Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
1.5
General Description
The CMX880 works as a signalling encoder/decoder, in conjunction with a micro-controller (µC) and Digital Signal
Processor (DSP). It is intended for use in half duplex, dual mode, analogue and digital two way land mobile radio
(LMR) equipment and is particularly suited to APCO-25 LMR terminal designs. The DSP will perform the high
level digital signal processing functions, such as forward error correction coding/decoding, voice
coding/decoding, station ID and data packet extraction, whilst the CMX880 provides A-to-D and D-to-A
conversion, plus a number of further radio signal encoder and decoder functions for Selcall, CTCSS, DCS,
FFSK/MSK, DTMF, C4FM, CQPSK and voice. It can also be used in analogue only radio applications, without
an external DSP. Power control facilities allow the device and external DSP to be placed in sleep mode, to
conserve power when the terminal is in standby mode. The CMX880 includes a crystal (xtal) clock generator,
with a buffered output to provide a common system clock if required. A block diagram of the CMX880 is shown
in Figure 1.
Analogue Input Signal Processing Path
The analogue signal source is selected from either one of the two low noise microphone input amplifiers (for
voice mode transmission) or the FM discriminator input amplifier (for signal reception). Analogue input selection
is set by “C-BUS” command. The gain and frequency characteristics of the inputs are set by the external
feedback networks shown in Figure 2. The configured input amplifiers act as anti-alias filters (AAF) for the ADC.
The FM_DISC input must be DC coupled for receiving C4FM and DCS signals; the VBIAS from the CMX880 can
be used to set the FM discriminator reference, but it must be buffered as shown in Figure 2. The input signal is
converted to a 16-bit word by the ADC before passing to the signal processing stage. The input stage gain can
be adjusted within the range 0dB to 22.4dB in steps of 3.2dB, set by a “C-BUS” command.
Analogue Output Signal Processing Path
The output signals from the signal processing stage are converted from 16-bit words to analogue signals by the
DACs. The signal outputs of each DAC pass through a reconstruction filter before being routed to the selected
output ports. Signals are routed either to the audio port during reception of a voice message or to the modulator
ports during transmit mode. The CMX880 has two modulator outputs, each with independently programmable
gains that simplify implementation of two point modulation schemes. The output destination, the gain of the
audio output amplifier and the gain of the modulator output amplifiers are set by “C-BUS” command.
Interfaces
The CMX880 has two digital interfaces, the “C-BUS” and the Fast Serial Bus (FSB). The “C-BUS” is used for
command, status and data transfers between the CMX880 and the host mC. An interrupt signal notifies the host
mC that a change in status has occurred. The mC should read the new status across the “C-BUS” and respond
accordingly. Note Interrupts only occur if the appropriate mask bits have been set. See section 1.5.2.1.
The FSB provides an independent full duplex data transfer link with the DSP.
Signal Processing Functions
The signal processing unit performs the various voice, in-band, sub-audio, FFSK/MSK and C4FM receive and
transmit processing tasks. A summary of these tasks is described here, with the detail provided in subsequent
sections.
Processing of signal data from the ADC:
·
Low pass filtering and decimation (by various function dependent ratios) of the signal from the FM
discriminator or from the microphone inputs.
·
Voice band filtering for analogue channel signals to suppress the sub-audio range (below 300Hz) and limit the
upper frequency range.
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Voice band filtering for APCO 25 digital channel signals, prior to digital voice encoding (by the DSP).
Low pass sub-audio filtering (0 to 300Hz) for CTCSS and DCS decoding.
CTCSS tone detection.
Selcall tone detection.
DCS filtering, data slicing and decoding - detect the single, pre-programmed 23 or 24-bit DCS code. The
polarity of the DCS code is set by the user.
ã 2001 Consumer Microcircuits Limited
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D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
