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Project Notes

#195 LCResonator

Measure the resonant frequency of an LC coil or inductance of an inductor



Again I’m inspired by one of w2aew’s videos. I really needed something like this, as my attempts at winding my own inductors (for some RF projects) have failed miserably, and it’s a really usefull thing to be able to measure the inductance of a custom coil (or unknown inductor), and/or resononant frequency of the resulting LC pair.

The circuit is used to measure the resonant frequency of an LC circuit. It works in two ways:

  • by attaching an LC circuit without the calibration capacitor
  • by inserting a known-value calibration capacitor, the inductance of the inductor can be derived (calculated)

If the capacitane is known, then the inductance may be calculated with a re-arrangement of the LC resonant frequency formula:

L = 1 / ( C * (2πf)^2 )

JFET Selection

The original circuit used a 2N5245 N-JFET with 470Ω R2, w2aew used a J310 with 1kΩ R2. I ended up comparing J201 and J310 qith a range of R2 values.

JFET Idss Vgs(off)
2N5245 5-15mA -1V to -6V
J310 24-60mA -2V to -6.5V
J201 0.2-1mA -0.3V to -1.5V

Breadboard Test Results

I did my first tests on a breadboard build:


Here are some measurements. Where “n/a” it means I was not able to provoke an oscillation.

JFET R2 C1 C2 f L(nom) L(calc)
J310 4.7kΩ 150pF 33pF 438kHz 1mH 0.88mH
J310 2.2kΩ 50pF 33pF 660kHz 1mH 1.16mH
J310 4.7kΩ 150pF 33pF 1.307MHz 100µH 99µH
J310 2.2kΩ 150pF 33pF 1.311MHz 100µH 98µH
J310 2.2kΩ 50pF 33pF 1.988MHz 100µH 128µH
J201 2.2kΩ 50pF 33pF 2.088MHz 100µH 116µH
J201 4.7kΩ 33pF 15pF 2.43MHz 100µH 130µH
J201 4.7kΩ 50pF 15pF 2.08MHz 100µH 117µH
J201 2.2kΩ 150pF 33pF 1.33MHz 100µH 95µH
J201 4.7kΩ 470pF 15pF n/a 100µH n/a
J201 2.2kΩ 470pF 33pF n/a 100µH n/a
J201 2.2kΩ 33pF 33pF 8.196MHz 10µH 11.4µH
J310 1kΩ 50pF 33pF 6.729MHz 10µH 11.2µH
J310 2.2kΩ 33pF 33pF 7.82MHz 10µH 12.5µH
J310 2.2kΩ 50pF 33pF n/a 10µH n/a
J310 1kΩ 150pF 33pF n/a 10µH n/a
J201 2.2kΩ 150pF 33pF n/a 10µH n/a
J201 2.2kΩ 50pF 33pF n/a 10µH n/a
J310 2.2kΩ 50pF 33pF n/a 1µH n/a
J310 2.2kΩ 33pF 33pF n/a 1µH n/a
J310 4.7kΩ 33pF 33pF n/a 1µH n/a
J201 2.2kΩ 33pF 15pF n/a 1µH n/a

Findings - Breadboard Tests

As long as the oscillation can be sustained, measured results and inferred inductance is surprisingly close to theory.

In summary, with a breadboard build and 9V supply:

  • 10µH is minimum inductance I could measure
  • more accurate results with C1=50pF or higher
  • if C1 too high (over 150pF), can’t sustain oscillation
  • R2 sweet spot is around 2.2kΩ

I hoped that a protoboard or PCB build would helps sustain oscillation and in particular measure inductors below 10µH. Which turns out to be true .. read on for the results - they are good!

Sample trace measuring a 100µH choke with J201, R2=2.2kΩ, and C1=50pF:


Sample trace measuring a 10µH choke with J201, R2=2.2kΩ, and C1=33pF:


PCB Test Results

So next I’ve copied w2aew and put the circuit on a copper PCB (with 6 islands), and the improvement is staggering. I’m able to measure much lower inductances without having to raise the power rail from 9V. I made both L1 and C1 interchangeable (with pins on the board to add random capacitors, inductors, coils or LC modules)

A few minor component switches/selections:

  • I put J310’s down on the board, even though J201 seemed to perform in the same ballpark
  • replaced “ordinary” ceramic disc capacitors with supposedly higher-quality multi-layer ceramics
  • C2 33nF replaced with a 30nF multi-layer ceramic
  • I settled on 2.2kΩ for R2 (FET source load)

Here are some measurements:

C1 f L(nom) L(calc)
50pF 6.802MHz 10µH 10.95µH
150pF 4.255MHz 10µH 9.33µH
150pF 12.608MHz 1µH 1.06µH
30pF 24.631MHz 1µH 1.39µH
30pF 34.094MHz hand wound #1 0.73µH

The hand wound #1 coil is simply an 8mm diameter coil, about 2mm long, with 4.5 turns of 0.2mm enamelled copper wire. According to this coil calculator the inductance should be ablout 0.28µH. My measurement says 0.73µH, and given the accuracy of the other readings with “known” inductors, I’m encouraged to go with my measured value. No wonder my RF circuits are all off - it seems coil inductance calculators need to be treated with a big pinch of salt.

Here’s a very nice trace measuring a 10µH choke and C1=150pF:


And finally I can crack the 10µH barrier. Here’s a measurement of a 1µH choke and C1=30pF:


And a sub-1µH custom coil with C1=30pF:

custom coil #1/30pF



The Schematic

Here’s the build on a breadboard:


And now on a chopped-up PCB:


Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.