Receiver Circuitry Operation - Motorola MTR2000 T5544 Instruction / Field Service Manual

Receiver Circuitry Operation

68P81096E36-H
06/28/05
Introduction
The Receiver Circuitry accepts receive rf signals from the site receive antenna, performs fil-
tering and dual conversion, and outputs a digitized receive signal to the Station Control Mod-
ule. The receiver module utilized may have either an internal varactor-tuned preselector filter,
or an external metal preselector filter.
Receiver Module Operation
The receive signal is input from the site receive antenna to the receiver module, or to an ex-
ternal preselector filter (a separate assembly attached to the rear of the station which provides
highly selective bandpass filtering). The signal is fed through a low-pass filter, varactor-tuned
preselector (if external preselector is not used), rf amplifier and image filter to the rf input of
the first mixer. The filtered signal is mixed with an injection signal generated by the receive
synthesizer/VCO, resulting in a first i-f (intermediate frequency) signal. The injection signal
frequency is determined by frequency programming data from the Station Control Module via
the SPI bus. The specific frequency of the first i-f depends on the frequency band of the sta-
tion.
The first i-f signal is filtered and input to a custom receiver IC. This component contains circuitry
for generating the second injection signal, mixing down the first i-f to 450 KHz, amplification and
A/D (analog-to-digital) conversion of the second i-f signal, resulting in a digitized receive signal.
This signal is fed as differential data to the Station Control Module.
Description
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