Am Output - HP 8901B Operation And Calibration Manual

Model
8901B
Operation
AM Output
Do not apply greater than
10 Vpk (ac+dc) into the rear-panel
AM
OUTPUT
connector or damage to the instrument may result.
DESCRIPTION
The rear-panel AM OUTPUT (dc coupled, 10
kR
output impedance) provides an auxiliary output for
the AM demodulated from the
RF
input signal. This output enables monitoring of AM during display
of other measurements (such as FM, cPM, or audio signals).
The output can
also
be used in conjunction with Special k c t i o n 6.2 (described in AM
A E
Response
Time on page 3-139) to measure AM depth at low modulation rates.
The output signal comprises both a dc voltage (related to the detected IF
level) and an ac voltage
(bandwidth 16
kHz,
one pole proportional
to
the AM depth). The dc component contains an offset
voltage (Voff) which must be subtracted out. The relationship between
9 6
AM and the signal levels
at
AM OUTPUT is:
% A M =
vpk
x l W %
IVdc - Vof
f
I
where: Vpk is the peak of the ac component,
Vdc is the t o t a l dc component, and
V o f f is the dc offset voltage.
When the AM ALC is on, the dc level at
AM
OUTPUT is held constant, thus,
% A M = K
x
Vpk,
where:
100%
Vdc
-
V o f f
K =
When the AM ALC is turned off (Special F'unction 6.2), the dc voltage
at
AM
OUTPUT varies with the signal level (although the offset remains constant), and
the
f u l l
formula must be used for each measurement.
PROCEDURE
To measure AM depth via AM OUTPUT, first determine the offset voltage:
Select AUTOMATIC OPERATION to clear any Special k c t i o n s in effect, then connect a dc voltmeter
to the AM OUTPUT connector and remove any signal at RF INPUT. Press MHz
to
fix the tuning,
and read the offset voltage on the voltmeter.
Disable the AM ALC loop (6.2 SPCL) and apply the carrier to the
RF
INPUT. Measure the dc and
peak ac signals at AM OUTPUT. Use the first equation in "DESCRIPTION" to compute
%
AM.
AM Output
3-147