THD meter circuit needed

[patchell]

    Most THD meters are simply a real deep notch filter.  The meter I have
is good to about .05% (an old heathkit).  The problem with measuring .005%
is that first, you need a sine that has much less that that to begin with,
no easy feat.  Second of all, the notch filter and associated electronics
needs to also be capable of a similar feat, also not teribly easy.  And
thirdly, the broad band noise must be lower than what would be .005%.  A
simple THD meter, after it notches out the fundamental mearly measures the
RMS value of the residual, which will include both noise and distortion.

    Another posible way is to use a A/D converter on your PC.  Again, you
will need to get a sound card that exceeds what you are attempting to
measure, but I am pretty sure you can get one that will be better that 100
dB S/N.  Once you A/D the signal, you can process it with an FFT.  This
would probably give much more acurate results.

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

--
 -Jim
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