Instruction manual

TM 11-6625-3017-14
Since the multivibrator runs at constant frequency,
the positive-going pulses from Q4 are at equal time
intervals. Q7 and Q8 divide these 4 kHz pulses by two
and produce an exactly 1:1 square wave output, via C11,
at about 2 kHz. Q5 and Q6 operate as a gating circuit
controlled at 2 kHz by this square wave.
Q3 is an oscillator accurately controlled at 400 kHz
by crystal XLI and gated on and off by Q5 and Q6 at 2
kHz. Thus the 400 kHz signal is divided into bursts of r.f.
at a repetition frequency of 2 kHz and a 1:1 on/off ratio -
see Fig. 3-4. This is fed to the discriminator via C9 when
SB is at position SET CAL and corresponds, for
calibration purposes, to a crystal controlled deviation of
±
200 kHz. It should be noted that, in this mode, the 15
kHz filter is always in circuit to reduce the tendency of
high frequency ringing due to the use of a square wave
for calibration.
Fig. 3-4. Derivation of standard
deviation signal from
400 kHz oscillator
The calibrator operates only in the SET CAL
position of the Function switch, being switched off at all
other positions. A single attenuator pad contains the
variable resistor AORV3 (see Fig. 7-1). This resistor
(fitted immediately behind the calibrator board on the
chassis) can be used to standardize the meter deflection
to the SET mark.
CAUTION Do not attempt to SET CAL on the 1.5 kHz
f. m. deviation range.
3.9 LOW-PASS FILTERS
(Units A6 and A8-Fig . 7-3)
200 kHz and 15 kHz
These filters are 3 dB down at 250 and 28 kHz
respectively to obtain optimum flatness over their pass
ands of 200 and 15 kHz. In order to accommodate the
channel separation requirements of stereo broadcast
monitoring, the filter designs are such as to provide a
linear phase frequency response.
The 200 kHz filter integrates the pulses from the
discriminator and rejects the i. f. signal; thus it produces
a low frequency voltage which is proportional in
amplitude to the deviation. The 15 kHz filter restricts the
bandwidth to the audio range and can be switched in as
required by switch SE.
3.10 1st L.F. AMPLIFIER
(Unit A7 Fig. 7-3)
Demodulated signals from the discriminator reach
the first 1. f. amplifier via the 200 kHz low-pass filter. The
amplifier has three stages, Q1 and 2 being a current
feedback pair, while Q3 and 4 are arranged as a special
low impedance configuration known as an ’emitter-
squared follower.
The amplifier has been designed to give high
stability, large bandwidth and constant gain with
immunity from h. t. changes, by virtue of its feedback
loops. C6 carries positive feedback over part df the
circuit to increase the gain, while R8 carries d. c. bias to
the first stage. C5 and R7 take a. c. negative feedback to
the first stage via the SET CAL F. M. control. The output
stage, Q3 and Q4, gives a very low impedance, so that
the output impedance of the amplifier is due almost
entirely to R14. A high stability resistor is used here to
give optimum matching to the following switched 15 kHz
filter. Similar considerations apply to the input stage,
where the impedance is effectively lowered by parallel
negative feedback, and R2 matches the preceding unit,
the 200 kHz low-pass filter
Range attenuator
(Chassis A0-Fig. 7-1)
Between the first and second 1. f. Amplifiers is the
range attenuator. The 1.5 and 5 kHz ranges are directly
connected, but for each succeeding range 10 dB
attenuation is switched into circuit. Two meter scales are
used alternately to give 10 dB steps.
3.11 2nd L.F. AMPLIFIER
(Unit A9-- Fig. 7-4)
The action of Q1 and Q2 in this unit is similar to that
of the first two transistors in the first 1. f. amplifier. RV1
in the feedback loop sets the gain. Two connections are
made after
3-5