THD meter circuit needed

[Harry Bissell]

I'd suggest you use a 1KHz NOTCH filter. Maybe even a passive filter if you
can stand the loss in gain.

Why...  It will be hard to have a highpass filter with .005% distortion
itself...

The notch topology is theoretically capable of infinite depth of notch...

I'd try a twin-tee filter, maybe 2 in series both tuned to 1KHz.  Maybe tune
your
generator watching the filter output for a null.

I really don't know if the figure (accuracy) you want to get is DIY
material. maybe
someone else will...

H^) harry

H^)  harry

Alex Stettler 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