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Agilent
4 Steps for Making Better Power Measurements
Application Note 64-4D
Four Steps for Power: The Fundamental
Making Better Power RF and Microwave
Measurements Measurement
Before you select a power meter and Even a cursory analysis will reveal that Power measurement is the
its associated sensors, make sure present power sensor technologies' fundamental parameter for
that you have taken the following four have considerable overlap in characterizing components and
steps, each of which can influence capabilities. New system technologies, systems at RF and microwave
the accuracy, economy, and technical such as wireless modulation formats frequencies. Above the range of
match to your application. and their associated production 30 MHz to 100 MHz, where the
test requirements, will often require parameters of voltage and current
1. Understand the characteristics of some combined measurements such become inconvenient or more difficult
your signal under test and how as time-gated peak parameters or to measure, microwave power
they interact with the power- computed data such as peak-to- becomes the parameter of choice.
sensing processes. average ratios. And you can be sure Power specifications are often the
that all that data will be required at critical factor in the design, and
2. Understand power measurement speeds that push the state of the art. ultimately the performance, of almost
uncertainties and traceability all RF and microwave equipment.
to a primary power standard at Your analysis might also include
a national laboratory, such as considerations of the installed base Power specifications are also
the U.S. National Institute of of other sensors and power meters in central to the economic concept of
Standards and Technology (NIST). your organization's inventory. And, it equity in trade. This simply means
should consider the traceability chain that when a customer purchases a
3. Understand the characteristics of your organization's metrology lab to transmission product with specified
and performance of available national standards. power performance at a negotiated
sensor technologies and operating price, the delivered product must meet
features of various power meters. This application note will provide you that specified power when installed
with a brief review of the four factors and qualified at a distant location,
4. Make the performance that influence the quality of your perhaps in another country. Accuracy
comparison and select the right power measurements. It will also offer and traceability of your power
product for your application. other suggested information sources instrumentation will help ensure this
with more technical details, such as measurement consistency.
Agilent Application Note, AN 64-1C,
"Fundamentals of RF and Microwave
Power Measurements", publication
number 5965-6630E.
2
STEP 1
Understanding Your Signal
Under Test
A world of signal formats
System technology trends in modern Some radar and EW (counter- To test overload and rejection
communications, radar, and navigation measures) transmitters have the characteristics of a receiver,
signals have resulted in dramatically traditional pulsed format, but many composites of out-of-channel
different modulation formats, some of new systems also use spread- interference signals are created
which have become highly complex. spectrum or frequency-chirped for use as test signals. Whenever
The objective of this section is to and complex phase-coded pulse such multiple signals are present,
briefly examine a range of typical configurations, which reveal more composite carriers can add random
formats to see how their spectrum precise data on the unknown target phases and create power "spikes".
characteristics interact with various returns. Thus, an application analysis is crucial
power sensor technologies. to understand these effects on the
Navigation systems such as the global power sensor.
Wireless and cellular systems depend positioning system (GPS) use complex
on digital I-Q (inphase-quadrature) phase-shift-keyed (PSK) formats to In the sensor technology section
modulations at high data rates and yield precision radiolocation. Other (Step 3), much more detail is given to
other spread-spectrum formats. navigation systems use pulsed peak detection. Briefly, the measuring
Because the final transmitted signal formats for distance or coded target principle is that an averaging sensor
combines multiple carriers, statistical identification. responds to the average value of
processes at work that can create any format as long as the signal
extremely high peak power spikes, Some signals under test comprise of peaks remain within the sensor's
based on a concept called crest factor, multiple test tones and others contain square-law range. But driving
described in the following paragraphs high harmonic content. Still others ordinary diode sensors into their
and in the section entitled "Digital and are generated by frequency-agile linear-detection ranges, even those
complex formats." synthesizers, which can simulate with compensation techniques, will
entire, full-channel communications cause errors. Peak and average diode
Wireless systems also contain traffic formats. These test signals detectors, specifically designed for
frequency-agile local oscillators which are used to characterize the real- peak excursions, generally do not have
"hand-off" the vehicle's signal as it life performance of transmitters and problems with any type of complex
moves from ground cell to ground cell receivers such as satellite transponder signal format.
and links up to each new base-station systems.
frequency. Sometimes the power
perturbations, which occur during
the frequency transition, need to be
characterized.
3
Pulsed formats AM/FM formats
Some modern radars used narrower Diode-based sensors and associated Not many systems are active these
pulses that permitted better separation power meters, which are designed days that are pure AM or FM, other
resolution of multiple targets. Their for peak detection, are ideal when than commercial broadcast, and
rise/fall times were proportionately the pulse-top characterization is perhaps amateur radio or "shortwave"
shortened as well, and the bandwidth required, or when the pulse envelope formats. Frequency modulation, since
of the radar receiver increased. Then must be profiled. These peak sensors its carrier amplitudes are relatively
came other technologies for pulsing feature wide-band amplification of constant, can be measured with
with longer phase-coded formats, the detected envelope, and permit simple averaging power sensors.
which made it possible to determine digital signal processing (DSP) to Amplitude modulation signals, on
factors such as the shape or size of measure and display the pulse shape the other hand, must be analyzed
a target. Multiple pulses and random and numerical parameters. Most to ensure that the peak modulation
pulse-repetition times are design modern radar and EW systems use swings always remain below the limits
strategies needed for resistance to complex and pseudo-random pulse- of the sensor's "square-law" range,
countermeasures jamming. rate configurations for immunity to since the modulation peaks result in a
jamming, and thus can't use simple (Vcarrier)2 effect on power.
All of these trends in pulse technology computations based on duty cycle.
mean that specifying a measurement They require specific peak-type
power meter requires a clear sensors.
knowledge of the key parameters that
need to be characterized. For some When measuring peak power, it is
test sequences, measurement of important to understand the specific
the numerous pulse power and time test requirements for characterizing
parameters performed by peak power the pulse parameters of a system or
analyzers may be needed. On others, component. For example, measuring
the pulse top and average power will the rise time or fall time of a radar
suffice. pulse might be crucial for testing
the power amplifier component. The
Design and production tests for pulsed reason is that short rise/fall times
systems often require measurements correlate with higher bandwidth of
of both peak pulse power (pulse the transmitted pulse and relate to
top) as well as average power for its ability to resolve targets. Yet, in
the transmitter and other system other production tests, perhaps on
components. Thermal sensors later subsystems, it may only be
inherently respond to total average necessary to measure the pulse-top
power, as long as the peak power power of the pulse. By knowing the
excursions do not exceed the peak precise measurement specification
ratings of the sensor. And given a required, a test engineer might use
pulsed waveform with a fixed duty a simpler and less expensive power
cycle (pulse width/total pulse period), meter to determine that the subsystem
its peak power can also be computed is operating within its proper
using the average power from a performance envelope.
thermal sensor.
Navigation systems such as air-traffic
control (ATC) or distance-measuring
transceivers (DME) also have non-
traditional pulse configurations, such
as pulse pairs or triplets. In that case,
peak-detecting power meter/sensor
combinations are appropriate, such
as the Agilent E4416/17A meters
and E9320A sensors as well as
P-Series power meter N1911/12A and
wideband power sensor N1921/22A.
4
Digital and complex formats
Terrestrial communication
Terrestrial communication systems TDMA (time division multiple But, just like white noise, the average
abound with design examples of the access) is the technology for time- power of the transmitted signal is
new digital phase modulation formats. sharing of the same base station only one of the important parameters.
Some early migrations to microwave channel. Encoded voice data and Because, statistically, multiple carrier
terrestrial links from traditional FDM new high-data-rate wireless links signal voltages can increase randomly,
(frequency-division-multiplex), used are modulated unto the transmitted instantaneous peak voltages can
64QAM (quadrature-amplitude- carrier in the phase plane. These approach ratios of 10 to 30 times the
modulation) formats. create "constellations" of bit symbol rms voltage, depending on formats
locations such as shown in the 3/8 and filtering. This ratio,calculated
shifted-8PSK configuration shown in with voltage parameters, is commonly
Figure 1. This particular modulation called crest factor, and is functionally
format is used in EDGE (Enhanced similar to a peak-to-average power
Wireless and PCS Data Rates for GSM Evolution) ratio that is measured by Agilent peak
systems that offer high-data-rate and average power meters.1
More recent wireless technologies transfer over mobile wireless
combined digital formats with channels. By packing 3 bits per System designers accommodate this
sophisticated carrier switching of symbol, it increases data information crest-factor effect by "backing off" the
transmitted signals to permit time- rates, but thereby increases amplitude power amplifiers from their maximum
shared information from thousands of swings up to 16+ dB, making amplifier peak ratings to ensure that signal peak
mobile subscribers, who were arrayed saturation more likely. power operation is always within their
in cellular geographical regions. linear range.
Each TDMA wireless subscriber's
share of time might allow a useful
data burst of 524.6