ALC means Automatic Level Control,
as far as I know to ensure that the output power
of the ESG is the one you typed, you must have the
ALC on.. always..
I don't think that your amplifier
is changing it´s gain, I think your generator is
diving a different power than the one you think,
and therefore, you are seeing that at the output
of your amplifier..
I also believe that if you
turn the ALC off, then an "unlevel" message appears
on the screen noting you that you have problem with
your output power... Once, the output power detector
of my E4438 had a failure, and in consequence, I
had the "unlevel" error and the output power was
always different that the one I entered.
Hope to have understood well your problem.
when I use the Agilent E4438
to generate a modulated signal, I find there
is a big difference wheather I use ALC or not。
if I put the ALC on, the gain of my amplifier
chain is much lower shown on the spectrum than
ALC off .
so, who can give my some idea about
ALC? thanks alot