Baseband Processor For Mixed Mode Land Mobile Radio
CMX880
1.5.1.4
Transmit Mode
The device only operates in half duplex, so when the device is in transmit mode the receive path (FM disc and
audio output amplifiers) should be disabled, and can be powered down, by the host mC.
Two modulator outputs with independently programmable gains are provided to facilitate two-point FM
modulation. If this is not used, either of the modulator outputs can be disabled to conserve power.
The output of the Delta-Sigma DACs can be switched between either the audio output or the RF modulator
outputs. To avoid erroneous transmission of out of band frequencies when changing the output path destination,
the DAC outputs are ramped to the quiescent modulator output level, VBIAS before switching. Similarly, when
starting a transmission, the transmitted signal strength is ramped up from the quiescent VBIAS level and when
ending a transmission the transmitted signal strength is ramped down to the quiescent VBIAS level. The ramp
rates are set by “C-BUS” command. When the modulator outputs are disabled, their outputs will be at the VBIAS
quiescent level. When the modulator output drivers are powered down, their outputs will be floating, so the RF
modulator will need to be turned off.
The CMX880 supports various transmit options operating concurrently. These are shown in Table 9.
Table 9 Concurrent Tx Modes Supported by the CMX880
Analogue Channel Modes
Digital Channel Modes
Sub-Audio
Voice band
Voice
C4FM
(Data)
CTCSS
CTCSS +
CTCSS +
CTCSS +
CTCSS +
DCS
C4FM
CQPSK
CQPSK
(Voice from MIC input)
(Data)
(Voice from MIC input)
Voice
Selcall
DTMF
FFSK/MSK
DCS +
Voice
DCS +
Selcall
DCS +
DTMF
DCS +
FFSK/MSK
Selcall
DTMF
FFSK/MSK
The following paragraphs describe the types of signal transmission modes the CMX880 can perform.
Processing Voice Data For Transmitting On Analogue Channels
One of two microphone inputs can be selected as the voice input source. The selected microphone input signal
is A-to-D converted and band pass filtered as described below. The digitised signal can then be routed directly
to the D-to-A converter and modulator outputs for analogue signal transmission, or over the “C-BUS” to an
external processor for further encoding.
If the voice signal from the microphone input is routed over the “C-BUS”, the mC must read it within 125µs of
notification (interrupt) of availability of the data sample to maintain throughput at 8000S/s, otherwise the data will
be overwritten. Similarly, voice data must be written back to the CMX880 at the 8000S/s rate.
A programmable gain stage in the microphone input path facilitates mC controlled VOGAD capability.
ã 2001 Consumer Microcircuits Limited
27
D/880/1
CMLMICRO [ CML MICROCIRCUITS ]
