CMX969 Consumer Microcircuits Limited, CMX969 Datasheet - Page 7

• Manufacturer Part Number: CMX969
• Description: RD-LAP/MDC4800/Motient/ARDIS4-Level FSK Packet Data Modem
• Manufacturer: Consumer Microcircuits Limited

RD-LAP/MDC4800 Motient/ARDIS
CMX969
FEC Encoder/Decoder
In transmit mode, this circuit adds Forward Error Correction information to the transmitted data. In RD-LAP
mode it also converts the binary data to 4-level symbols. In receive mode, this block translates received
symbols to binary data, using the FEC information to correct a large proportion of transmission errors.
Interleave/De-interleave Buffer
This circuit interleaves data symbols within a block before transmission and de-interleaves the received data
so that the FEC system is best able to handle short noise bursts or fades.
Auto Frame Sync Detect, Rx Level & Timing Extraction, Rx Data Symbol Extraction
This block, which is only active in receive mode, is used to look for the Frame Synchronisation pattern which
is transmitted to mark the start of every frame and to extract the received symbols from the received signal
using extracted signal level and timing information.
Rx Input Amp
This amplifier allows the received signal input to the modem to be set to the optimum level by suitable
selection of the external components R1 and R2. The dc level of the received signal should be adjusted so
that the signal at the modem's RXFB pin is centred around V
(½ V
). See section 1.7.1.3 for details of
BIAS
DD
the optimum levels.
Low Pass Filter
This filter, which is used in both transmit and receive modes, is a linear-phase lowpass filter having a
frequency response automatically switched to suit RD-LAP or MDC operation.
In transmit mode, the data symbols are passed through this filter to eliminate the high frequency components
which would otherwise cause interference into adjacent radio channels.
Figure 4 Generation of Filtered Tx Baseband Signal
In receive mode, the filter is used to reject HF noise and to equalise the received signal to a form suitable for
extracting the received data.
Tx Output Buffer
This is a unity gain amplifier used in transmit mode to buffer the output of the Tx low pass filter. In receive
mode, the input of this buffer is connected to V
unless the RXEYE bit of the Control Register is '1', when it
BIAS
is connected to the received signal. When changing from Rx to Tx mode the input to this buffer will be
connected to V
for 8 symbol times in RD-LAP mode, 2 symbol times in MDC mode while the low pass filter
BIAS
settles.
Note: The RC low pass filter formed by the external components R4 and C5 between the TXOP pin and the
input to the radio's frequency modulator forms an important part of the transmit out of band spurious signal
filtering. These components may form part of any dc level-shifting and gain adjustment circuitry. C5 should be
positioned to give maximum attenuation of high frequency noise into the modulator.
2001 Consumer Microcircuits Limited
7
D/969/5