Raymarine Limited. Registered in England. Company no. 1177969
Page 7
Registered Office: Anchorage Park, Portsmouth P03 5TD
The microcontroller uses a 14MHz crystal oscillator. Provision is made to fit an additional off-chip
EEPROM should expansion be required.
Reliable reset at power-on and during brown-out conditions is assured by the supervisor IC.
Radio Interface
The radio interface comprises a collection of signals that control the radio module i.e. transmitter, all-
channel receiver and Channel 70 receiver. The signals can be divided into groups:
•
Analogue inputs to microcontroller ADCs: indicating noise levels and received signal
strength from each of the receivers and transmitter power level.
•
SPI data link to the synthesiser, DAC and EEPROM on the radio board: the microcontroller
reads calibration data from the radio board EEPROM and sets the VCO frequency by
commanding the synthesiser; it coarse tunes the VCO and local oscillators by writing to the
DAC.
•
DC voltages that trim the receiver tuning filters: The microcontroller generates these
voltages based on calibration data read from the radio board EEPROM. The PWM signals
are converted to dc levels, with negligible ripple, by the radio trim filters.
•
Discrete signals: to enable and disable the transmitter and receivers, power down the radio
module, coarse trim and monitor the locked status of the VCO.
Traffic Switching and Audio Amplifiers
The traffic switching block routes audio signals between the “talkers and listeners” in the system: the
“talkers” are the audio sources i.e. handset microphones, radio receivers, hailer microphone amplifier,
FSK modulator and telephone; the “listeners” are audio destinations in the system i.e. handset
speakers, radio transmitter, hailer power amplifier, telephone.
Each listener has a multiplexer capable of selecting any one of the talkers as its audio source. This
flexible arrangement allows multiple audio paths to be selected concurrently; half duplex for radio
and hailer communications, full duplex for intercom and telephone.
Audio signals into and out of the processor board are buffered with audio amplifiers.
The audio output amplifiers mix the alert tones, generated by the microcontroller with the audio
signal, and apply the –6dB/octave de-emphasis characteristic.
The radio mod signal filter band limits the audio signal for transmission to 3kHz prior to modulating
the rf carrier.
DSC and ATIS Functions
ATIS and DSC signals are encoded using frequency shift keying (FSK), where data 1 = 1300Hz and 0
= 2100Hz. The FSK modulator takes three digital signals from the microcontroller, switched in a
pre-defined sequence, and converts them into a stepped approximation to a sinewave. The stepped
sinewave is filtered to produce a purer sinewave with typically 3% total harmonic distortion that then
modulates the rf carrier. The filtering also applies a +3dB/octave pre-emphasis characteristic. The
frequency of the FSK signal is determined by very accurately controlling the rate at which the digital
inputs to the FSK modulator are switched.
