DISCRIMINATOR (Unit A5-Fig. 7-3)

TM 11-6625-3017-14

a d. c. term, Vm, almost proportional to the repetition

3.5 I.F. AMPLIFIER (Unit A3-Fig. 7-2)

frequency. This d. c. is fed, in the SET FREQ positions

of the Function switch, directly to the meter which is

This board, in the narrow box alongside the local

arranged to indicate SET when the i. f. is correct.

oscillator and power unit, contains three amplifying

stages, each of two transistors, Q1 to Q6, the stages

The presence of any appreciable degree of carrier

being coupled via band pass filters. No limiting occurs in

shift when modulation is applied to the input signal will be

the amplifier, and linearity, frequency response and an

indicated by the meter in the SET FREQ positions; the

overall gain of 50 dB are stabilized by a negative

amount of shift may be measured with a counter at I. F.

feedback loop in each stage.

OUT provided that the gate time is long with respect to

the period of the modulating signal or is equal to an

The output of the i. f. amplifier is taken in parallel

integral number of periods.

paths to the Function switch, SB, and to the I. F. OUT

socket via resistor R1 where it is available for viewing on

3.7 DISCRIMINATOR (Unit A5-Fig. 7-3)

an oscilloscope or for counting to check carrier frequency

drift. Via SB1 F, the output is routed to the a. m.

The pulse counter type discriminator occupies the

Detector or, in the f. m. positions, to the limiter.

front half of the central compartment fitted beneath the

chassis, the rear half of this compartment being

occupied by the limiter. The overall operation of the

3.6 LIMITER (Unit A4 Fig. 7-3)

discriminator is described in section 3.1 and illustrated in

Fig. 3-2 (c), (d)' and (e).

Housed in the rear half of a compartment

underneath the centre of the chassis, the limiter consists

The limiter output drives a Schmitt trigger circuit, Q2

of three stages of emitter-coupled amplifiers, arranged

and Q3, to produce a large square wave output into C4

so that signal amplitudes of either polarity above a

with constant rise and fall times. The collector voltage

certain level are limited. The emitters are connected to

of QI, the trigger amplifier, is set by RV1 so that the

balancing potentiometers to equalize the excursion in

Schmitt circuit is on the point of regeneration.

each direction.

The square wave is differentiated and passed to a

Part of the i. f. signal is tapped off from the third

pulse generator, Q4 and Q5, which produces positive-

stage, differentiated by C7 and LI and detected by MIl1 to

going pulses. These pulses are clipped b3 Q6. The

produce the uni-directional pulses shown in Fig. 3-3

emitter of this semiconductor is taken to the -12 V line,

(c).These pulses have

so that it clips the bases of the positive-going pulses,

thus maintaining constant amplitude.

Q7 is an emitter follower which, unlike Q6, is

conducting continuously and presents a constant low

impedance to the 200 kHz low-pass filter. The low

frequency change in the mean value of the pulses is

therefore passed and the i. f. signal rejected.

3.8 CALIBRATOR (Unit A12-Fig. 7-5)

Q1 and Q2 form a multivibrator running at nominally

4 kHz. CG, R8 and Q4 differentiate and clip the negative

spikes of the square wave output from Q2 and feed them

from a low impedance to the binary divider, Q7 and Q8.

Fig. 3-3. Operation of limiter

3-4