GMSK Packet Data Modem and RF Transceiver
CMX990
Frame Assembly / Disassembly
Each of these blocks consists of 4 circuits which generate (in transmit mode) or check (in
receive mode) the bits of both short and normal Mobitex data blocks.
CRC Generator/Checker
A circuit which generates (in transmit) or checks (in receive) the CRC bits, which are included
in transmitted Mobitex data blocks so that the receive modem can detect transmission errors.
FEC Generator/Checker
In transmit mode this circuit calculates and adds the FEC (4 bits) to each byte presented to it.
In receive mode the FEC information is used to correct most transmission errors that have
occurred in Mobitex data blocks or in the Frame Head control bytes.
Interleave/De-interleave Buffer
This circuit interleaves data bits within a data block before transmission and de-interleaves the
received data block so that the FEC system is best able to handle short noise bursts or signal
fades.
Scramble/De-scramble
This block may be optionally used to scramble/de-scramble the transmitted and received data
blocks. It does this by modulating the data with a 511-bit pseudorandom sequence, as
described in section 5.1.5.4. Scrambling improves the transmitted spectrum, especially when
repetitive sequences are to be transmitted.
Frame Sync Detect
This circuit, which is only active in receive mode, is used to look for the user specified 16-bit
Frame Synchronisation pattern which is transmitted to mark the start of every frame.
Tx Modulator and Low Pass Filter
The filter is used in transmit mode and is a low pass transitional Gaussian filter having a 3dB
loss at 0.3 or 0.5 times the bit rate (BT=0.3 or 0.5). See figure 9. This filter eliminates the high
frequency components which would otherwise cause interference into adjacent radio channels.
The unmodulated baseband ‘eye’ diagrams of the transmitted signal is shown in Figure 10.
The Tx Modulator converts the baseband signal into an I and Q form which is passed to the Tx
IF stage.
Figure 9 Typical Tx Baseband Filter Response
ã 2004 CML Microsystems Plc
16
D/990/1
CMLMICRO [ CML MICROCIRCUITS ]
