LOW-PASS FILTERS (Units A6 and A8-Fig. 7-3)

TM 11-6625-3017-14

Since the multivibrator runs at constant frequency,

200 kHz and 15 kHz

the positive-going pulses from Q4 are at equal time

intervals. Q7 and Q8 divide these 4 kHz pulses by two

These filters are 3 dB down at 250 and 28 kHz

and produce an exactly 1:1 square wave output, via C11,

respectively to obtain optimum flatness over their pass

at about 2 kHz. Q5 and Q6 operate as a gating circuit

ands of 200 and 15 kHz. In order to accommodate the

controlled at 2 kHz by this square wave.

channel separation requirements of stereo broadcast

monitoring, the filter designs are such as to provide a

Q3 is an oscillator accurately controlled at 400 kHz

linear phase frequency response.

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.

The 200 kHz filter integrates the pulses from the

at a repetition frequency of 2 kHz and a 1:1 on/off ratio -

discriminator and rejects the i. f. signal; thus it produces

see Fig. 3-4. This is fed to the discriminator via C9 when

a low frequency voltage which is proportional in

SB is at position SET CAL and corresponds, for

amplitude to the deviation. The 15 kHz filter restricts the

calibration purposes, to a crystal controlled deviation of

bandwidth to the audio range and can be switched in as

200 kHz. It should be noted that, in this mode, the 15

required by switch SE.

kHz filter is always in circuit to reduce the tendency of

3.10 1st L.F. AMPLIFIER (Unit A7 Fig. 7-3)

high frequency ringing due to the use of a square wave

for calibration.

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

Fig. 3-4. Derivation of standard

give optimum matching to the following switched 15 kHz

deviation signal from

filter. Similar considerations apply to the input stage,

400 kHz oscillator

where the impedance is effectively lowered by parallel

negative feedback, and R2 matches the preceding unit,

The calibrator operates only in the SET CAL

the 200 kHz low-pass filter

position of the Function switch, being switched off at all

other positions. A single attenuator pad contains the

Range attenuator (Chassis A0-Fig. 7-1)

variable resistor AORV3 (see Fig. 7-1). This resistor

(fitted immediately behind the calibrator board on the

Between the first and second 1. f. Amplifiers is the

chassis) can be used to standardize the meter deflection

range attenuator. The 1.5 and 5 kHz ranges are directly

to the SET mark.

connected, but for each succeeding range 10 dB

attenuation is switched into circuit. Two meter scales are

CAUTION

Do not attempt to SET CAL on the 1.5 kHz

used alternately to give 10 dB steps.

f. m. deviation range.

3.11 2nd L.F. AMPLIFIER (Unit A9-- Fig. 7-4)

3.9 LOW-PASS FILTERS (Units A6 and A8-Fig. 7-3)

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