Bill,
Several years ago during a RTTY contest on 40 meters I noticed a weird
hash noise covering the entire band. It would come and go with a
timing like someone was calling CQ. Tuning around, I found a station
whose CQ was exactly in sync with the hash.
Later I looked him up on QRZ.com and lo and behold there he was
bragging about his Class E amplifier and how wonderful it was!
I have also heard such stations. For example, there is a station that transmits
high powered AM on 40m, about 1000km from my place, using a poorly made class D
transmitter. It's easy to get it wrong and create huge amounts of QRM, but it's
also possible to do it right.
I have a significant tolerance for such low quality signals, as long as a
station doesn't transmit it for long. If the operator/experimenter puts his
creation on the air, and after getting reports of a dirty signal, goes on to
improve it, I won't blame him for the trouble caused. It's part of
experimentation, which is what this hobby really is about! Only when someone
insists on placing a dirty signal on the air, time and again, and rejects any
reports that tell him there is a problem, then I get sour at that guy!
Digital AM transmitters are sensitive to any noise on the power supply, because
they operate driven into saturation, so the output amplitude is proportional to
the supply voltage. If a phase-shift transmitter uses a switching supply, and
that supply has even 1% of ripple at the supply's switching frequency, then this
will create noticeable sidebands at that frequency from the carrier. A 50kHz
dirty switching supply will create QRM every 50kHz across the whole band covered
by the bandpass filter of the transmitter. And a noisy supply, for example
caused by a poor control loop, will create broadband noise on the band.
In addition there are all the phase modulation issues. AM people are quite
unsensitive to phase modulation of their signals, because in an AM receiver the
signal will still sound fine - but it will splatter! And unwanted phase
modulation is very common in FET amplifiers.
I mentioned AM because it looks like the AM fans are the ones that most commonly
run homebrew solid state class D and class E equipment. But it also applies to
other modes, of course. Only that bad phase distortion in SSB is quite obvious
to anyone listening to the signal, while in AM it's not that obvious.
It's good to keep such dirty signals in mind, when developing equipment that
might produce them! My method is to first measure the quality of my signals as
best I can, when I have built something, and then when I go on the air with it
for the first time, I find some hams with adequate technical abilities to make a
meaningful appraisal of signal purity, and ask them to tune up and down while I
talk to someone else, and tell me if there is anything that shouldn't be there.
Manfred
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Visit my hobby homepage!
http://ludens.cl
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