455 Khz Filter And If Amplifier - Codan NGT 2010 RF Technical & Service Manual

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
ALTERNATIVE TEXT
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.
When the optional filter Z202 is installed, relays K201 and K202 are fitted and links
LK201 and LK202 are cut. The optional filter is selected when V207 is activated to
energise the relays.
Drawing
ALTERNATIVE TEXT
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.
When the optional 500 Hz filter is installed, the 500 Hz Filter PCB (Codan part number
08-05890-001) is added and R222 and R223 are removed. The optional filter is selected
when V207 is activated.
NGT Transceiver System Technical Service Manual
04-03106 (sheet 2)
04-03378 (sheet 2)
If the PCB issue status is -06 or earlier, see drawing
NGT—Technical description (2010)
page 80, Tuning).
04-03378 (sheet 2).
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