3-10 Chapter 3
Making GSM Base Station Measurements
Making the Phase and Frequency Error Measurement
Making the Phase and Frequency Error
Measurement
Purpose
Phase and frequency error measures the modulation quality of GSM
systems. Since GSM systems use relative phase to transmit
information, phase and frequency accuracy of the GSM transmitter is
critical to the system’s performance and ultimately affects 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 can often still 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.
There are two ways of showing the measurement results. The I/Q Quad
View (Figure 3-7) displays the numeric results and three graphical
displays of the same data - Phase Error, Phase Error with Frequency
and RF Envelope. The I/Q Measured View (Figure 3-8) displays the
numeric results and a graphical display of the I/Q Measured Polar
Vector.
Making the Measurement
1. Press the Measure key.
2. Press the
Phase and Frequency key.
Factory default parameter settings provide a GSM compliant
measurement. For special requirements, you may need to change
default settings:
• Mode setup and frequency/channel parameters.
Use the
Mode Setup and Frequency Channel keys to change these
parameters for all measurements made within the current mode. For
further information refer to Chapter 1 of this document.
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