User's Manual

68P81093C75-A 10-3
The filtered signal is then amplified and fed to the RF input of the 1st
mixer.
The 1st mixer combines the filtered signal with an injection signal
generated by the Synthesizer/VCO. This results in a 21.45 MHz
(VHF), or 73.35 MHz (UHF and 800 MHz) 1st IF (intermediate
frequency) signal. VHF and UHF receivers use high-side injection;
800 MHz receivers use low-side injection. (The injection signal is
determined by frequency programming data from the Station
Control Module via the SPI bus.)
The 21.45 or 73.35 MHz 1st IF is filtered and routed to a custom
receiver IC. This component contains the following circuitry:
• 2nd injection and mixing
• Amplification
• A/D (analog to digital) conversion
This results in a digitized receive signal. This signal is routed to the
Station Control Module as differential data.
Station Control
Module
Introduction
The Station Control Module (SCM) is the microprocessor-based
controller for the station. Major components include the following:
• MC68360 microprocessor,
• 56002 Digital Signal Processor (DSP)
• Two Application Specific Integrated Circuit (ASIC) devices
(host and DSP).
Station Control Module
Operation
The Host Microprocessor (µP) serves as the controller for the SCM,
operating from the station software stored in FLASH memory. This
software determines the system capabilities of the station (analog,
ASTRO , SECURENET , etc.).
The Host µP communicates with the station modules and the SCM
circuitry via address and data buses, a High-Level Data Link Control
(HDLC) bus, and a Serial Peripheral Interface (SPI) bus. External
communications is accomplished using a serial port on the top panel.