Re: Noob question about distortion

On Mon, 3 Mar 2008 03:49:24 -0800 (PST), distort10n@xxxxxxxxx wrote:

On Mar 3, 2:29 am, nos...@xxxxxxxxxx (Don Pearce) wrote:
On Mon, 3 Mar 2008 01:09:20 -0800 (PST), distort...@xxxxxxxxx wrote:
So measure the hum, measure the THD+N and provide individual figures
for them. That is what everyone else does, for good reasons.

Excellent.  Thank you for the clarifications.

Let me give one caveat before anyone else jumps in. Where an amplifier
has a balanced input (microphone preamplifiers are a common case),
there is a spec that is somewhat related to PSRR, and that is CMRR -
common mode rejection ratio. That defines the ability of the amplifier
to reject external interfering signals which appear on the two input
lines with the same polarity. CMRR will be specified for the better
grade of preamp.


Let me play devil's advocate one last time.

We agreed that with the ripple voltage on the power supply rail with
an input signal present is not a valid THD+N test. And we also said
that if sidebands are present then there will be intermodulation
distortion.

Sidebands are an indication that there is intermodulation. But that
doesn't necessarily mean that the amplifier itself will suffer
intermodulation of signals applied to the input - they do not have the
modulation mechanism available to a signal on the power supply. So do
not confuse the true IM spec of the amplifier with any IM you can
achieve through signals on the power line.

Now the noise portion of THD+N should include *everything* within the
tested bandwidth once the fundamental is removed with the notch: buzz,
hum, harmonics, noise, etc. Wouldn't the power supply ripple signal
be considered 'noise' or hum (although hum is usually mentioned to be
the mains frequency). So wouldn't this be a valid THD+N test;
although, the additional signal (noise) from the power supply should
raise the noise floor and mask the real distortion of the amplifier.


It depends on how you are measuring. If your test set simply nulls the
fundamental and measures the residual power, then yes, hum will be
included. Hopefully the design will be such that any residual hum is
well below other, unavoidable noise source. But a modern test set
would use an FFT analyser, which allows separate measurement of all
frequencies, so hum, harmonics, intermodulation and wideband noise can
be identified and measured separately.

I have seen an article on Wikipedia saying that THD+N includes
intermodulation distortion, but I will have to think about that
further. It is late here.


No. THD measurements are made with a single input signal. That means
that there is no mechanism for IM to occur. If you want to measure IM,
you need at least two input signals and tune your analyser to the sum
and difference rather than the harmonics of either.

d

--
Pearce Consulting
http://www.pearce.uk.com
.

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