Codan NGT 2010 RF Technical & Service Manual page 114

NGT—Technical description (2010)
The 455 kHz transmit signal is mixed with 44.545 MHz (LO2), which is applied to
IC103 pin 5 to produce a second IF output signal at pins 3 and 14 centred on 45 MHz.
The gain of the mixer/amplifier is approximately 20 dB.
45 MHz band-pass filter
Drawing
ALTERNATIVE TEXT
Drawing
NOTE
The 45 MHz IF signal is filtered through a 15 kHz-wide BPF consisting of T108, Z104,
L117, Z103 and T107. This filter removes unwanted signals produced by the first mixer.
The filtered output goes to the balanced input of the second mixer IC102 pins 12 and 13.
R138 is between T107 and IC102 and is used as a fine balance adjustment for the mixer
to remove unwanted spurious signals.
Second mixer and exciter output filter
Drawing
ALTERNATIVE TEXT
Drawing
NOTE
The second mixer IC102 is enabled by applying +10 VTx. This applies DC via R133 to
pin 4, and bias current via R134 to pin 11. The filtered +10 V rail is applied via L116 and
the transformer T106 to the mixer output from pins 3 and 14.
The 45 MHz transmit signal is applied to IC102 pins 12 and 13. It is mixed with the
output of the local oscillator LO1+, LO1– (applied to IC102 pins 5 and 7) to produce the
selected channel frequency at the mixer output pins 3 and 14. The gain of the
mixer/amplifier is approximately 20 dB.
The mixer output goes via a 30 MHz LPF to the transmit exciter output connector J2.
From there it is coupled via a coaxial cable to the PA assembly.
Tuning
Drawings
ALTERNATIVE TEXT
When the Tune command is activated, IC405 pin 7 (sheet 3) goes low. This switches off
V416 to allow the 456.5 kHz local oscillator carrier (453.5 kHz for LSB) to be passed to
V206 (sheet 2) via CAR INJ. V206 inserts carrier to the input transformer T202 of the
first Tx mixer IC103 (sheet 1).
At the same time, IC405 pin 5 (Tx MUTE) goes high and switches on V203 (sheet 2).
Thus, IC202/C and IC202/B open the noise gate V204, which stops the inserted carrier
from being loaded by the sideband filter Z201.
The carrier at the output of the second mixer IC102 (sheet 1) is at the frequency of the
selected channel (the SCF). This frequency is used for tuning automatic antennas.
80
04-03106 (sheet 1), or
04-03378 (sheet 1)
If the PCB issue status is -06 or earlier, see drawing
04-03106 (sheet 1), or
04-03378 (sheet 1)
If the PCB issue status is -06 or earlier, see drawing
04-03106 (sheet 3), 04-03106 (sheet 2)
04-03378 (sheet
04-03378 (sheet
and 04-03106 (sheet 1)
NGT Transceiver System Technical Service Manual
1).
1).