Yes, it really is just baseband audio into an SSB transmitter. And some of
the new rigs that run that process audio through A/D converters to digitize
if for DSP are just asking for trouble.
I guess I'm an "old fart", but I suspect that a 12AX7 triode audio preamp,
with its 3/2 power-law transfer characteristic and perhaps 150 volts on its
plates will withstand a bit of overdrive with a lot less harmonic distortion
than an A/D converter operating from 3- 5 volts and driven beyond its range
into saturation where it produces hard, "brick-wall" limiting! Not all
things are best done digitally! I suspect that some of these "digital" modes
are improperly adjusted, resulting in excessive audio drive into the SSB
transmitters. AND if the transmitter is using IF DSP and/or I/Q modulation
for sideband selection, there are more opportunities for spurious outputs.
FWIW
Good to turn down the drive in the audio inputs of the SSB transmitters!
73,
Charlie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Tom W8JI
Sent: Thursday, January 02, 2014 12:14 PM
To: topband@contesting.com; kr7o@vhfdx.com
Subject: Re: Topband: Digial mode spurious issues
> 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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