Detailed Functional Operation.

TM 11-5820-873-34

sum of the input frequencies and the difference between

selected front panel FREQUENCY switches and

the two input frequencies. Since the nominal frequency

determines the operating frequency of the radio. The

of the 1st local oscillator signal is 91.25 MHz plus the

2nd local oscillator frequency is 80.75 MHz And, the 3rd

setting of the selected FREQUENCY switches, the

local oscillator frequency is 10.5 MHz.

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

difference between the two input frequencies will always

be 91.25 MHz.

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

(9) The output of the 1st balanced mixer is

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

coupled to VHF filter 1A3A1FL1. The filter is a narrow

as a product detector injection signal.

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

band crystal filter with a center frequency of 91.25 MHz.

Therefore, only the difference frequency output from the

oscillator at a nominal frequency of 80.75 MHz. This

1st balanced mixer is passed by 1A3A1FL1.

frequency is used in VHF mixer 1A3A1 to convert the 1st

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

(10) Amplifier 1A3A1Q6 is enabled by the 12 volts

receive enable signal and amplifies the 91.25 MHz

local oscillator is not referenced to 1A8U1, a small

output from 1A3A1FL1. The output from amplifier

frequency error can exist. However, because of the

1A3A1Q6 is applied to the 2nd balanced mixer

mixing scheme used, this same error appears on the 1st

composed of diodes 1A3A1CR9 thru 1A3A1CR12.

local oscillator signal and is therefore canceled at the

(11) The nominal 80.75 MHz 2nd local oscillator

output of the VHF mixer.

signal from synthesizer 1A4 is coupled to the 2nd

balanced mixer through 2nd local oscillator amplifier

locked oscillator covering the frequency range of 91.25

1A3A1Q8. The 10.5 MHz (difference frequency) output

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

of the 2 balanced mixer is coupled to amplifier

the 1 local oscillator is equal to 91.25 MHz plus the

1A3A2Q3.

setting of the selected FREQUENCY switches and the

(12) Amplifier 1A3A2Q3 is enabled by the 12 volts

difference between the frequency of the 2

local

receive enable signal and amplifies the 10.5 MHz output

oscillator and 80.75 MHz. The 1 local oscillator is used

from the 2nd balanced mixer. The output from amplifier

to convert the incoming signal to 91.25 MHz.

1A3A2Q3 is applied to the Input of three diode gates.

(4) The incoming RF signal is coupled into the

(13) The diode gates at the Inputs and outputs of

RT-1277/URC-92 at ANT jack 1A8J1. From the ANT

filters 1A3A2FT1, 1A3A2FL2 and 1A3A2FL3 are

jack, the signal is coupled through the selected

enabled by + 12 volts from mode switch 1A1S13. The

bandpass filter in filter module 1A5 (para 2-10) to VHF

10.5 MHz signal from 1A3A2Q3 is coupled through the

mixer board 1A3A1 in the receiver/exciter.

selected filter to amplifiers 1A3A2U1 and 1A3A2U2.

(5) Diode gate 1A3A1CRJ is switched off by the +

(14) Because the RT-1277/URC-92 uses high side

12 volts receive enable signal and has no effect on

conversion, the sidebands are reversed in the IF

circuit operation.

In the transmit mode diode

circuits. That is, the lower sideband is at a higher

1A3A1CR1 would ground gate 2 of RF amplifier

frequency than the upper sideband.

1A3A1Q2, disabling the amplifier.

(a) With mode switch 1A1S13 set to LSB, diode

(6) The 1 6 to 29 9999 MHz output from

gate 1A3A2CR8, diode gate 1A3A2CR1, 1A3A3CR2

bandpass filter 1A5 is amplified by RF amplifier

and diode gate 1A3A2CR11, 1A3A2CR12 are enabled

1A3A1Q2 and coupled through diode gate 1A3A1CR3 to

The lower sideband signal is coupled through upper

the 1 balanced mixer composed of diodes 1A3A1CR4

sideband filter 1A3A2FL1. The center frequency of

thru 1A3A1CR7. Diode gate 1A3A1CR3 is enabled by

1A3A2FL1 is 10.5016 MHz.

the + 12 volts receive enable signal and allows the 1 6

(b) With mode switch 1A1S13 ste to USB or

to 29 9999 MHz signal to pass.

(7) The 91.25 MHz to 121.2499 MHz 1st local

CW, diode gate 1A3A2CR10, diode gate 1A3A2CR5,

1A3A2CR6 and diode gate 1A3A2CR15, 1A3A2CR16

oscillator signal from synthesizer 1A4 is coupled to the

are enabled. The upper sideband signal is coupled

1st balanced mixer through 1 local oscillator ampllfier

through lower sideband filter 1A3A2FL3. The center

1A3A1Q5. The nominal frequency of the 1st local

frequency of 1A3A2FL3 is 10 4984 MHz.

oscillator signal is 91.25 MHz plus the setting of the

selected FREQUENCY switches.

relay 1A8K1 not energized) diode gate 1A3A2CR7,

(a) If the selected FREQUENCY switches are

diode gate 1A3A2CR3, 1A3A2CR4 and diode gate

set to 16, the 1 local oscillator nominal frequency is

1A3A2CR13, 1A3A2CR14 are enabled. The signal is

92.85 MHz.

coupled through AM filter 1A3A2FL2 The center

(b) If the selected FREQUENCY switches are

frequency of 1A3A2FL2 is 10.5 MHz.

set to 29 9999, the 1 local oscillator nominal frequency

(15) The output from the selected filter is amplified

is 121.24999 MHz.

(8) The output of the 1st balanced mixer is the

2-3