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5-6
Moduiation and
Audio
5.
Press the RANGE HOLD key to hold all ranges and
tuning.
6.
Key i n
18."
SPCL to
set
the time delay. Where NN equals the
desired delay in milliseconds (from 1 to 99). (For instance, if a 5ms
delay
is
desired, enter 18.5 SPCL, if a 50ms delay is desired, enter
18-50 SPCL.) The speaal function value
will
remain in the display.
7.
Key
on the transmitter. The audio counter should display
the modulation rate present at the end of the delay period.
Residual Modu/afjo#
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 d B
referenced to 3
kHz
peak deviation.)
1 . Connect your transmitter to the
HP
8902A
and turn
off FM
modulation.
2. Press the 50 H t HP filter key. The 50
Hz
high-pass filter
is
recommended
for this measurement. The
H p
8902As 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 8902A.
3. Press the PRE-DISPLAY key and then the
750ps
key. The 750ps
filter attenuates a 1
kHz
signal by 13.66 dB (a factor of 0.2076).
4.
Press the FM key on the
HP
8902A to measure residual
FM.
5.
Press the Blue
S h i f t
key and the RMS (AVG) key. This selects the true
6.
Key in .44 and then the RATfO key. For standard modulation, the
7.
Use the LOG/LfN key to select dB units. The display now
RMS
detector.
ratio reference is 0.440 (that is, 3.0
X
1/fi
X 0.2076 = 0.440).
indicates FM hum and noise in dB relative to 3 kHz peak deviation.
-40
Gi
0
U
p
-50
2
k!
N
3
-M)
%
f
E),
W
-70
c d
I
-80
t
1
I
I
I
I
0
200
400
600
0
200
400
600
800
1000
1200
FREQUENCY (MHz)
FREQUENCY
(MHz)
15
kHz
Low-Pass
Filter
Diagram 5-2. Typical HP 8902A Residual FM Characteristics
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