It brightens my heart to see my employer's products
presented so favorably! However, a Hewlett-Packard
impedance meter is not required for measuring
impedance, Q or any other passive parameter. Our
impedance meters are actually not purchased by many
R&D departments. The main market for them is
manufacturers of the components. That's why our
impedance measurement handbook is so full of
examples of automated component handlers and
fixtures. Our R&D labs tend not to have them, and
we seem to be able to develop RF and Microwave
equipment adequately!
In the early 1970s, I attended my first ever ham
club meeting. The speaker that night showed how to
make complex impedance measurements using a grid
dip meter and some standard capacitors and
resistors. So it can be done. I'd surely be
figuring out a way if I didn't have network
analzers available!
A few years ago I had to make some measurements of
complex impedance. I already had an old HP 8405A
vector voltmeter ($200 at most flea markets), and I
blew the budget purchasing two Mini-Circuits BNC-
packaged directional couplers. Must have spent
$150 on them. I used a Yaesu FT-840 for the signal
source, but if I didn't have that available, I also
have a very old Heath RF generator that would have
worked.
Calibration of the system was easy enough, I just
applied a short circuit to my 'test port', and
tweaked the verniers on the 8405 to read 0dB and 0
degrees. Then remove the short, make sure I'm
reading 0 dB and 180 degrees and I have my one-port
network analyzer up and running. I could have used
an Excel spreadsheet to convert the reflection
coefficient readings to impedance, but I just used
a Smith Chart instead.
If, in fact, we're measuring devices that are
supposed to be low Q (say, below 50), there are
multiple techniques for doing so.
One other technique I use for measuring components
is to create a simple test circuit that places the
component in question in a situation not too
different from the intended purpose. A 50 ohm
transmission line (I use a 0.1" wide trace on a PC
board) is a wonderful tool. If I put a bypass cap
from that trace to ground, then sweep the circuit
(using an RF signal generator and a diode detector
or an oscilloscope), I'll see a deep notch at the
resonant frequency of the bypass capacitor. And,
from the depth of the notch, I can calculate the
ESR of the capacitor. Once I know those values, I
can calculate Q at any frequency below the resonant
point, and some distance above.
Were I measuring a suppressor, I'd place the device
in series in a 50 ohm line. Given the lower
likelihood of finding resonance, I'd need to make
phase measurements. A scope clipped to the input
and output of the line takes care of that, and the
rest is simple AC circuit analysis.
We get spoiled by the modern test equipment, and
I'm glad we have it, but these are all measurements
that were done before we had HP impedance meters
available. It just takes some good old ham
enginuity.
Dave W8NF
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