LC Resonant Frequency Meter

Posted on Apr 10, 2012

Recently, I have needed to measure inductances in the hundreds of microhenries to several millihenry range. Though I have a pretty good LRC meter and an excellent bridge on my workbench in Mesa, Arizona, I wanted to make these measurements in my home in Thailand, thus I decided to put something together. The RF Inductance meter on this website is good for low value rf inductors, but because of the way it works - putting a sharp-edged square wave through the inductor -its not suitable for inductors made with high permeability ferrites (Because of pulse shape distortion that results from high frequency losses in the core.) This meter operates at lower frequencies, and by careful selection of the resonating capacitor, the oscillator can be made to run anywhere from 100 kHz on down. This makes it possible to test near standard frequencies like 1 kHz and 400 Hz, to compare results with precision bridges.

LC Resonant Frequency Meter
Click here to download the full size of the above Circuit.

This is basically just an oscillator based on a comparitor and a frequency meter. The oscillator oscillates at the resonant frequency of an LC parallel tuned circuit. A really nice version of this was created by Chris Krah using an AT90S1200, including floating point math, etc. to display the L and C readings directly. I'll post a link to Chris' version once it is published on the web (it was posted on the AVRFreaks board). The LM393 comparitor only seems to work accurately a little past 100 kHz, but it is good enough for my purposes. Faster comparitors like the LM311 can work well at higher frequencies. The second comparitor in the package was left floating. A dual-row connector is used to connect the resonant circuit under test to the oscillator. It has two rows so there will be one pair of contacts for the reference capacitor, and a second pair of contacts to connect for inductor under test. Or, if one wants to measure capacitors, a reference inductor can be soldered to the connector and the other set of contacts used for the capacitor under test. I built this on a phenolic board with one pad per hole. As shown in the photograph above, the board is laid out pretty much the same as the circuit diagram. I made sure the oscillator components were on the opposite side of the LM393 from the AT90S2313 and its crystal oscillator to minimize coupling as a precaution because I was concerned that energy from the AT90S2313...

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