With frequencies soaring to new heights, wireless
and RF testing gets pushed to the limit in terms of complexity
and cost. In fact, higher frequencies in the gigahertz
range are now commonplace. Simple AM and
FM/PM have disappeared, replaced by the more complex
digital modulation methods.
Binary phase-shift keying (BPSK), quadrature phaseshift
keying (QPSK), and quadrature amplitude modulation
(QAM) are the norm these days. And, cell phones
extensively exploit spread-spectrum (CDMA) technology.
Meanwhile, other newer wireless methods have switched
to orthogonal frequency-division multiplexing (OFDM).
Specialized wireless techniques like software-defined
radio (SDR) and cognitive radio (CR), time-multiplexed protocols,
bursty transmissions like radar, frequency hopping,
wide-bandwidth technologies like Ultra-Wideband (UWB), and
adaptive modulation further complicate the testing process.
It isn't a pretty picture for the designer or the test engineer.
But that's not all. Testing speed is more important than
ever before. Time-to-market still rules the roost in engineering
today, and testing adds no value. It's just a cost borne
to ensure that the product works and conforms to the
guidelines in play. The more time spent in manufacturing
testing, the greater the cost and the lower the margin.
That's a grim scenario in a high-volume commodity
market like cell phones. With over 1 billion new phones
produced this year alone, just think of the hours that go
into testing. One manufacturer indicates that cutting the
test time for one measurement by 10 ms can save $1 million
in a production run.
On that front, though, there's some really good news. Test
equipment manufacturers, always on the leading edge of
technology anyway, recognized the problem and produced
some excellent solutions that simplify and significantly accelerate
testing procedures - at a price. Yet that price is a good
tradeoff because time is, after all, still money.
Common Tests
When planning your wireless testing, make sure the standards
of the technology you're using spell out the key
parameters of what you want to measure. Whether the
standard comes from an international standards organization
or an industry alliance that certifies products, you must
acquire that standard documentation and become aware of
all its gruesome details. Here, you'll find the specific tests
that need to be made as well as the required equipment.
Keep two facts in mind. First, RF measurements are
measurements of power, not voltage. Meters and displays
readout in power directly or, in some instances, in dBm (dB
referenced to 1 mW). Table 1 shows the relationship
between actual power and dBm. Since the goal in all cases
is maximum power transfer, proper impedance matching
within your circuits and between the test instruments and
the device under test (DUT) is critical. Most RF measurements
are made with a 50-Ω characteristic impedance.
Second, everything is a transmission line. If it isn't a coax
cable, it's a strip line or microstrip whose impedance is crucial.
Again, 50 Ω is the standard, and all impedances should
match up for maximum power transfer. Also, impedances
should match to minimize reflections and high voltage standing-wave ratio (VSWR), which can
lead to inefficiency and circuit damage.
Generally, RF tests divide into two categories:
those for transmitters (TX) and
those for receivers (RX). Many other special
tests exist, and beyond the ones listed
below, companies consistently develop
new tests to add to the mix (see "Six
New Measurements You're Going To
Need" at www.electronicdesign.com,
Drill Deeper 17102).
Transmitter Tests
Output Power:
The most important test
is power output from the final power
amplifier (PA). You can get a good measurement
by using a spectrum analyzer or vector signal analyzer,
though in most cases, greater accuracy of measurement
is essential. This requires an RF power meter. It will
give you the accuracy needed to ensure compliance with
whatever standard or regulation you must meet.
The two common power measurements are average and
peak. Your needs will be determined by the type of modulation
you're using. A further complication is the requirement of
a gated or timed power measurement in some applications.
For example, the GSM cell-phone standard that uses TDMA
requires you to measure the power in a burst of RF during the
524.6-µs time slot allotted. Another example of a pulsed RF
application is radar, which has very narrow pulses and random
and sometimes coded formats.
With CDMA, you will measure average
power because the signal is similar
to random or white noise. In a CDMA PA
that must handle multiple signals concurrently,
the signals (though random)
can add together and produce higher
power peaks 10 to 30 times that of one
signal. A key measurement in such
amplifiers is the crest factor, or the
peak-to-average ratio, which may be a
power or voltage ratio. Some RF power
meters will measure and calculate the
crest factor.
Another key measurement is the PA's
1-dB compression point. As the input
power to a PA increases, the output
increases linearly, up to a point. At some power level, the
output will saturate, meaning the output power will max out
and remain essentially constant despite increasing input
power (Fig. 1). The 1-dB compression point is the point
where the output power becomes 1 dB lower than what it
should be on a linear output scale.
Of course, driving an amplifier into saturation stresses it.
Even worse, the nonlinear response will produce harmonics
and spurious signals resulting from intermodulation-distortion
(IMD) effects. You can measure the harmonics and spurious
signals with a spectrum analyzer.
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