Specifications

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4.5. ACC-514 (FFSK MODEM CIRCUITS)
MAXON FFSK option modem boards uses a CMX469 IC to generate and decode FFSK signals.
The modem fully supports auto mode, dump mode (RTS mode) and extended dump mode
operation. This section describes only the operation of the circuit. A special characteristic of
FFSK modems is that they can apply the sub-audio (tone) squelch system (CTCSS, DCS) to
data communication.
TX-SIGNAL CIRCUIT
The transmit data (TXD) signal comes from Pin 1 of CON2 that is level shifted to the TTL levels
by Q1 and then it’s applied to the Pin 45(UART port) of U1 (slave MCU). The slave micro
controller unit stores received data in the memory buffer and releases stored data to U2
(CMX469A) by transmit clock pulses of the CMX469A. Random timed asynchronous input from
the external data terminal is synchronized by the MCU and synchronous information is sent to
U2 (CMX469A), which converts the logic 0 and 1 into a sine-wave signal with two different
frequencies. At this moment, frequency is decided by a transmit bit ratio. The converted signal
is fed into a gain amplifier and 5th order low pass filter (U5). The first stage of U5 is used as a
gain amplifier to supply sufficient amplitude level for modulation and the next two stages
comprise of a 5th order Butterworth LPF to guarantee maximum attenuation of high-frequency
noise before the modulation. The output of the LPF is then fed to the RF board for TX
modulation.
RX-SIGNAL CIRCUIT
Discriminated signal from the RF board is amplified by IC416-C in the digital board to have
proper level for data detection. The amplified signal is fed into the FFSK modem option board
and then inputted to Pin 16 of U2(CMX469A) through Pin 14 of CON1. The CMX469A detects
synchronous data and recovers receive clock and then each data and clock is sent into U1
(slave MCU) for conversion back into asynchronous data. This information is then sent out to
the data terminal through U3 (MAX234).
LEVEL CONVERTER
The MAX234 level converter (U3) and peripheral circuitry with Q1, Q2 convert TTL input/output
levels to the nominal +/- 10V,RS-232 input/output levels. If dump or extended dump mode is
used an additional handshake signal is required. PTT, Carrier Detect and Busy signal are used
as handshake signals. The PTT signal level is shifted from RS-232 to TTL level and Carrier
Detect and Busy signal are shifted from TTL to RS-232 level.