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How to Choose the Right Power Meter and Power Sensor

Application Notes

Introduction

Design and manufacturing of engineering processes for RF/microwave systems have reached a status undreamed of a decade ago. Wireless communication engineers especially, faced with aggressive project schedules, must quickly select and configure power-measurement equipment that provides the accuracy and repeatability required by their innovative new modulation formats. New wireless technologies needed to support wideband data transmissions now demand power measuring instruments and sensors for average power, as well as time-gated and peak power profiles and peak-to-average ratios, with all those measurements delivered at high measurement data rates.

 

In general, power sensors are designed to match user signals and modulation types. Power meters are designed to match the user’s measurement data requirements. That’s why you can choose from a versatile line of 33 different power sensors and six power meters from Keysight Technologies, as shown in Table 1. In addition, Keysight offers many custom configurations for ATE system applications and other calibration, traceability, and quality processes.

 

This product note outlines applications considerations and the newest sensor technologies available from Keysight Technologies. It includes Keysight’s new power meters and a family of peak and average sensors, designed for pulsed power and the complex modulation signals of wireless communications markets. It also reviews the families of thermocouple, diode, and two-path diode-attenuator-diode sensors. It discusses the advantages and disadvantages of each sensor technology as they apply to current and near-future wireless system advances.

 

Not discussed is the Keysight family of thermistor sensors and the associated Keysight 432A power meter. This venerable technology now is used almost exclusively for the standardization and traceability of power measurements by the U.S. National Institute of Standards and Technology and other international standards agencies. Because the Keysight 432A power meter and thermistor sensor technology is based on the highly precise DC-substitution method, the sensors are used as transfer standards, traveling between the user’s primary lab and the NIST measurement services laboratory.

 

Users interested in such metrology power-transfer processes may request Keysight’s AN 64-1, 5965-6630E and AN 64-4, 5965-8167E application notes.

 

Table of Contents

  • Power Measurements on Complex Modulation Wireless Signals
  • Understanding Sensor Technologies
  • Extended Dynamic Range Diode Sensors
  • Two-Path Diode-Stack Sensors
  • Peak and Average Power Sensors
  • Wideband Power Sensors
  • USB Power Sensors
  • Applications
  • Pre-Defined Measurement Setups
  • External Calibration-Free Measurement
  • Comprehensive Power, Time, and Statistical Measurements
  • Keysight Sensor Families
  • Keysight Power Sensor Characteristics

 

Power Measurements on Complex Modulation Wireless Signals

Digital vector modulation became the modulation of choice as the digital revolution swept over communication systems some 20 years ago. The need to pack the maximum amount of digital data into the limited spectrum of cellular and data transmission systems made it an obvious choice. RF power measurements for these new complex phase/amplitude formats call for careful application analysis of the test signals.

 

The advent of wireless communications technology accelerated the migration from analog to digital modulation formats. Soon came an alphabet soup of digital modulation formats: BPSK, QPSK, 8-PSK, 16 QAM, etc. Then came important variations such as pi/4-DQPSK. Many systems used data streams that depended on time-division multiple-access (TDMA) technology (example: GSM). Other system developers introduced a highly competitive code-division multiple access (CDMA) format (recent example; IS-95A).

 

Having transmitters at both the base stations and in the individual wireless handsets requires the most creative designs to preserve the frequency spectrum and reduce power drain. Whether you use a TDMA system, which feeds multiple carriers through a common output amplifier, or a CDMA system, which encodes multiple data streams onto a single carrier with a pseudo-random code, the resulting transmitted power spectrum features almost white-noise-like characteristics.

 

Just like white noise, the average power of the transmitted signal is only one of the important parameters. Because of the statistical nature of multiple carrier systems, the signal peak-to-average power ratio is crucial, because instantaneous peak powers can approach ratios of 10 to 30 times the average power, depending on formats and filtering.

 

Those high peak-to-average power ratios imply dangers in the saturation of the output power amplifiers. When saturation occurs, the outer symbol locations compress, increasing bit errors and system unreliability. System designers handle this effect by “backing off” the power amplifiers from their maximum peak ratings to assure that signal peak power operation is always within their linear range.

 

Therefore, all of these technologies require precise characterization of the pulse performance of their systems’ amplifier power output, including peak-to-average power ratios and time-gated parameters for profiling the pulsed signals, which ensure conformity to specified limits.

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