[sdiy] Measuring Audio Sine Purity

[Neil Johnson]

Hi Scott,

The answer really depends on how much you care about detail.  What
level of distortion are you looking for?

Here is a superb example of a DIY audio analyser:

http://www.users.on.net/~glenk/thd/thd.htm

You can also read about the Cordell analyser which predates the above
project and you can still download the articles describing it.

Or, as I did, buy an HP 8903B audio analyser which is rather easier to use :)

The trouble with using a PC card, or USB scope, or any other type of
scope, even a spectrum analyser, is that you're hitting the input with
both the fundamental signal and the distortion and noise.  So you're
relying on the dynamic range of the equipment alone.  A notch filter
removes the fundamental leaving only the harmonics and noise, which
you can then amplify if needed before feeding into your test
equipment.  That way you get the full benefit of the instrument's
dynamic range to make meaningful measurements.

For example, suppose you have a sinewave oscillator that puts out a 5V
peak-peak signal.  And lets say you're using one of those PicoScope
USB pods, with 8-bit converters (2000-series or 3000-series).  So you
set the input attenuator to the 5V range to avoid overloading the
input amplifier.  That gives you an LSB of 5/256 = 19.5mV.  Any noise
below that you just won't see.  In other words your dynamic range is
48dB, or a THD of about 0.4%.  With a notch filter you can remove the
fundamental and turn up the gain of the scope input and get much
better analysis of the residual.

An audio analyser has both a notch filter and a sensitive AC RMS
voltmeter, which is exactly what you need.  For really detailed
measurements you need a very deep notch (>90dB), which can be done in
various ways, e.g., state variable filter, twin-T, bridged-T, etc. One
nice thing the 8903B does is once it has locked onto the fundamental
it autotunes the notch filter, servoing out any small changes in the
behaviour of the notch filter (thermal drift mostly).

There is a good write-up here: http://www.moorepage.net/THD.html

Cheers,
Neil
-- 
Modules and more: http://www.cesyg.com
Homepage: http://www.njohnson.co.uk