On Sat, 8 Oct 2005 18:54:44 +0000 (UTC), "Reg Edwards"
wrote:


Automatic Level Control, ALC.

What characteristics of the load impedance are used to automatically
control the drive-level of the power amplifier?

Well, as you no doubt know Reg, in the early days of application of
ALC to amateur transmitters it was literally to control the drive
level of the PA, and often did that is a closed loop where excess
drive in a class AB1 PA was detected by occurrence of grid current,
which was used to reduce gain in prior stages to limit the drive level
to AB1 conditions. Such a control system is indirectly affected by the
PA load.

Today, modern transceivers have further developed that idea to
controlling the RF output voltage or (differently) PEP power out (ie
be detecting Vout or power out and adjusting gain in prior stages to
limit output. This might more loosely be called Automatic Power
Control. In this case, the measurement point of the control loop is
more directly affected by the load.

Many modern solid state transceivers also detect poor loading using a
reflectometer calibrated for the specified load for rated output
power, and reduce drive based on the measured "reflected power" in the
reflectometer to protect the PA (if that needs to be done anymore for
that purpose - transistors are more robust). This might be properly
described as PA Protection. The measurement is directly affected by
the load, and is highly responsive to load changes beyond a configured
threshold.

All of these things may be included in the readers understanding of
ALC. Certainly, many if not most, modern solid state transceivers
perform all three of these functions, and display the control voltage
as "ALC". And many current model external valve based high power PAs
(Linear Amplifiers) still use the grid current detection scheme (if it
is hooked back to the transceiver).


Whatever they are must apply to CW, SSB and FM modes of operation.

And AM?

Owen
----
Reg.

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