Inductance Meter

Building your own instruments to measure electrical quantities and parameters of various components can be a satisfying and worthwhile passtime. An inductance meter, for example.
There are a great variety of circuits posted online, some are wonderfully simple but perhaps less than ideal. They may not, for example, account for coil resistance or self capacitance. Each approach will have its pros and cons. An experimenter will experiment...This is what I invite you to do:



How it works:
-The unknown inductor is inserted into the oscillator circuit which then self adjusts to produce a precise stabilized amplitude sinusoid. To achieve this, the oscillator amplitude is fed back to control a negative resistance within the tank circuit.
-The output is integrated twice in succession.
-The resulting waveform is demodulated and averaged out to give a reading proportional to the inductance being measured.

Further details:
L1 is the inductance to be measured.
R13 sets the oscillator output amplitude, which should be at 400 mv peak.
The circuit uses a "home made" Led-Cadmium sulfid optocoupler consisting of a single Led illuminating two separate photocells. One of the cells is used to linearize the response, the other for controlling the oscillator.
The low frequency noise components tend to be greatly amplified in the process of integration, therefore a high pass filter is an absolute must before demodulation.
High pass filters 1 and 2 are identical except for an offset adjustment in filter 1. The purpose of filter 1 is to eliminate noise while filter 2 assures that the phase conditions for demodulation are correct.
Potentiometer R33 of the demodulator is meant to offset the U6 output, it should be adjusted to obtain a clean half wave rectified signal. U6 output is meant to be as large an amplitude as possible, without driving the LM318 into saturation. Saturating it will result in unwanted phase shifts.
Finally, a digital voltmeter is connected to the averaged out demodulated signal.
The circuit below will measure inductances up to about 20 milihenries. Meaningful readings can be taken even at the milivolt level. All op amps used here were TL072-s except for op amp U6 which is an LM318. The integrators can be scaled to any custom range desired. The circuits here are capable of accurate readings down to 10 microhenries.
I also include a clip showing the actual demodulated output and control voltage levels. The measurement consecutively displays outputs for 10, 20 and 100 uh inductors. The last measurement is then repeated with a 200 ohm resistor put in series with the inductor. Note that the output stays the same while the control voltage settles at a much higher value.