Topband: Digial mode spurious issues

[Joe Subich, W4TV]

> While we call these digital modes, they are really baseband audio
> transmissions run through SSB transmitters. They are subject to all
> the dynamic range problems and limitations of SSB transmitters
> processing the same types of audio tones.

Like it or not, all digital transmitters and all digital modes except
old direct FSK are essentially SSB transmitters with a subcarrier on
which the digital information has been modulated.  The SSB transmitter
is nothing more than a series of mixer stages and amplifiers designed
to put the subcarrier on the final at the necessary power level.

The issue is, and always has been (even with AFSK RTTY), that the
mixer/amplifier stages need to be distortion free and the conversion
oscillators need to be clean.  That means careful attention to
synthesizer phase noise (Icom, Flex Radio?), gain distribution,
linearity and proper interfacing.  There are too many poorly designed
"modern" rigs (noisy synthesizers), improperly maintained older rigs,
and technically unsophisticated users who don't understand that digital
subcarriers should be injected through the "line in" direct modulator
input of their transceivers rather than the mic jack and through audio
processing.

Of course it doesn't help that modern solid state amplifiers all seem
have all of 1 dB between the 1dB compression point and saturation with
ALC circuits that are designed to keep the average power at the 1 dB
compression point rather than keeping the PEP at the 1 dB compression
point.  Couple digital modes like PSK## and others with 6 to 14 dB
"crest factors" with ALC circuits designed to push the amplifier to
the average power level and IF gains as much as 20 dB "too hot", one
has an inherently unstable system that is beyond the ability of most
technically unsophisticated users to understand or control.

73,

    ... Joe, W4TV


On 1/2/2014 12:14 PM, Tom W8JI wrote:
>> I have been using WSJT since 2001 and cringe every time I hear it
>> called a "QRP mode".
>
> I want to put this to rest with just a few more comments.
>
> While we call these digital modes, they are really baseband audio
> transmissions run through SSB transmitters. They are subject to all the
> dynamic range problems and limitations of SSB transmitters processing
> the same types of audio tones.
>
> The usual "cure" offered for the poor or limited performance of the SSB
> systems that process the encoded audio is to turn the power down. That
> doesn't always work to eliminate problems, plus low power obviously
> limits the potential range.
>
> This stuff is really all just an audio signal processed by a SSB rig.
> That's why it can go into the SSB transmitter! SSB transmitters are just
> not that good, and that is why these systems create adjacent channel
> problems and will always create problems. They should be off someplace
> out of the way of weak CW signals, not nestled up against weak signal CW
> areas. A receiver cannot filter out transmitter flaws, no matter what
> some might claim or imagine. If a spurious signal is right on top of a
> weak station, it is on top of it.
>
> These are just the hard, cold, facts of life. It's just disappointing we
> have no technical people who think things through before picking
> frequencies for "new modes".
>
> 73 Tom
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