On Thu, 30 Sep 2004 00:29:35 -0700, Jerald Volpe wrote:
>On 20-meter PSK earlier today I received a report of unusual distortion
>on my signal.
This distortion is caused by overdrive somewhere. It could be in the audio card
in the
computer generating the PSK, it could be overdrive of the mic input of the
transmitter, it
could be overdrive of audio or rf chain in the transmitter, or of the output
stage in the
transmitter. If you are SURE that it only happens on 20 meters, the first thing
I would try is
backing off on the drive at the output of the audio chain and/ or to the output
stage.
I've done only a bit of PSK31. When I set it up, I found that I could reduce
the IM distortion
from about -40 dBC to about -55 dBC at the computer's output by backing it down
about 6
dB from maximum output and making up for the lower level by increasing the gain
at the
audio input of the transmitter. I had already put a 20 dB pad between the
computer and the
mic input. I haven't looked at transmitter output, but didn't have complaints
of a dirty signal.
Note also that nearly all transceiver mfrs recommend backing the output power
down by 3
dB for "carrier-down" modes like PSK and RTTY. Dissipation is one reason, but
reduced
distortion on PSK is another very good reason for being conservative here. That
3 dB
reduction in output could easily result in 10-15 less distortion.
Jim Brown K9YC
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