measuring impedance - was "Amplifier A7"

[Mike Morris WA6ILQ]

At 09:49 PM 09/21/2001 -0700, Murray Grandy wrote:
>Also, can someone please answer a
>question. What type of equipment is used to measure impedance of say a loud
>speaker. Seems all the bridges I see measure only L C and R.
>This is perhaps a silly question so, please don't any of you fall off your
>emails laughing.
>Murray.

One simple way to measure impedance is to take a AC voltage
source (I use an HP 200CD audio generator at 1kc) and place it
across a series string consisting of the device under test (DUT)
and a carbon potentiometer (connected as a variable resistor).
Then take 2 VTVMs and set them to AC volts, and clip one
across the DUT and the other across the pot.
Then twist the pot until the meters are equal (it helps if you have
some idea of the inductance/impedance of the DUT before you
start, so you can pick the proper value of pot out of the junkbox).
When you get a 50%/50% setting shut off the generator, unhook
the pot and measure the resistance using an ohmmeter.  The
value of the pot is the impedance of the item - at the test frequency.

Basically you are taking advantage of the fact that a carbon pot
has the same resistance value to DC as the impedance value is
at AC.

You can use this same trick on capacitors.  It's how I determine
the capacitance of unmarked variable caps - using an RF generator.
Determine the capacitive reactance at 1mc or 10mc and work the
formula backwards to get the capacitance value.

Mike WA6ILQ

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