Dual coil for mpp

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This implies that the in-circuit resonance is different to open coil resonance because of the Rx network and MOSFET.
The implication is profound. The Rd is a function of in-circuit resonance, which(the in-circuit resonance) can only be measured with an oscilloscope WHILE THE CIRCUIT IS OPERATING.
ALL OTHER SRF MEASUREMENTS METHODS ARE IRRELEVANT.(for PI)

godigit1, I think your intuition is correct!

This means that the value of C in the formula is indeterminable, except by means of solving for C after the in-circuit resonance has been measured. in other words, If we use just the coil C, then the calculated value of Rd will be way off what is actually required.

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This brings me to another conclusion, for the making of a fast coil, measuring the SRF by conventional method is sufficient.(sig gen and scope)
But for the fast coil to be effective, the capacitive loading of the coil in circuit must be kept as low as possible.

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Now I am more curious about the formula for Rd used in the Whites dual field patent. It must be a little different to take into consideration the coupling between the inner and outer coils. But it might be completely bogus for calculating a realistic damping resistor value for any other PI.
Has anyone tried a whites dual field coil on the MPP with the same value damping resitor as with the TDI?

[godigit1]

https://www.youtube.com/watch?v=6FOq7-WsL5c
Please see this video listen to the verbage "the industry has specific standards for measuring SRF not sRF"
he uses the word spacial resonance I think its spatial resonance by reading other stuff.

[godigit1]

https://www.youtube.com/watch?v=tjbK4LsOQRk
This video is a little long but he actually uses a spider web coil.
This method I get 3.333 mhz on the coil.
this is a convenient way as q can quickly be determined as well.

The most interesting part of this video is where he talks about being able to adjust the q seperatly.
I havent played with it yet I need a big varible cap.

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Oh my lord! Yet another wrench is thrown into the wheel!
Time to throw in the towel and look for a nice chacuzi.

[godigit1]

This video discusse the differential probe method.
https://www.youtube.com/watch?v=hqhV50852jA
With this method Im getting with 1 pf cap 6.288mhz with 5 pf 1.598mhz.
I dont see where he states his cap value. But at 5 pf my damping works without adding the 1k input resistor.
I really hate to say it but I think this coil is really 1.598 mhz I get this # 3 different test methods and its in the ball park of where im actually damping.
I was going to do a nice report but its all coming out in the thread, but I think you guys can now see my delima its not just two #s Im looking at and the lowest works math wise.
So what am I missing?

[dbanner]

My theory is that the "in-circuit" resonace is lower than the "open circuit" resonance because of the added capacitance presented by the Rx network and mosfet and other things like coil cable and connectors etc.
It is the frequency of the "in circuit" resonance (RINGING) that needs to be quantified in order to calculate an appropriate Rd using the formula where C is calculated by using the Frequency of the ringing oscillations (in-circuit resonance) and where L is the inductance of the coil.
The point is that the C in the above formula must not be taken to mean only just the coil self-capacitance!! Rather, it represents the total capacitance presented to the coil "in-circuit"

[green]

My theory is that the "in-circuit" resonace is lower than the "open circuit" resonance because of the added capacitance presented by the Rx network and mosfet and other things like coil cable and connectors etc.
It is the frequency of the "in circuit" resonance (RINGING) that needs to be quantified in order to calculate an appropriate Rd using the formula where C is calculated by using the Frequency of the ringing oscillations (in-circuit resonance) and where L is the inductance of the coil.
The point is that the C in the above formula must not be taken to mean the coil self-capacitance!! Rather, it represents the total capacitance presented to the coil "in-circuit"

Reply #35 Carl states you can't measure coil capacitance, need to calculate capacitance from resonance and inductance. You can calculate Rd=pi*L*frequency or you can make an extra calculation, calculate C and use your formula above. Same answer.

The Rx network is disconnected when measuring circuit resonance, Tx circuit only with Rd not connected. The Rx circuit can effect damping, better to find damping resistance with a pot. I like comparing calculated Rd to final Rd to try and learn something. Calculated and critical damped Rd aren't always the same with spice either. I wonder why.

My question is how to calculate internal damping resistor for a dual field coil. Example(120uH coil, 4MHz resonance, calculated capacitance 13.2pf). Rd=pi*120*4=1508, Rd=.5*sqrt(120uH/13.2pf)=1508 or pi*120uH*13.2pf(patent formula)

[godigit1]

the way im seeing the whites patent Rd=L*pi*c
Where c is in henries.
for your first example 120 uh 13.2 pf =.000119999999999999999999999
I get .0452 and move the dcimal over 4 places to get 452 ohm damping.
Dont know if thats correct.
For my coil 135 uh at 3.333mhz and 16.89 pf + .000135H
I get .0572 -572 ohms close to what im actually damping at with a damping set up . The math works that way for me what am I doing wrong?
gotta get to work.

[Carl-NC]

Some general points...

If you are getting 3-5pF for a scramble-wound coil (or almost any coil) I would suspect a foul somewhere.

I've never used a dip meter so I don't know what gotchas might be lurking there. But be aware that using a pickup coil can alter the results, just like using an oscope can also alter the results. For example, a pickup coil might alter the inductance of the main coil, similar to how a BFO detector works.

The oscope loading can be accounted for... you can measure the probe capacitance and subtract it from the calculated total.

A final SRF absolutely MUST include everything... coil, shielding, cable, connector, MOSFET switch, clamp, and preamp. Everything contributes to capacitance.

Ideally the final SRF is best measured by looking at the preamp ringing with no damping R. However, if you are using a MOSFET without a series drain diode, be aware that the FET body diode will truncate negative oscillations and alter the frequency. You either need to have the series diode, or look downstream in the oscillations to where the body diode no longer has an effect.

Non-final SRFs are still useful. I suggest measuring the SRF at every stage: raw coil, add the shielding, add the cable, add the connector, add the MOSFET, add the clamp, add the preamp. In this way you get to see the specific effects of each step, and it will suggest where to focus your efforts if you are looking for more speed.

I suspect the formula for RD in the White's patent is in error.

[green]

Some general points...

If you are getting 3-5pF for a scramble-wound coil (or almost any coil) I would suspect a foul somewhere.

I've never used a dip meter so I don't know what gotchas might be lurking there. But be aware that using a pickup coil can alter the results, just like using an oscope can also alter the results. For example, a pickup coil might alter the inductance of the main coil, similar to how a BFO detector works.

The oscope loading can be accounted for... you can measure the probe capacitance and subtract it from the calculated total.

A final SRF absolutely MUST include everything... coil, shielding, cable, connector, MOSFET switch, clamp, and preamp. Everything contributes to capacitance.

Ideally the final SRF is best measured by looking at the preamp ringing with no damping R. However, if you are using a MOSFET without a series drain diode, be aware that the FET body diode will truncate negative oscillations and alter the frequency. You either need to have the series diode, or look downstream in the oscillations to where the body diode no longer has an effect.

Non-final SRFs are still useful. I suggest measuring the SRF at every stage: raw coil, add the shielding, add the cable, add the connector, add the MOSFET, add the clamp, add the preamp. In this way you get to see the specific effects of each step, and it will suggest where to focus your efforts if you are looking for more speed.

I suspect the formula for RD in the White's patent is in error.

Thanks for the reply

The coils are spiral wound not scramble wound. https://www.geotech1.com/forums/atta...9&d=1570051535

[Carl-NC]

My mistake above... if you are looking at the output of the preamp, then the clamps will also affect the ringing. Not only the diodes turning on, but also the damping effect of the series R. In an inverting amp configuration you will not be able to look at the preamp ringing; in a non-inverting amp configuration you need to look beyond where the clamp stops clamping.

In any case, nix looking at the preamp output; probe the coil connection to the MOSFET. It still may be difficult if the clamp is causing damping action so you may only be able to measure SRF up to the clamp and not beyond. But, hopefully, clamp and preamp capacitance are nowhere close to being the dominant problem.

[godigit1]

Couldn't get to work fast enough.
I was I a hurry to get out the door. I converted the inductance to Henry's math is obviously bad on that last post. Opps

[godigit1]

Thanks Carl,
That's a very thorough explanation.
I have some more experimentation to do .