Half Duplex GMSK Modem

FX579

© 1996 Consumer Microcircuits Limited

17

D/579/4

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.)