coverstory By Dan Strassberg, Senior Technical Editor
Look
before
you leap
Taking the specsmanship
out of spectrum analysis
At a glance ..........................44
RF spectrum analyzers’
1-dB gain-compression
point......................................44
For more information ......46
Photo courtesy Bryan Leister
42 edn | October 17, 2002
www.edn.com
AMONG THE BIGGEST CHALLENGES RF AND MICROWAVE
SPECTRUM ANALYZERS PRESENT TO USERS IS FIGURING OUT
THE INSTRUMENTS’ MEASUREMENT ACCURACY.
R
F and microwave spectrum analyzers aren’t even conceptually simple. Calling them calibrated superheterodyne re-
ceivers merely hints at what they do and how they do it. Calling them
frequency-domain oscilloscopes reveals even less. Moreover, if you
take only a superficial view of the technology, you can incorrectly
conclude that the instruments haven’t changed much in the last
decade. In fact, however, in this increasingly wireless world, spectrum analyzers that display signals to 3 GHz and more—often to 7
GHz, and sometimes to more than 20
GHz—are changing significantly and
growing dramatically in importance.
What’s more, selecting the most appropriate analyzer for a task can present
major challenges, especially when your
managers don’t understand why the selection process should involve more than
just comparing prices and some numbers
from a couple of vendor data sheets. Few
engineers and even fewer managers understand that spectrum-analyzer specifications often conceal more than they reveal and that a data sheet’s most important information can be what the instrument manufacturer doesn’t say. For example, some manufacturers merely provide typical values of parameters that
their competitors supply as worst-case.
In addition, test conditions can profoundly affect specifications that some
vendors would like you to think are independent of those conditions. The sidebar,
“ RF spectrum analyzers’ 1-dB gain-comwww.edn.com
pression point,”discusses one such arcane
specification. Gain compression quantifies the effect of an interfering signal
whose amplitude and frequency you don’t
want to measure on the measured amplitude of a signal you are investigating.
Most suppliers understand that if
you’ve never used a spectrum analyzer
from the same model family as the one
you are thinking of buying, your evaluation of the product will take at least two
weeks, and is likely to take as long as a
month. Ideally, you would have in your
lab, side by side, for the entire month,
demonstration units of your two top
product candidates. You could then compare them under conditions that are not
only identical, but also pertinent to your
application. In practice, you sometimes
can obtain the necessary units for the
same lengthy period and conduct such an
evaluation. A facet of modern spectrumanalyzer design that simplifies evaluation
and use of the instruments—especially
October 17, 2002 | edn 43
coverstory RF spectrum analyzers
AT A GLANCE
for first-time users—is the inclusion of
automatic setups for performing tests defined by standards bodies for various
wireless-communication protocols (Reference 1).
컄 RF and microwave spectrum analyzers
are growing in importance to EEs who work
on a variety of projects—not just wireless
communications.
SERVICE COUNTS
Often, you will discover that evaluating and using the instrument requires accessories, such as splitters, directional
couplers, and the like. If you don’t have
the necessary items, and an accessory
manufacturer or distributor can’t deliver them quickly enough, an instrument
vendors’ field engineer in your area may
be able to lend them to you.
You might think that this sort of customer service is so costly, it would have
ceased to exist decades ago, or would be
available only to the largest companies.
However, spectrum-analyzer vendors say
that good support is good business, and
컄 Increased automation and the use of
new technologies, especially high-speed
ADCs and DSPs, are making the instruments more accurate and easier to use.
컄 For those who are unfamiliar with spectrum analyzers, making a wise choice of instruments is an engineering project in itself.
There is no substitute for extensive side-byside comparisons in the lab.
they have never stopped providing it.
Manufacturers include support costs in
the instruments’ price structure. Because
of the high probability that a company
will buy more than one analyzer but will
need support for only its first unit, the
manufacturer has multiple opportunities
to recoup the support costs or to offer attractive discounts to repeat buyers. Although portable units often cost less than
$10,000, and some handheld units cost
substantially less, high-performance RF
spectrum analyzers (that is, wideband,
low-noise instruments) tend to be fairly
expensive; many benchtop units cost
more than $30,000.
The difficulty of evaluating a unit
works to the selected vendor’s advantage.
The complexity of the product combined
with the likelihood of repeat sales creates
the potential for benefits to both the customer and the vendor. After investing so
much time in evaluating the product,
finding out how to take advantage of its
unique capabilities, and learning how to
deal with its eccentricities and quirks, a
customer who has received satisfactory
RF SPECTRUM ANALYZERS’ 1-dB GAIN-COMPRESSION POINT
By Joop Klaassen, Agilent Technologies
Gain compression is a known
phenomenon of four varieties—
single-tone, SSB (single-sideband), FM-sideband, and AMsideband—associated with mixers
and amplifiers.
SSB gain compression can exist
in an RF spectrum analyzer in
which a large interfering signal
outside the displayed frequency
range compresses a small test signal on the display. These signals
TABLE A—COMPRESSION MEASURED ON AGILENT
E4440A PRECISION SPECTRUM ANALYZER
Input to
spectrum
analyzer
(dBm)
ⳮ20
ⳮ10
ⳮ5
0
1
2
3
4
5
6
7
8
8.28
9
10
Interfering
signal
(dBm)
ⳮ18.04
ⳮ7.92
ⳮ2.92
2.08
3.08
4.1
5.1
6.1
7.1
8.1
9.1
10.1
10.38
11.1
12.1
Test
signal
(dBm)1
ⳮ25
ⳮ25
ⳮ25
ⳮ25
ⳮ25
ⳮ25
ⳮ25
ⳮ25
ⳮ25
ⳮ25
ⳮ25
ⳮ25
ⳮ25
ⳮ25
ⳮ25
Compression
(dB)
0
0
ⳮ0.04
ⳮ0.11
ⳮ0.12
ⳮ0.14
ⳮ0.16
ⳮ0.2
ⳮ0.26
0.36
ⳮ0.54
ⳮ0.87
ⳮ1
ⳮ1.49
ⳮ2.79
Compression
point
First-IF overload
First-IF overload
First-IF overload
First-IF overload
First-IF overload
1. To obtain a 25-dBm test signal at the combining network output,
you must apply a 8.64-dBm signal to the network's input.
2. Test-signal frequency=1000 MHz.
3. Interfering signal frequency=997 MHz.
4. Span=100 kHz; RBW=10 kHz.
44 edn | October 17, 2002
might be so small that
they are at or near the
analyzer’s noise floor, and
you need to lift them out of
the noise by setting the analyzer’s input attenuator to 0 dB.
Analyzer users need to
understand their instrument’s
behavior, so that they know
when to expect compression.
They also need to know which circuits the large signal affects and
how to interpret the warning indications on the analyzer screen. A
basic block diagram of Agilent
Technologies’ E4440A precision
spectrum analyzer shows the two
detectors that warn you when the
analyzer is overloaded and not
showing the proper signal (FFigure
A). To understand these warnings,
you must understand the compression measurement.
The measurement of the 1-dB
SSB compression point uses the
test signal and the interfering signal. You combine the two signals,
which you generate with two signal sources, using an RF directional coupler, to prevent them from
interfering with each other.
Because the shape of the com-
bined signal resembles that of an
SSB signal, the measurement
technique is called the SSB
method. The test signal is a
25-dBm carrier. The interfering-signal power level
varies from 20 to
10 dBm. You must calibrate
both signals over the entire range
using a power meter.
SEPARATING THE SIGNALS
The analyzer’s final IF (intermediate-frequency) filters determine
the amount by which you must
separate the signal frequencies to
enable the analyzer to produce
proper results. If the frequencies
are too close, the final IF stage
might already be overloaded, and
the signal presented to the analyzer’s display will be distorted.
A rule of thumb is to separate
the two carriers by at least 15
times the RBW (resolution bandwidth), so that the strong signal
does not influence the final IF
path. Thus, when using a 10-kHz
RBW, the spacing should exceed
150 kHz. Many analyzers have no
overload detectors, or they have
only one. In those cases, you
www.edn.com
service will probably buy many more of
the selected manufacturer’s products. A
change of vendors would require another large, time-consuming investment in
learning the fine points of a different
complex instrument.
MIXING
The concept that underlies the operation of spectrum analyzers is known as
mixing, heterodyning, or frequency conversion. Fundamentally, mixing is multiplication, an inherently nonlinear process. Spectrum analyzers mix the incoming
signals (whose frequency is fIN—shorthand for any frequency in a range of fIN1
to fIN2) with a signal from a variable-frequency LO (local oscillator) at frequency
fLO to produce a signal at an IF (intermediate frequency), fIF. Usually, fLOfINfIF;
that is, the frequency of the LO is higher
than that of the input signals, although
must observe the test signal to
ensure that it follows the compression curve and does not
deteriorate.
You start the test by measuring
both sources’ output with a
power meter. The test-signal frequency is F and the interferingsignal frequency is F-offset. You
then disconnect the power meter,
apply the signals to the analyzer,
and set the analyzer to the testsignal frequency with a 100-kHz
span, 10-kHz RBW, and 0-dB
attenuation. While the test signal
remains at 25 dBm, you raise
the interfering signal from 20
dBm to 5 dBm in 5-dB steps,
then to 10 dBm in 1-dB steps.
Each time, you verify that the test
signal is at its original 25-dBm
value and record the change in
decibels until you find a difference of 1 dB. You then select a
new RF frequency. For this test,
the frequencies were 50, 200,
1000, 1400, and 2500 MHz.
Table A and Figure B show the
measurements on an E4440A
analyzer at 1 GHz. At that frequency, the 1-dB difference
occurred at a spectrum-analyzerinput level of 8.28 dBm.
Combining-network losses
explain why the spectrum-analyzwww.edn.com
designs in which fLOfINfIF are also possible. A classical spectrum analyzer
sweeps fLO, so that, over a short time span,
the mixer output represents the input in
a band of frequencies from fIN1 to fIN2. The
analyzer then detects the mixer output’s
envelope (removes the output’s fIF component) and displays the envelope as a
function of time to create a picture of the
input-signal amplitude as a function of
frequency.
Much of spectrum analyzers’ architectural complexity results from an unavoidable aspect of mixing: The mixer
output at fIF represents not only the input at fLOfIN, but also the input at
fLOfIN. If you were to attempt to convert
in one step a band of frequencies from,
say, 30 MHz to 3 GHz to the very common IF of 10.7 MHz, you would probably have the LO sweep from 40.7 to 3010.7
MHz. (You could also have the LO sweep
Figure A
Even where their extreme portability is
nonessential, handheld spectrum analyzers
are becoming increasingly popular. By the
time you read this article, Rohde & Schwarz’s
100-kHz to 3-GHz FSH3 should be available
from Tektronix in North America at a US price
of less than $7000. When the unit debuted in
Europe a few months ago, its price was less
than €7000.
TEST
SIGNAL
TERMINATOR
POWER
METER
INTERFERING
SIGNAL
FINAL IF
OVERLOAD
FIRST IF
OVERLOAD
FFT
LOCAL OSCILLATOR
A spectrum analyzer can overload at several circuit points. Although not all analyzers do so, those that provide overload indications at several points make it easiest to notice conditions that cause measurement errors.
er-input values (left-hand column) are smaller than the corresponding interfering-signal levels
(second column).
0
0.5
1
COMPRESSION
OF TEST SIGNAL
(dB)
1.5
Author’s biography
Joop Klaassen is a technical-sup2
port engineer in Agilent Technologies’ signal-analysis division (San2.5
ta Rosa, CA). He earned a BSEE
3
from the Technische Hogeschool
20
5
1
3
5
7
9
(Arnhem, Netherlands) and for
INTERFERENCE INPUT LEVEL (dBm)
more than 15 years has specialized
As the level of the interfering signal increases, gain
in spectrum-analyzer measFigure B
compression increases monotonically and reaches 1 dB
urements for Agilent in the
Netherlands and the United States. with a spectrum-analyzer input of 8.28 dBm at 1 GHz.
October 17, 2002 | edn 45
coverstory RF spectrum analyzers
from 19.3 to 2989.3 MHz.) Were you to
choose the first alternative, the sigFigure 1
nal at the mixer output would represent not only the input over the desired
frequency band, but also, simultaneously, the input over the frequency band
from 51.4 to 3021.4 MHz. At any instant,
the mixer output would represent the
sum of the inputs at a pair of frequencies
separated by 21.4 MHz (fIF•2)—the desired frequency plus an undesired (in this
case, higher) frequency, called the image.
To further complicate the discussion, National Instruments’ PXI-5660 is a 2.7-GHzthe mixer output never represents the in- bandwidth signal analyzer embodied within
put at just one (or just two) frequencies; two 3U-high PXI modules—a triple-width freobtaining such an output would imply an quency converter and a single-width digitizer.
IF bandwidth of zero. The IF bandwidth, The display at the top center is a waterfall
or the analyzer’s RBW (resolution band- diagram, which depicts how a spectrum varies
width), is always greater than zero. The as a function of time. The time dimension is
minimum RBW that you can select is a normal to the plane of the screen.
figure of merit for a spectrum analyzer.
However, if you could select an RBW of of the highest to the lowest fLO. In this
zero, all swept-frequency spectrum case, the ratio is reduced from more than
analyses would require infinite time, be- six octaves (40.7 to 3010.7 MHz) to less
cause a zero-bandwidth bandpass filter than one (3.43 to 6.4 GHz).
needs an infinite amount of time to reTURNING FREQUENCY INTO TIME
spond to a change in its input.
At the input to the swept-frequency
A solution to the image problem involves multiple frequency conversions. It analyzer, signals at frequencies throughis common for spectrum analyzers to out the instrument’s input range can sihave three IF stages. An analyzer that cov- multaneously exist. At the mixer output,
ers 30 MHz to 3 GHz might first convert however, the range of frequencies is draits inputs to frequencies greater than 3 matically reduced, because instead of exGHz. This approach places all of the un- isting simultaneously, the signals—transdesired image frequencies above the an- lated to frequencies close to fIF—exist
alyzer’s input-frequency range, so that a sequentially in time, as the sweep tunes
fixed-cutoff-frequency lowpass filter can the instrument across the frequency
remove them. If the first IF were at, say, band of interest. Thus, after the first mix3.4 GHz, the LO would sweep from 3.43 er, the swept-frequency analyzer doesn’t
to 6.4 GHz, and the image band would need very wide bandwidth, which greatcover 6.83 to 9.8 GHz. Another benefit of ly simplifies the design of most of the inthis approach is that it reduces the ratio strument’s circuits. On the other hand,
modern communication signals often
exist for only brief instants and recur at
low duty cycles, requiring instruments
that can more or less continuously
examine wide frequency bands.
Such instruments exist. Manufacturers call them by a variety of names, including signal analyzer, vector-signal analyzer, and wireless-communications
analyzer. All instruments in this class
make extensive use of DSP technology—
but so do a growing number of spectrum
analyzers. Generally, though, DSP-based
spectrum analyzers and signal analyzers
differ considerably in specifications and
intended applications. The signal analyzers more quickly acquire data, and
they store long digitized records of timedomain data, handle vector quantities
(phase as well as magnitude), and perform complex analyses of signals that
have been digitally modulated in accordance with such formats as 64-QAM (64level quadrature-amplitude modulation). The spectrum analyzers are usually
more compact and less expensive than
the signal analyzers but offer significantly greater dynamic range.
Most DSP-based spectrum analyzers’
block diagrams at least superficially resemble the block diagrams of spectrum
analyzers that use classical analog signal
processing. Like their classically architected brethren, the DSP-based units
make extensive use of analog IFs. However, following the last mixer, you find not
analog filtering but a high-speed, highresolution ADC and a DSP that performs
digital filtering. The benefits are improved selectivity (narrower RBW) with
fewer compromises in sweep speed as you
reduce the RBW. Agilent, however, re-
FOR MORE INFORMATION...
For more information on RF and microwave spectrum analyzers and related instruments, go to www.edn.com/info and enter the reader service number. When you contact
any of the following manufacturers directly, please let them know you read about their products in EDN.
Advantest
1-732-346-2600
www.advantest.com
Enter No. 331
Anritsu Co
1-800-267-4878
www.anritsu.com
Enter No. 334
AeroComm
1-800-492-2320
www.aerocomm.com
Enter No. 332
HC Protek
1-201-760-9898
www.protektest.com
Enter No. 335
Agilent Technologies
1-800-452-4844
www.agilent.com
Enter No. 333
IFR Systems
1-800-835-2352
www.ifrsys.com
Enter No. 336
46 edn | October 17, 2002
Information Security
Associates Inc
1-203-329-8387
www.ISA-TSCM.com
Enter No. 337
JMS Consulting
1-503-292-7035
www.pcez.com/~jmsc/
Enter No. 338
Morrow Technologies Corp
1-727-531-3531
www.vigilcom.info
Enter No. 339
National Instruments
1-800-258-7022
www.ni.com
Enter No. 340
Science Workshop
1-516-731-7628
www.science-workshop.com
Enter No. 341
Tegam Inc
1-440-466-6100
www.tegam.com
Enter No. 342
Tektronix Inc
1-800-426-2200
www.tektronix.com
Enter No. 343
SUPER INFO NUMBER
For more information on
the products available
from all of the vendors listed in this box, enter no.
344 at www.edn.com/info.
www.edn.com
coverstory RF spectrum analyzers
cently announced (albeit with
almost no fanfare) vector-modulation-analysis capabilities for its
PSA series of DSP-based spectrum analyzers. These capabilities are a significant departure for spectrum analyzers,
which have heretofore made only
scalar (magnitude) measurements.
The day may not be too far off
when the familiar architecture undergoes significant changes. Instead of using
analog mixing, the analyzers may employ
the ADC as part of the mixer. ADCs that
take 64 million 14-bit samples per second
already exist. If you could precede such
an ADC with an appropriate T/H (trackand-hold) amplifier, you could intentionally undersample, with the requisite
accuracy, communications signals at frequencies well above the ADC’s 32-MHz
Nyquist frequency. Suppose that you
could sample a modulated 200-MHz carrier in this manner. If the sidebands did
not extend past half the sample rate on
either side of the carrier frequency, and
no other signals could be aliased into the
baseband, you would obtain an accurate
digitized replica of the modulation.
Of course, most new digital-communications systems operate at carrier frequencies of 2.35 to 5.8 GHz, so a signal
analyzer containing a T/H amplifier that
worked with modulated 200-MHz carriers would still have to incorporate mixing. Today, however, you can’t buy a
greater-than-200-MHz-bandwidth T/H
amplifier whose accuracy is commensurate with that of a 14-bit-resolution
ADC. Still, such devices, though not yet
in production, are allegedly now feasible.
When and if they reach the market, they
will enable architectural changes that will
eliminate one or more stages of frequency conversion from signal analyzers and
DSP-based spectrum analyzers.
SIGNAL ANALYSIS GOES MODULAR
A modular, 2.7-GHz signal analyzer
recently announced by National Instruments breaks many of the rules for instruments of this type. The PXI-5660
(Figure 1) is not a benchtop instrument
but a pair of 3U-height PXI modules—a
three-slot frequency-converter and a single-slot, 14-bit, 64M-sample/sec digitizer with 16M words (32 Mbytes) of digitizer memory (expandable to 32M words
for $2000 more). SFDR (spurious-free
dynamic range) is 80 dB, but you can
trade off measurement speed to achieve
48 edn | October 17, 2002
greater dynamic range. National Instruments claims that the measurement
speed that yields 80 dB of SFDR is 200
times as great as that of instrumentlevel signal analyzers that offer no
better SFDR.
The two modules together
are much smaller than instrument-level products that perform similar functions, but the modules don’t constitute a
complete analyzer. They plug into a PXI
card cage, which accommodates the necessary CPU module, includes the necessary power supply, and can accommodate
additional modules. The $12,995 base
price includes the two modules, a spectralmeasurement-tool-set software package,
modulation-analysis data, and several
drivers. Although the price is much lower
than that of instrument-level signal analyzers, the modular products also require
the purchase of the card cage, the CPU, a
keyboard, a pointing device, and a display.
The manufacturer also recommends that
you have either its LabView or LabWindows CVI development environments,
because you will probably want to develop your own custom applications.
The earlier discussion of the trade-off
between a swept-frequency spectrum analyzer’s RBW and sweep speed should remind buyers and users of spectrum analyzers and related instruments of an
important characteristic of these products’ design: Without implementing architectural changes, it is generally impossible for the instrument designers to
improve one characteristic or specification without adversely affecting another. Fortunately, the growing need for the
instruments—not just in the development, deployment, and maintenance of
wireless-communications systems, but
for purposes such as measurement and
control of EMI—is bringing about such
architectural changes.
However, spectrum-analyzer manufacturers point out that one kind of dramatic improvement requires no changes
to the instruments’ design. By carefully
analyzing their applications, users can often modify their test protocols to reduce
test times without sacrificing measurement accuracy. One manufacturer estimated that such improvements could
satisfy almost half of the requests he received for faster instrumentation. This
manufacturer suggested that, before they
contemplate the purchase of additional
instruments, companies that use spec-
trum analyzers in production test should
call in the supplier’s applications staff to
investigate the possibility of such improvements. Although manufacturers always hope to sell more instruments, they
view helping their customers to make the
most efficient use of their existing instruments as an important tool for maintaining customer loyalty.왏
References
1. Strassberg, Dan,“Advanced wireless
technology revamps spectrum analysis,”
EDN, March 1, 2001, pg 50.
2. Rauscher, Christoph, V Janssen, and
R Minihold, Fundamentals of Spectrum
Analysis, Rohde & Schwarz GmbH, 2001,
Available in North America from Tektronix.
3. Spectrum-analysis basics, Agilent
Technologies, AN 150. (To download any
listed Agilent application note, go to
www.agilent.com and enter the AN number in the search engine. Be sure to include a space between “AN” and the
number.)
4. Eight hints for making better spectrum-analyzer measurements, Agilent,
AN 1286-1.
5. Spectrum-analyzer measurements
and noise (Measuring noise and noise-like
digital-communications signals with a
spectrum analyzer), Agilent, AN 1303.
6. Optimizing RF and microwave spectrum-analyzer dynamic range, Agilent,
AN 1315.
7. Optimizing spectrum-analyzer amplitude accuracy, Agilent, AN 1316.
8. Optimizing spectrum-analyzer measurement speed, Agilent, AN 1318.
9. Making precompliance conducted
and radiated emissions measurements
with EMC analyzers, Agilent, AN 1328.
10. Barlowe, Murray and B Barlowe,
Build your own spectrum analyzer, Science Workshop, 1992.
Author’s bio graphy
Senior Technical Editor Dan Strassberg
has been covering
test-and-measurement topics at EDN
for 15 years. Before
that, he spent more
than a quarter of a
century designing and managing the design
of measurement instruments and systems.
You can reach him at 1-617-558-4205, email dstrassberg@edn.com.
www.edn.com