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Agilent
8 Hints for Making and Interpreting
EVM Measurements
Application Note
Introduction Defining EVM
Error vector magnitude (EVM) The error vector is the vector dif-
measurements can provide a great ference at a given time between
deal of insight into the perfor- the ideal reference signal and
mance of digital communications the measured signal. Expressed
transmitters and receivers. With another way, it is the residual
proper use, EVM and related mea- noise and distortion remaining
surements can pinpoint exactly after an ideal version of the signal
the type of degradations present has been stripped away. EVM is
in a signal and can even help the root-mean-square (RMS) value
identify their sources. of the error vector over time at
the instants of the symbol (or
Primarily a measure of signal chip) clock transitions.
quality, EVM provides both a sim-
ple, quantitative figure-of-merit
for a digitally modulated signal,
and a far-reaching methodology
for uncovering and attacking
the underlying causes of signal
impairments and distortion. EVM
measurements are growing rapid-
ly in acceptance, being already
the required modulation quality
measurement in such important
technology standards as 3GPP
W-CDMA and IEEE 802.11a/b/g
WLAN, and they are poised to
appear in several upcoming
standards.
This application note provides
useful tips that will assist in accu-
rately making and understanding
EVM measurements.
Depending on the technology, EVM The magnitude of the error vec- The spectrum of the error vec-
is reported as a percentage of the tor versus time measurement tor (or error vector spectrum)
square root of the mean power of shows the error vector magni- is the frequency spectrum of
the ideal signal, as a percentage of tude variations as a signal the error vector time.
the square root of the average sym- changes over time--that is, at
bol power, or as a percentage of and between symbol decision The EVM troubleshooting tree
the peak signal level, usually timing points. shown in Figure 2 is a useful
defined by the constellation's cor- tool for analyzing vector modu-
ner states. The EVM value can lated signals with EVM mea-
also be reported in units of dB and surements.
some wireless networking stan-
dards use the term
"relative constellation error"
(RCE) instead of EVM. EVM troubleshooting tree
While the error vector has a phase Measurement 1
value associated with it, this angle phase vs. mag error
generally turns out to be random,
because it is a function of both the
error itself (which may or may not phase error >> mag error phase error mag error
be random) and the position of the
data symbol on the constellation Measurement 2 Measurement 3
(which, for all practical purposes, IQ error phase vs.time constellation
is random). A more useful angle is waveshapes asymmetric
Residual PM I-Q imbalance
symmetrical
measured between the actual and
ideal phasors (I-Q error phase or
phase error), which contains infor- noise tilted Quadrature
Phase noise
mation useful in troubleshooting error
signal problems. Likewise, I-Q Measurement 4
error magnitude, or magnitude EVM vs. time
error, shows the magnitude differ- error peaks Amplitude
uniform noise
ence between the actual and ideal non-linearity
signals.
(setup problem clues)
Setup problems
Magnitude error (IQ error mag) Measurement 5
Q
error spectrum
Error vector discrete signals
Spurious
flat noise
l
na
sig
d
ure
sloping noise Adj. chan.
as
nal
Me
sig
interference
ce)
ren Measurement 6
refe
al ( freq response
Ide
distorted shape
Phase error (IQ error phase) Filter distortion
I
Figure 1. Error vector magnitude (EVM) and related
quantities flat
SNR problems
Figure 2. Flow chart for analyzing vector modulated signals with EVM measurements
2
Hint #1
EVM and the various related EVM and the various related dis-
plays, I/Q constellation, I/Q polar
measurement displays are or vector diagram, magnitude of
sensitive to any signal flaw the error vector versus time, the
spectrum of the error vector (error
that affects the magnitude vector spectrum), I/Q error phase
and phase trajectory of a versus time, and I/Q error magni-
signal for any digital tude versus time, etc are sensitive
to any signal flaw that affects the
modulation format. magnitude and phase trajectory of
a signal for any digital modulation
format. Large error vectors, both at
the symbol points and at the tran-
sitions between symbols, can be
caused by problems at the base-
band, IF or RF sections of the
transmitter. Different modulation
quality displays and tools can help
reveal or troubleshoot various
problems in the transmitter. For
instance, the I/Q constellation can
be used to easily identify I/Q gain
imbalance errors. Small symbol
rate errors can be easily identified
by looking at the magnitude of the
error vector versus time display.
The error vector spectrum can
help locate in-channel spurious.
3
Hint #2
Measurements of EVM to quantify the errors in digital
demodulation can provide powerful insight into the
performance of a digital radio receiver.
BER is the preferred measure- In addition, the steep slope of 10◦ Jabse Service Manual Search 2026 ◦ Jabse Pravopis ◦ onTap.bg ◦ Other service manual resources online : Fixya ◦ eServiceinfo