal-generator to messure the self ressonance. I have an ossziloscope but no signal generator.

[baum7154]

Yes you can watch the ringing frequency of the coil and try to determine it with your scope but it is less accurate and includes the capacitance contributed by the electronics of the detector...but in the end that is a good thing!

[mschmahl]

Can you give me some hint´s how i can watch this ringing frequency? Where should i connect the probe? Sorry for my stupid question, but i´m lost there.

[Tec]

With a typical coil with a gain of around 1000 amplifiing the discharge signal:
I have found that a small piece of Al foil will add 2uSec to this curve and a 20Cent coin (fairly large nickel copper coin) will add 14uSec.

The later the sample, the less you will detect.

Where the sample is and how much the curve is increased, depends on the gain of the amplifier also.
You could adjust the gain so you are in a good range about 15uSec after switch off. And then you can sample at any point after that.
eg If you turned the gain right down, you would have no signal after 5uSec

[Tec]

An important point if it hasn't already been made, is the target only feels the Switch Off of the field.

It is 'Hit' by the change in field, It doesn't matter how long the pulse is on for, this is only increasing the current in the coil and making the Switch-off more abrupt.

[green]

Can you give me some hint´s how i can watch this ringing frequency? Where should i connect the probe? Sorry for my stupid question, but i´m lost there.

I disconnect the damping resistor and lay the scope probe near the fet driver, not connected. Maybe near the coil might be better.