Codan NGT 2010 RF Technical & Service Manual page 145

Transmit modulator
Drawing
NOTE
The audio goes to the Tx Level potentiometer R256 to set the exciter output level at J2 to
650 mV p–p. It is passed to the input of the balanced modulator IC204 pin 1. The
modulator is enabled when +10 VTx is applied via R254 to the bias input pin 5.
The audio is mixed with the local oscillator of 456.5 kHz (453.5 kHz for LSB). The local
oscillator is applied to IC204 pins 8 and 10. This produces a DSB output at pin 6. From
there it is passed via D205 to the 455 kHz sideband ceramic filter Z201 pin 1.
455 kHz filter and first mixer
Drawings
NOTE
(sheet 2) The 455 kHz sideband ceramic filter Z201, with a bandwidth of 2.5 kHz, filters
out the unwanted sideband and leaves only the wanted sideband at pin 3. From there it is
passed via the noise limiter gate V204 to a tuned auto transformer T202.
(sheet 1) The signal is passed from T202 via a resistor-matching attenuator network
R144, R145 and R146 to the input of the first balanced amplifier/mixer IC103 pins 12
and 13. IC103 is enabled by applying +10 VTx. This applies DC via R142 to pin 4, and
bias current via R143 to pin 11. The filtered +10 V rail is applied to the mixer output,
pins 3 and 14, via the centre tap of the output transformer T108.
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
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.
NGT Transceiver System Technical Service Manual
04-03378 (sheet 2)
If the PCB issue status is -06 or earlier, see drawing
04-03378 (sheet 1) and
If the PCB issue status is -06 or earlier, see drawings
and 04-03378 (sheet
04-03378 (sheet 1)
If the PCB issue status is -06 or earlier, see drawing
NGT—Technical description (2011/2012)
04-03378 (sheet 2)
2).
04-03378 (sheet 2).
04-03378 (sheet 1)
04-03378 (sheet 1).
111