Owen,
You have obviously have extensive knowledge and familiarity with some
equipment on this subject. I haven't studied this aspect of the radios I
have yet, but offer the following from my design experience.

There are several relatively independent considerations which have been
mentioned in this thread and I hope this clears up any confusion about which
is doing what.

My understanding of the true purpose of (what we call) ALC is to prevent
distortion in the final amplifier and the resulting "splatter" it causes,
while allowing the maximum power out. As you so correctly point out, this
originated in the linear amplifiers of old. Distortion at this point causes
increased intermodulation (mixing) of the desired signal components and the
resulting products appear out of the normal bandwidth = "splatter". There
is always SOME of this going on, but as long as it is below about -30 dBc
this is considered acceptable.
Because there is no Grid current to sense in solid state amplifiers, some
other method of detecting the onset of distorting is required. Not knowing
what _IS_ done, I can only speculate that knowing the highest level of
output power which causes only a small, allowable increase in distortion, an
output level detector can be designed which begins to provide feed back
which cuts drive when this point is reached; thus paralleling the function
of the grid current detection method of old. Because this is an
instantaneous feedback system with a relatively high gain, the feed back
voltage will exhibit a considerable rise as the device is driven closer and
closer to the undesirable point. This must be what we are looking at that
is called ALC on the front of the radio. As long as the level of this
signal is within the manufacturer's limits, the distortion is limited to the
specification and you are assured of the highest power with in-spec
distortion.

With solid state comes other considerations which are actually separate
from the distortion issue although from a circuitry standpoint they may seem
identical. Because these devices have been and are still somewhat sensitive
to mis-treatment and can be destroyed instantaneously, various methods are
available to help save the devices. However, had they been easily available
in tube days, they would have been equally applicable.

Sensing and placing a limit on collector/drain current is one method.
Some load conditions can cause excessive current resulting in device
destruction, so sensing over-current electronically and reducing drive can
prevent this.

With the advent of simple reverse power sensing circuitry, another fault
detection scheme can be used to reduce drive and protect the final power
device. Sensing SWR and reducing drive can prevent high device current and
destruction, as well.

Heat rise can also destroy the device, so sensing device temp and
reducing drive controls this also.

Because a design has one or more of these, the identity of each could be
confused by combining the feedback signal and calling it only "ALC" when in
actuality, there are two or more protection schemes present along with the
(distortion limiting) ALC.

73, Steve, K'9;D.C,I



snipping quite a bit:

"Owen Duffy" wrote in message
news
On Sun, 9 Oct 2005 20:11:09 +0000 (UTC), "Reg Edwards"
wrote:


"Owen Duffy" wrote various ways and means of automatically
controlling the power output of transmitters. Thanks very much.

Presumably control is needed to limit the power dissipated ...

Another method of PA protection is thermal protection of the PA...


In my Icom 735 transceiver the first control requirement appears to be
met by by placing a limit on the DC collector current or power input.

You are right, some radios do incorporate over-current protection ...

Whilst probably intended for PA protection, it has the great benefit
that the radio is less likely to draw excessive current while
adjusting an ATU, ...

As I described, and for example, from the IC706IIG service manual:
"The reflected wave signal appears ...the antenna is mismatched. The
...signal level is detected ...and
applied to the ALC circuit as the reference voltage." ....

Owen
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