I'm afraid I must disagree in some cases. I know nothing about the
Icom ALC design but I'll be willing to bet it is typical of modern ham
SSB transceiver design attempts at a cheap way of preventing power
output excursions above some front panel set limit.
With modern SSB transmitters what is driving all the stages up to and
including the PA is the AF current from the audio source coming into
the rig. Designers usually put some kind of high vswr protection on
the PA output so the transistors are not damaged by high vswr. This
is often done with a reflected power coupler on the PA output that
produces some control voltage that is fed back to a low level voltage
controlled amplifier at some IF stage, 455 KHz perhaps, that performs
a gain reduction to fold back drive to the PA and reduce output to
protect the final transistors. This design works pretty well. It
doesn't cost much to add a forward power coupler engineered to produce
a certain voltage level such that with a certain setting of the power
out pot (PA collector current pot) that voltage fed back to the VCA at
a previous IF stage, performs the needed gain reduction to hold the PA
to some set forward power limit.
Obviously, while this is great for VSWR protection, it is a really bad
way to limit forward power output peaks and spikes for the simple
reason that once that horse is out the barnyard gate, it's too late to
close it. Most ALC circuits employ some delay time constants so once
a power spike is sensed and a gain reduction voltage fed back to the
VCA it holds for a few seconds and continues to be effective if there
is high average audio driving the RF but don't count on placing your
SSB rig in transmit, turning your PA power pot to 100 w. and expect
that not very well thought out design to trap every initial breath
blast or sudden impulsive sound force at 100 w. Once that spike is
out, it's on the way to the amp and everyone in radio land and your
ALC can say "oopsie" and send a gain reduction v. back to the VCA and
deal with any other pops and blasts for a few seconds but that first
one has left the station. And far too many get out the gate in
typical operation to make ALC something to depend on for power output
limiting.
So, since ALC is a bad scheme to rely on for peak power limiting.
What to do? A far better solution is to employ a device that
completely controls audio level peaks being fed into the ham rig as is
done in broadcasting, so ALC is not even necessary and does not have
to intervene. Why isn't this done? Because a zero attack time
extremely fast gain reduction limiter on the audio input (i.e. mic
jack) isn't cheap. And hams who do not understand how ALC works and
realize that it is inferior as far as what it is supposed to do is
concerned, won't want to pay for something that gets the job done.
So, we're left with a sort of half assed method and too many hams
overdriving their amps (those cheap s.s. ones are the worst)
unfortunately.
73
Rob
K5UJ
<<<<There are two kinds of ALC and I think Roger is talking about the kind
of ALC that is fed back from the amplifier to the transceiver. With
that kind, I fully agree with the above.
But there is also ALC that is generated entirely within the
transceiver (such as my IC-756Pro3), and is completely independent of
the amplifier. With that kind you can safely use ALC to limit the
drive and it does an excellent job, I might add.
And if you use the internal type of ALC, you don't need the fed-back
kind at all.
73, Bill W6WRT>>>>
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