* For *many years the AADE L-C meter (http://www.morsex.com/aade/lcmeter.htm) was the ‘gold standard’ bit of test equipment for radio home brewers. At about $100 it was a bit pricey considering that you could get a Chinese clone on Ebay for about 1/4 the price. I wish I had bought one before Neil Hecht passed away, his device was more accurate, and had a wider measurement range than any of the clones.

The principle of operation for these L-C meters is well known. The frequency of an L-C oscillator is constantly being measured under micro processor control. The oscillator has known fixed values of L and C, and there is provision to introduce either an unknown value of capacitance in parallel with the known fixed value, or an unknown value of inductance in series with the known value. In either case the frequency will go down by an amount proportional to the added inductance or capacitance.

Calculating the value of the unknown part is a simple matter of applying the math. The formula for finding the frequency of an LC oscillator when both L and C are known can be modified to solve for either L or C when the other and the frequency are known. The accuracy of this measurement will depend on how accurately we can measure the frequency, and how accurate are the measurements of the internal ‘standard’ inductor and capacitor are. It is possible to calibrate the meter by measuring a precision capacitor or inductor (1.0% or better). Usually precision capacitors are easier to find than inductors.

A few years ago Phil Rice, VK3HBR, published a design for an easy to build L-C meter based on the AADE circuit, but using his own software. Like the AADE meter, Phil’s design used a PIC microcontroller as a frequency counter, doing the necessary calculations in software. A open source floating point library provided by Microchip was used to perform the necessary computations. The original version of this meter used a separate LM311 comparator for the oscillator, a newer design made use of the PIC16F628 which has that function built in. Other than a slight cost reduction, the two designs work about the same. Since circuit board artwork was available for the newer design, that is the one that I chose to build.

You can find all of the details on construction from Phil’s website: https://sites.google.com/site/vk3bhr/

Here are photos of the unit as I built it:

Being able to measure the inductance of home wound coils is very useful to the amateur radio builder. While formulas, computer programs, and on line calculators are available to compute the number of turns required for a desired inductance when winding toroids or slug tuned coils, being able to measure the result will save hours of troubleshooting later on. Of course, one can also measure the resonant frequency of tuned circuits with a dip meter, and thereby indirectly measure the inductance of a coil (assuming one knows the circuit capacitance). I’ve already described a home built dip meter in a previous post.