Exploring other LC measurement techniques

Glen Chenier's article posted last week got me thinking about other LC measurement techniques, including an inductance checker I built about 40 years ago and haven't used in nearly as long.

The inductance tester in question isn't exactly your standard piece of test gear. It's basically an LC oscillator in a box – without the resonant components. The idea is basically the same as Glen's approach: Resonate a known capacitor with an unknown inductor (or vice versa if you have a known coil). But in this case, the resulting tank circuit isn't rung – it oscillates continuously. The original circuit was meant to be used in conjunction with a radio receiver! Yes – tune the radio until you find the dead air generated by your LC combo, and you've got your value after some simple calculation.

Figure 1: Oscillator-based LC meter.

Of course, now I can use a scope instead of wasting my time with a radio.

Figure 2: That's 20ns/div – a period of about 69ns (14.4MHz). It's quicker – but less fun – than a radio.

A few years ago, I purchased a nice little LC tester kit from AADE. It basically automates the process Glen describes in his article: ringing the unknown L or C with an internal, accurate, complementary reactance. However, instead of scoping the signal, the microcontroller measures the frequency and does the calculation for you. I've found it quite trustworthy.

Figure 3: A 278nH inductor on the AADE.

It's funny: Since I hadn't used my old tester for years, I wasn't sure if it still worked. At first it didn't, until I adjusted the component values to within its operating frequency range. Then I compared the results with the values obtained from the AADE meter. There was a significant discrepancy. I figured the most likely culprit was stray capacitance in the circuit, so I crunched some numbers and tried a few more values. Sure enough, there's an internal capacitance of about 34pF that needs to be accounted for. That's no biggie, but I think I'll stick to my cushy microcontroller-based gear.

Figure 4: A 389pF cap. Add in the stray oscillator (34pF) and the scope probe (15pF) capacitance, and the resulting 438pF matches what I saw using the oscillator-meter.

About the author
Michael Dunn has been messing with electronics almost as long as he's been walking. He got his first scope around age 15, and things have been going downhill ever since. The scopes now vie with wine racks, harpsichords, calculators, and 19th century pianos for space. Over the years, he's designed for the automotive, medical, industrial, communications, and consumer industries, as both freelancer and employee, working with analogue, digital, micros, and software.