NGT—Brief description (2010)
60 NGT Transceiver System 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 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 signals.
Audio Interface PCB
The audio is passed to the Audio Interface PCB where it divides into two paths. One path
is the unprocessed (raw) audio on one TDM channel. The other path is passed to the
Application Processor PCB where the audio is enhanced (noise-reduced). This audio is
then passed back to the Audio Interface PCB and becomes another TDM audio channel.
The audio is then passed to the junction box via the CIB.
Junction box receive path
The audio from the RF unit is decoded and either raw audio or noise-reduced audio (as
selected by the user) is passed to the speaker.
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