Transmit Signal Path

TM 11-5820-509-35

the grid circuits of rf amplifiers V1 and V2 in signal data

dard signal from the frequency control circuits. The

translator A3.

resulting output from the balanced modulator is a

double-sideband, suppressed-carrier signal that is

2-6. Transmit Signal Path (fig. 7-3)

amplified by alc amplifier Q1 and if amplifier Q2 before

being applied to bandpass filter FL1. Only the lower

sideband remains after this signal is applied to FL1 and

a. In the transmit mode, the audio signal is

this single sideband is further amplified by Q3 through

obtained from the microphone of Handset H 33G/PT or

Q5 before being applied to signal data translator A3. If.

from the transmit output circuits of an external fsk unit.

amplifiers Q3 through A5 and bandpass filter FL1 are

This signal is connected to the transmit audio input of

shared with the receiver circuits of the receiver-

audio frequency amplifier A1 where it is amplified by

transmitter, but during the transmit period the agc bias

microphone amplifier Q1 and Q3, clipped by limiters

otherwise applied to Q2 and Q3 is removed so that the

CR5-CR6, further amplified by output amplifier Q4, and

if. gain of this circuit will not vary as a function of

applied to the audio input of the balanced modulator of

modulation.

amplifier-modulator A2. Part, of the amplified audio

output from Q4 is also applied to sidetone gate CR10.

d. Transmit relay A2K1 fin amplifier-modulator A2)

This gate is enabled during tansmit periods by the bias

is energized for transmit operation by closure of vox

from sidetone rectifier CR101 in the power amplifier

relay A1K1. The output of if. amplifier Q5 is routed

compartment so that the sidetone audio from Q4 is

through the closed contacts of A2K1 to the transmit

routed through driver Q10, audio output amplifier Q1

mixer (CR3-CR8) in signal data translator A3. The 2.5-

1W-QI2X and VOLUME control R11-R12 to the

to 12.499-MHz hf oscillator signal from the frequency

connected audio output device. The sidetone audio is

control circuits is applied to the transmit mixer through

then available for monitoring purposes.

follower Q13. The difference-frequency obtained by

heterodyning this signal with the 500-kHz if. output of

b. Another part of the output signal from Q4 is

Q5 is a 2.0to 11.999-MHz single-sideband rf signal that

applied to vox detector CR7-CR8. When this signal

is buffered by follower Q14 before being applied to

level exceeds a minimum threshold value, vox relay K 1

amplifier QI5 and subsequently to rf amplifiers V1 and

transfers circuit functions in the RT-671/PRC-47 from

V2. The amplified hf output of V2 is further amplified by

the receive mode to the transmit mode. It the audio

driver V3 and power amplifier V101. The output circuit

signal drops below this threshold level, or disappears

of the power amplifier is connected to the antenna

completely for a predetermined period of time, these

system when t/r relay K101 is closed. This stage is

circuits will automatically return to the receive

tuned to the operating frequency shown on the

configuration.

(Where the vox mode is not

KILOCYCLES indicator and is loaded to the antenna

implemented, vox relay K1 cannot cause operation of

system by the adjustment of POWER AMPLIFIER

the ptt relay (A5K1) in power supply module A5. This

TUNE and POWER AMPLIFIER LOAD controls on the

relay operates when the ptt switch on the operator's

front of RT-671/PRC-47. Optimum adjustment of these

handset is pressed and energizes the high-voltage

controls is indicated by maximum deflection (maximum

power supply circuits.) If continuous wave (cw)

power output) of XMTR OUTPUT meter M 101 on the

telegraphy is desired, the circuit functions described

front of the receiver-transmitter.

above apply except that Telegraph Key J-45 turns on,

and shuts off, the audio tone generated by oscillator Q5.

This tone replaces the voice audio at the input of

2-7. Frequency Control Circuits (fig. 7-4)

microphone amplifier Q3, and is amplified by the

remaining circuits of audio frequency amplifier A 1. In

a. The temperature-compensated crystal-oscillator

the cw mode, the CW-FSK/VOICE switch on the front

of radio frequency oscillator A6 is an extremely stable

panel of RT-671 /PRC-47 is placed in the CW-FSK

standard that provides a 3-MHz output signal for the

position so that vex relay A1K1 and ptt relay A5K1 are

frequency divider (locked oscillator) circuits of this

both energized The CW-FSK/VOICE switch must be

module. The crystal is contained in a temperature-

returned to the VOICE position when it is desired to

controlled oven to minimize frequency drift caused by

receive incoming telegraph or teletypewriter signals (i.e.,

changes in ambient temperature, and the circuit is

break-in operation is not possible).

equipped with an adjustment that permits compensation

c. That part of the audio signal from Q4 that is

for long-term crystal aging effects. Transistor Q1 is the

applied to the balanced modulator of amplifier-

oscillator, and transistors Q2 and Q3

modulator A2 is mixed with the 500-kHz stan-

2-9