Second Mixer; Noise Limiter; 455 Khz Filter And If Amplifier - Codan NGT 2010 RF Technical & Service Manual

Second mixer

Drawings
NOTE
Mixer IC101 is a combined amplifier/balanced mixer. The 45 MHz signal is mixed with
the local oscillator LO2 (44.545 MHz) applied to pin 5. The mixing produces a second
IF centred at 455 kHz.
DC to the mixer output is provided by the +10 V rail, which is connected to the centre
tap of the 455 kHz output transformer T202 (sheet 2). In Receive mode, the +10 VRx
rail provides DC via R132 to IC101 pin 4, and bias via R131 to IC101 pin 11.

Noise limiter

Drawings
NOTE
The 455 kHz output of IC101 pins 3 and 14 (sheet 1) is applied to the balanced input of
the high-gain differential amplifier IC201 pins 2 and 13. The balanced outputs at pins 7
and 8 are connected to a tuned auto transformer T201 (455 kHz). The output of T201
goes to the base of the active rectifier V201. Noise bursts produce positive-going pulses
at the collector of V201, and via V202, trigger the monostable flip-flop IC202/B/C. The
pulse width is determined by C210, R210 and R211 (nominally 250 µs).
The outputs of IC202/B/D pins 11 and 4 produce complementary pulses that are
connected to the FET gates of V204 and V205. These gate out the noise bursts from the
455 kHz signal. With V204 on and V205 off, the IF signal is passed to the sideband filter
Z201. When a noise burst is present, V204 is switched off and V205 is switched on,
blocking the IF signal for the period of the gate pulse.
The average DC component of the collector current of V201 is used as an automatic gain
control to IC201. It is applied via R206 and IC211 to pins 6 and 11. This ensures that
only the noise burst signal triggers the flip-flop IC202/B/C. The AGC decay constant is
set by the components C206 and R204.
In Transmit mode, IC202 is disabled (see

455 kHz filter and IF amplifier

Drawing
NOTE
The IF signal from the noise blanker gate V204 is passed to the 2.5 kHz-wide ceramic
filter Z201. The filter removes the unwanted sideband, adjacent channels and other
unwanted products generated by the second mixer. The wanted sideband, centred at
455 kHz, is passed by the filter to a 3-stage gain-controlled IF amplifier comprising
V210, V211, V212 and associated components. The IF amplifier is broadly tuned to
455 kHz by the tuned circuits L201 and C229, L202 and C232, and L203 and C236.
NGT Transceiver System Technical Service Manual
04-03378 (sheet 1) and
If the PCB issue status is -06 or earlier, see drawings
and 04-03378 (sheet
04-03378 (sheet 1) and
If the PCB issue status is -06 or earlier, see drawings
and 04-03378 (sheet
04-03378 (sheet 2)
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)
2).
page 112, Tuning).
04-03378 (sheet 1)
04-03378 (sheet 1)
04-03378 (sheet 2).
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