FX579 Datasheet, PDF (17/23 Pages) CML Microcircuits – HALF DUPLEX GMSK MODEM

Half Duplex GMSK Modem
FX579
1.6 Application Notes
Radio Channel Requirements
To achieve legal adjacent channel performance at high bit rates, a radio with an accurate carrier
frequency and an accurate modulation index is required.
For optimum channel utilization, (eg. low BER and high data rates) attention must be made to the
phase and frequency response of both the IF and baseband circuitry, and to the accuracy of the Tx/Rx
carrier frequencies.
Bit Rate, BT and Bandwidth
The maximum data rate that can be transmitted over a radio channel depends on the following:
Channel spacing
Allowable adjacent channel interference
Tx filter bandwidth
Peak carrier deviation (Modulation Index)
Tx and Rx carrier frequency accuracies
Modulator and demodulator linearity
Receive IF filter frequency and phase characteristics
Use of error correction techniques
Acceptable error-rate
As an example, for MOBITEX operation, a raw data rate of 8kbits/sec at 12.5kHz channel spacing may
be achievable - depending on local regulatory requirements, using a ± 2kHz maximum deviation, a BT
of 0.3, and no more than 1.5kHz discrepancy between Tx and Rx carrier frequencies.
Forward error correction (FEC) could then be used with interleaving to reduce the effect of burst errors.
Reducing the data rate to 4800bits/sec would allow the BT to be increased to 0.5, thus improving the
error rate performance.
For CDPD operation, a 19 200 bits/sec raw data rate at 30kHz channel spacing may be utilised with a
± 8kHz maximum deviation, a BT of 0.5, and no more than 3kHz discrepancy between Tx and Rx
carrier frequencies.
These examples should be used for guidance purposes only. Regulatory compliance of a design
should be verified.
FM Modulator, Demodulator and IF
For optimum performance, the 'eye' pattern of the received signal (when receiving random data)
applied to the FX579 should be as close as possible to the transmit 'eye' pattern examples shown in
Figure 5.
Of particular importance are general symmetry, cleanliness of the zero crossings, lack of overshoot
and, for a BT of 0.3, the relative amplitude of the inner eye opening.
To achieve this, attention must be paid to:
a)
Linearity and frequency/phase response of the Tx frequency modulator.
(Unless the transmit data is especially encoded to remove low frequency components,
the modulator frequency response should extend down to a few hertz. Hence two
point modulation is necessary for synthesised radios.)
© 1996 Consumer Microcircuits Limited
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D/579/4