THD meter circuit needed

[Alex Stettler]

Hello anyone!


Does someone of you know a circuit to build a simple THD meter, which
can measure THD as low as about 0.005%?

The equipment I have:
-2 channel oscilloscope
-1 digital volt meter
-a lot of electronics experience

I propose a such circuit:
Feed a 1kHz signal into the circuit to be tested, then run the output of
this circuit into
a highpass filter, which filters out the 1kHz fundamental, and the
measure the remaning voltage.

1kHz sine wave ======> circuit to be tested ========> Highpass Filter
(1kHz cut-off) ======> harmonics which can be measured


Is it possible to measure the THD with a such circuit, if a pure
sinewave and good Op-Amps are used?

The problem is, that I don't have the money to buy a commercial audio
precision analyzer or a THD meter.


Many thanks for any help,
Alex