Specifications

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4.4. ACC-513 (GMSK MODEM CIRCUITS)
MAXON GMSK option modem boards uses a CMX589A IC to generate and decode GMSK
signals. The modem fully supports auto mode, dump mode (RTS mode) and extended dump
mode operation. This section describes only the operation of circuit. A special characteristic of
GMSK modems is that they can use a high data throughput in limited channel space.
TX-SIGNAL CIRCUIT
The transmit data (TXD) signal from Pin 1 of CON2 is level-shifted to TTL levels by Q1 and then
applied to the Pin 45(UART port) of U1 (slave MCU). The slave micro controller stores received
data in the memory buffer and releases stored data to U2 (CMX589A) by the transmit clock
pulses of the CMX569A. Random timed asynchronous input from the external data terminal is
synchronized by the MCU and then synchronous information is sent into U2 (CMX589A), which
filters the data stream through a Gaussian filter in order to reduce transmission bandwidth. The
cut-off frequency of the Gaussian filter is decided by a transmit bit ratio. Output signal of the
GMSK modem is fed into a RC network formed by R12 and C10 that is positioned to reduce
high-frequency noise. To adjust the cut-off frequency of the RC network, R22and U6, is used.
The filtered signal is inputted to the gain amplifier and the 3rd order low pass filter (U5) through
a buffer. The first stage of U5 is used as a buffer and the second stage is a gain amplifier to
supply sufficient amplitude level for modulation and the third stage comprises of an additional
3rd order Butterworth LPF to guarantee maximum attenuation of high-frequency noise before
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 a
proper level for data detection. The amplified signal is fed into the GMSK modem option board
and then inputted to Pin 11 of U2(CMX589A) through Pin 14 of CON1. The CMX589A 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 the 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. The 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 is shifted from TTL to RS-232 level.