Re: Noob question about distortion

On Sun, 2 Mar 2008 17:54:16 -0800 (PST), distort10n@xxxxxxxxx wrote:



It depends on the amplifier, the frequency and all sorts of stuff.
What should happen is that the ripple signal you put on the power
supply simply appears at the output in diluted form. If the amplifier
is not designed well, it is possible that the ripple will instead
amplitude modulate the audio you feed in. If that happens you will see
the signal frequency plus two more sidebands at signal +/- ripple
frequency. But that has nothing to do with the actual intermod spec of
the amplifier per se. It is really better to keep these measurements
apart.

When you say "if the amplifier is not designed well [...] the ripple
will instead amplitude modulate the audio..." are you impling the
specification at fault here would be the PSRR of the amplifier?
Also, if I see the signal frequencies applied to the DUT and the
sidebands, is this not intermod?
I am not disagreeing here; rather, I want to ensure I understand.


In a well designed amplifier, the gain is set by two resistors, and
nothing else. If the design is poor - a lack of open loop gain for
example - some of that gain figure will depend on device current,
which is determined by the rail voltage. This will allow ripple on the
rail to modulate the audio signal, rather than simply appearing as a
low level interference. And yes, if you see the sidebands, that is
intermod.

measurement.

Ok, I did not think such a setup was a valid THD+N measurement either;
however, can you elaborate a little on why it is not?
My answer would be that what is being measured are not harmonic
components of the input signal. With the Audio Precision, I believe
you can move the frequency of the notch filter but only one signal
would be removed. Meaning the fundamental of the input signal or the
power supply signal would be removed. Since one signal frequency
remains then this is not a THD+N.

THD+N stands for total harmonic distortion plus noise. What you are
measuring is total harmonic distortion plus noise, plus some random
other signals you are choosing to inject. Stop injecting those other
signals and you will have a THD+N measurement.


You say that *in general* if THD+N is good, then so is intermod.  Ok,
so it is a Rule of Thumb.  But can one be converted to another?

Probably, but I haven't done the maths so I can't say for sure.


Not a problem.

Let me give you a little bit more background on this issue. The
reason I am asking these questions deals with the difficult I have had
with a PSRR test. I completely understand that the current set up
(input signal + signal on power supply) is *NOT* a PSRR test. Rather
it is a substitute, and some of the senior apps say that seeing a "THD
+N" vs. Power Supply Frequency curve would serve customer's better.
Simply put, "We cannot give you a PSRR curve, but there is this THD+N
curve vs. Power Supply Frequency" which we think is better. I, the
junior engineer, am having trouble justifying this hence this
discussion thread.

Well, it isn't better. It is a totally unrealistic and meaningless
measurement. PSRR is a parameter you need during design and
development. It is the parameter that allows you to design your power
supply - size of decoupling caps etc. It has no place in a finished
amplifier spec because the customer has no access to, or ability to
inject signals into, the power supply rails. All the customer needs to
know about is the resulting level of hum (and that is generally
nothing to do with PSRR, but usually sub-optimal design or layout).

As for a PSRR curve - what for? 50 or 60Hz mains is the customer's
interface with the power supply. He doesn't connect signals there. As
I say, it is necessary to know and understand the PSRR of the
amplifier (internal component) as a DESIGNER. A PSRR spec for the
amplifier (whole system) is irrelevant to the customer.

So measure the hum, measure the THD+N and provide individual figures
for them. That is what everyone else does, for good reasons.

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

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