FX929A Datasheet, PDF (38/44 Pages) CML Microcircuits – 4-Level FSK Modem Data Pump

4-Level FSK Modem Data Pump
FX929A
1.6.5 Radio Performance
The maximum data rate that can be transmitted over a radio channel using these modems depends on:
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RF channel spacing.
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Allowable adjacent channel interference.
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Symbol rate.
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Peak carrier deviation (modulation index).
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Tx and Rx reference oscillator accuracies.
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Modulator and demodulator linearity.
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Receiver IF filter frequency and phase characteristics.
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Use of error correction techniques.
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Acceptable error rate.
As a guide, 4800 symbols/sec can be achieved - subject to local regulatory requirements - over a system with
12.5kHz channel spacing if the transmitter frequency deviation is set to ±2.5kHz peak for a repetitive '+3 +3 -3
-3 ....' pattern and the maximum difference between transmitter and receiver 'carrier' frequencies is less than
2400Hz.
The modulation scheme employed by these modems is designed to achieve high data throughput by exploiting
as much as possible of the RF channel bandwidth. This does, however, place constraints on the performance
of the radio. In particular, attention must be paid to:
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Linearity, frequency and phase response of the Tx Frequency Modulator. For a 4800
symbols/sec system, the frequency response should be within ±2dB over the range 3Hz to
5kHz, relative to 2400Hz.
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The bandwidth and phase response of the receiver's IF filters.
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Accuracy of the Tx and Rx reference oscillators, as any difference will shift the received signal
towards the skirts of the IF filter response and cause a dc offset at the discriminator output.
Viewing the received signal eye - using the Mode Register RXEYE function - gives a good indication of the
overall transmitter/receiver performance.
Figure 20 Typical Connections Between Radio and FX929A
© 1996 Consumer Microcircuits Limited
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D/929A/4