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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
(c)  The VCO (1st local oscillator) is a phase
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
setting of the selected FREQUENCY switches and the
(12) Amplifier 1A3A2Q3 is enabled by the 12 volts
difference between the frequency of the 2
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
(c)  With mode switch 1A1S13 set to AM (and
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

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