Hi, Jim
Well as one who has been an RF and radio engineer and designer for 40+
years, I have to agree with most all of your points. Great deal of truth in
there, but so many guys don't appreciate all those things and their
inclination is "crank it to the right" and "the "louder you shout, the
further you get"! And they are looking for large meter excursions. To
appreciate the tendency to overdrive transmitters and amplifiers. One need
only listen to the the prevalence of awful key clicks and SSB splatter in
contests!
(And "real men" use vacuum tubes to develop "real power"! :-) )
73,
Charie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Jim Brown
Sent: Monday, December 30, 2013 6:52 PM
To: topband@contesting.com
Subject: Re: Topband: Digital mode spurious issues
On 12/30/2013 3:13 PM, Charlie Cunningham wrote:
> I would think that IMD products in a high-level PA that is over-driven
> beyond good linearity limits could add some junk in the "undesired
> sideband"? FWIW
Yes. Indeed, any IMD would do that. K6XX is an Elecraft engineer who
worked on their KPA500, among other things, and looked at a lot of
competing power amps in preparation for doing so. Bob recently did an
excellent tutorial presentation to a meeting of the Northern California
Contest Club about the root causes of sideband trash, the general
properties of various amplifier types, and how to minimize the trash.
In general:
Distortion products increase when the antenna is poorly matched to the
amplifier That's true whether it's a tuned tube amp or a fixed tuned
solid state amp -- in other words, the tube amp must be carefully tuned,
and the solid state amp should be used with a tuner if the antenna is
not an ideal match.
Distortion products increase as power supply voltage decreases. In other
words, a rig designed to run on 13.8 volts will be much cleaner at 13.8
volts than at 12V.
Most solid state output stages are cleaner at half power than at full
power. That means that a rig will be cleaner driving a power amp at 50W
than at 100 W.
Using AGC between the power amp and the rig to set output level is a
recipe for sideband trash.
A properly tuned hollow state power amp is typically 8-10 dB cleaner
than the best solid state amps.
Fast rise time of the keying waveform is the major cause of clicks W8JI
and others long ago identified this as the cause of the FT1000-series
rigs awful clicks, and fixed them. The rise time of some rigs (notably
the IC7600) is adjustable, and only the slowest rise time is acceptable.
The K3 uses an optimally shaped keying waveform (which designer N6KR
calls "sigmoidal") to minimize clicks, and it is not user adjustable.
Most ICOM rigs have overshoot that also causes clicks.
Something I learned from N6KR a few days ago is that the very low level
of sideband trash from a K3 is the result of two design elements. First,
the synthesizer is very clean.. Second, they run it through the TX
crystal filter, which gets rid of trash more distant from the carrier.
73, Jim K9YC
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