Topband: Digital mode spurious issues

[Jim Brown]

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