Topband: Digital mode spurious issues

[Richard (Rick) Karlquist]

On 12/31/2013 11:21 AM, Tom W8JI wrote:

> I've never looked at the FT1000, but I doubt that is a clean system. It
> would still be like a SSB transmitter with audio signals if that is how
> they do it. It would be much better to actually shift a carrier, and not
> do the baseband audio signal run though a balanced modulator and then
> through a sideband filter.

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.  I imagine I could make a synthesized
RF FSK system if I wanted to go to enough trouble,
but obviously Yaesu's resources were limited.

Rick N6RK