Data Sheet
4-Level FSK Modem Data Pump Page 39 of 47 MX919B PRELIMINARY INFORMATION
©2001 MX•COM, INC. www.mxcom.com Tele: 800 638 5577 336 744 5050 Fax: 336 744 5054 Doc. # 20480170.003
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All trademarks and service marks are held by their respective companies.
5.5 Radio Performance
The maximum data rate that can be transmitted over a radio channel using these modems depends on:
• RF channel spacing.
• Allowable adjacent channel interference.
• Symbol rate.
• Peak carrier deviation (modulation index).
• Tx and Rx reference oscillator accuracy.
• Modulator and demodulator linearity.
• Receiver IF filter frequency and phase characteristics.
• Use of error correction techniques.
• 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:
• 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.
• The bandwidth and phase response of the receiver's IF filters.
• 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
CIRCUITS
Tx
CIRCUITS
D0 - D7
A0 - A1
CS
RD
WR
IRQ
D0 - D7
A0 - A1
CS
RD
WR
IRQ
TXOUT
SIGNAL AND
DC LEVEL
ADJUSTMENT
SIGNAL LEVEL
ADJUSTMENT
RXIN
RXAMPOUT
Rx FREQUENCY
DISCRIMINATOR
Tx FREQUENCY
MODULATOR
DC LEVEL
ADJUSTMENT
MX919B MODEM
µC
µC
Figure 26: Typical Connections between Radio and MX919B