I year or so ago, I built a Class E AM transmitter with a Class D pulse
width modulator. The PA had a 94% efficiency and the modulator had a
90% efficiency. The modulator ran at 70 kHz. Both the modulator and
the PA used a single IRF640 Mosfet. The transmitter put out 100 Watts
on 160 meters. I never got used to the small heatsinks running very cool.
Tom W0IVJ
On 10/21/2014 12:28 PM, Manfred Mornhinweg wrote:
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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