Topband: Digital mode spurious issues

[Tom W8JI]

> Well, I certainly have to agree, Tom, if the signal on the desired 
> sideband
> is just a single shifting tone. Might get messier if an sudio stage or A/D
> is driven into limiting and producing harmonic distortion at audio, I 
> guess.
>

The entire thing for digital modes was poorly planned. I'm surprised no one 
objected to the frequency choices (of Europe), because they are technically 
always going to be a long term problem. The frequency range really could not 
have been more poorly planned for future long-term band use.

Any digital mode piped into the SSB transmitter system, like it or not, is 
really SSB modulated. It has all the noise, carrier, and opposite sideband 
suppression issues, as well as sensitivity to levels. If they transmit 
1835-40 using USB on the radio, the opposite sideband falls in the 1832 and 
upward range. IMD can be anywhere, if it is a multiple tone at the same time 
mode. Harmonic distortion is upward from the carrier on USB.

Collins got burned by this. They tried running a pure audio tone into the 
SSB transmitter of the early S line to generate CW. The FCC (back when they 
did enforcement) starting handing out pink slips left and right to Collins 
owners. Any carrier, noise, or harmonic distortion at all resulted in an FCC 
citation, so Collins had to convert transmitters back to a keyed carrier.

Anything converted to RF in a SSB transmitter system really belongs off by 
itself, well away from other operation. It's really a technical issue that 
will always exist, because the basic RF generation system or idea is flawed.