Topband: Digital mode spurious issues

[Tom W8JI]

> AFAIK, all non-DSP rigs with synthesizers work this way.
> Assuming you want the output frequency to be derived
> from the master clock frequency, there is no easy
> way to shift an RF carrier.  You can't use a free
> running oscillator, because it won't be derived
> from the master clock frequency.  You can't switch between
> a mark synthesizer and a space synthesizer because of
> transients.  If you try to key the programmed frequency
> of a BFO synthesizer, the PLL will probably go out
> of lock momentarily, producing garbage.  Also, it
> may not be fast enough to keep up with RTTY.  After
> considering all this (as a very experienced synthesizer
> designer) it is hard for me to blame the designers
> for using AFSK.

It is easy to do it much better, and it would only cost pennies extra at the 
most. The signal could be generated by the normal SSB system and then run 
through a narrow IF filter. Problem solved. They could have done CW the same 
way, or in a similar fashion, with an unshaped off-on carrier through a 
narrow filter. They just didn't think to use the parts they already had in 
the radio.

But that isn't this issue. This issue is they run baseband audio from a 
computer into a SSB transmitter to generate TX signals. This means it is 
really a SSB transmitter processing the tones, and they don't even restrict 
bandwidth with a narrow filter. That is really the entire issue. Instead of 
a narrow filter cleaning up stuff, it all goes through a SSB filter. Anyone 
with a computer and a little skill can invent a "new mode". It's just bad 
engineering to stick that stuff near weak signals, because the problem can 
only be fixed at the transmitter.

73 Tom