GMSK Modem Data Pump
Page 31 of 37
MX909A PRELIMINARY INFORMATION
5.5 Radio Performance
The maximum data rate that can be transmitted over a radio channel using this modem depends on:
RF channel spacing.
Allowable adjacent channel interference.
Bit rate.
Peak carrier deviation (modulation index).
Tx and Rx reference oscillator accuracy’s.
Modulator and demodulator linearity.
Receiver IF filter frequency and phase characteristics.
Use of error correction techniques.
Acceptable error rate.
As a guide, 8kbits/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 ± 2kHz peak for a repetitive ' 1100... ' pattern
and the maximum difference between transmitter and receiver 'carrier' frequencies is less than 1500Hz.
The modulation scheme employed by this modem is designed to achieve high data throughput, by exploiting
as much as possible, the RF channel bandwidth. This does, however, place constraints on the performance
of the radio. In particular, attention must be paid to:
Linearity, frequency and phase response of the Tx Frequency Modulator.
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 received signal eye pattern, using the output of the frequency discriminator, gives a good
indication of the overall transmitter/receiver performance.
Tx FREQUENCY
MODULATOR
Rx FREQUENCY
DISCRIMINATOR
SIGNAL LEVEL
ADJUSTMENT
DC LEVEL
ADJUSTMENT
SIGNAL AND
DC LEVEL
ADJUSTMENT
RXIN
RXAMPOUT
TXOUT
Rx
CIRCUITS
Tx
CIRCUITS
µC
D0-D7
A0-A1
CS
D0-D7
A0-A1
CS
RD
WR
RD
WR
MX909A GMSK MODEM
IRQ
IRQ
Figure 19: Typical System Installation
¤2001 MX-COM, Inc.
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480134.005
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