Coil tester SRF/Inductance/distributed capacitance

[Skippy]

Once I'm able to print out that new Latex crib sheet (I didn't find George's original one that helpful), I'll make the effort to learn it. For now, I'll manage with what I know. What I've just typed above is pretty legible.

I have the maths for this problem written down somewhere, as many years ago I reverse-engineered my Bounty Hunter / Fisher coil, with the internal capacitor on the TX. I drove it from a sig-gen, until it peaked, and noted the freq. Then added various extra caps, and repeated. Then used maths to find L and internal C. But I couldn't find the notes, of course, so have just re-worked it out. Harder than I thought, and still doesn't look quite right ... that single 2*PI looks out of place, but I tested it, and it seems OK.

[green]

Thanks JL for tip on displaying frequency. Better resolution if I took more cycles.

Tried formula with my circuit since wasn't sure about 113 fets in other circuit.

Not what I was expecting, worked good.

[dbanner]

I found another quick way for displaying the frequency of the waveform:
Click on part of the cycle and drag your mouse to the exact same point to the next cycle - the bottom of the waveform window will show the frequency of the distance between these two points.

[green]

I found another quick way for displaying the frequency of the waveform:
Click on part of the cycle and drag your mouse to the exact same point to the next cycle - the bottom of the waveform window will show the frequency of the distance between these two points.

Works good

[green]

Thanks JL for tip on displaying frequency. Better resolution if I took more cycles.

Tried formula with my circuit since wasn't sure about 113 fets in other circuit.

Not what I was expecting, worked good.

Tried with oscillator circuit. Bottom row way off, maybe something I did wrong. Could someone could see what they get using oscillator circuit. Worked with my test circuit reply #32.

[Skippy]

It looks like there's about 6 pF of stray capacitance in the circuit, presumably from things like the FET gate ? Maybe a different FET would give different values, eg choose an RF fet for your model, which would I assume to have lower C values.
The last line is clearly an error. Changing the tuned C from 400pF to 550pF would only drop resonant freq to sqrt (40/55) = 85% approx.

I wonder about how high the Q of that oscillator is. It looks as thought the jfet gate is forwards conducting for half the oscillation cycle. If it's running at a low Q value, then the oscillation freq will be lower (than a hi-Q oscillator ), which may be why there appears to be stray capacitance. It may not actually be stray capacitance.

[KingJL]

It looks like there's about 6 pF of stray capacitance in the circuit, presumably from things like the FET gate ? Maybe a different FET would give different values, eg choose an RF fet for your model, which would I assume to have lower C values...

The J113 Cgd is ~6.46pf, Cgs is ~5.74pf. 2N4393 Cgd is ~4.57pf, Cgs is ~4.04pf.

[green]

Tried with oscillator circuit. Bottom row way off, maybe something I did wrong. Could someone could see what they get using oscillator circuit. Worked with my test circuit reply #32.

Did some lawn work and tried again. Lot better this time. Wasn't awake earlier.

[dbanner]

I stumbled across this article while browsing through the pages of Nuova Elettronica magazine. It involves some sort of inductor measurements method.

Unfortunately the article is not written in english. The Google translation is atrocious. Anyone knows how to get a decent translation on this document?

[6666]

I found another quick way for displaying the frequency of the waveform:
Click on part of the cycle and drag your mouse to the exact same point to the next cycle - the bottom of the waveform window will show the frequency of the distance between these two points.

Thanks that works good.

[dbanner]

I was looking for an RF generator circuit for conducting srf experiments. Came across this one. 3N211 is strange. Range purported to be from 50KHz to 30MHz
http://www.qrp.gr/rfgenerator/index.htm

[AK48]

Anyone knows how to get a decent translation on this document? try this :

Attached Fileshttps://www.deepl.com/translator

[green]

I was looking for an RF generator circuit for conducting srf experiments. Came across this one. 3N211 is strange. Range purported to be from 50KHz to 30MHz
http://www.qrp.gr/rfgenerator/index.htm

Hi dbanner, wondering if you have tried exciting the test coil with a PI to compare with the other methods of measuring SRF you have tried?

[dbanner]

Hi dbanner, wondering if you have tried exciting the test coil with a PI to compare with the other methods of measuring SRF you have tried?

No, I haven't tried that method. I'm sure it works as others will atest.

I'm exploring alternative methods which do not involve the use of an oscilloscope, but measuring the peak resonance response by means of a detector circuit driving a meter.

In the end the goal is to have an accurate and simple means of measuring coil parameters without counting cycles on a ring down waveform off of an old analog scope.

A learning experience in old methods.

Here is a schematic of a Q meter, useful device which has alternative uses in measuring Inductance and self resonance, notice the metering circuit and how it is coupled, very important to avoid loading as this will adversely affect the results.

[dbanner]

Here is a document on self resonance and self capacitance.