THD meter circuit needed

[Harry Bissell]

Hi Rene (et al)

I'm missing something here... don't you still need a "perfect" sine-wave to
drive
the system... if not you measure the sine distortion too...

Or maybe its ratiometric... how much WORSE is the circuit than the
generator...?

??? (puzzled)

H^) harry

René Schmitz wrote:

> Hi!
>
> There is a circuit on Steve Benchs website, which can probably used.
> http://members.aol.com/sbench101/
> Its at the very bottom of the page. Lots of interesting tube content there
> as well!
>
> It uses a notch filter, so that you don't need a low THD source for the
> measurement. But I don't know if you can measure as low as 0.005% with it.
>
> Bye,
>  René
>
> At 23:36 05.08.00 +0200, you wrote:
> >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
> >
> >
> --
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
>
>