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 kction 6.2 (described in
AM
AE
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
96
AM and the signal levels
at
AM OUTPUT
is:
%AM=
vpk
xlW%
IVdc
-
Vof
f
I
where:
Vpk
is
the peak of the ac component,
Vdc
is
the
total
dc component, and
Voff
is
the dc offset voltage.
When the AM ALC
is
on, the dc level
at
AM
OUTPUT
is
held constant, thus,
%AM
=
K
x
Vpk,
where:
100%
Vdc
-
Voff
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
full
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 kctions
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
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