Emery's Projects

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.

A Fast Precision Rectifier

Opamp rectifiers are not particularly fast.

-Take for example a 15mhz bandwidth TL072CP with the basic diode rectifier configuration as in the circuit displayed on the bottom. At an input of 100mv 100khz signal the output will show no rectification at all.
-The more standard circuitry (in the middle) employs an extra diode and a configuration that will not let the opamp saturate in the half cycle. The results are visibly better, but still quite weak at the low amplitude and high frequency employed here.
-Take a look at the top circuitry. The extra components added will further reduce the opamp output swings so that it can respond much faster during the zero transitions. The resulting scope image speaks for itself. In this setup the diodes will yield full temperature compensation as well.
-outputs 8,17 and 30 are signal averaging outputs. For any single type of waveform, they will produce dc levels proportional to the input amplitude. The resistors in these low pass RC elements must be kept at a high value so that they will not interfere with rectified signal. This part of the circuit could also be separated by an opamp buffer if necessary.

A Simple Regenerative Receiver

It is hard to resist the simplicity of these receivers as well as their potential. Charles Kitchin, among others, has published fine working models that are worth looking at.
After building some of these over the years, I settled on a list of what I wanted in a regenerative receiver:

1-There should be no external coupling inductors, be it for antenna signal or regeneration. A single small diameter coil only.
2-To be able to tune through the bands without plugging in a variety of inductors.
3-Proximity of objects should not detune the receiver to a noticeable extent.
4-An antenna of a relatively short length, no more than 1.5 meters but still sufficient to rival the sensitivity of commercial single transposition superhets. In the entire frequency range of reception.
5-Regeneration should be smooth, without any hysteresis.
6-Bandwidth of reception should be narrow enough even at the higher frequencies.
7-Simple to build.

The design below aims to achieve all of the above.

-The single most important performance determinant feature of this circuit is the separate demodulator. Demodulation is not done in the oscillator stage, as is most common. The large source resistor shunt capacitor, used to obtain the demodulated signal in the majority of regenerative receivers, will necessarily cause non-linearities in the drain current. This limits the achievable Q. Comparing the total harmonic distortion figures in the drain current (at a 100mv amplitude sinusoidal gate signal) will yield .06% without and 3% with a 1nf source resistor shunt. Leading to a 4 times increase in the minimum bandwidth figures! Here lies the problem as well as the solution based on the addition of a separate fet demodulator stage that will not degrade bandwidth. This demodulator is biased to very low currents to enable great sensitivity.
-The grounded base antenna matcher is standard, its other function is to minimize antenna proximity effects and power radiation. What is different is the capacitive coupling.
-The throttle capacitor is connected in a way to current feed directly into the tank inductor's grounding capacitance. Its done on the inductance side to invert the phase. The parallel choke provides the dc path. (simply grounding the gate with a high valued resistor instead of using the choke, is not a good idea, leaves the gate vulnerable to AC fields)
-The Tank is slug tuned with a high-frequency ferrite rod of 10mms diameter and 7cms length. The coil's maximum inductance is about 7uh, its diameter is 12 mm, 11 turns, length at 30 mm for my particular rod. I was able to receive on frequencies from 5.5 to 20 Mhz or so. Fine-tuning is with a 25pf variable capacitor.

A little help for the build:

-The fequency compensator source resistor shunt capacitance C3 is there to enable oscillation at the higher frequency bands.
-Tank inductance grounding capacitor C5 should be chosen so that the feedback is in the proper range, to a value that would just start oscillation with the throttle capacitance at middle setting and the tuning slug fully inserted.
-All 3 RF chokes are identical, 3mh on small 16 mm dia. toroidal cores.
-If any longer antenna is to be attached, an attenuator would be a good idea. I found the 1.5 meter length more than sufficient to match the sensitivity of my Realistic DX-370 digital receiver.