Receive Path - Codan X-2 Service Manual

X-2 Technical Service Manual

2.4 Receive Path

PA and Filter PCB
The received signal from the antenna passes through a PA low-pass filter to
the transmit/receive relay and broadcast filter on the PA PCB to the receiver
input on the main PCB.
Rx/Exciter & Control PCB
From the receive input, the signal is fed via a second 20 MHz low-pass filter
to the input of the first balanced mixer. Here it mixes with the local
oscillator VCO1 and converts to 45 MHz.
A 15 kHz roofing filter filters this 45 MHz signal before being applied to the
second balanced mixer. The signal mixes with a second local oscillator
VCO2 producing an IF signal centred on 455 kHz.
The output of the second mixer divides into two paths:
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The amplified 455 kHz signal is demodulated to produce an audio signal and
amplified. The amplified audio signal operates an AGC circuit. This
controls the IF amplifier gain to prevent overloading when receiving strong
signals. It is also used to maintain a constant audio output with changing
input signals.
After amplification the signal passes through a mute that removes the
receiver noise from the speaker when enabled. When speech is detected, the
gate in the audio line closes to allow the signal to be heard. The signal feeds
the volume control on the Front Panel PCB.
Front Panel PCB
The signal from the volume control is applied to a power amplifier IC2 to
drive the transceiver's loudspeaker.
Publication No: 15-02047
the main path passes through a noise gate to a 2.5 kHz sideband filter
where only the wanted sideband passes to the high gain AGC controlled
IF amplifier.
the second path passes the signal through an amplifier which detects
noise and controls the noise gate to remove impulse noise such as car
ignition from the 455 kHz signal.
Brief Description 2-3