Receiver, Radio R-1467(P)A/GRC-144 (V) Demodulator 2A4 Circuit Functioning

TM 11-5820-695-35

from the resonant frequency of transformer T1, a dc

b. Limiter input matching circuitry, consisting of

component is present in the output of the frequency of

inductor L6, resistor R20 and common emitter amplifier

transformer T1, a dc component is present in the output

Q3, provides impedance matching between the low pass

of the frequency discriminator at the junction of R40 and

filter and the limiter stages. Test point E8 serves as a

R41. The dc level/ depends on the frequency offset,

matched impedance point for alignment checking.

and the polarity on whether the if frequency is above or

Transistors Q3, Q4 and Q5 are common emitter limiter-

below the resonant frequency of transformer T1. This

amplifiers. Diodes CR2 through CR7 serve as low

dc component is extracted by RC low pass filter R42

impedance shunt limiters. Emitter follower stage Q6

and C47, and can be monitored at J4 (TP4 DISCR), thus

provides low impedance (75 ohms) coupling into the low

determining whether the frequency discriminator is

pass filter L11, L12 and L13. These variable inductors

properly tuned, or conversely, whether the if frequency

are adjusted for amplitude flatness and group delay

is properly centered.

characteristics. A 3 dB resistive pad (R44 through R46)

is used to improve the 75-ohm impedance match.

e. Video Output Amplifier.

The frequency

c. Discriminator driver amplifiers Q7 and Q8 use

discriminator output is applied through capacitor C48 to

emitter by-passing to produce a flat amplitude response

the base of Q8, which together with Q9 comprises the

(+0.2 dB) and a differential group delay of less than 1

video output amplifier. This circuit provides isolation

nanosecond over the 60 to 80 MHz frequency range.

between the frequency discriminator and 1.0 MHz low

Capacitor C42 in the emitter circuit of Q7 is adjusted for

pass filter 2A3. Resistors R43 and R44 set the base

flat response in the 63 to 77 MHz range. Capacitor C49

bias for Q8, while R45 limits the collector current of

in the emitter circuit of Q8 is adjusted for 13 dB

video output amplifier Q8 and Q9. Capacitor C51

minimum gain. The discriminator is a standard Crosby

bypasses the collectors of video output amplifier Q8 and

type. Tank circuit L19/C51 is tuned to 93 MHz and

Q9. The output is developed across resistor RH5 and

L20/C52 is tuned to 47 MHz. Capacitors C53 and C54

applied to the 1.0 MHz low pass filter 2A3 through

are charged with opposite polarities through

capacitor C52, resistors R47 and R48 and connector J6

discriminator/rectifier diodes CR8 and CR9.

The

VIDEO OUTPUT.

charging time constants are short and the voltages

across C53 and C54 track frequency variations. The

algebraic sum of these voltages is developed across

2.68.1

Receiver, Radio R-1467(P)A/GRC-144 (V)

R66, R67 and R68. R67 is adjusted for average (center

Demodulator 2A4 Circuit Functioning (fig. 5-54.1)

frequency) level and the signal is fed through low pass

filter L21/L22 are adjusted for flat response (+1 dB)

Demodulator 2A4 converts the 70 MHz IF signal (from

below 30 MHz. The low pass filter rejects harmonics of

2A5) to the baseband (composite data/orderwire) signal.

the video signal. Common emitter video amplifier

stages Q9 and Q10 use inverse feedback and provide a

a. The amplifier-low pass filter stage Q1-Q2

gain of 25 dB. Emitter follower stage Q11 provides low

includes an input matching section (R1, R2, R3, L1)

impedance coupling of the video signal to low pass filter

required to meet a 26 dB return loss limit. Buffer

2A3.

amplifiers Q1 and Q2 provide isolation between the

input section and the low pass filter. Variable inductors

L4 and L5 in the low pass filter are adjusted for

2.69 70 MHz Intermediate Frequency Amplifier 2A5

frequency response flatness. The cutoff frequency of

Circuit Functioning (figs. 5-55 and 5-55.1)

the filter is 90 MHz and it rejects harmonics of the 70

The 70 MHz intermediate frequency amplifier 2A5

MHz IF signal. A rectifier-filter network monitors the

amplifies the low level 70 MHz signal to a sufficient

input signal and supplies a test point voltage at J2 (TP2

level

drive

the

demodulator

circuits.

INPUT LEVEL) .

2-112

Change 6