HP 8901B Operation And Calibration Manual page 122

5-6
Modulation and Audio
Residual Modulation Residual modulation is a measure of the hum and noise of the unmodulated
transmitter. For FM mobile transmitters, hum and noise is defined
as the ratio of the output of a standard receiver with de-emphasis
when the transmitter
is
modulated and then unmodulated. (Hum and
noise is usually expressed in dB referenced to 3 kHz peak deviation.)
1. Connect your transmitter to the
H p
8901B and turn off
FM
modulation.
2 . Press the
50
H r HP
filter key. The 50 Hz high-pass filter is recommended
for this measurement. The HP 8901B's residual
FM
is low enough
that it can measure the hum and noise of most transmitters directly.
Diagram 5-2 shows the typical residual FM characteristics for the HP 8901B.
3.
Press the
PRE-DISPLAY
key and then the 750ps key. The 75011s
filter attenuates a 1
kHz
signal by 13.66 dB (a factor of 0.2076).
4.
Press the FM key on the
HP
8901B to measure residual
FM.
5 . Press the Blue
Shift key and the RMS
(AVG)
key. This selects the true
6.
Key in .44 and then the
RATIO
key. For standard modulation, the
7. Use the
LOG/LIN
key to select dB units. The display now
R M S
detector.
ratio reference is 0.440
(that is, 3.0
X
l / f i
X
0.2076
indicates FM hum and noise in dB relative to 3 kHz peak deviation.
=
0.440).
-40
w^
a
a
P
0
z
Y
-50
w
-60
-
w
;
4
2
-70
I
-an
I "
0 200 400
600
800
I
1000 1200 0 200 400 600
800
1000 1200
FREQUENCY
(MHz)
FREQUENCY
(MHz)
15 kHz Low-Pass Filter
Diagram
5-2. Typical HP 8901B Residual
FM
Characteristics