Optimizing Target Responses

[Carl-NC]

Waveform drawings are in Visio, graphs are in Excel.

[alex---1967]

Let's look at a realistic example...

Measured coil parameters:
L = 300uH
Rs = 5 ohms (total series R)
SRF = 500kHz (inc. cable etc)

I have a question - when measuring the natural frequency of a coil, is only a coil with a cable used?
Does the transistor capacity not count? This is right?
It seems to be logical, yes.
But then, when determining the coil time constant tau_RX (in a real metal detector), it seems to me that the capacitance of the switching transistor should also be taken into account.
That is, it is necessary to use in the calculations not the capacity of the coil, but the total capacity of the coil + cable + transistor ...
Am I right or wrong?

[green]

I have a question - when measuring the natural frequency of a coil, is only a coil with a cable used?
Does the transistor capacity not count? This is right?
It seems to be logical, yes.
But then, when determining the coil time constant tau_RX (in a real metal detector), it seems to me that the capacitance of the switching transistor should also be taken into account.
That is, it is necessary to use in the calculations not the capacity of the coil, but the total capacity of the coil + cable + transistor ...
Am I right or wrong?

Fet capacitance can have a large effect on SRF. Adding a fast diode between fet and coil minimizes the effect, small effect.

[alex---1967]

What is SRF?
I am not very good at English abbreviations.

How does a fast diode minimize this effect? By what means?

[Qiaozhi]

What is SRF?
I am not very good at English abbreviations.

How does a fast diode minimize this effect? By what means?

SRF = Self-resonant frequency

https://www.geotech1.com/forums/show...e-series-diode

Also, Appendix D of The Voodoo Project gives a more detailed analysis.
https://www.geotech1.com/forums/show...d-October-2020

[Mr.Jaick]

What is SRF?
I am not very good at English abbreviations.

How does a fast diode minimize this effect? By what means?

SRF = Self-Resonant Frequency
is the frequency of undamped coil(aka ringing), which is determined by inductance and over all parasitic capacitance.

A fast reverse recovery diode with low junction capacitance in series with the drain of you power MOSFET will reduce your overall solid state parasitic capacitance seen from coil.
how?
your diode capacitance will be in series with Coss of your MOSFET thus two caps in series, and I suppose you know what happens to the overall value?
but the story has not finished yet
since capacitors hold charge and there are two of them in series one smaller(diode) and the other larger(MOSFET) they're gonna trap part of the flyback charge and distribute between themselves after turn off, and the major chunk of this charge will be on the MOSFET's Coss obviously because larger capacitor eats more.
that charge or voltage whatever you're comfortable with will remain there and part of it slowly discharges due to Leakages(Drain-Gate, Drain-Source, P-N, PCB) all the OFF time when you're busy with signal processing, now what that remaining charge does is it will put these P-N junctions in Hard reverse bias which will decrease their capacitance by orders of magnitude (good news).
for example your typical MUR460 will have around 100pF for 1V of reverse bias but for 200V it'll be lower than 10pF which is great.
circuit

blue trace is Drain(where you trap the charge)
Red is RX


as you can see the blue trace basically starts at 626V and discharges down to 273V in 400us
you may say why doesn't it go all the way to the 900V flyback then decay form there?
the answer to that is my MOSFET can take 900V flyback but not my 600V rated diode hence starting form there, the blue trace actually does go to 900V for a moment but diode breaks down fast and continues from 600V
if the MOSFET was 600V as well then the flyback wouldn't go that far.

[alex---1967]

Thanks. I'll think about it.
663

[bbsailor]

Thoughts to consider when trying to reduce coil capacitance.

Sources of coil- seen capacitance.

1. The coil windings, turn to turn capacitance governed by the dielectric constant of the wire insulation and the insulation thickness and coil to shield capacitance.
2. MOSFET and other diode capacitance as seen by the coil.
3. COAX cable capacitance based on the length between the coil and the control box that is about 100pf.

We can reduce number 3 by being creative and building a remote circuit located at the end of the aluminum shaft, near the coil that puts the necessary circuits in the available space to reduce or eliminate the coil- seen coax capacitance.

The less capacitance that the coil sees, the higher value the damping resistor can be and the steeper the coil discharge sloop will be to better stimulate smaller targets. However, smaller targets may be parts of the coil that retain eddy currents such as coil wire, shield, or other metal parts that were not previously detected.

These thoughts are just to get the creative minds of this forum to push forward.

Thanks

Joseph J. Rogowski

[ivconic]

Thoughts to consider when trying to reduce coil capacitance.

Sources of coil- seen capacitance.

1. The coil windings, turn to turn capacitance governed by the dielectric constant of the wire insulation and the insulation thickness and coil to shield capacitance.
2. MOSFET and other diode capacitance as seen by the coil.
3. COAX cable capacitance based on the length between the coil and the control box that is about 100pf.

We can reduce number 3 by being creative and building a remote circuit located at the end of the aluminum shaft, near the coil that puts the necessary circuits in the available space to reduce or eliminate the coil- seen coax capacitance.

The less capacitance that the coil sees, the higher value the damping resistor can be and the steeper the coil discharge sloop will be to better stimulate smaller targets. However, smaller targets may be parts of the coil that retain eddy currents such as coil wire, shield, or other metal parts that were not previously detected.

These thoughts are just to get the creative minds of this forum to push forward.

Thanks

Joseph J. Rogowski

Obviously the technology is having its limits.
Change the technology if possible.
If you can't squeeze more juice from a lemon: change a lemon... or take a orange!


Purely for the sake of theoretical discussion; how would it be if instead of a cable we connect the coil to the TX output with the Lecher line?

[Carl-NC]

Ha, I haven't seen Lecher lines mentioned in over 35 years, back to my e-mag lab in college. I actually own a metal detector that almost uses that concept, a Roth BFO. The coil is connected to the control box by two regular 70s-style telescoping antennae, and each antenna is a conductor for the coil. One of the most impractical designs I've ever seen.

Back to practical matters, a higher impedance cable has lower capacitance. So 75Ω is better than 50Ω. I have seen stranded-core coax up to 125Ω but can't seem to find any for sale. And then there is 300Ω twin-lead which looks more like a Lesher line, but it's not shielded.

[ivconic]

... One of the most impractical designs I've ever seen...

Wait a minute! The conductors can be "impregnated" into a strong plastic material, in the form of a rod that will go between the detector and the coil.
With a very simple connection system. It doesn't have to be impractical that much.


"...So 75Ω is better than 50Ω..."


And 300Ω is better than 75Ω !
So called "twin lead" (but not twisted) Tv antenna cable is practically a form of a Lecher line!
(i haven't read your post completely, haven't seen that you also mentioning it)
Wink! Wink! Wink!
Does it have to be shielded at all?
FelezJoo works better if not shielded. Much calmer and more accurate.
And yes it can be additionally shielded if needed.
"...The conductors can be "impregnated" into a strong plastic material..."
The very same material with conductive compound used in common search head shielding.
And if give more freedom to my imagination; the connecting system can be made with two small neodymium pairs, one pair also "impregnated" into rod and another pair into detector enclosure.
(Similar to Apple charging&data transfer connectors)
Give your imagination total freedom to figure out how actually easy it can be done!


"...So called "twin lead" (but not twisted) ..."
Yes there is also a twisted one version too.
Come to think... why they also made a twisted one?

Go figure...

[waltr]

Another way is to put the RX front end and the TX pulse transistor into the detector head right at the coil. Then the cable has no effect.

[bbsailor]

Another way is to put the RX front end and the TX pulse transistor into the detector head right at the coil. Then the cable has no effect.

Yes, Here is where experimentation is very important. If the active components are inside the coil housing, might they be seen as targets at certain delays. However, if these active components were located at the end of the plastic coil stem, they might be far enough away and outside an area where these components are detected as targets.

These are just thoughts to eliminate or minimize coil seen capacitance to sample faster and better detect smaller targets. Good experiments can lead to better performance!

Joseph J. Rogowski

[Carl-NC]

One of the problems with putting components down below is that now you have to provide power and clock over the cable. In a traditional mono coil PI, you have to put both the TX and preamp down below so you might have TX power, VCC & VEE for the preamp, ground, and TX clock. Plus the RX signal, so a minimum of 6 lines. The TX power will have large transient currents unless you also place the tank caps down below.

It becomes a logistical challenge which is probably why no one has done it. And the only reason for doing it is to squeeze out a lower sample delay. But then all that metal close to the coil becomes a problem. And it's not the absolute decay deflection that's a problem; that just becomes an offset that can be ignored. Rather, it's the motion over variable mineralized ground (esp hot rocks) that causes variations in the TX field which then induces a transient response in the circuit components that looks just like a good target. This doesn't show up in bench testing or even in the usual backyard testing. Only when you finally get to a real gold field.

[ivconic]