Topband: Digital mode spurious issues

[Grant Saviers]

An interesting point from Tom re SSB modulation added problems.

A really informative website re "plain old RTTY" is
http://www.frontiernet.net/~aflowers/k3rtty/k3rtty.html
and is a good introduction to the technical complexity of digital 
modulation systems and what to do to make "better" RTTY signals.

The W7AY work is referenced w7ay.net but the site was not available when 
I tried it.

Is there any good testing posted for other modes and on other radios and 
software generators?

Grant KZ1W



On 12/31/2013 7:13 AM, Tom W8JI wrote:
>> 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.
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