Codan NGT 2010 RF, Technical & Service Manual

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NGT—Brief description (2010)
NGT Transceiver System Technical Service Manual 59
Transmit path
Handset transmit path
The microphone audio is amplified in the handset and passed to the junction box via the
handset bus.
Junction box transmit path
The audio from the handset is amplified and gated to produce TDM channel audio on the
Audio PCB in the junction box. There are eight time slots for the TDM, comprising one
synchronisation channel and seven audio channels.
The audio is passed to the RF unit via the CIB.
RF unit transmit path
Audio Interface PCB
The audio from the junction box connects to the Audio Interface PCB. This PCB
decodes the TDM audio to normal audio.
Application Processor PCB
The transmit audio is passed to the Application Processor PCB for compression, which
converts the signal to a fixed level before passing to the RF/IF PCB.
RF/IF PCB
The audio is mixed with the local oscillator to produce double sideband. It is then filtered
by a 2.5 kHz sideband filter centred on 455 kHz, so that only the wanted sideband is
passed to the first mixer. It is then mixed with the reference oscillator to produce an IF
signal centred on 45 MHz.
The transmit signal is filtered by a BPF with a bandwidth of 15 kHz, before being passed
to the input of the second mixer.
At the second mixer, the signal mixes with the local oscillator VCO to produce the
required channel frequency. The signal is passed to the PA PCB.
PA PCB
On the PA PCB, the signal is amplified then passed through the transmit/receive relay to
the selected LPF.
The output from the LPF is passed via the VSWR detector to the RF output connector.
From there it is connected by coaxial cable to the antenna.