THD meter circuit needed

[Martin Czech]

A good sine wave generator is possible, ->Wien.

Usually a notch t-network is used to eliminate the fundamental.
What you are plannig seems to be very demanding though.

And THD is a figure that won't tell much about t6he actuall
so8und of the distortion components. Are the odd or even
and so on.

I'd get a audio card with optical in and connect that to my DAT.
After that you have a very good A/D system (as long as the audio
card produces no artefacts like dropouts, which some do!)

And I'd burn an audio CD with different sine waves on it
and play that back with some CD-player, e.g. my portable.
Then you only need to do a FFT to see what's going on.

btw. I currently run into the question why nobody seems
to use LDRs for Wien amplitude regulation.


m.c.

:::From: "Alex Stettler" <x_ray at hanfi.com>
:::To: "DIY Synth List" <synth-diy at node12b53.a2000.nl>
:::Subject: THD meter circuit needed
:::Date: Sat, 5 Aug 2000 23:36:24 +0200
:::Content-Transfer-Encoding: 7bit
:::Importance: Normal
:::
:::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