Figure 2-1. Radio Receiver-Transmitter Simplified Block Diagram.

TM 11-5820-873-34

Figure 2-1. Radio Receiver-Transmitter Simplified Block Diagram.

fed to a 600 ohm line driver and to a fixed audio

oscillator signal from synthesizer 1A4. The resultant

amplifier. The envelope detector detects the amplitude

signal is filtered m a narrow band filter which passes

of the received signal reproducing the audio on a

only the 91.25 MHz 15 kHz portion of the signal. The

transmitted AM signal. The resultant audio is also fed to

output from the filter is amplified and mixed with the 2

the 600 ohm line driver and to the fixed audio amplifier.

local oscillator signal from synthesizer 1A4, to produce

The output of the fixed audio amplifier is connected to

10.5 MHz. The 10.5 MHz output form the VHF mixer is

the VOLUME control, then to the speaker driver (located

coupled into IF filter 1A3A2. The IF filter amplifies the

on the front panel), which supplies up to five watts of

signal and it is fed to a diode gating network The diode

audio to the speaker. The AGC voltage is also used to

gating network selects the upper sideband filter (USB),

vary the current through the front panel meter to give a

lower sideband filter (LSB), or amplitude modulation

visual indication of relative signal strength received.

filter (AM) in accordance with the setting of the mode

switch. These filters determine the receiver's bandwidth

The front panel RF GAIN control acts to vary the gain of

and reduce the interference form adjacent channels The

an amplifier which reduces the AGC voltage, reducing

the gain of the RF amplifier and the IF amplifiers. This

upper sideband, lower sideband, or double sideband

greatly reduces background noise when receiving strong

output from the filters is further amplified and coupled

signals.

into audio board 1A3A4 where it is detected and

b. Detailed Functional Operation.

amplified In the audio board the signal is coupled to a

product detector when receiving sideband signals and to

(1) Power supply 1A6 (para 2-7) provides the

a fixed amplifier and an envelope detector for receiving

regulated DC voltages required to operate the circuits in

amplitude modulated signals. The fixed amplifier also

the RT-1277/URC-92.

feeds into the AGC detector which develops a DC

(2) TCXO 1A8U1 is an extremely stable,

voltage proportional to the received signal amplitude.

temperature controlled, crystal oscillator and provides

The AGC voltage is amplified and used to control the

the 5 MHz reference signal to the synthesizer.

voltage gain of the RF amplifier and the IF amplifiers.

(3) Synthesizer 1A4 (para 2-8) generates three

The product detector combines the signal with the 3rd

local oscillator frequencies The 1st local oscillator

local oscillator (10.5 MHz), giving an audio signal which

frequency of 91.25 to 121.2499 MHz is controlled by the

reproduces the original transmitted audio. The audio is

2-2