Fig. 4-5. F.M. distortion measurement

TM 11-6625-3017-14

Fig. 4-4. F.M. noise

measurement

To ensure that the latter output is due to the modulation

meter alone it is important that the source should be

(2)

On the modulation meter set the Function switch to

exceptionally free from spurious noise deviation.

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

(1)

Apply an f.m. input with 5 kHz deviation from the

LEVEL control for a reading within the black arc.

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.

Fig. 4-5. F.M. distortion measurement

(3)

Note the reading on the voltmeter - this should be

about 0 dBm if the modulation meter is terminated

(3)

Switch to F. M. SET FREQ and adjust c frequency

in 600 n.

to bring the TF 2300A meter reading to the SET

mark.

(4)

Replace the signal generator with the external

crystal oscillator set to the same output level and

(4)

Switch to DEV+ and turn the MAX MOD FREQ

note the reduction in volt-meter reading. This

switch to 15 kHz and the DEV RANGE switch to

should be at least 50 dB, which is equivalent to a

150 kHz.

noise level of -70 dB with reference to 50 kHz

deviation in a 15 kHz bandwidth.

Apply external modulation to the two signal generators

as follows:

(5)

Note that the oscilloscope trace is free from hum,

external field and noise. If not, check the power

TABLE 4.4

supply ripple - see Sect. 5.4.3.

External Modulation

Sig. gen.

Mod. freq.

Deviation

4.3.6

F.M. distortion

16 kHz (fl)

37.5 kHz

Test equipment:- b, c, 2f and i.

14 kHz (f2)

37.5 kHz

(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

Switch off b and set up the level meter to give a

frequency Fc1. Adjust b to give a c. w. output of 50

reference level of -6 dB at 150 kHz. Switch on b again.

mV at frequency (Fc1- 1.5 MHz).

Measure the levels of the following components: -

4-7