GMSK Packet Data Modem
CMX909B
1.5.1.6 CRC Generator/Checker
A circuit which generates (in transmit mode) or checks (in receive mode) the Cyclic Redundancy
Checksum bits, which are included in transmitted Mobitex data blocks so that the receive modem
can detect transmission errors.
1.5.1.7 FEC Generator/Checker
In transmit mode this circuit calculates and adds the Forward Error Correction (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.
1.5.1.8 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.
1.5.1.9 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.
1.5.1.10 Rx I/P 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 value of R1 should be calculated to
give 0.2 x V pk-pk at the RXFB pin for a received ‘...11110000...’ sequence.
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A capacitor may be fitted if ac coupling of the received signal is desired (see section 1.6.4),
otherwise 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 ).
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1.5.1.11 Tx/Rx Low Pass Filter
This filter, which is used in both transmit and receive modes, is a low pass transitional Gaussian
filter having a loss of 3dB at 0.3 times the selected bit rate (BT = 0.3). See Figure 4.
In transmit mode, the bits are passed through this filter to eliminate the high frequency components
which would otherwise cause interference into adjacent radio channels.
In receive mode this filter is used with an increased BT factor (0.56) to reject HF noise, so that the
signal is suitable for extraction of the received data.
1.5.1.12 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 . When changing from Rx to Tx mode
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the input to this buffer will be connected to V
for 2 bit periods to prevent unwanted signals, from
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the low pass filter, at the output. When the modem is set to power save mode, the buffer is turned
off and the TXOP pin connected to V via a high value resistance. When exiting from power
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save mode the Tx output is only reconnected to the buffer after 2 bit periods, to prevent unwanted
signals, from the low pass filter, at the output.
Note: The RC low pass filter formed by the external components R4 and C5 between the Tx Output
Buffer and the input to the radio's frequency modulator forms an important part of the transmit signal
filtering. These components may form part of any dc level-shifting and gain adjustment circuitry.
The ground connection to the capacitor C5 should be positioned to give maximum attenuation of
high frequency noise into the modulator. R4 and C5 should be chosen so that the product of the
resistance of R4 (in Ohms) and capacitance of C5 (in Farads) is 0.34/bit rate (bit rate in bits per
second). R4 should be not less than 47kW and the value used for the external capacitor should
take into account parasitic capacitance.
ã 2001 Consumer Microcircuits Limited
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D/909B/1
CMLMICRO [ CML MICROCIRCUITS ]
