[[ Most recent Update: 28 September 2012: ]]
A device for measuring inductance
Over the years, articles describing devices for measuring inductance have appeared in QST, and elsewhere, but none of them were “just right” for me, so I have developed my own and have named it the Calibrated Variable Capacitor (CVC).
The photo, below, shows the underside of the CVC, which was built (almost) entirely with parts from the junk box.
Perhaps it seems strange to call a device intended to measure inductance a CVC, but it works for me, and I think it will make sense for anyone interested enough to read the next few paragraphs.
The CVC will measure inductance from about 0.5 uH to about 100 uH in four ranges, and shows the value on a direct-reading analog dial. This covers virtually all inductance values you are likely to encounter in HF resonant circuits. A fifth scale on the dial shows capacitance in pF.
In addition, you can determine the tuning range for a given coil/capacitor pair within the frequency limits of the CVC. For example, suppose you want to know the tuning range for a 4.5 uH coil and a 50 to 150 pF variable capacitor.
Easy with the CVC. Simply attach the coil to the CVC and tune the dial from 50 to 150 pF while monitoring on your receiver.
Monitoring on your receiver ?!
Your receiver is the read-out device that is used in conjunction with the CVC to determine the values you are seeking. Clever, don’t you think?
“How in the world do you do that?” (You might ask.)
This simplified diagram shows how it works . . .
An unknown inductance, in parallel with the CVC, creates a resonant circuit. The resonant circuit determines the frequency of the oscillator, which you are monitoring on your receiver. By using coils of known inductance to calibrate the dial in a few points, you can determine the approximate value of any coil within the range of the CVC. The inductance values are read from the dial at zero beat with your receiver in CW mode.
Notice, I said “approximate value”. This is not a laboratory instrument, but it is perfectly adequate for most HF Ham Radio work, with accuracy similar to the venerable grid-dip meter, or grid-dip oscillator (GDO) as they are commonly referred to these days.
The schematic diagram shown here is the circuit I used for the prototype CVC. Capacitors marked with an asterisk are type NPO or COG. The line marked “C” on the 2N4416 is connected to the transistor case. I have found that an MPF-102 transistor works equally well in this circuit, and it is much less expensive than the now obsolescent 2N4416.
A drawing of a raw (uncalibrated) dial is shown below.
Notice that there are six spaces in which to place calibration, one space for each of five high frequency bands, 160 through 15 meters, plus one band for calibration of capacitance in pF . . .
. . .which brings us to calibrating of the CVC (in case you want to build your own).
Calibration of the CVC is relatively easy. Use at least three coils of known inductance to calibrate the high end, middle, and low end of each band on the CVC dial, then interpolate values between the known calibration points.
Using more than three known inductance values will, of course, improve the accuracy of your CVC.
Suggested values are shown below.
Mhz High Middle Low
21 0.5 uH 0.2 uH 0.1 uH
7 5.0 uH 2.0 uH 1.0 uH
3.5 25.0 uH 10.0 uH 5.0 uH
1.8 88.0 uH 40.0 uH 30.0 uH
The value of inductance you use for calibration is relatively unimportant, but you should know what the specific values are in order for your calibration to be accurate. 5% tolerance RF chokes suitable for calibration purposes are readily available in values ranging from 0.1 uH to 220 uH.
In the event you may be inspired to build your own CVC -YOU MUST CALIBRATE YOUR OWN DIAL – in order for your CVC to be useful. My dial on your CVC would be accurate only if you duplicate, exactly, the instrument I built, which is highly unlikely. That is why I have shown only the raw (uncalibrated) dial, above.
The calibration technique I used is shown below. I will use the high inductance (low frequency) band as an example.
 Set your receiver to CW mode.
 Tune your receiver to 1.800 MHz.
 Connect a coil of known value to the “unknown inductance” terminals. (A value of about 45 uH will put you somewhere near the mid-point of the 1.8 MHz band on the dial.)
 Tune the CVC until you hear zero beat in your receiver.
 Mark the dial and write “__” (whatever value of inductance you used).
 For a calibration point near the low end of the 1.8 MHz band, use an inductance of 80 to 90 uH and repeat steps 3, 4, & 5.
 For a calibration point near the high end of the 1.8 MHz band, use an inductance of about 30 uH and repeat steps 3, 4, & 5.
Repeat this procedure for at least three points on each band (the more calibration points, the better).
Once you have completed the inductance calibration, you can record the capacitance values “pF” band of your dial. For example, assuming you used a 56 uH coil as your known inductance, we know (by calculation or other method) that it takes about 140 pF to resonate at 1.800 MHz, so make a mark on the “pF” band that lines up with “56” on the 1.8 MHz band and write “140” on the pF band of the dial. This may sound complicated, but it is easier and quicker to do than to read about it. After calibrating a few points, your speed and ease of calibration will improve greatly.
I know, I KNOW! The exact capacitance required to resonate a 56 uH coil at 1.8 MHz is a little less than140 pF, but I said “about”, and we only need accuracy roughly equal the average GDO.
Also, there is the stray capacitance and wild inductance within the oscillator circuitry that will be included in the settings being calibrated.
I have built a couple of versions of the CVC (maybe more by the time you see this). The prototype was built from salvaged parts, except for the dial blank, which I created on the computer. The second version looks better because I put it into a brand new box and used (mostly) new parts. The ‘ pretty ‘ version performs no better than the original, and I actually prefer using the original on my workbench, mainly because it is smaller. Everyone has a ‘favorite child‘, I suppose.
OTHER USES FOR THE CVC
The CVC can also be used for purposes other than measuring inductance; three are listed below.
 A quick check for the tuning range of an L / C circuit (as described earlier in this article).
 RF Signal Generator
 Workbench VFO for testing homebrew transceiver, receiver, or transmitter
– END OF CVC –