User Manual

SECTION 1: THEORY OF OPERATION
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For up-link transmissions, the microcontroller receives data from the receive modem chip, checks for
errors, adds framing and optionally signal strength information, and sends it out the serial port.
Synthesizers
The microcontroller is responsible for keeping the base station on channel by programming the digital
PLL (phase-locked-loop) frequency synthesizers. This task is accomplished via a serial communications
interface consisting of three signals PLLDATA, PLLCLOCK, and PLLENABLE. The PLLDATA signal
provides the PLL data for the synthesizer, the PLLCLOCK signal clocks the data into the synthesizer, and
the PLLENABLE signal enables synthesizer programming. After initially programming the synthesizer,
the microcontroller monitors the synthesizer's lock detect signal. Upon detecting an unlocked condition,
the microcontroller will re-program the synthesizer to lock the base station.
4-Level FSK Modem Section
The system controller contains two (2) internal 4-Level FSK modems for receiving and one (1) for
transmitting for reliable, high-speed transfer of data over-the-air. The modems interface with the
microcontroller and the base station’s modulation / demodulation circuits to deliver reliable two-way
transfer of high-speed application data over a wireless link.
The modem assembles application data received from the microcontroller, adds FEC (forward error
correction) and error detection (CRC - cyclic redundancy check) information and interleaves the result for
burst-error protection. After automatically adding symbol and frame sync code words, the data packet is
converted into filtered 4-Level analog signals for modulating the base station’s transmitter.
In the receive mode, the modems perform the reverse function using the analog signals from the
receivers discriminators. After error correction and removal of packet overhead, the recovered application
data is supplied to the microcontroller. CRC detected residual errors will be flagged.
The modems use data block sizes and FEC/CRC suitable for high-speed transfer of data over narrow-
band radio links. The modem can be operated at 4.8 kbps, 9.6 kbps, and 19.2 kbps under software
control. The modem IC is a highly integrated device incorporating many advanced features.