Receive Path; Rf Unit Receive Path - Codan NGT 2010 RF Technical & Service Manual

Receive path

RF unit receive path

PA PCB
The receive signal from the antenna passes through a PA LPF to the transmit/receive
relay. It is then passed to the receiver input on the RF/IF PCB.
RF/IF PCB
From the receiver input, the signal passes via a selectable pre-amplifier to the input of the
first balanced mixer. Here it mixes with the local oscillator VCO to produce an IF signal
centred on 45 MHz.
The 45 MHz signal is filtered using a BPF with a bandwidth of 15 kHz, before being
passed to the second balanced mixer. The signal mixes with 44.545 MHz to produce an
IF signal centred on 455 kHz.
The output from the second mixer passes through a 2.5 kHz sideband filter so that only
the wanted sideband is fed to the high-gain AGC IF amplifier.
The amplified 455 kHz signal is demodulated to produce an audio signal and also to
operate an AGC circuit. This circuit controls the gain of the IF amplifier to prevent
overloading when strong signals are received. It also maintains constant audio output
with changing input levels.
Audio & Interconnect PCB (2011)/Audio Interconnect & Data I/O PCB (2012)
The audio is passed to the Audio & Interconnect PCB (2011)/Audio Interconnect & Data
I/O PCB (2012) where it divides into two paths. One path is the unprocessed (raw) audio.
The other path is passed to the Application Processor Handset I/F 3 V PCB
(2011)/Application Processor Handset & Data I/F PCB (2012) where the audio is
enhanced (noise-reduced) before returning to the Audio & Interconnect PCB
(2011)/Audio Interconnect & Data I/O PCB (2012). The required audio path is switch-
selected and passed to the audio amplifier, then to the speaker via the handset and
speaker connection cable.
NGT Transceiver System Technical Service Manual
NGT—Brief description (2011/2012)
67