PI Monocoil - sampling under 4us almost impossible?

[nick_f]

Hi All,

I've played with LTSpice simulation, using all kind of components and selecting coil capacitance to much less than what we can obtain in real life. Based on these experiments, my conclusion is that is virtually impossible to obtain sampling under 4us, using a 300uH coil and a similar circuit with Hammerhead (including the MOSFET series diode).
If some people got such results with their detectors, I would be very interested to hear about the Transmitter circuit details. I am talking about monocoil circuits.
Also, if somebody got better results in the Spice simulation, let's compare the circuits.
The selected components have rather good characteristics among the components that can be purchased at a decent price and my next board will be based on them.

I attach in here the Spice file for the circuit.

Regards,
Nicolae

[okantex]

if you like reading , you will find ns timing in the following topics
regards

[nick_f]

if you like reading , you will find ns timing in the following topics
regards

Hi Okantex,

What topics? Some links? Do they address the specific configuration I am talking about?

Regards,
Nicolae

[Esteban]

Ultrashort pulses by tunnel diode less than 1 us. This deppend how rapid is the tunnel diode. You can inject a senoidal wave and at output is present square wave. A common tunnel diode conmuts in 600 ps, but a rapid at 20 ps! But I suspect that the short pulses convert in long pulses when travel via the cable and coil.

http://www.geotech1.com/forums/showthread.php?t=15305

[nick_f]

Ultrashort pulses by tunnel diode less than 1 us. This deppend how rapid is the tunnel diode. You can inject a senoidal wave and at output is present square wave. A common tunnel diode conmuts in 600 ps, but a rapid at 20 ps! But I suspect that the short pulses convert in long pulses when travel via the cable and coil.

http://www.geotech1.com/forums/showthread.php?t=15305

Hi Esteban,
My question has nothing to do with the fastest pulse we can generate.
It's about the specific circuit used in Hammerhead, where the effect of coil inductance, capacitance of coil and transistor limit the minimum turnoff time of the voltage on the limiting diodes. I was curious if anybody can claim to obtaining a delay of less than 4us, because I consider it would be very difficult to obtain, no matter what components and coil construction would be used. Tinkerer got to less than 4us using a concentric coil (not monocoil as I specified), which is a great accomplishment in itself.

Regards,
Nicolae

[Tinkerer]

Hi Esteban,
My question has nothing to do with the fastest pulse we can generate.
It's about the specific circuit used in Hammerhead, where the effect of coil inductance, capacitance of coil and transistor limit the minimum turnoff time of the voltage on the limiting diodes. I was curious if anybody can claim to obtaining a delay of less than 4us, because I consider it would be very difficult to obtain, no matter what components and coil construction would be used. Tinkerer got to less than 4us using a concentric coil (not monocoil as I specified), which is a great accomplishment in itself.

Regards,
Nicolae

You are right, my posted results refer the an IB-PI coil arrangement. It is like comparing apples with beet roots.

However, if your priority is to have the shortest possible delay with a mono coil, you have to make the compromises needed to achieve that.
It is a question of priority.
If you make a coil with AWG 30, Teflon insulated wire (use bbsailors instructions)
8mm spacer between winding and shield,
47 Ohm series resistor between coil and Drain,
low Coss Mosfet,
an ultrafast low capacitance diode,
a damping resistor right at the coil and a Cermet trim pot to adjust for precise damping on the PCB
a fast opamp in the non inverting mode with low gain,

You will end up with a front end with a very short delay, well suited for finding small gold.
The trade-off is that you will not have so much depth penetration.

Tinkerer

[nick_f]

Hi Tinkerer,

Your advice is very sound and I learnt this is what I have to do in order to obtain the lowest time delay. My question is more theoretical than practical, I just want to find the physical limits, based on real components for this delay.
I was curious if I can trust the results of my Spice simulation, or if I had some flaws in judgement and some people actually got real results better than that (or better simulations).

Regards,
Nicolae

[Esteban]

I think that for to minimize very great the capacitance in cable, for example, you can use this arrangement: divide the PI detector in 2 parts. I think this model can be useful for many of us in the forum.

[Max]

I think that for to minimize very great the capacitance in cable, for example, you can use this arrangement: divide the PI detector in 2 parts. I think this model can be useful for many of us in the forum.

The best solution is potting all with epoxy... that way he'll use then e.g. nylon screws and nuts to keep it fixed on the pole...

I see that stuff also in some unusual vlf thing.

Also ML did similar stuff... but potting front-end inside coils...

Kind regards,
Max

[Esteban]

The best solution is potting all with epoxy... that way he'll use then e.g. nylon screws and nuts to keep it fixed on the pole...

I see that stuff also in some unusual vlf thing.

Also ML did similar stuff... but potting front-end inside coils...

Kind regards,
Max

Yes... right! But are the pulse gen. and mosfet into the coil? Don't remember... But in this model the mosfet and pulse gen. is near the coil.

Regards

Esteban

[nick_f]

Shortening the cable helps with the pulse width. However, I don't think is a good idea to place the board on the coil, because all the metal from the board, large capacitor, heatsink of transistor, PCB tracks can be detected and they increase the pulse width. The lower cable has to have some length, to keep the tiny board at a safe distance from the coil detecting range.

Regards,
Nicolae

[Esteban]

Maybe designing a PCB with thiny tracks and study disposition of components. Also you can use here SMD components, only electrolitic can be the big problem.

[Woody.au]

Sound to me that you want to detect Pin Heads but there sure is merit getting the dead time down.

[nick_f]

Sound to me that you want to detect Pin Heads but there sure is merit getting the dead time down.

Hi Woody,
I think lowering the dead time improves sensitivity to all small targets (this includes 0.5g...5g gold nuggets). Tinkerer explained to me about the response of small targets. A 1g gold nugget's response decreases a lot with every microsecond and there are two ways to increase the sensitivity of the detector: either shorten the dead time, either increase the gain of the circuit. There are physical limits to both of these.
Increasing the gain also increases susceptibility to electrical noise.
Overall, I would like to make a circuit that can detect a target with a TC=5us (that's a 0.5 square inch thin aluminium foil) to over 15cm and a target with TC=10us (1 square inch alu foil) to over 25cm.
Could you tell us what's Minelab sensitivity to these targets in the standard (unmodified) version, and after your mods? I know that in the domestic environment you're using the fig 8 coil, that has reduced sensitivity, but I can also use a similar setup and we can compare the results. You'll have to bear with me though, it will be some time until I get my electronic components to do some improvements to my circuit.
When you get out in the field with a Minelab SD2000 or higher, maybe you can make a short movie about its discrimination abilities. I haven't seen such a movie on YouTube.

Regards,
Nicolae

[simonbaker]

Hi All,

I've played with LTSpice simulation, using all kind of components and selecting coil capacitance to much less than what we can obtain in real life. Based on these experiments, my conclusion is that is virtually impossible to obtain sampling under 4us, using a 300uH coil and a similar circuit with Hammerhead (including the MOSFET series diode).
If some people got such results with their detectors, I would be very interested to hear about the Transmitter circuit details. I am talking about monocoil circuits.
Also, if somebody got better results in the Spice simulation, let's compare the circuits.
The selected components have rather good characteristics among the components that can be purchased at a decent price and my next board will be based on them.

I attach in here the Spice file for the circuit.

Regards,
Nicolae

Hi Nicolae:

My LTSpice (ver 4.02c) does not have 1N4937 diode. Where do we get that?

Regards,

-SB