OPTION
"651
LOW-RATE Fm
REQUIREMENT
FM Droop is less
than
15%
with
lO
Hz external square wave modulation.
REMARKS
When using the RF
Spectrum
Analyzer as an FM demodulator using slope detection, it is
important to operate
the
RF Spectrum Analyzer detector in a linear range. This can be
checked
by
stepping the UUT
frequency
up
5
kHz
and
then down 5 kHz from the
operating point
and
noting that the display moves equal amounts. If it
doesn’t, tune
the
RF
Spectrum Analyzer slightly and
check
for linearity again.
TEST
EQUIPMENT
Low-Frequency
Synthesized
Signal Generator (LFSSG)
RF
Spectrum Analyzer
PROCEDURE
1. Remove the Generator
top and
the Synthesizer module plate covers,
2 . Connect the LFSSG
TTL output through a 604-ohm resistor and a 320
uF,
6V
capacitor to the UUT MOD INPUT.
Thi.s
provides a square wave
approximately 2V
p-p
at the Generator's MOD input.
.1 Program the LFSSG to 10
Hz and any
level
around
IV.
4.
Program the UUT to the [RCL]
[9][8]
and 3-kHz deviation,
This
provides a
300-
MHz
signal
at
-10
dBm.
5.
Connect the input of the RF Spectrum
Analyzer
to
the
UUT
RF OUTPUT, Set
the
RF
Spectrum Analyzer so that the signal response is at
the top
of the display
using linear
detection.
6. Program UUT for EXT FM.
7. Using a lO-kHz
Resolution
Bandwidth and zero Span/Div, adjust either
the
Generator frequency or the RF Spectrum Analyzer
tuning
for slope detection to
obtain
a .square-wave display. Adjust Time/Div and Trigger
a.s
nece.ssary to
obtain a
stable
square-wave display.
8. Verify that the droop
of the demodulated FM is less than 1
5%.
For example,
if
the displayed
square-wave
amplitude (vertical edge)
is
3.4 divisions, then the
droop should be less than 0.51
divisions
(0,15
x
3,4).
651-5.
ADJUSTMENT
TEST
EQUIPMENT
DMM
PROCEDURE
The Offset adjustment (A2A9 R4) is set to OV,
1.
Program
UUT to [RCL]
[9][8]
and 9.99 kHz deviation,
and
set the rear panel
EXT/INT
FM switch
to EXT, with no external modulation
signal
applied.
651-2
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