AW: THD meter circuit needed

[Alex Stettler]

Hi Rene,Harry and all

Instead of a perfect sinewave I think you can overcome this problem by
using a second notch filter. One after the
circuit to be tested and one directly after the sine generator. Then you
put this two signals to the +/- Inputs of an
Opamp. As the harmonics of the sinewave are phase (only if the test
circuit has 0° phase shift), it is canceled out by the opamp, while the
harmonics from the test circuit remain, because they are on only one
input of the opamp.
But I think it will be difficult to measure the THD of filter and other
circuit, because phaseshift isn't 0° or not constant.

Alex

> -----Ursprüngliche Nachricht-----
> Von: Harry Bissell [mailto:harrybissell at prodigy.net]
> Gesendet am: Sonntag, 6. August 2000 02:02
> An: René Schmitz
> Cc: Alex Stettler; synth-diy at node12b53.a2000.nl
> Betreff: Re: THD meter circuit needed
>
> 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
> >
> >
>