On Jul 30, 2004, at 12:32 AM, Ian White, G3SEK wrote:
Will Matney wrote:
"I'd like to make a modest proposal here: how about not feeding a
"dirty" supply to *either* of the grids?".......... Ian G3SEK
Ian, that's exactly the point I'm trying to get across.
Agreed, we're mostly dickering about different ways of saying the same
thing.
The only other thing I mentioned was that voice modulation creates
more IMD than does the two tone test, and by what I've read, we are
all in agreement it does.
Again, agreed.
But taking up Peter's point, even a tone spacing as narrow as 30Hz
don't come anywhere close to a realistic test for an amplifier that is
supposed to be used with human voice modulation.
The nasty feature of real voice signals - ordinary spoken words - is
that they also contain a *very* strong pulsing VLF component at a
syllable/word rate of only a few Hz.
Not to mention the additional problem caused when nasty words are
spoken.
This VLF component drags down the power supply, and makes the amp very
vulnerable to the large 300-3000Hz components that are often happening
at the same time. No two-tone test can simulate this, and even
three-tone or pulsed two-tone tests are only tinkering with the
problem.
If you want to measure the SSB IMD performance, then the *only* truly
valid test signal is... the human voice.
The question is, how to develop a reproducible voice test?
Virtually everybody has one built-in. And, as a free added bonus,
it's the very same one that is going to be used on the air.
Well, a standard voice signal is very easy to generate, in these days
of digital audio (I'll come back to the question of *whose* standard
voice).
But it's not the same one that will be in use on the air.
....
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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