WILLTEK 4400 Mobile Phone Tester Series, Руководство пользователя

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Chapter 4 EDGE Call Mode Operation
EGPRS basics
4460 GSM System Option and 4468 EDGE System Option Version 12.20
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Modulation quality 8-PSK requires modulation measurements different from GMSK. The beauty of
these measurements is that they provide parameters which are more likely to
allow you to track down the source of any potential.
The key measurement is that of the error vector magnitude or EVM for short,
which is the distance in the I/Q diagram between the measured signal and the
ideal one (see figure below); the EVM is measured separately for each symbol.
Standard measurement results to observe are the RMS-averaged EVM (for all the
symbols of a burst), the maximum EVM within the burst, and the 95% EVM. The
latter is the error vector magnitude not exceeded by 95% of the symbols within
a burst; the parameter effectively disregards the highest 5% of all error vectors.
The error vector can be broken down into two components; one component is
radial i.e. in the direction of the circle (phase error) and the other is vertical to
the circle (magnitude error). These parameters are not part of the mandatory
measurements but can be helpful in tracing a problem with the modulator.