DB929B Datasheet, PDF (42/51 Pages) CML Microcircuits – Full Data Packet Framing

4-Level FSK Modem Data Pump
Page 42 of 50
MX929B PRELIMINARY INFORMATION
5.5 Radio Performance
The maximum data rate that can be transmitted over a radio channel using this modem depends on:
x RF channel spacing.
x Allowable adjacent channel interference.
x Symbol rate.
x Peak carrier deviation (modulation index).
x Tx and Rx reference oscillator accuracy.
x Modulator and demodulator linearity.
x Receiver IF filter frequency and phase characteristics.
x Use of error correction techniques.
x Acceptable error rate.
As a guide, 4800 symbols/sec (9600bps) 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. However, this does place constraints on the
performance of the radio. Particular attention must be paid to:
x Linearity, frequency, and phase response of the Tx Frequency Modulator. For a 4800 symbols/sec
(9600bps) system, the frequency response should be within ±2dB over the range 3Hz to 5kHz, relative to
2400Hz.
x The bandwidth and phase response of the receiver's IF filters.
x 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 equalized received signal eye diagram, using the Mode Register RXEYE function, provides a good
indication of the overall RF transmitter/receiver performance.
Rx FREQUENCY
DISCRIMINATOR
DC LEVEL
ADJUSTMENT
µC
D0 - D7
A0 - A1
CS
RD
WR
IRQ
SIGNAL LEVEL
ADJUSTMENT
RXIN
D0 - D7
A0 - A1
CS
RD
WR
IRQ
RXAMPOUT
Rx
CIRCUITS
Tx
CIRCUITS
MX929A MODEM
Tx FREQUENCY
MODULATOR
SIGNAL AND
DC LEVEL
ADJUSTMENT
TXOUT
Figure 26: Typical Connections between Radio and MX929B
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