> From: yo3ctk <yo3ctk@alltrom.ro>
> Subject: RE: [AMPS] IMD measurement scheme
To: <amps@contesting.com>
> Date: Thu, 24 Jul 97 07:28:49 +0000
Good to have someone who makes measurements on, instead of using
over the air splatter reports....hi.
> During my career as a R&D engineer for military equipment in the
> eighties, I used the following setup for measuring IMD performance of
> power amplifiers:
>
> A pair of HP signal generators (non-synthesized, for low phase
> noise) feeding two power amplifiers (Marconi, 150W) connected to the
> tested amplifier's input via a -3 dB hybrid combiner. This combiner must
> have at least -40 dB crosstalk between its two inputs.
With a class AB PA stage, such as in transceivers, and measuring
the typical -20 to -40 dB third order of a PA, that much isolation
probably isn't necessary.
I do use class A PA's for my generators, driving six dB pads driving
the combiner driving a 3 dB pad driving the PA. That way the
generator to generator isolation is 12 DB plus the combiner
isolation (easy to make 30 dB or more), and the pad between the amp
and combiner guaranties at least six dB of return loss in the
combiner's termination.
But a Ham needn't go through all that bother. Just a simple magic T
(or for monoband use a 75 ohm line hybrid) will be enough if the PA's
input VSWR is low.
Linear PA's are very poor mixers, even if a little RF from one makes
it back to the other.
This can be checked before doing the PA test by measuring IMD from
the generators (radios). If the IMD is -50 or more dB WITHOUT the PA
on, the results will be pretty good. Especially since most radios are
only 30 dB or less down!
No reason to have a PA ten or twenty times cleaner than the trashy
exciters out today.
By the way, all radios are not cleaner at low power. Nearly all of
the solid state rigs I've tested get dirtier as the power level is
increased or decreased from a median value of power.
Running a 100 watt radio at ten watts to drive a tetrode will
almost certainly make the signal trashier than running the
same radio at normal power.
73, Tom W8JI
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