Coil tester SRF/Inductance/distributed capacitance

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[LEFT]Does anyone now how to display frequency with spice? No replies

To measure waveforms frequency I right click on the waveforms title and select 1st and 2nd cursor from the drop down box. I then place the cursors on the successive peaks of waveform.

By the way green, you've been playing with ltspice for years, it is I who should be asking you my friend!

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Dbanner did you run it with 18 v ?

I used a 13.8 volt power supply.

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Trying to understand how the circuit can work. Somewhere I'm missing something. Did another simulation. Coil by it self has a SRF of 1MHz. Both testers measure resonant frequency only. When I calculate inductance, I add a 10n or 100n capacitor in parallel with the coil. A lot higher than coil capacitance. Measure resonance, then calculate inductance using capacitance and resonance. The other tester does the same but uses 150p and resonance to calculate inductance. When I use 150p inductance calculates over 500uH. When I use 10nf, inductance calculates 341uH close to the coil inductance of 338uH. Don't see how to get capacitance or inductance for coil only just knowing resonance. Maybe someone could explain what I'm missing.

I think I understand what you are getting at.
At low tank capacitance of say 150pF, the distributed(coil)capacitance is more of a factor in determining the overall tank resonance. Remember the circuit is for calculating distributed capacitance as well. 150+75=225pF in your example. So you have to make your total capacitance 225pF for calculating inductance from the resonant frequency.

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I found another application for this little circuit. Used as an rf generator.
https://www.zl2pd.com/HFRFgen.html

Interesting here is the replacement of 10k resistor with a 1mH choke.

[green]

I think I understand what you are getting at.
At low tank capacitance of say 150pF, the distributed(coil)capacitance is more of a factor in determining the overall tank resonance. Remember the circuit is for calculating distributed capacitance as well. 150+75=225pF in your example. So you have to make your total capacitance 225pF for calculating inductance from the resonant frequency.

My point is, the only measurement I have is resonance. To calculate inductance need resonance and capacitance and to calculate capacitance need inductance and resonance. Think the only way to calculate inductance is to add a large enough capacitance that coil capacitance doesn't matter. Still might be missing something?

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My point is, the only measurement I have is resonance. To calculate inductance need resonance and capacitance and to calculate capacitance need inductance and resonance. Think the only way to calculate inductance is to add a large enough capacitance that coil capacitance doesn't matter. Still might be missing something?

That is precisely what the coil tester circuit in conjunction with the program does. It measures the characteristics of an unknown coil by taking 2 measurements. First with 150pF and second without 150pF.
The two results are plugged into the program which spits out the Inductance and distributed capacitance.

If you have a look at the math equations contained in the program, it becomes obvious what is being done.

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Opportunity to practice LateX arises.

Cd=distributed capacitance

L= inductance of coil.(L x 1E6)uH

F2 =frequency with capacitor

F1=frequency without capacitor.

150E-12 is capacitance in pF(150pF).

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I hope my translation of the computations in the program are correct. I will have to plug in the numbers.

[KingJL]

I think I understand what you are getting at.
At low tank capacitance of say 150pF, the distributed(coil)capacitance is more of a factor in determining the overall tank resonance. Remember the circuit is for calculating distributed capacitance as well. 150+75=225pF in your example. So you have to make your total capacitance 225pF for calculating inductance from the resonant frequency.

A few tweaks and the circuit will oscillate with 10n in parallel with the coil under test. The 10n allows for fairly accurate coil inductance determination as it minimizes the effect of coil and stray capacitances. I found that the 2N4393 gave the most accurate results as the cdg and cds are a little lower than for the J113. The calculated inductance is 300.5uH versus the actual inductor being 300uH. After you determine the coil inductance, take out the 10n and calculate coil capacitance.
The way I measure the frequency in LtSpice (and for that matter most USB oscilloscopes) is with the FFT function. Grab about 10 cycles from the display, select View->FFT, select "Current Zoom Extent",OK. Now you can measure the frequency. On USB oscilloscopes set your horizontal timing to show 10-15 cycles and enable your FFT view.

[Qiaozhi]

Opportunity to practice LateX arises.

Cd=distributed capacitance

L= inductance of coil.(L x 1E6)uH

F2 =frequency with capacitor

F1=frequency without capacitor.

150E-12 is capacitance in pF(150pF).

I see that you're getting to grips with LaTeX syntax.

A couple of small points:

When writing equations in LaTeX, I usually put 150p in place of 150E-12.



Alternatively you could use engineering notation:



Hint: You can also use C_d, F_1, and F_2 to enable subscript.

[Skippy]

And to calculate the inductance, using the two frequency measurements ( with/without test jig cap):

L = ( F_H² - F_L² ) / ([2 * PI * F_H * F_L ]² * C_T)

Where F_H is the high freq ( eg 1MHz in Greens sim )
And F_L is the low freq ( with the 150p cap, = 577 kHz )
And C_T is the test jig capacitor ( 150pF here)

[dbanner]

I see that you're getting to grips with LaTeX syntax.

A couple of small points:

When writing equations in LaTeX, I usually put 150p in place of 150E-12.



Alternatively you could use engineering notation:



Hint: You can also use C_d, F_1, and F_2 to enable subscript.

I'll definitely take those advice. Thanks.

[dbanner]

And to calculate the inductance, using the two frequency measurements ( with/without test jig cap):

L = ( F_H² - F_L² ) / ([2 * PI * F_H * F_L ]² * C_T)

Where F_H is the high freq ( eg 1MHz in Greens sim )
And F_L is the low freq ( with the 150p cap, = 577 kHz )
And C_T is the test jig capacitor ( 150pF here)