64 Chapter3
Making EDGE (with GSM) Measurements
Making the Phase and Frequency Error Measurement
Making the Phase and Frequency Error
Measurement
Purpose
Phase and frequency error are the measures of modulation quality for
GSM systems. Since GSM systems use relative phase to transmit
information, the phase and frequency accuracy of the GSM transmitter
are critical to the systems performance and ultimately affect range.
GSM receivers rely on the phase and frequency quality of the 0.3
GMSK signal in order to achieve the expected carrier to noise
performance. A transmitter with high phase and frequency error will
often still be able to support phone calls during a functional test.
However, it will tend to provide difficulty for mobiles trying to maintain
service at the edges of the cell, with low signal levels or under difficult
fading and Doppler conditions.
Measurement Method
The phase error of the test signal is measured by computing the
difference between the phase of the transmitted signal and the phase of
a theoretically perfect signal.
The instrument samples the transmitter output in order to capture the
actual phase trajectory. This is then demodulated and the ideal phase
trajectory is mathematically derived. Subtracting one from the other
results in an error signal.
This measurement allows you to display these errors numerically and
graphically on the analyzer display. It also allows you to view a binary
representation of the demodulated data bits.
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