Transmitter Functional Operation

TM 11-5820-873-34

operating the press-to-talk switch on the handset, by

by amplifiers 1A3A2U1 and 1A3A2U2 and coupled to

grounding the keyline at AUDIO connector 1A8J2, or by

the inputs of amplifier 1A3A4Q1 and product detector

operating the CW key. Operating the CW key energizes

1A3A4Q6, 1A3A4Q8, 1A3A4Q9.

the CW tone oscillator, which produces a 1 kHz audio

(a) The output from amplifier 1A3A4Q1 is

signal. Input audio from the handset, the 600 ohm

coupled to emitter follower 1A3A4Q2 and AM detector

balanced line, or the CW tone oscillator is amplified and

1A3A4Q7, 1A3A4Q10.

used to modulate the 10.5 MHz, 3rd local oscillator

1. The output from emitter follower

signal from synthesizer 1A4. The resultant double

1A3A4Q2 is coupled to AGC detector 1A3A4Q3. The

sideband suppressed carrier signal is amplified and

DC output from 1A3A4Q3 is coupled to emitter follower

filtered in accordance with the setting of the mode

1A3A4Q4. The output of 1A3A4Q4 is the AGC voltage

switch. The remaining upper or lower sideband signal

RF gain control amplifier 1A3A4Q12 is connected

(AM transmission is actually upper sideband plus

across the AGC line, RF GAIN control 1A1R55 controls

carrier) is amplified and, if the mode switch is set to AM,

the gain of 1A3A4Q12 and established the minimum

mixed with the 10.5 MHz carrier signal. The lower

level of AGC. With 1A1R55 fully counterclockwise, it

sideband, upper sideband, or upper sideband plus

should be possible to completely disable the receiver.

carrier signal is mixed with the 80.75 MHz of the 2nd

The AGC voltage is also used to drive S meter amplifier

local oscillator signal from synthesizer 1A4.

The

1A3A4Q5.

The output of 1A3A4Q5 drives meter

resultant signals are amplified and passed through a

1A2M1.

91.25 MHz (sum frequency) filter. The 91.25 MHz

2. When extremely high signal levels are

signal is mixed with the 91.25 to 121.2499 MHz, 1st

present, peak signal level AGC amplifier 1A3A5Q2

local oscillator signal, from synthesizer 1A4.

The

limits the level of input signal that is applied to RF

difference frequency (1st local oscillator frequency

amplifier 1A3A1Q2.

minus 91.25 MHz) is amplified to the 100 watt level and

3. With mode switch 1A1S13 set to AM

passed through a low pass filter to ANT jack 1A8J1.

(and relay 1A8K1 not energized) AM detector 1A3A4Q7,

b. Detailed Functional Operation.

1A3A4Q10 is enabled. The 10.5 MHz signal from

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

1A3A4Q1 is detected by AM detector 1A3A4Q7,

regulated DC voltages required to operate the circuits in

1A3A4Q10. The audio signal from the AM detector is

the RT-1277/URC-92.

coupled to the inputs of 600 ohm audio line driver

(2) TCXO 1A8U1 is an extremely stable,

1A3A4U1 and audio amplifier 1A3A4Q11.

temperature controlled, crystal oscillator and provides

(b) With mode switch 1A1S13 set to LSB, USB

the 5 MHz reference signal to the synthesizer.

or CW product detector 1A3A4Q6, 1A3A4Q8, 1A3A4Q9

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

is enabled. The 10.5 MHz 3rd local oscillator signal

local oscillator frequencies. The 1st local oscillator

from the synthesizer is mixed with the 10.5 MHz signal

frequency of 91.25 to 121.2499 MHz is controlled by the

from amplifier 1A3A2U1, 1A3A2U2 in product detector

selected front panel FREQUENCY switches and

1A3A4Q6, 1A3A4Q8, 1A3A4Q9. The audio signal from

determines the operating frequency of the radio. The

the product detector is coupled to the inputs of 600 ohm

2nd local oscillator frequency is 80.75 MHz. And, the

audio line driver 1A3A4U1 and audio amplifier

3rd local oscillator frequency is 10.5 MHz.

1A3A4Q11.

(a) The 3rd local oscillator signal is derived by

(16) The audiot signal from 600 ohm audio line

direct synthesis techniques (i.e. by dividing and mixing

driver 1A3A4U1 is coupled to AUDIO connector 1A8J2.

the output of 1A8U1). This local oscillator signal is used

The audio signal from audio amplifier 1A3A4Q11 is

as a carrier generator.

coupled through VOLUME control 1A1R51 to speaker

(b) The 2nd local oscillator consists of a crystal

driver 1A1A1. The output from 1A1A1 is coupled

oscillator at a nominal frequency of 80.75 MHz. This

through the normally closed contacts of PHONE jack

frequency is used in VHF mixer 1A3A1 to convert the

1A1J3 to speaker 1A1LS1.

1st IF of 91.25 MHz to the 2nd IF of 10.5 MHz. Since

the 2nd local oscillator is not referenced to 1A8U1, a

2-5.

Transmitter Functional Operation

small frequency error can exist. However, because of

(Fig. FO-2).

the mixing scheme used, this same error appears on the

1st local oscillator signal and is therefore canceled at

The RT-1277/URC-92 can be used in the transmit mode

the output of the VHF mixer.

in any setting of the mode switch. This paragraph

describes functional operation as a transmitter with the

locked oscillator covering the frequency range of 91.25

mode switch set to LSB, USB, AM, or CW For functional

to 121.2499 Hz in 100 Hz steps. The exact frequency of

operation with the mode switch set to CPLR TUNE KW

the 1st local oscillator is equal to 91.25 MHz plus the

refer to paragraph 2-6.

setting of the selected FREQUENCY switches and

a. Overall Functional Operation.

The RT-

1277/URC-92 is placed in the transmit mode by

2-4