Description
68P81096E36-H
7
06/28/05
Receiver Circuitry Operation
Introduction
The Receiver Circuitry accepts receive rf signals from the site receive antenna, per-
forms filtering and dual conversion, and outputs a digitized receive signal to the Sta-
tion Control Module. 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 external 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 (in-
termediate frequency) signal. The injection signal frequency is determined by fre-
quency 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 station.
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.
