First, it ignores all the empirical evidence to the contrary. This
alone would make the statement suspect.
Second, it ignores the different methods and implementations of ALC in
different transmitters.
One of the first effects of ALC is the imposition of an amplitude
modulation on the RF envelope. This looks like a sawtooth whose shape
is determined by the ALC characteristics of attack and delay time and
whose amplitude is determined by the amount of ALC. The end result
are pairs of sidebands that look like IMD. The stronger you drive ALC
the stronger the sidebands will be. This is taken from HF Radio
Systems Circuits by Sabin and Schoenike.
Another effect is that ALC occurs after the fact. This means a sudden
pulse like a data tone may very well overdrive any gain controlled
stages before rectified and amplified ALC voltage is applied.
Depending on the decay time, this can occur on each off/on transition.
Lastly, ALC's main purpose is to prevent overdriving amplifiers, not
as some say to increase average power output. You can achieve an
increase in average power, but only by depending upon ALC to act as an
RF clipper. Any time clipping is done, you introduce distortions that
can severly impact the quality of the output. This also requires that
ALC be adjusted properly so that RF amplifiers are kept in their
linear region. This alone is probably not the case in many ham
transmitters that haven't been aligned since their date of manufacture.
By using very low levels of ALC or none at all, you eliminate all of
the factors above insuring a good signal. Of course, even that
assumes everything in your transmitter is operating linearly and as
designed.
Jim
WA0LYK
--- In digitalradio@yahoogroups.com, Andrew O'Brien [EMAIL PROTECTED]
wrote:
I found this quote today, anyone care to comment ?
Although it is a common belief to the contrary, it is in fact quite
alright to let the
transmitter ALC line operate on PSK31. The ALC line will control the
drive
level without
clipping in the same way that it does on voice operation
Andy K3UK
Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org
Other areas of interest:
The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/
DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion)
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