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Agilent PN 89400-14
Using Error Vector Magnitude
Measurements to Analyze and
Troubleshoot Vector-Modulated
Signals
Product Note
Error vector magnitude (EVM) measurements Vector signal analyzers such as the Agilent 89400
can provide a great deal of insight into the per- perform the time, frequency, and modulation domain
analyses that provide these insights. Because they
formance of digitally modulated signals. With
process signals in full vector (magnitude and phase)
proper use, EVM and related measurements form, they easily accommodate the complex modu-
can pinpoint exactly the type of degradations lation formats used for digital RF communications.
present in a signal and can even help identify Perhaps most importantly, these analyzers contribute
their sources. This note reviews the basics of a relatively new type of measurement called "error
EVM measurements on the Agilent Technologies vector magnitude," or EVM.
89400 vector signal analyzers, and outlines a
Primarily a measure of signal quality, EVM pro-
general procedure that may be used to methodi- vides both a simple, quantitative figure-of-merit
cally track down even the most obscure signal for a digitally modulated signal, and a far-reaching
problems. methodology for uncovering and attacking the
underlying causes of signal impairments and dis-
The diverse technologies that comprise today's dig- tortion. EVM measurements are growing rapidly
ital RF communications systems share in common in acceptance, having already been written into
one main goal: placing digital bit-streams onto RF such important system standards as GSM1, NADC 2,
carriers and then recovering them with accuracy, and PHS 3, and they are poised to appear in several
reliability, and efficiency. Achieving this goal demands upcoming standards, including those for digital
engineering time and expertise, coupled with keen video transmission. This note defines error vector
insights into RF system performance. magnitude and related measurements, discusses
how they are implemented, and explains how they
are practically applied in digital RF communica-
tions design.
1. Global System for Mobile Communications
2. North American Digital Cellular
3. Personal Handyphone System
Understanding error vector magnitude Figure 1 defines EVM and several related terms.
EVM defined As shown, EVM is the scalar distance between
Recall first the basics of vector modulation: digital the two phasor end points (the magnitude of the
bits are transferred onto an RF carrier by varying difference vector). Expressed another way, it is
the carrier's magnitude and phase such that, at the residual noise and distortion remaining after
each data clock transition, the carrier occupies any an ideal version of the signal has been stripped
one of several specific locations on the I versus Q away. By convention, EVM is reported as a percent-
plane. Each location encodes a specific data sym- age of the peak signal level, usually defined by
bol, which consists of one or more data bits. A con- the constellation's corner states. While the error
stellation diagram shows the valid locations (for vector has a phase value associated with it, this
example, the magnitude and phase relative to the angle generally turns out to be random, because it
carrier) for all permitted symbols, of which there is a function of both the error itself (which may or
must be 2n, given n data bits transmitted per sym- may not be random) and the position of the data
bol. Thus, to demodulate the incoming data, one symbol on the constellation (which, for all practi-
must accurately determine the exact magnitude and cal purposes, is random). A more useful angle is
phase of the received signal for each clock transition. measured between the actual and ideal phasors
(I-Q phase error), which will be shown later to
The layout of the constellation diagram and its contain information useful in troubleshooting
ideal symbol locations is determined generically signal problems. Likewise, I-Q magnitude error
by the modulation format chosen (BPSK, 16QAM, shows the magnitude difference between the
/4DQPSK, etc.). The trajectory taken by the signal actual and ideal signals.
from one symbol location to another is a function
of the specific system implementation, but is readily Magnitude Error (IQ error mag)
Q
calculated nonetheless.
Error Vector
At any moment in time, the signal's magnitude
and phase can be measured. These values define Measured
the actual or "measured" phasor. At the same time, Signal
a corresponding ideal or "reference" phasor can
be calculated, given knowledge of the transmitted
data stream, the symbol clock timing, baseband
filtering parameters, and so forth. The differences
between these two phasors form the basis for the
EVM measurements discussed in this note. Ideal (Reference) Signal
Phase Error (IQ error phase)
I
Figure 1. Error vector magnitude (EVM) and related
quantities
2
Making EVM measurements Step 3. Complex comparison
The sequence of steps that comprise an EVM Taking the calculated reference waveform and
measurement are illustrated in Figure 2. While the the actual incoming waveform (both now existing
Agilent 89400 Vector Signal Analyzers perform as blocks of digital samples), the two need only
these steps automatically, it is still useful to under- be subtracted to obtain the error vector values.
stand the basic process, which will aid in setting This is only slightly complicated by the fact that
up and optimizing the measurements. Practical both waveforms are complex, consisting of I and Q
steps and hints for making these measurements waveforms. Fortunately, the 89400's DSP engine
are provided in the Appendix, "Ten Steps to a has sufficient power to handle this vector subtrac-
Perfect Digital Demodulation Measurement." tion and provide the desired measurement data.
Step 1. Precision demodulation A final note
Following analog-to-digital conversion of the The analyzer's ADC samples the incoming signal
incoming signal, a DSP1 based demodulator recov- asynchronously, so it will generally not provide
ers the transmitted bitstream. This task includes actual measured data points at the exact symbol
everything from carrier and data symbol clock times. However, a special resampling algorithm,
locking to baseband filtering. The 89400's flexible applied to the incoming ADC samples, creates an
demodulator can demodulate signal formats rang- entirely new, accurate set of "virtual samples,"
ing from BPSK to 256QAM, at symbol rates from whose rate and timing are precisely in sync with
hundreds to several megahertz, yet can be config- the received symbols. (This is easily seen on the
ured for the most common signal types with a 89400 by observing the time-sample spacings of
single menu selection. an input waveform, first in standard vector mode,
and then in digital demodulation mode.)
Step 2. Regenerating the reference waveform
The recovered data bits are next used to create
the ideal reference version of the input signal. This
is again accomplished digitally with powerful DSP
calculating a waveform that is both completely
noise-free and highly accurate.
Figure 2. Block diagram of the EVM measurement process
1. Digital Signal Processing
3
EVM Troubleshooting Tree
Measurement 1
Phase vs. Mag Error
phase error >> mag error phase error mag error
Measurement 2 Measurement 3
IQ Error Phase vs.Time Constellation
waveshapes asymmetric
Residual PM I-Q Imbalance
symmetrical
noise tilted Quadrature
Phase Noise
Error
Measurement 4
EVM vs. Time
error peaks Amplitude
Non-Linearity
uniform noise
(setup problem clues)
Setup Problems
Measurement 5
Error Spectrum
discrete signals
Spurious
flat noise
sloping noise Adj. Chan.
Interference
Measurement 6
Freq Response
distorted shape
Filter Distortion
flat
SNR Problems
Figure 3. Flow chart for analyzing vector modulated signals with EVM measurements
4
Troubleshooting with error vector measurements Setup: from digital demodulation mode, select
Measurements of error vector magnitude and related
quantities can, when properly applied, provide MEAS DATA Error Vector: Time
insight into the quality of a digitally modulated sig- DATA FORMAT Data Table
nal. They can also pinpoint the causes of any prob-
lems uncovered during the testing process. This Observe: When the average phase error (in degrees) is
section proposes a general sequence for examining larger than the average magnitude error (in percent)
a signal with EVM techniques, and for interpreting by a factor of about five or more, this indicates that
the results obtained. some sort of unwanted phase modulation is the
dominant error mode. Proceed to measurement 2
Note: The following sections are not intended as to look for noise, spurs, or cross-coupling problems
step-by-step procedures, but rather as general guide- in the frequency reference, phase-locked loops, or
lines for those who are already familiar with basic other frequency-generating stages. Residual AM is
operation of the 89400. For additional information, evidenced by magnitude errors that are significantly
consult the instrument's on-screen Help facility, larger than the phase angle errors.
the appendix to this note, or the references in the
bibliography. In many cases, the magnitude and phase errors
will be roughly equal. This indicates a broad cate-
gory of other potential problems, which will be
Measurement 1
further isolated in measurements 3 through 6.
Magnitude vs. phase error
Description: Different error mechanisms will affect
Measurement tip
a signal in different ways, perhaps in magnitude
1. The error values given in the data table summary
only, phase only, or both simultaneously. Knowing
are the RMS averages of the error at each displayed
the relative amounts of each type of error can quickly
symbol point (except GSM or MSK type I, which
confirm or rule out certain types of problems. Thus,
also include the intersymbol errors).
the first diagnostic step is to resolve EVM into its
magnitude and phase error components (see Figure 1)
and compare their relative sizes.
16QAM Meas Time 1
1.5
I - Eye
-1.5
-1 Sym 1 Sym
EVM = 248.7475 m%rms 732.2379 m% pk at symbol 73
Mag Error = 166.8398 m%rms -729.4476 m% pk at symbol 73
Phase Error = 251.9865 mdeg 1.043872 deg pk at symbol 168
Freq Error = -384.55 Hz
IQ Offset = -67.543 dB SNR = 40.58 dB
0 1110011010 0110011100 0110011010 0100101001
40 0010100110 1000010101 0010010001 0110011110
80 1001101101 0110011001 1010101011 0110111010
120 1000101111 1101011001 1001011010 1000011001
Figure 4. Data table (lower display) showing roughly similar amounts
of magnitude and phase error. Phase errors much larger than magnitude
errors would indicate possible phase noise or incidental PM problems
5
Measurement 2 Examples:
IQ phase error vs. time
Description: Phase error is the instantaneous angle
difference between the measured signal and the
ideal reference signal. When viewed as a function
of time (or symbol), it shows the modulating wave-
form of any residual or interfering PM signal.
Setup: from digital demodulation mode, select
MEAS DATA IQ Error: Phase
DATA FORMAT Phase
Observe: Sinewaves or other regular waveforms
indicate an interfering signal. Uniform noise is a
sign of some form of phase noise (random jitter,
residual PM/FM, and so forth).
Figure 5. Incidental (inband) PM sinewave is clearly
Measurement tips visible--even at only 3 degrees pk-pk.
1. Be careful not to confuse IQ Phase Error with
Error Vector Phase, which is on the same menu.
2. The X-axis is scaled in symbols. To calculate
absolute time, divide by the symbol rate.
3. For more detail, expand the waveform by reduc-
ing result length or by using the X-scale markers.
4. The practical limit for waveform displays is from
dc to approximately (symbol rate)/2.
5. To precisely determine the frequency of a phase
jitter spur, create and display a user-defined
math function FFT(PHASEERROR). For best
frequency resolution in the resulting spectrum,
reduce points/symbol or increase result length.
Figure 6. Phase noise appears random in the time domain.
6
Measurement 3 Examples:
Constellation diagram
Description: This is a common graphical analysis
technique utilizing a polar plot to display a vector-
modulated signal's magnitude and phase relative
to the carrier, as a function of time or symbol. The
phasor values at the symbol clock times are partic-
ularly important, and are highlighted with a dot.
In order to accomplish this, a constellation analyzer
must know the precise carrier and symbol clock
frequencies and phases, either through an external
input (traditional constellation displays) or through
automatic locking (Agilent 89400).
Setup: from digital demodulation mode, select
Figure 7. Vector display shows signal path (including
MEAS DATA IQ Measured Time peaks) between symbols.
DATA FORMAT Polar: Constellation
(dots only)
or
Polar: Vector
(dots plus intersymbol
paths)
Observe: A perfect signal will have a uniform con-
stellation that is perfectly symmetric about the ori-
gin. I-Q imbalance is indicated when the constella-
tion is not "square," that is when the Q-axis height
does not equal the I-axis width. Quadrature error
is seen in any "tilt" to the constellation.
Measurement tips Figure 8. Constellation display shows symbol points only,
1. Result length (number of symbols) determines revealing problems such as compression (shown here).
how many dots will appear on the constellation.
Increase it to populate the constellation states 3. To view the spreading of symbol dots more
more completely. closely, move the marker to any desired state,
2. Points/symbol determines how much detail is press mkr