Instruction manual

TM 11-6625-3017-14
To ensure that the latter output is due to the modulation
meter alone it is important that the source should be
exceptionally free from spurious noise deviation.
(1) Apply an f.m. input with 5 kHz deviation from the
signal generator. Connect the oscilloscope, filter
and voltmeter to the OUTPUT terminals as shown
in Fig. 4-4.
(2) Set the modulation meter for deviation
measurement as in Sect. 2.5, with the local
oscillator crystal-controlled (see Sect. 2.15). Switch
the DEV RANGE to 5 kc/s and the MAX MOD
FREQ to 15 kHz.
(3) Note the reading on the voltmeter - this should be
about 0 dBm if the modulation meter is terminated
in 600 n.
(4) Replace the signal generator with the external
crystal oscillator set to the same output level and
note the reduction in volt-meter reading. This
should be at least 50 dB, which is equivalent to a
noise level of -70 dB with reference to 50 kHz
deviation in a 15 kHz bandwidth.
(5) Note that the oscilloscope trace is free from hum,
external field and noise. If not, check the power
supply ripple - see Sect. 5.4.3.
4.3.6 F.M. distortion
Test equipment:- b, c, 2f and i.
(1) Set up the equipment as shown in Fig. 4-5. Adjust
c to give a c. w. output of 200 mV at any convenient
frequency Fc1. Adjust b to give a c. w. output of 50
mV at frequency (Fc1- 1.5 MHz).
(2) On the modulation meter set the Function switch to
TUNE and the oscillator RANGE switch to EXT.
Adjust the frequency of c for maximum reading on
the TF 2300A meter and adjust the TF 2300A
LEVEL control for a reading within the black arc.
Fig. 4-5. F.M. distortion measurement
(3) Switch to F. M. SET FREQ and adjust c frequency
to bring the TF 2300A meter reading to the SET
mark.
(4) Switch to DEV+ and turn the MAX MOD FREQ
switch to 15 kHz and the DEV RANGE switch to
150 kHz.
Apply external modulation to the two signal generators
as follows:
TABLE 4.4
External Modulation
Sig. gen. Mod. freq. Deviation
c 16 kHz (fl) 37.5 kHz
b 14 kHz (f2) 37.5 kHz
Switch off b and set up the level meter to give a
reference level of -6 dB at 150 kHz. Switch on b again.
Measure the levels of the following components: -
Fig. 4-4. F.M. noise
measurement
4-7