GMSK Packet Data Modem
CMX909B
1.6.5
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 accuracies.
Modulator and demodulator linearity.
Receiver IF filter frequency and phase characteristics.
Use of error correction techniques.
Acceptable error rate.
As a guide, 8000 bits/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 of 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.
Figure 19 Typical System Installation
ã 2001 Consumer Microcircuits Limited
42
D/909B/1
CMLMICRO [ CML MICROCIRCUITS ]
