Overview of how to do RF and Microwave Power

Agilent
Fundamentals of RF and Microwave Measurements (Part 4)
An Overview of Agilent Instrumentation for RF/Microwave Power Measurements
Application Note
Table of Contents
I. Introduction
...................................................................................................................... 3
II. A Review of Various Power Measuring
Instrumentation ............................................................................................................. 4
Instrument alternatives for measuring RF/microwave power............................................... 4
Types of superheterodyne instruments for measuring power ............................................... 5
Power measurement considerations for superheterodyne
instruments .................................................................................................................. ............... 6
Power measurement considerations for test-set-type instruments ..................................... 8
III. Power Sensor/Meter Methods and
Comparisons...................................................................................................................... 9
Accuracy vs. power level .............................................................................................................. 9
Frequency range and SWR (reflection coefficient) ................................................................ 12
Waveguide sensor calibration.................................................................................................... 13
Speed of response at low signal levels ................................................................................... 13
Automated power measurement ............................................................................................... 14
Susceptibility to overload............................................................................................................ 14
Signal waveform effects.............................................................................................................. 16
Computed data and analyzer software package .................................................................... 17
IV. Capabilities Overview of Agilent Sensors
and Power Meters ................................................................................................... 18
An applications overview of Agilent power sensors ............................................................. 18
A capabilities overview of Agilent power meters .................................................................. 19
For user convenience, Agilent’s
Fundamentals of RF and Microwave Power
Measurements, application note 64-1,
literature number 5965-6330E, has been
updated and segmented into four technical
subject groupings. The following abstracts
explain how the total field of power
measurement fundamentals is now
presented.
Fundamentals of RF and Microwave Power Measurements
Part 1: Introduction to Power, History, Definitions, International Standards, and Traceability,
AN 1449-1, literature number 5988-9213EN
Part 1 introduces the historical basis for power measurements, and provides definitions for
average, peak, and complex modulations. This application note overviews various sensor
technologies needed for the diversity of test signals. It describes the hierarchy of
international power traceability, yielding comparison to national standards at worldwide
National Measurement Institutes (NMIs) like the U.S. National Institute of Standards and
Technology. Finally, the theory and practice of power sensor comparison procedures are
examined with regard to transferring calibration factors and uncertainties. A glossary is
included which serves all four parts.
Part 2: Power Sensors and Instrumentation, AN 1449-2, literature number 5988-9214EN
Part 2 presents all the viable sensor technologies required to exploit the users’ wide range
of unknown modulations and signals under test. It explains the sensor technologies, and
how they came to be to meet certain measurement needs. Sensor choices range from the
venerable thermistor to the innovative thermocouple to more recent improvements in diode
sensors. In particular, clever variations of diode combinations are presented, which achieve
ultra-wide dynamic range and square-law detection for complex modulations. New
instrumentation technologies, which are underpinned with powerful computational
processors, achieve new data performance.
2
Fundamentals of RF and
Microwave Power
Measurements, continued
Part 3: Power Measurement Uncertainty per International Guides AN 1449-3, literature
number 5988-9215EN
Part 3 discusses the all-important theory and practice of expressing measurement
uncertainty, mismatch considerations, signal flowgraphs, ISO 17025, and examples of typical
calculations. Considerable detail is shown on the ISO 17025, Guide for the Expression of
Measurement Uncertainties, has become the international standard for determining
operating specifications. Agilent has transitioned from ANSI/NCSL Z540-1-1994 to ISO
17025.
Part 4: An Overview of Agilent Instrumentation for RF/Microwave Power Measurements, AN
1449-4, literature number 5988-9216EN
Part 4 overviews various instrumentation for measuring RF and microwave power, including
spectrum analyzers, microwave receivers, network analyzers, and the most accurate method,
power sensors/meters. It begins with the unknown signal, of arbitrary modulation format,
and draws application-oriented comparisons for selection of the best instrumentation
technology and products.
Most of the note is devoted to the most accurate method, power meters and sensors. It
includes comprehensive selection guides, frequency coverages, contrasting accuracy and
dynamic performance to pulsed and complex digital modulations. These are especially crucial
now with the advances in wireless communications formats and their statistical
measurement needs.
I. Introduction
The purpose of the new series of Fundamentals of RF and Microwave Power Measurements
application notes, which were leveraged from former note 64-1, is
1) Retain tutorial information about historical and fundamental considerations of RF/
microwave power measurements and technology which tend to remain timeless.
2) Provide current information on new meter and sensor technology.
3) Present the latest modern power measurement techniques and test equipment that
represents the current state-of-the-art.
Fundamentals Part 4, Chapter 2 presents an overview of various instrumentation for
measuring RF and microwave power. Those methods include spectrum analyzers, microwave
receivers, vector signal analyzers, and wireless and cellular test sets, among others.
Naturally, it also includes the most accurate method, power sensors and meters. It begins
with the unknown signal of arbitrary modulation format and draws application-oriented
comparisons for selection of the best instrumentation and technology.
Chapter 3 reviews other applications and measurement considerations of power sensors
and meters not covered in the technology presentations of Fundamentals Part 2. These
include such matters as susceptibility to overload, automated data functionality, etc.
Chapter 4 provides an overview of the entire line of Agilent sensors and meters. It includes
a functionality chart for compatibility of sensors with meters. Some early sensor
technologies like thermocouples work with all Agilent meters, while new peak and average
sensors are only compatible with the EPM-P meter. Signal application charts and frequency
and power range capabilities are all presented in tabular format.
Note: In this application note, numerous technical references will be made to the other
published parts of the series. For brevity, we will use the format Fundamentals Part X. This
should insure that you can quickly locate the concept in the other publication. Brief
abstracts for the four-part series are provided on the inside front cover.
3
II. A Review of Various
Power Measuring
Instrumentation
Instrument alternatives for measuring RF/microwave power
Ingenuity has dominated the inventive progress of RF/microwave power measurements.
One clever method mentioned in Fundamentals Part 1, was the World War II (WWII) legend
of Russell Varian drilling a tiny hole in his experimental klystron cavity and using a
fluorescent screen to indicate whether the oscillation was on or off. Other non-instrument
methods followed, such as the “water-load” calorimeter, which threaded a glass tube
diagonally through a rectangular waveguide. By measuring the heat rise and flow rate of the
water stream, the transmitter power could be computed. That method served both as a
high-power termination for the tube as well as a power measuring process.
Serious power measuring instrumentation came out of the WWII developments of radar,
countermeasures and communications system demands. Crystal detectors furnished a crude
method of indicating and metering power, but since they were fragile, the high power
signals required precision attenuation before applying to the sensor. Bolometers, which
utilized tiny power-absorbing elements, terminated the unknown power and heated up. By
monitoring or sensing the heat buildup, highly accurate measurements could be realized on
unknown power over wide frequency ranges.
Microwave superheterodyne-type receivers were always capable of sensing RF/microwave
power because their inherent purpose was the detection and display of power versus
frequency. Some called them “frequency-domain oscilloscopes.” While most such receivers
were used for system purposes, some were used for research and instrumentation. The
main advantage of using superheterodyne-type instruments was and is the ability to obtain
power over a specified and tuned bandwidth, whereas power sensors measure total power
across their entire specified frequency range.
Early spectrum analyzers were basically uncalibrated for absolute power. An unknown signal
under test could be measured by comparison with a known power from a calibrated signal
generator, where the microwave receiver was used only as a comparison sensor. The
calibrated reference signal generator signal would be adjusted to be equal to the unknown.
4
Types of superheterodyne instruments for measuring power
In 1964, Agilent introduced the HP 851/8551 as the first “power-calibrated” spectrum
analyzer, available as a commercial product. This offered a considerable advance in
frequency and power characterization of unknown signals. While we look back now at such
relatively crude instruments with relatively poor accuracy specifications, they were the
wonders of their time.
Enormous progress has been made in improved accuracy and functionality in the intervening
years. Spectrum analyzers have gained digital precision in both frequency and power level
because of sophisticated digital signal processing (DSP) microcircuitry and more precise
components such as highly-linear amplifiers. Meantime, a number of other types of
instrumentation have also been configured to make excellent measurements of power
levels, not just for simple modulated signals, but for all of the new modulation formats
common to the modern communications and wireless systems.
Here is a list of typical instrument types, based on a superheterodyne block diagram, that
are designed to make signal power measurements:
•
•
•
•
spectrum analyzers
vector signal analyzers
calibrated microwave test receivers
other instruments
Another variety of instrumentation for signal power measurements is the popular and
ubiquitous wireless, cellular, and communications test sets. For the purposes of this
application note, a test set is considered to contain an array of test functions that can
characterize a complete operating communications system, both transmitter and receiver. It
provides precision calibrated and adjustable test signals with system-specific modulation
formats to test the system’s receiver portion, and it contains power measurement capability
to characterize the performance of the system’s transmitter.
A typical wireless test set would be the Agilent E5515C mainframe and E1962B test
application software. For the most precise power measurement, such a test set uses
directional bridges at its input to feed the power to a thermal power detector as well as a
“fast power detector.” Other portions of the test set feature demodulation downconversion
and measurement downconversion, which utilize the superheterodyne processes.
5
Power measurement considerations for superheterodyne
instruments [1]
As with most things in life, a required measurement of a system’s output power comes with
predictable tradeoffs. Simply stated, the power sensor/power meter method always offers
the best measurement accuracy, but it measures all the power at the input to the sensor - it
is not frequency-selective. Further, the power meter method measures true average power,
even of complex digital modulation formats, some of which look like noise. Even peak power
sensors, which are based on detection curves ranging from square law to linear, are digitally
compensated to present full averaging of power. Power meter instrumentation now also
provide time-selective measurements, meaning the user can set time gates for bracketing
the time period over which a power measurement is made.
Superheterodyne-type instruments, on the other hand, offer versatile frequency selectivity,
as well as considerable flexibility on the measurement of signal power, also including all the
newer and complex modulation formats. In fact, one of the main reasons that
superheterodyne-type instruments are selected is to provide a selectable bandwidth for a
power measurement. A typical requirement would be for measuring integrated channel
power in the presence of other system channel power.
Since their block diagrams are typically double or triple downconversion, there are important
measurement considerations of resolution bandwidth, types of final detection and, moreimportantly, the particular DSP algorithms used to furnish output data for the power
level. [2]
Figure 2-1 shows a block diagram of a typical modern spectrum analyzer. The unknown
signal receives a user-set attenuation, usually has some pre-selection filtering, then gets
downconverted to an IF (intermediate frequency) amplifier. Modern analyzers are designed
with less and less IF amplification and more and more powerful DSP microcircuits further
forward in the IF signal path. Those DSPs can now sample the IF signals and their
modulation envelope with extremely high sampling rates.
Whatever signal conditioning strategy is used, the end result is a display presentation of
the modulation envelope of the signal under test. It should be noted that this is all carried
out as linear detection, meaning the display is a voltage-related parameter. Logarithmic
amplifiers or logarithmic data processing, as shown in Figure 2-1, converts the display to a
dB display at the user command. Such display formats are especially useful for ultra-wide
dynamic amplitude ranges, for example 10 dB per division.
RF input
attenuator
IF gain
IF filter
Detector
Mixer
Input
signal
Log
Amp
Pre-selector
or low pass
filter
Video
filter
Local
oscillator
Sweep
generator
Crystal
reference
CRT display
Figure 2-1. Block diagram of a typical traditional superheterodyne spectrum analyzer.
6
To measure power, consider the CW power spectrum of Figure 2-2(a). The display is
produced by sweeping frequency (horizontal scale) across the CW unknown power. If the
resolution bandwidth (RBW) set by the user is wider than the spectral components of the
unknown CW, then the highest point on the display will represent the true CW power.
Power
RBW
CW signal
(a)
Frequency sweep
Power
3-MHz
RBW
8-MHz QAM
signal
(b)
Frequency sweep
Power
3-MHz
RBW
2-MHz QAM
signals
(c)
Frequency sweep
Figure 2-2. A spectrum analyzer’s resolution bandwidth setting can be adequate for a single
CW power measurement (a), but for digitally modulated signals it is either inadequate (b) or
erroneously adds power in from the adjacent channel (c).
Now, consider an unknown signal as in Figure 2-2(b), which is a QAM (quadratureamplitude-modulation) format with an 8 MHz bandwidth. This is considerably wider than the
filter’s RBW of 3 MHz, and therefore cannot integrate all the 8 MHz power.
If you select a RBW setting that is larger than the unknown signal, will that integrate the
unknown broadband signal format? Well, no, because the RBW
filter is Gaussian shape, and while it may be wide enough to enclose the unknown signal at
its –3 dB points, out at the noise floor of the RBW, its width will enclose a lot of noise as
well as perhaps other adjacent channels of power. This is shown in Figure 2-2(c) as a QAM
signal of 2 MHz bandwidth, easily enclosed by the 3 MHz RBW of the analyzer, but also
allowing in unwanted sideband channels and noise.
7
The upshot of this analysis is that the measurement needs to be made with a narrower
RBW, such that the skirts of its filter are steep and help define the skirts of the power
spectrum of the unknown signal under test. Powerful software computation routines of
modern analyzers take all this into consideration, realizing that the user simply wants to
measure the integrated “channel power” of a wireless channel, for example 8 MHz, and
reject the channel power of the adjacent channel which is also on the air.
The analyzer performs this task internally by using the proper RBW settings, and making
corrections for what is called the equivalent noise power bandwidth (ENPBW) using a
correction factor for Agilent ESA analyzers of 1.128. The internal computations for pre-set
channels can also be pre-determined since the performance parameters for industry
standard wireless formats are known, for example CDMA, TDMA, etc. Suffice to say that
instrumentation analyzers based on the superheterodyne principle have powerful
characterization capabilities, and generally can provide channel power readings with
accuracies on the order of ±1 dB or less.
When considering the tradeoffs in specifying power instrumentation, a lot will depend on
the needs of the measurement requirement and the condition of the unknown signal. Is it
accompanied by multiple channels? Is the lowest uncertainty required? Does the unknown
signal format contain time-domain characteristics that need to be time-selectively sorted out
by the Agilent EPM-P and P-Series gated power capabilities?
Power measurement considerations for test-set-type instruments
A modern wireless test set provides two paths for measuring unknown power. The first was
mentioned above, whereby the unknown signal is directionally-bridged off right at the input
terminal and applied to either a thermal detector or a peak detector. This assures that the
best possible accuracy is maintained for characterizing the unknown. Actual performance
specifications are quoted in considerable detail for various frequency bands and power
levels. For a general ballpark figure, specified uncertainties range from ±4.2 percent to
±7.5 percent (typical ±3.0 percent). Such specifications, of course, state only the instrument
performance, other uncertainties such as mismatch and power reference details would have
to be computed for given measurement environments and requirements. Specifications differ
for the thermal detector and the peak detector.
The other power measurement function of test sets is the “tuned channel power
measurement.” This is the one that utilizes the complete superheterodyne downconversion
signal chain, such that it measures and computes “channel power,” by rejecting adjacent
channel power in operating systems. Typical measurement uncertainty for such system
signal characterization is stated in the ± 1 dB ranges. Calibrating against a known power
level signal furnished from within the test set enhances accuracy for this function. Again,
additional uncertainties would need to be considered for mismatch and other additives.
[1] Mill, Alistair, Measuring Digital Carrier Power with a Spectrum Analyzer, Test & Measurement World
Europe, April/May 2000.
[2] Agilent Technologies, Spectrum Analyzer Basics, Agilent Application Note 150, literature number
5952-0292.
8
III. Power Sensor/Meter
Methods and
Comparisons
Assume that the user’s power measurement requirements have been analyzed. The
outcome of that analysis shows that power sensor/meter is the preferred method. All of the
previous discussion in Fundamentals Part 2 on sensor and meter technology still leaves
choices for which power meter and sensor will provide the best or fastest or most accurate
solution. As was seen, each average or peak and average sensor and meter technology has
some advantages over the others, yet there is an optimum choice, and that is the purpose
for this chapter.
Factors such as cost, frequency range, the range of power levels to be measured, the
importance of processing and capturing data, accuracy, speed of measurement, and the skill
of the personnel involved take on varying degrees of importance in different situations. This
chapter compares the measurement systems from several aspects to aid in the decisionmaking process for any application. At the end, a signal applications chart profiles sensors
best suited for particular modulation formats. Other charts briefly overview the measurement
capabilities of the sensor and power meter families now available from Agilent.
Accuracy vs. power level
This comparison of power measuring systems demonstrates the measurement uncertainty
and power range of several equipment selections. The EPM Series power meters and
E Series sensors are emphasized, although several existing sensors are included. The EPM-P
meters and E9320A peak and average sensors were not included in this comparison exercise
since they require other considerations outlined in Fundamentals Part 2, Chapter V.
Figure 3-1 shows plots of the root-sum-of-squares (RSS) uncertainty when measuring power
for a common condition at various levels from –70 to +20 dBm. The measurement conditions
were assumed for a CW signal at 2 GHz and a source SWR of 1.15, and data sheet
specifications.
The three parts of this figure show a comparison of three common combinations of power
meter and sensor:
a) Agilent 432A analog power meter plus 8478B thermistor sensor.
b) Agilent E4418B digital power meter plus 8481A thermocouple and 8484D diode sensor.
c) Agilent E4418B digital power meter plus E4412A extended dynamic range power sensor.
The data for Figure 3-1 was computed using a commercially-available mathematics
simulation software product called MathCad. To present these operating performances
under typical present-day conditions, the ISO uncertainty combining process of
Fundamentals Part 3 was used for the MathCad calculations. Results are approximate,
although they are entirely suitable for these comparison purposes.
The reason for presenting these overall measurement uncertainties in this format is that, as
far as the user is concerned, there is little need to know whether the sensor works on the
diode principle or on the thermocouple principle. With the introduction of the new extendedrange PDB diode sensors, a single E4412A sensor can achieve the –70 to +20 dBm power
range, which previously required a combination of diode and thermocouple sensors.
9
The top graph of Figure 3-1 describes the thermistor sensor/meter combination and is
shown mostly for reference. With the decreasing applications of thermistor-type sensors, the
primary need for understanding their theory and practice is that they are used as power
transfer devices for metrology round robins. They also find use in transferring a power
reference from a higher-accuracy echelon or national standards labs to operating labs. In the
DC substitution process, 432 instrumentation error is substantially reduced because the
substitution DC power can be measured with precision digital voltmeters.
A comparison of the top two graphs of Figure 3-1, (a) and (b), shows that the uncertainties
of the thermocouple and diode-based systems (b) are somewhat less than the thermistorbased systems (a). At this 2 GHz calculation frequency, the thermocouple and diode sensors
have the better SWR (see Figure 3-2), but the thermistor system, being a DC substitution
system, does not require a power reference oscillator and its small added uncertainty. These
two effects tend to offset each other for this application. The significant advantage of the
E4418B power meter measurement is the performance flexibility of being able to use the
large installed base of all the other Agilent family of thermocouple and diode sensors.
The third graph of Figure 3-1, (c), for the E4418B power meter and E4412A extended dynamic
range sensor, immediately shows that even with the sensor’s wide dynamic measurement
range from –70 to +20 dBm, it provides approximately equivalent uncertainties. The dashed
portion of the E Series sensor curve (0 to +20 dBm) represents nominal high-power cal
factor uncertainty limitations imposed by the sensor calibration system. Refer to the latest
sensor technical specifications to determine actual uncertainties for your particular
application.
20
15
(a)
Uncertainty %
10
5
0
-70
-60
-50
-40
-30
-20
Power dBm
-10
0
+10
+20
20
15
(b)
Uncertainty %
10
5
8481A
8481D
0
-70
-60
-50
-40
-30
-20
-10
Power dBm
0
+10
+20
20
15
(c)
Uncertainty %
10
5
0
-70
-60
-50
-40
-30
-20
Power dBm
-10
0
+10
+20
Figure 3-1. RSS uncertainty vs. dynamic power range from data sheet specs for source SWR
= 1.15 (ρs =0.07) and f = 2 GHz: (a) Analog thermistor mount system(432A plus 8478B). (b)
E4418B digital power meter system using 8481D diode and 8481A thermocouple sensors. (c)
E4418B digital power meter and E4412A PDB extended-range sensor. RSS-combining
method is the same as used in Fundamentals Part 3.
10
While most modern power meter designs have utilized digital architectures, analog-based
meters, such as the 432A, are still available. Analog meter measurements are limited by the
mechanical meter movement of the instrument that requires uncertainty to be stated in
percent of full scale. Thus, at the low end of each range, the uncertainty becomes quite
large when expressed as a percent of the reading. Digital systems are free of those
problems and, with proper design and an adequate digital display resolution, provide better
accuracy.
The instrumentation accuracy for a digital meter is specified as a percent of the reading
instead of as a percent of full scale. This means that at the point of each range change,
there is not a big change in uncertainty for the digital meter. This effect can be seen in the
max-min excursions of the sawtooth-like curves of the analog meter shown in Figure 3-1 (a).
For this reason, the digital power meter does not need as many ranges; each digital range
covers 10 dB with little change in accuracy across the range.
One application advantage attributed to analog meters is the “tweaking” functions where an
operator must adjust some test component for optimum or maximum power. Digital displays
are notoriously difficult to interpret for “maximizing or minimizing” readings, so the display
of the E4418B power meter features an analog scale in graphic display format, which
provides for the “virtual-peaking” function.
It should be recognized that the accuracy calculations of Figure 3-1 are based on
specification values. Such specifications are strongly dependent on the manufacturers’
strategy for setting up their specification budget process. Some published specifications are
conservative, some are less so. Manufacturers need to have a good production yield of
products for the whole family of specifications, so this often leads to a policy of writing
specifications that have generous “guard bands” and thus are more conservative.
Further, a particular measurement configuration is likely to be close to one specification
limit, but easily meet another specification; a second system might reverse the roles. By
using the new ISO uncertainty-combining method, this takes advantage of the random
relationship among specifications and the uncertainties tend to be smaller, yet realistic.
A second reason to observe is that the Figure 3-1 calculations are done for one
particular frequency (2 GHz) and one particular source SWR (1.15). A different
frequency and different source match would give a different overall uncertainty. Sources
frequently have larger reflection coefficient values that would raise the overall uncertainty
due to usually-dominant mismatch effects.
11
Frequency range and SWR (reflection coefficient)
All three types of power sensors have models that cover a frequency range from 10 MHz to
18 GHz, some higher, with coaxial inputs. A special version of the thermistor mount operates
down to 1 MHz (see Fundamentals Part 2) and the 8482A/B/H thermocouple power sensors
operate down to 100 kHz. The effective efficiency at each frequency is correctable with the
Calibration Factor dial or keyboard of the power meter, so that parameter is not particularly
critical in deciding on a measurement system.
In most analyses, the sensor’s SWR performance is most important because mismatch
uncertainty usually contributes the largest source of error, as described in Fundamentals
Part 3. Figure 3-2 shows a comparison of the specification limits for the SWR of a thermistor
mount, a thermocouple power sensor, an 8481D PDB diode power sensor, as well as the
E Series power sensors.
It should be recognized that published SWR specifications are usually conservative and that
actual performance is often substantially better, yielding lower uncertainty in practice. That
fact argues for a preferred practice that measures actual source SWR for situations where
highest accuracy is important.
These graphs indicate that over the bulk of the frequency range, the thermocouple and
diode sensors have a considerably-lower SWR than the thermistor sensor. It also shows that
the E4412A sensor, even with its superior dynamic range, still provides a satisfactory SWR.
1.7
1.6
8478B
8481D
8481A
E4412A
1.5
1.4
SWR
8478B thermistor
8481D diode
1.3
E4412A diode
1.2
8481A thermocouple
30
50
30
50
2
4
2
4
12.4
18
1.1
12.4
10 MHz
100 MHz
1 GHz
10 GHz
30 GHz
Frequency
Figure 3-2. A comparison of specified SWR limits for the 8478B thermistor mount, 8481A
thermocouple power sensor, 8481D PDB power sensor, and E4412A PDB sensor.
12
Waveguide sensor calibration
Power measurements in waveguide present several special considerations. Waveguide
thermocouple and diode sensors must have the usual 50 MHz reference oscillator to adjust
for calibration factor from one sensor to another. Such a low-frequency signal cannot
propagate in a waveguide mode. Agilent waveguide thermocouple sensors (26.5 to
50.0 GHz) and waveguide diode sensors (26.5 to 110 GHz) all utilize a special 50 MHz
coaxial injection port that applies the reference oscillator output to the sensor element in
parallel to the usual waveguide input. This permits the meter-sensor system to be calibrated
at their waveguide operating frequencies.
Speed of response at low signal levels
To measure the lowest power ranges with optimum accuracy, power meters are designed
with a highly-filtered, narrow bandwidth compared to most other electronic circuits. Narrow
band circuits are necessary to pass the desired power-indicating signal but reject the noise
that would obscure the very weak signal. Narrow bandwidth leads to the long response
time. For heat responding power sensors, like the thermistor and thermocouple, response
time is also limited by the heating and cooling time constants of the heat sensing element.
The typical thermistor power measurement has a 35 ms time constant and 0 to 99 percent
response time of about five time constants or 0.175 s. The power meters for thermocouple
and PDB sensors have 0 to 99 percent response times of 0.1 to 10 s, depending on the
range of the power meter. The more sensitive ranges require more averaging and hence
longer settling times.
For manual measurements, the speed of response is seldom a problem. By the time the
observer turns on the RF power and is ready to take data, the power meter has almost
always reached a steady reading.
For analog systems applications, where rapid data acquisition is required, or where the
power meter output is being used to control other instruments, the power meter acts like a
low pass filter. The equivalent cutoff frequency of the filter has a period roughly the same as
the 0 to 99 percent response time. For signals where the signal power changes too rapidly
for the power meter to respond, the power meter averages the changing power. When a
power meter is being used to level the output of a signal generator whose frequency is
being swept, the speed of the frequency sweep may have to be reduced to allow the power
meter time to respond to the power level changes.
There is no clear-cut advantage with regard to speed of one power measurement system
over another. In some power ranges one system is faster, and in other ranges another
system is faster. If response time is critical, manufacturers’ data sheets should be compared
for the particular application.
13
Automated power measurement
Recognizing that a large percentage of digital power meters are used in production test and
in automated systems, it is reasonable to assume that digitizing measurement speed is
critical in at least some of those applications. Digital power meters programmed for
automatic operation gather data rapidly and with minimum errors. The data can be
processed and analyzed according to programmed instructions, and the system can be
operated with little process attention. Even in a manual mode, digital indications are less
prone to the human error of misinterpreting the meter scale and using wrong range
multipliers. In the case of power measurement, there are additional advantages to automatic
systems. Successive data points can be compared mathematically to assure that the power
measurement has reached steady state and multiple successive readings can be averaged
to statistically reduce the effects of noise.
The Agilent EPM Series power meters have been optimized for maximum digitizing speed.
Since its architecture is totally DSP-based, and it is married to a new E Series diode
sensors, circuit decisions were made to increase the digitizing speed to maximum. For
example, output filtering on the sensor is smaller, which provides faster response. On the
lower power ranges, this smaller filtering might cause an increase in measurement noise,
but the power meter itself provides for digital averaging up to 1,024 readings to minimize
noise effects. The meter is specified to provide up to 20 readings per second and 40 per
second in the X2 mode. The specification for the FAST range in the free-run trigger mode,
using the binary output format, is 200 readings per second. For that function, circuit settling
times are 5 mS for the top 70 dB power ranges.
Agilent’s EPM-P power meters have advanced their measurement data output speed
another step. Their peak and average power sensors have a wider video bandwidth, and
their DSP-based circuitry is designed for 20 megasamples per second (Msa/s) data sampling
in the analog-to-digital converter section. See Fundamentals Part 2, Chapter V for complete
details.
This permits data outputs up to 1000 corrected readings per second, which can be ideal for
certain production test situations. Further, because they can internally compute combined
parameters like peak-to-average ratio on the fly, important production test requirements are
easier to meet.
Agilent P-Series power meters have the fastest measurement speed of 1500 corrected
readings/second, which is ideal for manufacturing applications. There is an EEPROM in the
sensor that stores the 4-D calibration model. This model is generated in the factory during
the calibration process by measuring the input power, frequency, temperature, and output
voltage. While performing normal measurements, this model evaluates the current
temperature and input frequency to determine the correction factor. It does not require the
meter to perform interpolation of calibration factors and linearity curves. Therefore, this
results in quick and more accurate measurements.
Susceptibility to overload
The maximum RF power that may be applied to any power sensor is limited in three ways.
The first limit is an average power rating. Too much average power usually causes damage
because of excessive accumulated heat. The second limit is the total energy in a pulse. If
the pulse power is too high for even a very short time, in spite of the average power being
low, the pulses might cause a temporary hot spot somewhere in the sensor. Damage occurs
before the heat has time to disperse to the rest of the sensor. The third limit is peak envelope
power. This limit is usually determined by voltage breakdown phenomena that damages
sensor components.
Overload limits are usually stated on the manufacturer’s data sheet. None of the three limits
should be exceeded. The power limits of any sensor may be moved upward by adding an
attenuator to pre-absorb the bulk of the power. Then the power limits are likely to be
dictated by the attenuator characteristics, which, being a passive component, are often fairly
rugged and forgiving.
14
Table 3-1 shows that the 8481H power sensor, which consists of a 20-dB attenuator
integrated with a thermocouple sensor element, excels in resistance to overload. One
characteristic, which might be important but not obvious from the chart, is the ratio of
maximum average power to the largest measurable power. The 8481D PDB sensor can
absorb 100 mW (+20 dBm) of average power, yet the high end of its measurement range is
10 µW (–20 dBm). This means that the PDB diode is forgiving in situations where the power
level is accidentally set too high. A mistake of 10 dB in setting a source output attenuator,
during a measuring routine will merely cause an off-scale reading for the 8481D. The same
mistake might damage the other sensors. Excessive power is, by far, the primary cause of
power sensor failure.
The diode-stack-attenuator-diode stack topology of the Agilent E9300A average power
sensors provides a maximum average power specification of 316 mW (+25 dBm) and peak
power specification of 2 W (+33 dBm) for less than a 10 µS duration. These specifications
allow the E9300A sensors to handle the large crest factors typical of the newest signal
formats, such as W-CDMA and orthogonal frequency division multiplexing (OFDM), while
still maximizing the dynamic range.
Although intended for handling pulsed signals, peak and average sensors, typified by the
E9320A sensor, are not necessarily more immune to overload limits. In fact, it might be
argued that the user needs to exert even more caution when using pulsed power signals,
especially if the actual peak power is unknown. One simple way to do this is to insert a step
attenuator between the unknown pulsed power and the peak and average sensor and set in
an appropriate amount of attenuation. Since peak and average sensors have an excellent
dynamic range, the meter will indicate some peak power on the high sensitivity ranges. At
that point, the user can determine whether the test signal peak power will do damage to the
sensor.
Table 3-1. Overload characteristics for various types of power sensors.
Maximum
average power
8478B
8481A
Thermistor Thermosensor
couple
sensor
8481H
Thermocouple
sensor
8481D
Diode
sensor
E4412A
Extendedrange diode
sensor
E9300A
Two-path
diode sensor
E9320A
Peak and
average
diode sensor
N1921A/22A
Peak and
average
diode sensor
30 mW
3.5 W
100 mW
200 mW
316 mW
200 mW
200 mW
300 mW
Maximum
energy per pulse 10 W • µS
30 W • µS 100 W • µS See Footnote 1 See Footnote 1 See Footnote 1 See Footnote 1 See Footnote 1
Peak power
15 W
200 mW
100 W
100 mW
200 mW
2 W (< 10 µS) 1 W (< 10 µS) 1 W (< 1 µs)
1. Diode device response is so fast, device cannot average out high-energy pulses
15
Signal waveform effects
While the waveform considerations were fully covered in Fundamentals Part 2, Chapter IV, it
is well to consider the waveform factor as a differentiator for the various meters and sensor
technology. Briefly, the thermistor is a totally heat-based sensor and therefore the
thermistor sensors handle any input waveform with any arbitrary crest factor, that is, they
are true square law sensing elements.
Thermocouple sensors are full square law sensing for the same reason, but thermocouples
operate beyond the thermistor high power limit of 10 mW, all the way to 100 mW and 3 W
for the 848X H-models, which have the integrated fixed pads. The 8481B features a 25-watt
external characterized attenuator and operates from 10 MHz to 18 GHz for medium power
applications.
PDB-diode-based sensors of the 8481D family feature full square-law performance because
their operating power range is limited to a top level of –20 dBm, thus restricting their meter
indications to the square-law range of diodes. The user should assure that peak power
excursions do not exceed –20 dBm.
The E Series diode sensors (E441XA CW, E9300 average and E9320 peak and average)
require simple attenuation to their input signal characteristics. CW signals may be applied
all the way from –70 to +20 dBm with confidence and accuracy, using the E441XA sensors.
E932X peak and average sensors are intended for characterizing the power of complex
modulation formats. Thus their main purpose is to high-speed sample the detected power
envelope and compute various types of power parameters. When used with their companion
EPM-P power meter, the 20 Msa/s data sampling rates permit fast data acquisition and
combined parameter outputs, such as peak-to-average power ratios for specified time-gated
periods as defined by wireless system specifications.
The N1921/22A peak and average power sensors, when used with the N1911/12A power
meters, provide up to 30 MHz of video bandwidth. They are intended to measure output
power and timing parameters of fast radar pulses and wide bandwidth wireless signal
formats such as W-CDMA, WLAN, OFDM, WiMAX, and others ranging from -35 to 20 dBm.
16
Computed data and analyzer software package
As fully described in Chapter V of Fundamentals Part 2, the design strategy for the EPM-P
power meters includes highly-versatile user-selectable data computation and display
features. Gated data features the ability to set specific gate periods for time-selective power
periods, and then combine several data points into more desired system parameters such as
peak-to-average power ratios.
The EPM-P power meters support the innovative and powerful Agilent VEE analyzer
software package, which places the meters totally in the control of the user’s PC or laptop.
This VEE software package is available free of charge.[1] It operates via the GPIB, and
provides the statistical, power, frequency, and time measurements that are required for
CDMA and TDMA signal formats. The CD-ROM package includes a VEE installation program.
The statistical package includes the ability to capture
1. cumulative distribution function (CDF)
2. complementary CDF (CCDF or 1-CDF)
3. probability density function (PDF)
These are crucial diagnostic parameters for system signals like CDMA formats. For example,
analyzing such power distribution computations can reveal how a power amplifier may be
distorting a broadband signal that it is transmitting. Or a baseband DSP signal designer can
completely specify the power distribution characteristics to the associated RF subsystem
designers.
For traditional pulse work, the analysis package also includes a powerful pulse
characterization routine. It computes and displays the following power parameters: pulse
top, pulse base, distal, mesial, proximal, peak, average, peak/average ratio, burst average,
and duty cycle. It does the same for these time and frequency parameters: rise time, fall
time, pulse repetition frequency (PRF), pulse repetition interval (PRI), pulse width and off
time. All of these pulsed power parameters were originally defined with the 1990
introduction of the Agilent 8990A peak power analyzer, and are described in Chapter II of
Fundamentals Part 1.
[1] CD-ROM: EPM and EPM-P Series Power Meters, part number E4416-90032.
This CD-ROM contains the power meters and sensors Learnware (User’s Guides, Programming Guides,
Operating Guides and Service Manuals). The CD-ROM also contains technical specifications, data
sheets, product overviews, configuration guides, application and product notes, as well as power meter
tutorials, analyzer software for the EPM-P power meters, IVI-COM drivers, IntuiLink toolbar for the EPM
power meters and VXIplug&play drivers for the EPM power meters.
This versatile CD-ROM package is shipped free with every EPM and EPM-P series power meter. Most of
the information is also available at www.agilent.com/find/powermeters.
17
IV. Capabilities Overview
of Agilent Sensors and
Power Meters
An applications overview of Agilent sensors
In general, power sensors are designed to match user signal formats and modulation types.
Similarly, power meters are designed to match the user’s testing configurations and
measurement data requirements. Thus, it is the user’s responsibility to understand the test
signals in detail, the technology interaction with the sensor capabilities, and combine those
results with the optimum power meter to match the data output needs of the test
combination.
Table 4-1 presents an overview of the most common signal formats in various industry
segments and suggests appropriate sensor technologies that can characterize them. (Since
Agilent thermistor sensor/meter technology is almost uniquely metrology-and traceabilitybased, they are not included in Table 4-1.)
Table 4-1. Agilent sensor vs signals application chart
Signal characteristics
CW
CW
Pulse/
averaged
Pulse/
profiled
AM/FM
Metrology
lab
Radar/
navigation
Radar/
navigation
Mobile
radio
TDMA
GSM
EDGE
NADC
IDEN
cdmaOne
Bluetooth™
W-CDMA
cdma2000
802.11a/b/g
MCPA
HiperLan2
WiMAX
Thermocouple
sensors
•
•
•
•
Avg. only
•
Avg. only
•
Avg. only
•
Avg. only
Diode sensors
•
•
•
•
Avg. only
•
Avg. only
•
Avg. only
•
Avg. only
Diode sensors
compensated
for extended
range
•
Two-path diodestack sensors
•
•
•
•
Avg. only
•
Avg. only
•
Avg. only
•
Avg. only
Peak and
average diode
sensors (video
BW)1
•
•
•
(30 MHz)
•
(300 kHz)
Typical applicatioan
examples
Sensor technology
Modulated
Wireless standards
FM only
•
(30 MHz)
1. The video bandwidth is sometimes referred to as the modulation bandwidth.
18
(1.5 MHz)
peak, avg,
peak/ avg
(5 MHz)
peak, avg,
peak/ avg
(30 MHz)
peak, avg,
peak/ avg
A capabilities overview of Agilent power meters
Once the signal and modulation format leads you to the best sensor choice, the power
meter decision is straightforward. Table 4-2 compares the performance of Agilent’s present
power meter line of products. Generally the Agilent EPM, EPM-P, and new P-Series power
meters are completely backward compatible with all diode and thermocouple sensors. This
includes sensors of a vintage from several decades back. So, the power meter decision
becomes mostly a matter of single vs. dual channel capability. The VXI power meter
(E1416A) would be chosen for installations which use the plug-in instrumentation concept.
Table 4-2. Agilent’s family of power meters
Agilent model Name
Remarks
Peak and average power meters P-Series
N1911A
Single-channel
Digital, programmable, peak, and average
measurements, uses N1921/22A sensors. Perform
accurate and repeatable power, time, and
statistical measurements up to 300 MHz (video
bandwidth).
100 Msamples/sec. Continuous sampling.
N1912A
Dual-channel
Two channel version of N1911A. Measures and
computes parameters between the two sensors.
Peak and average power meters EPM-P series
E4416A
Single-channel
Digital, programmable, peak and average measurements, uses E9320 series sensors. Innovative
time-gated pulse-power measurements. 20 M
samples/sec.
E4417A
Dual-channel
Averaging power meters EPM series
E4418B
Single-channel
E4419B
Dual-channel
System power meter
E1416A
VXI power meter
Thermistor power meter
432A
Thermistor power
19
Two-channel version of E4416A, plus measures
and computes parameters between the two
sensors.
Digital, programmable, uses E-series and 8480
series sensors, reads EEPROM-stored sensor cali
bration factors of E-series sensors.
Two-channel version of E4418B, plus measures
and computes parameters between the two
sensors
Has functional performance features of previous
model 437B; uses all 8480-series sensors
DC-substitution, balanced-bridge technology, ideal
for meter reference power transfers
Table 4-3 presents a compatibility chart for combinations of sensors and meters.
Table 4-3. Agilent power meter/sensor compatibility chart
Agilent power meters
Type >
Agilent
power sensors
Thermocouple
8480A/B/
H-family
R/Q8486A W/G
(11 models)
Diode
8480D-family
8486A/D-W/
G-family
(7 models)
Diode sensors
with extended
range
E4412A/13A
(2 models)
Two-pathdiode-stack
E9300 family
(7 models)
Peak and
average
sensors
E9320 family
(6 models)
P-Series
peak, average,
and time gating
N1911A single Ch
N1912A dual Ch
EPM-P series
peak, average and
time gating
E4416A single Ch
E4417A dual Ch
EPM series
averaging
E4418B single Ch
E4419B dual Ch
Thermistor
power meter
432A
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Thermistor
sensors
478 coaxial
486 waveguide
(6 models)
Peak and
average
sensor
N1921A/22A
30 MHz video
bandwidth
(2 models)
System power
meter
E1416A VXI
•
•
20
Finally, the last user-selection step is to decide on the specific power sensor, which
matches the signal format, power dynamic range and frequency range of the application.
The technology considerations of Fundamentals Part 2 might be consulted for more detail
on performance and functionality. The five charts of Table 4-4 classify the various sensor
technologies and their frequency and power level coverage.
Table 4-4. Agilent’s five families of power sensors
Thermocouple sensors
Sensor family
Technology
Max. dynamic
range
Frequency range1
Power range1
Signal type
8480 series
Thermocouple
50 dB
100 kHz to 50 GHz
–30 to +44 dBm
All signal types,
unlimited bandwidth
Thermocouple sensors
Max. measurement
speed (rdgs/sec)
40 (x2 mode)
8482B
8481B
25 W,
0 to +44 dBm
3 W,
–10 to +35 dBm
8482H
8481H
8482A
8481A
8485A
100 mW, –30 to +20 dBm
Option 33
8487A
8483A .75 Ω
100 kHz
10 MHz
50
100
500 MHz 1 GHz
R8486A
W/G
Q8486A
W/G
4.2
2
Frequency
33
26.5
18.0
40
75
50
110 GHz
Diode sensors
Sensor family
Technology
Max. dynamic
range
Frequency range1
Power range1
Signal type
8480 series
Diode
50 dB
10 MHz to 110 GHz
–70 to –20 dBm
All signal types,
unlimited bandwidth
40 (x2 mode)
8481D
Diode sensors
10 μW,
Max. measurement
speed (rdgs/sec)
8485D
Option 33
–70 to –20 dBm
8487D
W/G
W8486A
R8486D
W/G
Q8486D –30 to +20dBm
V8486A
–30 to
+20dBm
100 kHz
10 MHz
50
100
500 MHz 1 GHz
2
4.2
Frequency
1. Sensor dependent
21
18.0
26.5
33
40
50
75
W/G
W/G
110 GHz
Extended range diode sensors
Sensor family
Technology
E-series: CW
E4412A
E4413A
Single diode pair
Max. dynamic
range
Frequency range1
Power range1
Signal type
90 dB
10 MHz to 26.5 GHz
–70 to +20 dBm
CW only
Extended dynamic
range diode sensors
Max. measurement
speed (rdgs/sec)
200 (fast mode)
E4412A
100 mW, –70 to +20 dBm
E4413A
100 mW, –70 to +20 dBm
100 kHz
Option H33
10 MHz
50
100
500 MHz
1 GHz
2
4.2
Frequency
18.0
26.5
33
40
50
75
110 GHz
Two-path diode stack sensors
Sensor family
E-series:
average power
sensors E9300
Technology
Diode-attenuatordiode
Max. dynamic
range
Frequency range1
Power range1
80 dB
9 kHz to 18 GHz
–60 to +44 dBm All signal types
unlimited bandwidth
Max. measurement
speed (rdgs/sec)
Signal type
200 (fast mode)
Two path diode
stack sensors
100 mW, –60 to +20 dBm
E9300A
Option H24
E9301A
100 mW, –60 to +20 dBm
E9304A
100 mW, –60 to +20 dBm
Option H18
E9300H
1 W, –50 to +30 dBm
E9300A Option H25
1 W, –50 to +30 dBm
E9301H
1 W, –50 to +30 dBm
E9304A Option H19
1 W, –50 to +30 dBm
E9300B
25 W, –30 to +44 dBm
E9301B
25 W, –30 to +44 dBm
9 kHz 100 kHz 1 MHz
10
50 MHz
100 MHz 500
1 GHz
Frequency
1. Sensor dependent
22
6
18.0 24.0 26.5
33
40
50 GHz
Peak and average sensors
Sensor family
E9320-series2
peak and average
E9321/22/23A
E9325/26/27A
N1921/22A3
peak and average
sensors
Technology
Max. dynamic
range
Frequency range1
Power range1
Signal type
Max. measurement
speed (rdgs/sec)
Single diode
pair, two-path
85 dB
50 MHz to 18 GHz
–65 to +20 dBm
CW, avg, peak,
pk/avg, TDMA,
W-CDMA
Up to 1000
Single diode pair
built-in voltage
reference for
internal zero and
calibration
55 dB
50 MHz to 40 GHz
–35 to +20 dBm
CW, avg, peak,
pk/avg, TDMA,
W-CDMA, radar
Up to 1500
E9321A 300 kHz
100 mW,
Avg. only: –65/60/60 to +20 dBm
Normal –50/45/40 to +20 dBm
E9322A 1.5 MHz
E9323A 5 MHz
E9325A 300 kHz
100 mW,
Avg. only: –65/60/60 to +20 dBm
Normal –50/45/40 to +20 dBm
E9326A 1.5 MHz
E9327A 5 MHz
N1921A 30 MHz
100 mW,
–35 to +20 dBm
N1922A 30 MHz
100 kHz 1 MHz
10
50 MHz
100 MHz 500
1 GHz
6
18.0
26.5
33
40
50 GHz
Frequency
1. Sensor dependent
2. Peak and average sensors must be used with an E9288A, B, or C sensor cable, and only operate
with the E4416A/17A power meters
3. E9320 Series sensors, when used with the E4416A/17A power meters, must be used with E9288A/
B/C sensor cables. When used with the N1911A/12A power meters, they must be used with the
N1917A/B/C sensor cables.
23
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