You can use a GDO to measure impedance, although you need some stuff to go
with. Make a coupling loop fixture, so the coupling is constant: put the
impedance to be measured across the loop, set GDO to frequency (using a
receiver or better, a counter), note grid current. remove the impedance. If
frequency goes up, it's inductive, and down, it's capacitive. Note how far the
frequency has changed, and place capacitors or inductors as appropriate across
the coupling loop to get the same frequency as with the unknown in place. In
practice, get values that straddle the frequency. Now add either series or
shunt resistance. If the load is capacitive, you may be better off with high
values in shunt, although higher value resistors tend to be capacitive. Find a
resistance which gives the same grid current as the unknown at as close to the
same frequency as possible. You now have an approximation in terms of parallel
R and C or series R and C (or L). The method will work for Q, but I
suspect, won't be very sensitive. By getting values that straddle the
frequency and grid current, you can interpolate to find the unknown.
This isn't going to be anything other than a rough and ready method, but it
should get one into the ballpark.
As far as measuring inductors is concerned, a comment that 'such and such
measures to 1%' is pretty meaning less without defining the conditions, such as
frequency and range of measurement. Apparent inductance and Q both vary wildly
with frequency. The measurement methods vary with value and frequency too, so
for example, trying to measure a 50nH chip inductor at 1kHz is not likely to
give a very good answer. Terman lists the formula for the relationship between
frequency and apparent inductance and Q for frequencies less than 80% of the
SRF.
73
Peter G3RZP
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