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Accurate Absolute and
Relative Power Measurements
Using the Agilent N5531S
Measuring Receiver System
Application Note
Table of Contents Introduction
Introduction .................................................................................2 The Agilent N5531S measuring receiver system is the
N5531S Measuring Receiver System ....................................3 successor to the venerable (and now discontinued) 8902A
N5532A/B sensor module ...................................................3 measuring receiver. The center piece of the N5531S is
Two-resistor power splitter .............................................3 an optional firmware personality module for the Agilent
3 dB pad .............................................................................3 PSA Series spectrum analyzers (Option 233). The N5531S
Power sensor.....................................................................4 measuring receiver system matches or outperforms the
Functional summary ..............................................................5 8902A in every operating mode, while extending the
fundamental operating frequency range from 1.3 GHz
RF Power Measurement...........................................................6 (8902A) to as high as 50 GHz.
Example: Verifying the power-level accuracy of
a signal generator ..................................................................6 Accurate power-level measurement was a key capability in
Measurement procedure .................................................6 the 8902A that has been carried over to the N5531S. This
Applying the specification...............................................7 capability is essential for the calibration of signal-generator
output level and step-attenuator accuracy.
Tuned RF Level Measurement .................................................8
Specifications .........................................................................8
Among the more important performance parameters for any
Supporting information for specifications .........................9
signal generator are the absolute and relative accuracies of
Range-to-range cal factor correction .................................9
its output power level. The highest-accuracy measurement
Absolute TRFL measurement ............................................11
of the output level of a signal generator requires a power
Measurement procedure ...............................................11
meter and sensor. However, modern power sensors can
Applying the specification.............................................11
measure only part of the output range of modern signal
Relative TRFL measurement ..............................................11
generators. Below -70 dBm, some other method must
Measurement procedure ...............................................12
be used. The traditional approach has been to check the
Applying the specification.............................................12
relative accuracy of the generator's output level for power
Appendix A: Derivation of Absolute RF Power Accuracy...13 levels below the range of power sensors. The power meter
Measurement equation ......................................................13 is employed to establish a reference power level, and a
Uncertainty equation ...........................................................14 sensitive, linear measuring receiver is used to verify level
accuracy relative to that reference down to -140 dBm.
Appendix B: Derivation of Tuned-RF-Level (TRFL)
Amplitude Uncertainty ............................................................22
Detector linearity .................................................................24
Residual noise ......................................................................27
Range-to-range cal factor...................................................29
Combined uncertainty for PTRFL measurement .............30
Appendix C: Two-Resistor Versus
Three-Resistor Power Splitter Choice ................................33
2
N5531S
Measuring Receiver System
As shown in Figure 1, the N5531S measuring receiver
system consists of an E444xA PSA Series high-performance
spectrum analyzer (with measuring receiver personality,
Option 233), a P-Series power meter (N1911A or N1912A)
or an EPM power meter (N1913A or N1914A), and an
N5532A/B sensor module.
Agilent P-Series
power meter
N5532A/B sensor module
HWCI 2
3-dB pad Power
Splitter sensor
RF LAN cable
input
HWCI3
Figure 1: N5531S measuring receiver system Agilent PSA Series
spectrum analyzer
N5532A/B sensor module HWCI 1
The sensor module includes a low-frequency, multi-signal
cable between the power sensor output and the power
meter, and a coaxial RF cable between the sensor module
and the spectrum analyzer input. Figure 2 shows the
internal block diagram of the N5532A sensor module. The
newer N5532B sensor module is the "drop-in" replacement
of the N5532A, using the N848X power sensor in lieu of the
848X.
Two
resistor
splitter
3 dB Pad 848x power sensor
Figure 2: N5532A sensor module (N5532B has similar architecture but uses an N848x power sensor instead)
Two-resistor power splitter
The details of the two-resistor power splitter, especially
why it was chosen over a three-resistor power splitter, are
covered in Appendix C.
3 dB pad
At the expense of 3 dB of signal loss, this pad improves
the match between the output of the power splitter and the
input of the power sensor. It does this by attenuating the
reflected voltage waves moving in either direction (toward
or away) from the power sensor.
3
Power sensor
The N5532A sensor module contains an Agilent 848XA In addition, they are more rugged than thermistors, measure
power sensor. The N5532B sensor module contains an power levels as low as 1.0 W (