High Speed GMSK Modem 4k to 64kbps
Page 14 of 20
MX589
5
Application
5.1
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, (e.g. low BER and high data-
rates) attention must be paid to the phase and frequency response of both the IF and baseband circuitry.
5.1.1 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
RX IF filter frequency and phase characteristics
Use of error correction techniques
Acceptable error-rate
As a guide to MOBITEX operation, a raw data-rate of 8kbps 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 & 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 4.8kbps would allow the BT to be increased to 0.5, improving the error-rate
performance.
5.1.2 FM Modulator, Demodulator and IF
For optimum performance, the eye pattern of the received signal (when receiving random data) applied to the
MX589 should be as close as possible to the Transmit eye pattern examples shown in Figure 10.
Of particular importance are general symmetry, cleanliness of the zero-crossings, and for a BT of 0.3, the
relative amplitude of the inner eye opening.
To achieve this, attention must be paid to:
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. This is because two-point modulation is necessary for synthesized
radios.
Bandwidth & phase response of the RX IF filters.
Accuracy of the Tx and Rx carrier frequencies -any difference will shift the received signal towards
one of the skirts of the IF filter response.
Ideally, the Rx demodulator should be DC coupled to the MX589 RX Signal In pin (with a DC bias added to
center the signal at the RX Feedback pin at VDD/2 [VBIAS]). However, AC coupling can be used provided that:
The 3dB cut-off frequency is 20Hz or below (i.e. a 0.1µF capacitor in series with 100kΩ).
The data does not contain long sequences of consecutive ones or zeroes.
Sufficient time is allowed after a step change at the discriminator output (resulting from channel
changing or the appearance of a RF carrier) for the voltage into the MX589 to settle before the
RXDCacq line is strobed.
2001 MX-COM, Inc.
www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054
Doc. # 204800103.011
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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