Fig. 3-2. Operation of discriminator

TM 11-6625-3017-14

the carrier level at the detector, while in the latter

position, when the i. f. is fixed, it is used to set the mixer

input to the correct level by adjustment of the LEVEL

attenuator and also for oscillator peaking adjustment

during initial setting up of the modulation meter.

A.M./F.M. sections

The A. M. /DEV RANGE attenuator has a total

attenuation of 40 dB. The attenuation in the 1.5 and 5

kHz positions is zero, and in the other deviation positions

increases by 10 dB per step, providing the deviation and

modulation depth ranges on f. m. and a. m. On a. m. only

the 10 and 20 dB steps are used.

5 to 500 kHz deviation ranges:

After attenuation, the signals are passed to the 2nd

1. f. amplifier which contains an output amplifier

arranged to supply approximately 0 dBm into 600 E2 to

the OUTPUT terminals. This stage effectively isolates

the meter circuits from the OUTPUT terminals and

permits de-emphasis networks to be switched in to the

output terminals when required, without affecting the

meter reading. The peak reading meter circuit consists

of an amplifier with push-pull pair to operate the meter

diode in its most linear region.

1.5 kHz deviation ranges:

Fig. 3-2. Operation of discriminator

The signal is passed to the 2nd 1. f.

amplifier which supplies approximately -10 dBm into 600

From this point onwards, all sections are common

Ω to the output terminals. Unlike the other deviation

to f. m. and a. m. and it is necessary to return to the i. f.

ranges the meter circuits are connected to the output of

amplifier and consider the a. m. sections of the

the amplifier and thus the de-emphasis networks will

modulation meter before proceeding to describe these

affect the meter reading.

final sections.

Calibration circuit

A.M. sections

In the SET CAL position of the Function switch, the

From the i. f. amplifier, a. m. signals are passed in

calibrator produces a standard crystal controlled

the appropriate position of the Function switch to the a.

deviation signal, i.e., it produces a similar signal to that

m. detector. A diode detector is employed, preceded by

from the limiter, with a peak-to-peak amplitude

an i. f. amplifier stage, the gain of which is variable by the

corresponding to a 200 kHz deviation. For a more

SET A. M. control to standardize the carrier level at the

complete description of the calibrator, refer to sect. 3. 8.

detector. When the carrier level is correct, the d. c.

output from the detector produces a reading at the SET

It thus provides a means of standardizing the

line on the meter.

discriminator and 1. f. circuitry in order that accuracy can

be maintained and also a means of checking for all

After detection, the signal is fed via a 50 kHz low-

possible sources of drift.

pass filter to the A M. /DEV RANGE switch and other

common a. m. /f. m. sections.

3.2 POWER UNIT (Unit A13--Fig. 7-6)

The d. c. component of the detected a. m. signal is

Occupying the rear corner behind the oscillator

fed direct to the meter in the SET A. M. and TUNE

compartment, the power unit includes two power

OSCILLATOR positions of the Function switch. In the

transistors fitted to heat sinks on the chassis, a printed

former position the d. c. component, which is

board and electrolytic capac-

independent of a. m., is used as already stated to set

3-2