RF Induction Meter

Posted on Feb 5, 2014

This instrument measures inductances from 0. 1 microhenry to 3 millihenries divided into four ranges set by a switch. It operates from 12 volts, and is powered by eight AA type cells attached to the unit. I initially referred to Drew Diamond`s unit in Amateur Radio, November 1992 to see if it fitted my needs. Drew used a fixed crystal oscillator at around 3. 5 MHz to source a bridge

RF Induction Meter
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where he compared the unknown inductor against a known 5 uH inductor. The bridge was balanced by adjusting a potentiometer which had its scale calibrated in terms of inductance. The meter measured a range of 0. 5 to 20 uH. I needed a wider inductance range than this. Also, I was a bit concerned that no provision had been made in Drew`s circuit to balance out the resistance components of the reference and unknown inductors. If the two resistance components were largely different, and particularly if one of them (the unknown) was fairly low in Q, the dip shown in the balance meter would occur with the potentiometer reading offset from the calibration. I guess I could have modified the bridge to include resistance balance but I decided to operate my circuit in a different way. I have used a fixed frequency source as in Drew`s circuit but extended this to four frequencies to expand the inductance range. Instead of using the bridge, the unknown inductor is resonated by adjusting a variable capacitor in parallel with the inductor. The parallel tuned circuit is energised from the oscillator source via a meter which monitors the current into the circuit. The system is illustrated in Fig 1. Resonance is indicated by a dip in current as shown on the meter. A dial attached to the variable capacitor is calibrated in terms of inductance. Influenced by Drew`s crystal controlled, Colpitts type oscillator circuit, I wired up the circuit...

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