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Agilent PN 89400-8
Using Vector Modulation Analysis in the
Integration, Troubleshooting, and Design of
Digital RF Communications Systems
Product Note
Introduction
The Agilent Technologies 89400 Series vector signal analyzers (VSAs) with
vector modulation analysis (Option AYA) provide the numerical and visual
tools to help quickly identify and quantify impairments to digitally modulated
signals, whether using standard or several nonstandard modulation formats.
Measurements are possible on continuous or burst carriers (such as TDMA)
at baseband, IF, and RF locations throughout a system block diagram. There
is no need for external filtering, coherent carrier signals, or symbol clock tint-
ing signals. The 89400 Series VSAs with Option AYA have built-in Gaussian,
raised-cosine, root-raised-cosine, and user-definable filters (with adjustable
alpha or BT) and lock to the carrier and a defined symbol rate.
Although it is useful to measure the signal being transmitted, imagine the ben-
efit of being able to detect, quantify, and locate the errors in the transmitted
signal when compared to an ideal reference signal. Common vector modulation
analysis tools such as eye and constellation displays are supported as well as
new analysis tools including the ability to compare measured signals to ideal
signals. Measurements update as fast as two times per second, allowing the
effects of changes to an active system to be quickly analyzed. Advanced spec-
trum analysis rounds out the 89400 Series VSAs' measurement contributions
(additional information is in the reference list). By reducing the amount of
external equipment or the need for developing custom solutions, accuracy is
preserved and system development time can be simplified and minimized.
Basic vector modulation and measurement concepts are presented, followed by
example measurements and setups. A transmitter system is used as the foun-
dation for the measuements; however, any system with I and Q signals can use
the principles and tools presented. These applications are intended to serve
as examples of the measurement power and ease of use of the analyzer.
Contents
1 Introduction
2 Background
3 Modulation concepts
4 Measurement concepts
8 Problems that can occur during
transmission, integration, and design
9 Identifying problems: Using
measurements and display tools
15 Setting up the measurements
20 Generating digitally modulated signals
20 Creating the arbitrary source
21 Measuring a signal (using the
arbitrary source)
21 Other tools
22 Summary
23 Configuration guide
24 Glossary
27 References
28 Index
Background privacy, there has been increased need signal with a code sequence that
Modulation is a method of carrying for more efficient and smarter use of results in spread spectrum or code-
information from a transmitter to one the already crowded RF frequency division multiple access (CDMA) sig-
or more receivers. Communication spectrum. Digital modulation has nals. CDMA signals simultaneously
systems use modulation to superim- been used for many years, however, occupy the same spectrum and affect
pose or "piggyback" low-frequency now it is being used more widely to each other like broadband noise.
voice or data signals onto an RF wave address the aforementioned needs in
(high-frequency carrier) which can be a variety of applications. Digital (also Figure 1 shows the basic block dia-
transmitted long distances. The infor- called complex or IQ) modulation gram of a digital RF communications
mation (voice or data) is used to uses a combination of amplitude and system, which could be the core of
modulate the carrier, usually by alter- phase modulation. personal communications systems,
ing its phase, frequency, amplitude, cordless telephones, digital special
or some combination of these. The Many digital modulation communica- services systems, cellular telephones,
receiver extracts or decodes the mod- tions schemes make better use of the pagers, wireless LANs, private trunked
ulation from the incoming signal to available spectrum by allowing multi- mobile systems, satellite communica-
recover the desired information. ple users per carrier frequency (i.e., to tions services, global positioning, digi-
access the same portion of spectrum) tal audio broadcast, fleet dispatching
Due to the propagation characteristics and take advantage of compression networks, digital video, and radar
of electromagnetic waves with different gains in digital signal processing (DSP) systems. Developing, testing, integrat-
frequencies, certain portions of the portions of the systems. Time-division ing, and troubleshooting these new
spectrum are more desirable for some multiple access (TDMA) is one method systems requires flexible testing
applications. With the applications of spectrum sharing. It uses burst capabilities to locate the probable
explosion in consumer communica- carriers which only transmit for short causes of any signal degradation.
tions, a desire for direct and reliable periods and are off while other users In many cases, modulation formats
transmission of digital information, occupy the channel. A second method that are not standardized are used.
and a drive for improved quality and of sharing is to combine the desired
2
Modulation concepts the I-Q plane, we can then transmit impairments and noise that cause a
In most digital radio systems, the encoded information. Each position spreading of the states (a dispersal
frequency of the carrier is fixed so or state (or transitions between the of dots around each state).
only phase and magnitude need to be states in some systems) represents a
considered. The phase and magnitude certain bit pattern that can be decoded An example modulation format is 16
can be represented in polar or vector at the receiver. The mapping of the QAM (16-state quadrature amplitude
coordinates as a discrete point in states at each symbol timing instant modulation). This format takes four
the I-Q plane (Figure 2). I represents (when the receiver interprets the sig- bits of serial data and encodes them
in-phase (phase reference) and Q nal) on the I-Q plane is referred to as as single amplitude/phase states, or
represents quadrature (90