Instruction manual

TM 11-6625-3017-14
3.5 I.F. AMPLIFIER
(Unit A3-Fig. 7-2)
This board, in the narrow box alongside the local
oscillator and power unit, contains three amplifying
stages, each of two transistors, Q1 to Q6, the stages
being coupled via band pass filters. No limiting occurs in
the amplifier, and linearity, frequency response and an
overall gain of 50 dB are stabilized by a negative
feedback loop in each stage.
The output of the i. f. amplifier is taken in parallel
paths to the Function switch, SB, and to the I. F. OUT
socket via resistor R1 where it is available for viewing on
an oscilloscope or for counting to check carrier frequency
drift. Via SB1 F, the output is routed to the a. m.
Detector or, in the f. m. positions, to the limiter.
3.6 LIMITER
(Unit A4 Fig. 7-3)
Housed in the rear half of a compartment
underneath the centre of the chassis, the limiter consists
of three stages of emitter-coupled amplifiers, arranged
so that signal amplitudes of either polarity above a
certain level are limited. The emitters are connected to
balancing potentiometers to equalize the excursion in
each direction.
Part of the i. f. signal is tapped off from the third
stage, differentiated by C7 and LI and detected by MIl1 to
produce the uni-directional pulses shown in Fig. 3-3
(c).These pulses have
Fig. 3-3. Operation of limiter
a d. c. term, Vm, almost proportional to the repetition
frequency. This d. c. is fed, in the SET FREQ positions
of the Function switch, directly to the meter which is
arranged to indicate SET when the i. f. is correct.
The presence of any appreciable degree of carrier
shift when modulation is applied to the input signal will be
indicated by the meter in the SET FREQ positions; the
amount of shift may be measured with a counter at I. F.
OUT provided that the gate time is long with respect to
the period of the modulating signal or is equal to an
integral number of periods.
3.7 DISCRIMINATOR
(Unit A5-Fig. 7-3)
The pulse counter type discriminator occupies the
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
discriminator is described in section 3.1 and illustrated in
Fig. 3-2 (c), (d)’ and (e).
The limiter output drives a Schmitt trigger circuit, Q2
and Q3, to produce a large square wave output into C4
with constant rise and fall times. The collector voltage
of QI, the trigger amplifier, is set by RV1 so that the
Schmitt circuit is on the point of regeneration.
The square wave is differentiated and passed to a
pulse generator, Q4 and Q5, which produces positive-
going pulses. These pulses are clipped b3 Q6. The
emitter of this semiconductor is taken to the -12 V line,
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.
3-4