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Keysight Technologies
Techniques for Precise Power
Measurements in the Field
Using FieldFox Handheld Analyzers




                                        Application Note




           Abstract
           This application note will discuss techniques for measuring average and peak power
           and the associated equipment options available for field testing. As there is a range
           of instrument types that can measure power, this application note will focus on the
           two most widely employed RF/microwave instruments, namely, the power sensor
           and the spectrum analyzer. Measurement examples will include signals such as CW,
           pulsed and digitally modulated waveforms. It is important to understand the accuracy
           and limitations of each instrument including factors that can affect the instruments'
           performance in the field. Comparisons between different power sensors and spectrum
           analyzers will be presented.
Introduction

     The output power level of a system is the critical factor in the design and performance of almost all radio
     frequency (RF) and microwave equipment [1]. The transmitted and received power levels determine the overall
     performance of any communication or radar system. The measurement of signal power is so important that it is
     measured at every stage of a system beginning with the initial design and prototyping of individual components,
     during system manufacturing and qualification testing, during on-site system installation and lastly as part of
     periodic maintenance and troubleshooting in the field.

     Given that power measurements have important ramifications to the performance and compliance of a system, it
     is important that power measurements can be duplicated at different times and at different places. This requires
     high performance instruments (accuracy) delivering measurements that are stable under various environmental
     and operating conditions (repeatability). It is also very important that all measured results, regardless of the
     equipment, have a common agreement as to what is considered an absolute value for the power measurement
     (traceability).

     This application note will discuss techniques for measuring average and peak power and the associated
     equipment options available for field testing.
Power Measuring Equipment Configurations

There are a variety of instruments capable     Selection of the power measurement
of measuring RF and microwave power            equipment involves a trade-off between
including power sensors and meters,            accuracy, frequency range, dynamic
signal analyzers, spectrum analyzers, and      range, portability, durability and warm-up
network analyzers [2]. This application        time. This application note will review the
note will discuss field-capable instruments,   benefits and tradeoffs when using a power
namely, the power sensor and the spec-         sensor and the analyzer's CPM to measure
trum analyzer. There are several configura-    the power of simple and complex signals
tions for connecting a power sensor to a       in the field.
measurement and display system including
a traditional power meter, a laptop and a
spectrum analyzer. For example, Figure 1
shows different configurations when
using combinations of power sensors and
spectrum analyzers to accurately measure
average and peak power in the field. The
power sensor can be configured with
a separate power meter, as shown in
Figure 1a, or connected to a laptop using
a power sensor having a USB port, as
shown in Figure 1b. The meter and laptop
                                               A) Power meter and sensors                           B) Power sensor and laptop
configurations can support a large number
of measurement channels, up to 4 sensors
when using a power meter and up to 20
sensors when using a laptop running the
appropriate power analysis software.

Another option, shown in Figure 1c,
connects a USB power sensor directly to
a spectrum analyzer. In this configuration,
the spectrum analyzer displays the power
measurement thus eliminating the need
to carry a power meter or laptop into the      C) Power sensor and analyzer                         D) Spectrum analyzer
field. This spectrum analyzer configura-
tion is very convenient when spectrum          Figure 1. Equipment configurations for measuring average and peak power
testing is also required as part of the
system installation, maintenance and/or
troubleshooting. One last option, shown in
Figure 1d, uses a spectrum analyzer with
a built-in channel power meter (CPM) to
directly measure the signal power without
the need for an external power sensor.
In this configuration, the analyzer's tuned
receiver measures the average power and
a short jumper cable is used to connect
the analyzer to the system's test point.




                                                                    3
Power Sensors and Signal Processing

A typical configuration of a power sensor          When using a power sensor and separate        and aging of the sensor element. Keysight
and meter includes a sensor element                power meter, the signal processing            power sensors and meters include a preci-
(detector) followed by analog and digital          functions are contained in the meter, as      sion 50 MHz reference oscillator whose
signal processing components. Figure 2             shown in Figure 2 (red box). When using       output power is controlled with great
shows a simplified block diagram of a              a USB-based power sensor, the signal          precision (

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