Transmitting Signal Path

bility of the sso approximately 40 times

f. The receiver audio amplifier output,

including the 150-cps tone, is applied to

relative to its free-running state.

the squelch input amplifier. The squelch

c. The frequency-modulated (fm) output

input amplifier with the emitter -follower

of the sso is coupled through the sso

and squelch feedback amplifier passes

buffer amplifier to the transmitter mixer.

The sso buffer prevents loading of the sso

the 150-cps tone, which is rectified in the

by the transmitter mixer. The transmitter

squelch direct current (dc) detector am-

mixer heterodynes the 11.5-mc fm signal

plifier. When the squelch relay driver

from the sso with the vfo signal to produce

receives a signal from the dc detector

amplifier, it provides the current neces-

the transmitter output frequency. The vfo

is stabilized by application of its output

sary to energize K3. When K3 is energized,

frequency through the frequency synthe-

the ground is removed from the audio

sizer buffer to the frequent y synthesizer

output amplifier to allow the audio sig-

system (fss) (para 5). The fss will correct

nal to be fed to the handset.

any vfo frequency drift.

g. In addition to the function of squelch

d. The transmitter mixer output is ap-

relay K3 described in f above, relay K3

plied to the transmitter first rf amplifier.

provides a transmitter keying function

The output of the transmitter first rf

when two AN/PRC-25's are used for re-

amplifier is applied to the transmitter

transmission. During this type of service,

second rf amplifier, which is cascaded with

t h e receiving AN/PRC-25 is set for

squelch operation. When a signal is re-

the intermediate power amplifier and the

ceived, energizing K3, a ground circuit

power amplifier. These amplifiers in-

is completed, through the interconnecting

crease the transmitted signal level to

cable, to key the transmitting AN/PRC-

the proper amplitude. The transmitter

25.

power amplifier output is coupled through

the power amplifier tank and antenna

loading network to the antenna. During

4. Transmitting Signal Path

transmission, the input to the receiver

(fig. 83)

is grounded by relay K1 which is energized

by receive-transmit relay K2. Relay K2

a. The audio signal developed in the

also energizes the dc-to-dc converter,

handset microphone is amplified in the

which supplies the operating voltages

first and second speech amplifiers, and

(B+ and bias) for the power amplifier

limited in the output limiter stage. The

tube.

output limiter stage has two outputs: it

applies the audio signals to the receiver

first audio amplifier as sidetone, and

5. Frequency Synthesizer System

applies the audio signal to the modulator.

(fig. 84)

The modulator also has a 150-cps tone

The frequency synthesizer system (fss)

input from the 150-cycle tone oscillator,

is a closed loop, automatic phase control

through the feedback amplifier. The mod-

(apc) system which locks the vfo on fre-

ulating voltages applied to the modulator

quency. The fss is used both during recep-

cause the sidestep oscillator (SSO) output

tion and during transmission. During

frequency to vary at the input audio-

transmission, the vfo output frequency is

frequency (af) rate from the microphone

shifted 50 kc lower than it is during recep-

and at the 150 -cps rate. The output from

tion. The frequencies shown on figure 84

the 150-cps tone generator is also applied

represent the fss frequencies associated

to the squelch input amplifier (para 3f).

with two typical received frequencies,

b The frequency stability of the sso is

and are used to illustrate the frequency

greatly improved by the quarter-wave net-

relationships. The two typical received

work. The quarter-wave network is a

frequencies are 33.10 mc (low band) and

crystal-controlled passive network con-

nected across the sso tuned circuit. The

68.45 mc (high band). All frequencies used

in a through i below can be converted to

quarter-wave network improves the sta-