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:[email protected]] On Behalf Of Jim Brown Sent: Monday, December 30, 2013 6:52 PM To: [email protected] 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 _________________ Topband Reflector Archives - http://www.contesting.com/_topband _________________ Topband Reflector Archives - http://www.contesting.com/_topband
