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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>
>
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