Model
8901B
Operation
ALC
off
AM
ALC
Response
Time
6.2 SPCL
I
6.2SP
FUNCTIONS
Special bction
6
(Automatic Level Control)
DESCRIPTION
The Modulation Analyzer normally uses
a
slow-responding AM automatic level control (ALC) circuit,
allowing AM rates
as
low
as
20
Hz
to
pass unaffected by the leveling loop. (Refer
to
the figure in
AM,
“Measurement Technique.”)
Select a
fast
ALC response time
to
speed up the measurement settling time required
for
varying carrier
levels. The
faster
ALC response time degrades
AM
accuracy
at
rates less than
1
kHz.
The ALC can be disabled. This feature, used in conjunction with the rear-panel AM Output connector,
is
useful for measuring AM
at
very low modulation rates. Disabling the ALC also permits measuring
absolute changes in depth without regard
to
changes in carrier level
(a
feature
that
is
useful
in
characterizing amplitude modulators).
PROCEDURE
The instrument normally operates with a slow AM ALC response time. To change the response time,
or
to
disable the ALC, enter the corresponding Special bction code, then select SPCL.
AM
ALC
Response
Special
Function Code
I
6.0 SPCL
I
6.0SP
I
Slow
ALC
I
6.1 SPCL
I
6.1SP
Fast ALC
I
(AM rates
>1
kHz
I
HP-IB PROGRAM CODES
For
HP-IB
codes, refer
to
“PROCEDURE”.
COMMENTS
Default Condition
When the instrument
is
first
turned on, or when Automatic Operation or Instrument Preset
is
selected,
the slow ALC response is selected.
IF
Level Dependency
The displayed modulation depth and the sensitivity
of
the Modulation Output connector for AM
demodulation are incorrect when the ALC
is
disabled unless the
IF
level
is
100%.
(Refer
to
IF
Level
on page
3-93.)
AM ALC Response Time
3-139
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