Am Alc Response Time - HP 8901B Operation And Calibration Manual

Model
8901B
Operation
ALC off
AM ALC Response Time
6.2 SPCL
I
6.2SP
FUNCTIONS
Special b c t i o n 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 b c t i o n 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