CMX969D5 MX-COM, Inc., CMX969D5 Datasheet - Page 8

• Manufacturer Part Number: CMX969D5
• Description: MOTIENT/ARDIS RD-LAP MDC4800 Modem
• Manufacturer: MX-COM, Inc.

MOTIENT
4.1.8
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.
4.1.9
This block, which is only active in receive mode, is used to look for the Frame Synchronization 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.
4.1.10 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 centered around V
optimum levels.
4.1.11 Low Pass Filter
This filter, which is used in both transmit and receive modes, is a linear-phase low pass 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
that would otherwise cause interference into adjacent radio channels.
In receive mode, the filter is used to reject HF noise and to equalize the received signal to a form suitable for
extracting the received data.
4.1.12 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
is connected to the received signal. When changing from Rx to Tx mode the input to this buffer will be
connected to V
filter settles.
Note: The RC low pass filter formed by the external components R4 and C5 between the TXOUT 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.
4.1.13 Clock Oscillator and Dividers
These circuits derive the transmit symbol rate (and the nominal receive symbol rate) by frequency division of
a reference frequency which may be generated by the on-chip Xtal oscillator or applied from an external
source.
Note: If the on-chip xtal oscillator is to be used, then the external components X1, C3, C4 and R3 are
required. If an external clock source is to be used, then it should be connected to the XTAL input pin, the
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4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
XTAL pin should be left unconnected, and X1, C3, C4 and R3 not fitted.
SM
Interleave/De-interleave Buffer
Auto Frame Sync Detect, Rx Level & Timing Extraction, Rx Data Symbol Extraction
/ARDIS
encoding
Data
BIAS
SM
RD-LAP
for 8 symbol times in RD-LAP mode, 2 symbol times in MDC mode, while the low pass
Binary
data
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TM
Figure 4: Generation of Filtered Tx Baseband Signal
MDC4800 Modem
binary -
symbol
Modem
2 or 4-level
BIAS
symbols
unless the RXEYE bit of the Control Register is '1', when it
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8
BIAS
Transmit
filter
(V
DD
/2). See Section 6.1.3 for details of the
baseband
Filtered
signal
CMX969 Advance Information
Frequency
modulator
Doc. # 20480211.002