User Manual

SECTION 1: THEORY OF OPERATION
IP4HPVGPS-MRFCCRpt.doc Page 6
Modem
The single-chip modem circuit converts parallel data to an analog audio waveform for transmission and
analog audio from a receiver to parallel data. In addition to the modem functions, the chip provides
forward error detection and correction (FEC), bit interleaving and Viterbi Soft Decision Algorithms for
more robust data communications.
The microcontroller section controls the modem operation. Address bus, address/data bus, and control
lines operate the modem chip. The modem circuitry is also run by a crystal-controlled clock, which
consists of crystal Y1 and an internal Pierce oscillator.
The received audio signal is demodulated into digital data appearing on the AD0-AD07 lines when the
MODEMCS* and RD* lines are low. The data goes to the microcontroller section for futher processing,
and then to the input/output section for conversion to RS232 or Ethernet signal levels.
During a transmission, outgoing data appearing on the AD0-AD07 lines is converted into a 4-level FSK
analog signal by the modem chip. This operation takes place when the MODEMCS* and WR* lines are
low. Data from the user’s MDC or VIU passes through the input/output section and microcontroller
section to the AD0-AD07 bus. After processing, data passes through a root raised cosine filter and is
output to TXMOD.
This modem supports 115.2 KBPS (serial port) and 19.2 KBPS (over-the-air) data transmission rates.
VLogic and Digital Ground
The VLogic and Digital Ground section consists of a pulse-width modulation (PWM) step-down DC-DC
converter (U20) that provides an adjustable output. It also reduces noise in sensitive communications
applications and minimizes drop out voltage.
An external Schottky diode (D2) is required as an output rectifier to pass inductor current during the
second half of each cycle to prevent the slow internal diode of the N-channel MOSFET from turning on.
This diode operates in pulse-frequency modulation (PFM) mode and during transition periods while the
synchronous rectifier is off.