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Agilent PN 89400-3
CDMA Measurements with the 89400 Series
Vector Signal Analyzers
Product Note
Measurement challenges in CDMA interest. Power in adjacent channels
In the precision RF front-end of
Code division multiple access (CDMA) makes it impossible to use power or
the Agilent Technologies 89400
communications systems combine peak-power meters without external
Series VSAs, a block downconverter
digital modulation and spectrum- filtering. You need an analyzer that
tunes to a 7-MHz span anywhere
spreading encoding techniques to makes combined time- and frequency-
within a 2.65-GHz range for the
create broadband signals which are selective measurements.
89441 and 1.8-GHz range for the
immune to noise, and which them- 89440A. The down-converted sig-
selves look like noise. Their natural Other measurements of interest to
nal is filtered and digitized by a
noise immunity allows multiple CDMA system designers include digital
high-speed, high-resolution A to D
CDMA signals to overlay each other modulation analysis, filter frequency
converter. Subsequent digital mix-
in a common band. Measurements of response, amplifier intermodulation
ers and filters allow the span to
CDMA signals present new challenges and compression, identification and
be narrowed to as little as 1 Hz at
to test instruments. quantification of AM or PM spurious
arbitrary center frequency. High-
sidebands, and phase noise character-
performance processors provide
In the world of CDMA, a key measure- ization. These common measurements
fast spectrum, time domain, and
ment is the power spectrum within a of digital and analog radio systems
demodulation analysis, with flexi-
communications channel. This power have heretofore required a number of
ble measurement displays.
is broadband, noise-like, and varies different analyzers.
over time. Both short-term and long-
term average power spectra are of
The Agilent 89400 Series vector sig- Fourier transform (FFT), and inten- These features are all available in the
nal analyzers (VSAs) bring to bear on tional or unintentional modulation analyzers' vector mode. In scalar
these problems an impressive array can be characterized using computed mode, multiple spans are pieced
of analysis tools. Their primary con- demodulation. (AM, FM, and PM, and together by the analyzers to show
tribution is the ability to isolate and optional digital formats can be ana- spectra with displayed spans of up to
digitize a frequency band up to 7 MHz lyzed.) Markers on the time display 2.65 GHz.
wide, in a range of 0 to 2.65 GHz on can be used to limit the portion of the
the 89441A and 0 to 1.8 GHz on the time record for which the spectrum Let's see how these features can be
89440A. (Up to 10-MHz spans are or demodulation is calculated. used to make your CDMA measure-
available in baseband mode.) The dig- Flexible digital filtering eliminates ments:
itized waveform can be displayed and unwanted signals from time and
analyzed in time, the frequency spec- demodulation analyses. Band-limited power measurements
trum can be calculated using the fast One class of CDMA measurements for
which the 89400 Series VSAs are
right at home is power measurements
within and adjacent to a CDMA chan-
nel. Suppose you were to set up the
analyzer in vector mode with a fre-
quency span of 5 MHz, and set the
center frequency of the analyzer to
the carrier frequency of a CDMA cel-
lular base station. You would see a
spectrum display similar to Figure 1.
You are looking at the 1.23 MHz wide
CDMA modulation and some adjacent
sideband energy. The display would
change rapidly due to the pseudo-
random nature of a CDMA waveform.
To calculate average power over time,
you would turn on RMS averaging.
The speed of the 89400 Series VSAs
allows you to average hundreds of
spectra in less than a second and
display the result, which will look
Figure 1. CDMA spectrum similar to Figure 2.
The FFT calculation used in the
89400 Series VSAs is like measuring
the output power of hundreds of nar-
rowband filters with center frequen-
cies distributed across the span, all
operating on the same signal. Because
of this, spectrum averages are accu-
rate at all frequency points regardless
of power changes during the measure-
ment. By contrast, a swept analyzer
calculates the average power at a sin-
gle frequency at a time, and moves
on. If the signal's power changes dur-
ing the sweep, the resulting average is
inaccurate.
Band-power markers allow easy calcu-
lation of the power within the 1.23-MHz
band. The result is displayed at the
lower left of the screen.
Figure 2. Averaged CDMA spectra
2
The band-power markers can now be The spot marker on the time display response, which happens over the
moved to a span of 30 kHz centered shows peak power versus time. You course of 100 ms, you can use the
750 kHz above or below the CDMA can also place band-power markers time capture feature of the 89400
carrier frequency and the power in on a time display and display average Series VSAs. This simply stores the
that band calculated. The ratio of power during a selected time period. output of the digitizer and digital fil-
these two band measurements, each ter in memory: up to 1-million sam-
normalized to a 1-Hz bandwidth, con- The markers shown on the time dis- ples with the extended capture mem-
stitutes an occupied bandwidth test. play, called gate markers, demonstrate ory (Option AY9).
The 89400 Series VSAs make this test an important feature of these analyz-
accurately and with ease. ers: the ability to do time-selective fre- You can then analyze any part of the
quency analysis. The spectrum shown data after capture as if it were your
Time-variant power measurements on the top of the display consists of signal source. External triggering
Another common measurement in the FFT of the data within the gate allows you to synchronize the start of
CDMA is to observe the power varia- markers only. This calculation may be the time-capture measurement with
tions of a mobile phone over time. windowed with any of the three avail- the increase in base-station power.
Power in a mobile CDMA phone is able windows, or left unwindowed. Then you can look at the mobile
gated on and off in 1.25-ms bursts You can adjust the gate width and transmitted power at any time
when the data rate is less than the position to observe the spectrum between the start of the measurement
full 9600 bits per second. before, during, and after the tran- and the end of the captured data. The
sient. For example, you may wish to base-station power is excluded from
To measure the power during such a position the gate on the transient to the measurement because of the digi-
burst, you set the span of the analyz- see if the spectrum splatters signifi- tal filtering. For more details on fre-
er to 2 MHz and the center frequency cantly out of the assigned channel quency and time-selective measure-
to the channel of interest. Increase during turn-on. In this display the ments, see product note 89400-1
the number of measurement points gate is shown well within the CDMA (5091-7194E).
from the default of 401 to 3201. This burst, and band-power markers on
allows a maximum time record length the top display show the in-channel Other system measurements
of 1.6 ms. IF triggering allows the power. CDMA systems consist of digital mod-
measurement to start based on when ulators, filters, amplifiers, oscillators,
the signal energy within the measure- Time capture for analysis of long and a host of other components com-
ment frequency span crosses a CDMA transients mon to radios, all of which must work
threshold. You set the threshold to a A power control circuit in the mobile together to successfully generate a
few dB below the peak power in the unit is required to change its output CDMA signal. If bit- and frame-error
burst and select pre-trigger delay to power in response to a change in the tests reveal transmission of the
view the transient. If the analyzer is power received from the base station. CDMA signal is impaired, the 89400
set to trigger continuously, this trig- To measure this power-control Series VSAs can help determine if
ger setup is interactive, much like set- any of these analog components are
ting the triggering on an oscilloscope. at fault.
After setup, you can set the analyzer
to trigger once and retain the data for
post-measurement analysis.
Figure 3 shows a split display with
the time record on the bottom. The
ordinate of the display is the magni-
tude of the time record in dBm. Note
that this is similar to a zero-span
measurement done with a swept
spectrum analyzer, with an important
difference. The 89400 Series VSAs
capture all information within the
span, and weights it equally.
Information outside the span is
sharply attenuated. The 3-MHz wide
Gaussian resolution-bandwidth filter
of a swept analyzer attenuates the
CDMA signal at either side of the
channel because the filter is not flat,
and includes much more out-of-band
noise because its rolloff is more grad-
ual. This makes accurate power meas- Figure 3. Measurement of a time-variant CDMA signal, showing peak power versus time
urements difficult. on the lower trace and the spectrum of a selected time span on the upper trace.
3
Vector modulation analysis (Option The outstanding linearity of the Agilent Technologies' Test and Measurement
AYA) allows you to analyze QPSK, 89400 Series VSAs makes them well Support, Services, and Assistance
Agilent Technologies aims to maximize
and in future versions, offset QPSK suited to perform amplifier compres-
the value you receive, while minimizing
signals. These modulation formats are sion and intermodulation tests. To your risk and problems. We strive to
used, for example, in a base-station support such testing in vector mode ensure that you get the test and measure-
pilot signal or a mobile access pream- (within 7-MHz bandwidths), the RF ment capabilities you paid for and obtain
ble. The analyzer acquires a time source (Option AY8) provides sinu- the support you need. Our extensive sup-
port resources and services can help you
record representing up to 4096 sym- soidal, pseudo-random, chirp, or user- choose the right Agilent products for your
bols, determines the symbol clock, defined arbitrary signals. applications and apply them successfully.
and calculates the error between the Every instrument and system we sell has
received vector trajectory and the With a second 10-MHz input channel a global warranty. Support is available
for at least five years beyond the produc-
ideal. This analysis requires that the (Option AY7), the analyzers can per-
tion life of the product. Two concepts
ideal filter shape be defined. form frequency response measure- underlie Agilent's overall support policy:
ments on two-port networks in the "Our Promise" and "Your Advantage."
The analyzer can use standard raised- frequency range of 0 to 10 MHz. The Our Promise
cosine, root-raised-cosine, Gaussian, internal source provides single tone "Our Promise" means your Agilent test
and user-defined filter shapes. Note: and several wideband signals to sup- and measurement equipment will meet its
This analysis is not a complete CDMA port such tests. advertised performance and functionality.
When you are choosing new equipment,
demodulation in that the bits recov- we will help you with product informa-
ered are simply the PRN code, but it Complex measurements can be easily tion, including realistic performance spec-
is sufficient to determine the health automated using Instrument BASIC ifications and practical recommendations
of the modulator. Available displays (Option 1C2). A "keystroke recording" from experienced test engineers. When
include I/Q pattern, constellation, eye feature records your setups as you you use Agilent equipment, we can verify
that it works properly, help with product
diagrams on I or Q channel, vector make them and turns them into operation, and provide basic measurement
magnitude error, and vector phase BASIC I/O statements. A PC keyboard assistance for the use of specified capabil-
error. attached to the analyzer allows you to ities, at no extra cost upon request. Many
further edit the program, adding self-help tools are available.
AM, FM, and PM demodulation of branching, calculation, I/O, and dis- Your Advantage
oscillator signals can reveal the play enhancements. "Your Advantage" means that Agilent
offers a wide range of additional expert
nature and magnitude of spurious sig-
test and measurement services, which you
nals and noise. Time and frequency The 89400 Series VSAs perform can purchase according to your unique
displays of the demodulated data are noise-power measurements required technical and business needs. Solve prob-
available. The use of the PM demodu- in CDMA with ease and calibrated lems efficiently and gain a competitive edge
lation analysis to characterize phase accuracy not found in swept analyz- by contracting with us for calibration, extra-
cost upgrades, out-of-warranty repairs, and
noise is detailed in product note ers. Their combined time and fre- on-site education and training, as well
89400-2 (5091-7193E). quency selectivity, together with as design, system integration, project man-
demodulation analysis, allow you to agement, and other professional services.
Despite their advanced features, the see details of signals in ways never Experienced Agilent engineers and techni-
cians worldwide can help you maximize
89400 Series VSAs were designed to before possible. In addition, these
your productivity, optimize the return on
be familiar to users of traditional analyzers bring a broad array of tools investment of your Agilent instruments and
spectrum analyzers. In scalar mode, to assist you in CDMA system design systems, and obtain dependable measure-
the 89441A can display a power spec- and troubleshooting. ment accuracy for the life of those products.
trum over any frequency span up to Get assistance with all your
2.65 GHz (89440A up to 1.8 GHz). test and measurement needs at:
With arbitrary resolution bandwidths www.agilent.com/find/assist
down to 312.5 mHz, you can easily Product specifications and descriptions in this
find low-level spurious signals that document subject to change without notice.
otherwise would be obscured by Copyright