> I do get your point regarding the circuit but the point we are trying to
> make is that despite the fact that you and others try to tell us it
> doesn't work it does work.
I keep saying, and I'll say it again, I've even seen class C PA's that
"work" on SSB.
I would not say a class C circuit is a "good idea" based on the fact
some can indeed produce, in some cases, acceptable IMD
performance.
WITH respectable IMD, the measurements that
> were carried out is proof of this. I think we have done far more extensive
> testing of these amplifiers than many others have done with their homebrew
> stuff. Far more..
Maybe the tests were "extensive" compared to others, but I'm sure
we all agree the test was not conclusive about the system in other
applications...which is the point I keep trying to make.
> Mr DAF might very well have the wrong numbers in his article, I will not
> and have not contested you on that as you are much more capable to check
> them out than me. But that is another story.
When the voltage across a resistor does not equal the PEP power
applied, someone missed something pretty basic. My only point
with that is we can't rely on the numbers for understanding what is
going on when the numbers are so obviously wrong.
When the loading on the exciter varies by around a 2:1 ratio over
the RF cycle and envelope cycle, that usually isn't good. It doesn't
mean the circuit will not "work" in some cases, it just means the
design isn't very good. However, I can take that varying load and
find some exciter that is happier with it than a constant load.
That's why ANY IMD test requires an almost perfect source. For
broadband tests, I use an exciter that is more than 60 dB down in
out-of-passband crud. For two or three tone tests, I mix single tone
transmitters in an almost perfectly IMD-free system.
Such measurements are repeatable, when the source is much less
distorted than the DUT. I can send a PA out to the someone else
not doing that, they can measure it five times, and get five different
numbers.
The same thing typically happens with antennas. Someone will
install an antenna that really doesn't work like they claim, but in
their environment they are quite happy with the antenna. That's
because they stick the antenna in a cluttered environment,
compare it to another antenna in a cluttered environment, and
reach a conclusion that "it works this way".
Coaxial dipoles are one example, quads are another, where the
claims don't agree with the facts. Like with an antenna, there can
be a lot going on in any IMD test. The user can be as happy as a
pig in slop, but that doesn't mean the system works as claimed.
One fellow in Ohio was going to patent an amplifier that REDUCED
exciter IMD based on his extensive IMD tests with a two-tone
signal fed into the microphone input of a solid-state amateur
transceiver. I understand quite well how in one particular test setup,
it is possible to add a PA that actually improves IMD of an exciter
with IMD.
I can add a FET amplifier behind my FT-1000D, and it actually
cleans my FT-1000D up a noticeable amount. If I add that same
PA behind a KWM-2, it gets notably WORSE.
The problem is if that PA is moved to another exciter, it can make
IMD significantly worse. After I suggested he move the PA to
another few exciters and do the same test, he became totally
silent about his future IMD reducing patent.
My only point is the G2DAF system is poorly engineered, and is
more hopeful thinking than good circuitry. There certainly might be
cases where it works, I'm not disagreeing with that.
73, Tom W8JI
w8ji@contesting.com
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