Electronic Frequency Converter 3A2 - TM-11-5820-695-350232

TM 11-5820-695-35

has been momentarily turned off. Capacitor 3A6A3C5

3A6A3U1. This transistor is normally biased off by the

suppresses transient noise spikes which put transistor

voltage at the junction of divider resistors 3A6A3R1 and

3A6A3Q1 into conduction, thereby eliminating

3A6A3R2, applied at pin 3A6A3U1-2.

When an

undesirable crow-barring. Resistor 3A6A3R6 limits the

overload occurs, the increased voltage drop across

base drive current' through transistor stage 3A6A3Q1.

current sensing resistor 3A6R3 causes the current

limiting transistor in 3A6A3U1 to conduct, and bias the

series pass transistors toward cut off. As a result, the

b. +28 V Rectifier Circuits. The 28 Vdc rectifier

voltage drop across the series pass transistors increases

circuit and its regulator control printed wiring board Al

greatly, causing the output voltage to drop to a very low

(fig. 5-104), except for part values and designations

level and thereby limit the load current. When the

function the same as the corresponding portions of the

cause of the overload is removed, the current limiting

high voltage regulator-rectifier described above.

transistor cuts off and the circuit automatically resumes

Operating power for the 28-volt rectifier is obtained from

regulation at the preset level.

Bypass capacitors

the center-tapped winding (terminals 5, 6 and 7) of

3A6A3C3 and 3A6A3C3 and 3A6A3C6 limit the circuit

transformer 3A7T1. The 28-volt rectifier is full-wave

response to short duration spurious noise pulses.

using diodes 3A6CR5 and 3A6CR6. Transistors 3A6Q4,

Capacitor 3A6A3C2 introduces a rolloff in the error

3A6Q5 and 3A6Q6 form the 28-volt regulator series

amplifier high frequency response and thereby prevents

pass stage, and SCR 3A6Q7 provides the crowbar

parasitic oscillation.

protection. Winding terminals 10 and 11 of transformer

3A7T1 provide operating power for control board 3A6A1

(3) Overvoltage protection is provided by the

which functions the same as board 3A6A2. The +28

crowbar control circuit (3A6A3Q1 and SCR 3A6Q3).

Vdc output and return is made available through pins 1,

Stage 3A6A3Q1 senses the output voltage between

3, 2 and 4 of connector 3A6P2.

terminal 3A6A3E5 and 3A6A3E6, and triggers

SCR.3A6Q3 into conduction when the overvoltage limit

2-85. Electronic Frequency Converter 3A2

is reached.

The overvoltage limit is set by

potentiometer 3A6A3R3, which controls the bias applied

Electronic frequency converter 3A2 (fig. 5-100) performs

to transistor 3A6A3Q1. When an overvoltage condition

as a K-band to C-band down converter, converting the

develops, transistor 3A6A3Q1 becomes forward biased

incoming 14.4 to 15.0 GHz band frequency signal from

and conducts through pulse forming network 3A6A3C4-

Filter BP, 3A3 to 4.4 to 5.0 GHz.

3A6A3R9, which forms the trigger pulse.

Once

triggered, SCR 3A6Q3 impresses a heavy load at the

a. The input signal at RF IN jack 3A2 A1J3 is a

circuit output terminals and thereby causes current

receive frequency in the K-band (14.4 to 15.0 GHz)

foldback through the limiting circuit described above.

range. The signal level at A1J3 is a nominal -40 dBm.

Because of the nature of SCR 3A6Q3, the CV-

Tunnel diode amplifier A1 provides a gain of 20 dB

3633/GRC-144(V) power supply must be momentarily

(min) to 23 dB (max) over the K-band range. There are

turned off to reset the SCR crowbar circuit. If the cause

three stages of amplification consisting of negative

of the overvoltage condition is a shortcircuited series

resistance tunnel diode amplifiers. The input and output

pass transistor (3A6Q1 or 3A6Q2), the triggering of SCR

impedances of Al are 50 ohms and input

3A6Q3 will cause fuse F1 to open. This prevents the

power supply from being reset even through the supply

Change 6 2-134