Section II. THEORY OF OPERATION

TM 11-5825-270-23

Section II. THEORY OF OPERATION

3-3. Switch Bracket Assembly

output for use by the receiver circuitry. The ac input is

applied, through power ON/OFF switch S4 and fuse F1,

(fig. 3-2 and fig. FO-3)

to the primary of transformer T3. Two primary windings

a. The switch bracket assembly houses a variable

of T3 are connected in series for 230 Vac operation and

in parallel when operation from 115 Vac input is

attenuator and a front-end tuning circuit for the receiver.

required. The secondary of transformer T3 provides 24

The rf signal from the antenna matching module is

V (rms) output which is full-wave rectified by a bridge

coupled through C3 to the attenuator circuit formed by

comprising diodes CR1 thru CR4. Filtering is provided

S1-a, S1-b and resistors R1 through R9. The amplitude

by C2, resulting in approximately 34 Vdc at the collector

of the incoming rf signal is proportional to the signal

of Q1. The dc voltage at this point is unregulated and

picked-up by the antenna and, under normal operation,

varies with the load current as well as the line voltage

remains constant. Depending upon the amplitude of the

fluctuations.

rf input, appropriate attenuation is introduced to the

b. The 28 Vdc regulator circuit is formed by Q1,

signal so as to establish a suitable reference signal level

Q2, CR5, C1, R1 and R2. The base of Q1 is held at a

for the receiver.

b. The attenuator consists of two 20 dB pads and

constant potential of 30 V by zener diode CR5. The

voltage at the emitter of Q1 therefore equals the

one 40 dB pad. Rotary switch S1 permits selection of

breakdown voltage of CR5 less the base emitter voltage

one or more sections, permitting up to 80 dB of

drop of Q1.

This voltage is constant (regulated)

attenuation in 20 dB of attenuation in 20 dB steps.

regardless of the load current and the Q1 collect voltage.

c. Transistor Q2, together with resistor R2, provide

NOTE

When RF ATTEN switch S1 is set to 0

the current limit function for the +28 V output. If a

dB position, the attenuator circuit is

malfunction within the receiver causes the current drain

by passed.

on the +28 V bus to exceed 0.5 amps, the voltage drop

across R2 is sufficient to turn Q2 on. AS a result, the

c. Resistors R1, R2 and R3 form the first 20 dB

base drive current for Q1 is partially diverted to Q2 and

the current through Q1 is limited to a safe value.

pad while R4, R5 and R6 make up the second 20 dB

d. The regulated +28 V output from Q1 is used to

pad. The 40 dB pad is formed by R7, R8 and R9. The

operate relay K1, POWER lamp DS1, ALARM lamp

input impedance and output impedance of the switched

DS3, NORM lamp DS2, if amplifier assembly A2, and

attenuator is 50 ohms.

d. The output from the switched attenuator is

the audio amplifier assembly A2. In addition, the +28 V

output is fed to the +12 V regulator comprising Q3, Q4,

applied to a tuning circuit formed by T1, L2, T2 and C1.

CR6 and associated components. The +12 V regulator

Transformer T1 is a step up transformer which raised the

circuit operates as follows:

output impedance of switched attenuator from 50 ohms

(1) The emitter of Q4 is held at a voltage of 6.2

to 450 ohms. The secondary of transformer T1 feeds a

Vdc by zener diode CR6. The base of Q4 senses the

series tuned circuit comprising L1, T2 and variable

+12 V bus through divider R4 and R5.

capacitor C1. The tuned circuit output is taken from T2

(2) Any change in the +12 V bus causes a

which also serves to restore the 50 ohm impedance.

corresponding change in the collector current of Q4 and

Note that stepping up of impedance by T1 is performed

hence the base drive for Q3. The conduction of Q3 is

to permit use of a convenient value for tuning capacitor

therefore automatically controlled so as to produce

C1 (6.7 pF to 140 pF); if transformer T1 was not used for

constant + 12 Vdc output at its emitter.

impedance transformation, the value of C1 would be

(3) As the input to the +12 V regulator is the

impractically large.

e. Variable capacitor C1 is adjusted at the time of

output of the 28 V regulator, the over-current protection

provided by R2 and Q2 also safeguards the +12 V

installation to tune the receiver to its operating

supply.

frequency. The tuned circuit has a high Q, thus rejecting

all unwanted signals that may be present at its input.

3-5. RF PCB Assembly

3-4. Power Supply

(fig. FO-1 and FO-3)

(fig. 3-3 and fig. FO-3)

a. The RF PCB performs the following functions:

a. The power supply operates from 115 Vac (or

(1) Accepts rf input from the tuning circuit on

230 Vac) input and provides +12 VDc and + 28 Vdc

3-4