THD meter circuit needed

Tim Ressel Tim_R1 at
Mon Aug 7 18:58:32 CEST 2000

Many moons ago I was tasked with building a distortion meter. We did the usual:
low-distortion sine, notch filter, etc. Some interesting facts came out of the
effort. For example, the notch filter was imperfect because all caps have this
"power factor" thing, which means their phase shift is not quite 90 degrees. Our
solution was to find caps with the lowest power factor possible. Also the sine
generator can be cleaned up with really good op amps, and filters to lower
harmonics and reduce low freq noise.

However, that was a long time ago. Very recently I took my Analog Devices DSP
development system and programmed it to measure distortion using the FFT method.
Got down to .005% easily. And when the DSP generates the signal, you get a bonus
of having a source sync'd to the sample rate. You can really pull out those
distortion products!

Tim Ressel--Compliance Engineer
Verifone Division
3755 Atherton Rd.
Rocklin, Cal
timothy_ressel at

-----Original Message-----
From: Alex Stettler [mailto:x_ray at]
Sent: Saturday, August 05, 2000 2:36 PM
To: DIY Synth List
Subject: THD meter circuit needed

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,

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