The majority of IMD problems in PSK-31 are a result of trying to drive a
microphone-level input on a transceiver with a speaker-level output from a
computer sound card. That's about 40 dB too much audio, which will result
in major IMD regardless of ALC action or output level setting.
Since PSK-31 works by shifting phase, the amplitude variations caused by ALC
shouldn't have any significant effect on either the IMD or readability of
this signal.
Ron N6IE
www.N6IE.com
(Formerly N6AHA)
----- Original Message -----
From: "Merle Bone" <merlebone@charter.net>
To: <tentec@contesting.com>
Sent: Friday, December 29, 2006 2:40 PM
Subject: Re: [TenTec] TX EQ = -14 dB for PSK31
Mark said:
" So, it would seem adjusting the mic gain with full output would be more
accurate for psk31 (or other data) work."
---------------------------------------------------------------------------------------------------------
As was indicated earlier, a major issue with PSK31 transmissions is
"self-inflicted" IMD (Intermodulation
Distortion). Significant IMD (Created at the transmission point) can make
an otherwise strong signal difficult or impossible to copy. Consequently,
you really don't want the output power level of the Orion anywhere near
the ALC trigger point. The Orion power set adjustment on the front panel
sets the ALC trigger point. Therefore, setting that level high - as in
100W - and then adjusting the audio signal drive level to something like
25W output on the Orion will ensure that the Orion is operating at a very
linear point of operation (No ALC action is even close to occurring) and
the PA has plenty of headroom - of course no signal processing on.
Another very useful meter facility for PSK31 is PSKMeter
http://www.ssiserver.com/info/pskmeter/ . This allows "measurement" of the
transmission envelope on idle. You can see the waveform, against a correct
pattern overlay, and automatically measure the IMD. This allows you to use
higher power levels for DX but still insure the IMD level is very low.
This is the kind of capability that would be "a natural" for the Orion and
its signal processing and display facilities. Even with the growth in
popularity of new digital modes, today's transceivers are essentially
"without tools" to correctly set signal levels. Maybe in a future release
of the V2 firmware :-)
73, Merle - W0EWM
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