Electronic Frequency Converter 1 A14A2 Circuit Functioning

TM 11-820-695-35

assembly DS1 is non-conducting. Bias diode CR2 is a

vco bias voltage is between 5.0 and 16.0 volts dc. The

temperature-compensating diode cluster which varies

rf output is applied to board A4 (buffer amplifier) through

the base voltage on QS in accordance with the voltage

terminal E18 and to board A2 (digital amplifier) through

variations between the base emitter junctions in DS1

terminal E3 and R8.

and Q8 caused by temperature variations.

For

example, if the temperature decreases, the base voltage

c. The rf signal applied to board A4 is applied to

of Q3 could become positive and Q3 may then conduct.

the base of transistor Q4 through the isolating network

which could cause the transistor in assembly DS1 to

consisting of resistors R19, R20, and R21, and coupling

conduct, and light the lamp. If the rf output signal from

capacitor C14. The level of this signal is set by

board A2 is decreased, the bias on Q8 becomes positive

selecting the value of R20, the nominal value of which is

and it conducts, causing the transistor in assembly DS1

270 ohms. The amplified rf signal is direct coupled from

to conduct and light the indicator lamp. When the TEST

the collector of Q4 to the base of transistor Q5 where it

pushbutton on the main chassis is pressed, a 5 volt

is further amplified to the required level (20 milliwatts).

lamp test signal is applied via pin P1-4, terminal E16,

The output of Q5 is developed across autotransformer

and resistor R16, to base of the transistor in DS1,

T2 and coupled through capacitor C19 to the isolating

causing it to conduct and light the indicator lamp.

network consisting of resistors R29, R31, and R32. The

rf output (high) to the main chassis is developed across

2-58.

Electronic Frequency Converter 1 A14A2

R32 and routed through terminals E21 and E22 and

Circuit Functioning (fig. 547 and 5-48)

coaxial connector P1-A1. The nominal rf output into a

Electronic frequency converter 1A14A2 is comprised of

50 ohm load is 20 milliwatts. The sinusoidal rf signal

printed wiring board assemblies (1A14A2A1 and

frequency is between 284.3750 and 303.1250 MHz.

1A14A2A2). Board 1A14A2A1 contains the if amplifier-

Part of the rf output developed by Q5 is rectified by

mixer circuits and is shown in figure 5-47. Board

diode CR3 and filtered by capacitor C20 to supply the

1A14A2A2 contains the amplifier-multiplier circuits (fig.

meter current which is applied to meter panel assembly

5-48) and provides a 128 MHz signal to 1A14A2A1.

1A15A8 through the main chassis. The nominal meter

current is 25 microamperes for 20 milliwatts into 50

ohms. The value of resistor R33 is also determined

a. The rf output (low) frequency from radio

during final test and its nominal value is 62K ohms.

frequency oscillator 1A14A1 is applied to board

1A14A2A1 (fig 547) through coaxial connector P1-Al,

terminals E7 and ES, capacitor C2, and steering diodes

d. The rf signal applied to board A2 is routed to the

CR1 and CR2 to a high-speed divide-by-two circuit

base of transistor Q1 through resistor R8, transformer

consisting of transistors Q1, Q2, and Q3. The divide-by-

T1, and capacitor C8. Transformer T1. and capacitor

two circuit operates as an astable multivibrator. The

C7 comprise a bandpass filter so that signals outside the

free running frequency of the astable multivibrator is

284.375 to 303.125 MHz range are excluded. The

locked to the first subharmonic of the rf input frequency.

signal is amplified by Q1 and is capacitively coupled by

The switching is performed by Q1 and Q2; transistor Q3

C4 to the base of emitter follower Q2. An emitter

provides a constant current input to the emitters of the

follower output, rf output (low), is developed across

switching transistors. The output frequency from the

resistor R12 and routed through capacitor C10, terminal

divide-by-two circuit Q1 through Q3 is amplified by rf

E7 and E8, and coaxial connector P1-A2 to electronic

amplifiers Q4 and Q5. Diode CR3 provides temperature

frequency converter 1A14A2. The nominal sinusoidal rf

compensation for the base-emitter junction of rf

output from the emitter follower is 225 millivolts rms into

amplifier Q4. The output from rf amplifier Q5 is coupled

50 ohms. The collector signal of Q2 is rectified by lamp

through capacitor C17 to the local oscillator input port

detector diode CR1 and filtered by capacitor C6.

(terminals) of mixer Z1. A 128 MHz signal form board

Resistor R7 provides the operational indicator lamp bias

1A14A2A2

also

applied

at terminal E6 which is applied to board A3.

e. The normal lamp bias voltage applied to

terminal E13 is negative, keeping transistor Q8

nonconducting.

Likewise, the transistor in lamp

2-95