With the discussion of what frequency to measure an inductor and what
sort of meter to use, I thought I would toss this tidbit out for the
wolves to gnash out. If we are probably using the inductor in a radio
frequency circuit, then knowing what it really is at the frequency in
question is entirely appropriate. Hence, the use of an MFJ or other
RF-analysist isn't a bad start. It is elementary to reconvert the
impedance back to inductance at the operating frequency, if that
number is desired. As you all know, inductors and capacitors (and
resistors) are their stated value at low enough frequencies that
parasitic effects don't alter them.
I find that using the old Hewlett Packard vector impedance meters is
very useful for in-circuit measurement on frequency, also for testing
components out of the box. The hp 4815A covers 450 KHz up to about
110 MHz and the 4800A covers audio up to 450. The later hp 4193A is a
modern microprocessor version of the 4815A. hp discontinued all of
them before it became Agilent, so they can be found on EPAY and
surplus. I picked up the 48xx pair for about $200 each over the
years, and they are excellent for checking things like plate RFCs,
loop antennas, bypass caps, once you make a few copper strap fixtures
and other tricks to adapt the probe. You can calculate Q of a
resonantor by looking at the resonant frequency at the +/- 45 deg
phase angle. If you hook a HPIB bus computer to the 4193A you can
even do error correction (short/open) to make it read impedances down
to an ohm and up to 100K ohms with high accuracty. Only problem is
the higher frequencies, where the antenna analyzers take over.
Network analyzers can be pressed to measure components, by placing
them across a 50 ohm circuit, and looking at the Smith Chart.
However, these boxes are optimized for 50 ohm measurements, and
accuracy falls off rapidly at the extremes of impedance that the
other boxes can measure. Agilent sells an expensive new box that does
components up to the GHz range, for a lot of cash, but it is, I
believe, based on a network analyzer internally. You can't beat the
old vector Z meters, if you can grab one. After all, its the
impedance (and phase angle if the circuit is not a pure inductor or
capacitor) in which you are really interested for RF circuit work.
The hp meters have become the GDO of another generation of RF
engineers.
73
John
K5PRO
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