2007-09-16
Nano-Henry Meter Counter Adapter
This project was inspired by the Tone Dipper (et al), and was in fact built on the same day on a whim. The unit is not as expedient to use as my other nH meter, but it has the advantage of higher potential precision and repeatability due to its better layout and the read-out mechanism.
The RF oscillator is the emitter coupled one from several of the last projects. It was laid out with high frequency performance in mind using BF199 transistors. A fragment of machined-pin IC socket makes the LC connector, soldered directly across the devices.
The buffer to drive the frequency counter is a differential amp, again using BF199 transistors. It is surprisingly well behaved, I'll likely use it again. I did experience a collector resistor resonance around 240 MHz, but it isn't especially annoying or damaging, so I've left it alone.
The oscillator supply voltage is Zener stabilised for good repeatability. Calibration at a particular capacitance is easily achieved by a few known inductances. Once done, keep the capacitor with the unit (or solder it across the circuit permanently. Larger capacitances are better, they tend to swamp the rather variable capacitances of the oscillator. Having the capacitor swappable is useful, it lets you change the range of inductance the device is capable of measuring with your counter. Although the device is optimised for VHF frequencies it works OK at lower ones.
Notes
The device is equally a capacitance meter if you use a calibrated inductor. It can measure minor variations in small value capacitors and temperature stability in particular, try warming a cap with your fingers or the soldering iron while watching the counter. Use a good inductor for this purpose, preferably on a type-7 material or air core, with the windings secured mechanically (potted in wax works well, glue a fragment of IC socket to a piece of PCB material and solder on the inductor then pot the works or otherwise arrange a nice solid inductor module).
The oscillator capacitances seem to vary significantly with oscillation frequency. Bear this in mind when you calibrate the unit. I used 100 pF most often, a value for which it is reasonably accurate over 100 nH to 1uH. If your counter goes to UHF it can measure hairpins, but accuracy won't be especially good.
My unit topped out near 400 MHz, but in practice I wouldn't use it much beyond 150 MHz. Buffer performance becomes a problem at extreme frequencies, and the tank to buffer coupling capacitor at lower ones.
Unfortunately it requires some maths to be done on the calculator or PC, and calibration is non-trivial, but a microcontroller could be used to make something similar into a direct-reading device...
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