On 12/29/2011 11:09 AM, richard courtway wrote:
> Personally am a cw operator - and hear quit a few very wide and
> splattering cw signals .
Me too.
> Thinking that most of this is caused by
> operators trying to get a kw out of a 800 watt rig. Overdriving - etc.
As has been noted by those who understand it, the fundamental causes are
phase noise (from the exciter, amplified by the power amp) and IMD in
both the exciter and the power amp. IMD (intermodulation distortion)
happens when there is ANY non-linearity in the system. And it is the
TRANSMITTER that is getting wide, NOT the receiver.
Sure, a receiver CAN be overloaded and IMD can happen there, and
receivers have phase noise, but that's NOT what we're talking about
here. I can have GOD'S receiver, with zero IMD and ZERO phase noise,
and if a transmitter is putting out trash on my frequency, there's
nothing I can do about it. And THAT'S what we're talking about.
The same thing happens whether it's CW or SSB or RTTY or PSK or JT65.
The only difference is that a CLEAN CW signal will show up at about 300
Hz wide to the 50dB down points on the P3 spectrum display of my K3, and
a dirty one can be 3 kHz wide at only 30dB down. A CLEAN SSB signal
will be about 3 kHz wide, assuming audio with 400 Hz - 2.8 kHz passband,
while a dirty one can be much wider. It's also common to hear splatter
from audio distortion caused by over-driving anywhere in the audio chain
and excessive signal processing. I occasionally hear splatter from a
dirty SSB signal, then look at the display and see both peaks of a
signal 10kHz away that exactly correspond with peaks on my frequency,
and with splatter that I hear. That's not my receiver, it's his dirty rig.
As only one example of a rig I've owned, there are four DSP settings in
the FT1000MP, one of which can be made to sound very good if you don't
overdo it, but awful if you DO overdo it, and the others sound awful no
matter what you do. And when they sound awful, they also splatter like
crazy. And it's well known that all versions of the FT1000 series rigs
produce massive clicks if not modified.
So -- bottom line: transmitters DO generate trash in their sidebands,
the causes can be both in the exciter and in the amp, careful tuning,
setting drive levels, and audio setup makes a HUGE difference, and some
rigs are inherently dirty.
73, Jim K9YC
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