On Tue, 22 May 2007 16:03:55 +0100, Jim Lesurf
wrote:

In article , Serge Auckland
wrote:


"Jim Lesurf" wrote in message
...


I'd also tend to use a THD+Noise value as otherwise effects like PSU
intermod might be missed as their components don't crop up at
harmonics of the test frequency in most cases. I've seen amps where
the THD value was low, but where there was much more LF garbage due to


Indeed, and in my previous post of the criteria, it was stated that THD
should be measured at all frequencies 20-20k and refers to all powers
and all loads for which the amplifier was designed. In practice, the
measurements are actually THD+N as this is what distortion meters
actually measure. Of course the use of a harmonic analyser for
distortion measurement won't pick up the +N component, but as a
practicing engineer, I found the use of such an instrument to be
tedious in the extreme, and unnecessary when an overall THD+N figure
was so easily achieved.

Well, I've used both 'traditional' THD+N kit that works by nulling or
filtering the test sinusoid, and a specan/generator that autofinds the
harmonics and works out THD. The second didn't seem at all 'tedious' to use
as it automated the process. It was also faster than the old distortion
kits I used to use that took some seconds to null down for low distortion.

I suspect that people use either form of kit, whichever is to hand. The
difficulty with this being what we discuss above.

Slainte,

Jim

There is a distinct advantage to the second, in that it gives you
information about the nature of the harmonics. If you are making this
measurement as part of a development process, that can be invaluable.

d

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