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Fundamentals of RF and Microwave Power Measurements (Part 2)

Application Notes

Table of Contents

  • Introduction
  • Thermistor Sensors and Instrumentation
    • Thermistor sensors
    • Coaxial thermistor sensors
    • Bridges, from Wheatstone to dual-compensated DC types
    • Thermistors as power transfer standards
  • Thermocouple Sensors and Instrumentation
    • Principles of thermocouple
    • The thermocouple sensor
    • Linearity characteristics for thermocouple and thermistor sensors
    • Power meters for thermocouple sensors
    • Traceable power reference oscillator
    • EPM Series power meters
  • Diode Sensors and Instrumentation
    • Principles of diode detectors
    • Using diodes for sensing power
    • Wide-dynamic-range CW-only power sensors
    • Wide-dynamic-range average power sensors
    • Traceable power reference oscillator
    • Signal waveform effects on measurement uncertainty of diode sensors
  • Peak and Average Diode Sensors and Instrumentation
    • Traditional pulsed modulation formats 
    • Complex modulation of wireless formats 
    • Other modern signal formats 
    • Peak and average power sensing
    • EPM-P Series power meters
    • Computation power-gated data concept
    • Video bandwidth considerations
    • Versatile user interface
    • Measurement considerations for traditional peak pulses
    • Analysis software package for complex modulation manipulations
    • P-Series power meter
    • Internal zero and calibration
    • Bandwidth considerations
    • Versatile user interface
    • Applications
    • Flexible configurations
    • Predefined measurement setups
    • Measurement speed
    • Theory of operation
    • CCDF measurement
    • Special peak sensor calibration for temperature and range
    • Recent research on linearity and pulse-shape characterization of peak and average sensors


The purposes of Keysight Technologies, Inc. new series of Fundamentals of RF and Microwave Power Measurements application notes, which were leveraged from former note AN 64-1, is to:

  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 a new meter and sensor technology.
  3. Present the latest modern power measurement techniques and test equipment that represents the current state-of-the-art.

Part 2 of this series, Power Sensors, and Instrumentation, is a comprehensive overview of the broad array of power sensors and instrumentation available today.

Chapter 2 starts with a brief look at the classic thermistor sensor and dc-substitution meter technology. Since thermistors remain the predominant method to trace standard power from National Measurement Institutes such as the National Institute of Standards and Technology (NIST), Keysight maintains a significant line of such products.

Chapter 3 introduces the thermocouple sensors and instruments that came on the scene in the early 1970s. They featured a truly dramatic increase in range and lowered uncertainty of measurements because the sensors featured full square-law conversion and a wider dynamic range. They had significantly lower SWR, which reduced measurement uncertainty greatly. And they were far more rugged than thermistors. Soon, Keysight enhanced them by combining them with integral, calibrated attenuators for accepting input powers up to 25 watts.

Chapter 4 describes the theory and practice of diode-based power sensors. Although diodes were used for power detection as far back as World War II, it took Keysight’s introduction of the 8484A diode power sensor in 1975 to provide broadband matching to the coaxial structure and temperature isolation from external elements to make the technology succeed. Since diodes were 30 dB more sensitive, and also exhibited true square conversion for about the lowest 50 dB of range, they quickly achieved an important place in user requirements.

Chapter 5 presents the latest technology for applying diode sensors to the characterization of RF/microwave signals with complex modulation typical of wireless systems, or pulsed formats typical of radar and navigation systems. Such complex modulations required wider-bandwidth instrumentation to accommodate the fast pulses and wideband digital modulations of the wireless technologies. But, in addition, new microprocessor-based technology plus the digital sampling data capture of the new instrumentation permitted dramatic expansion of computed characterization of power, such as peak-to-average ratios. New research data is provided for peak and average sensor linearity and pulse shape characterization.

Note: In this application note, numerous technical references will be made to the other published parts of the Fundamentals of RF And Microwave Power Measurements series. For brevity, we will use the format Fundamentals Part X. This should ensure 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.

Thermistor Sensors and Instrumentation

Bolometer sensors, especially thermistors, have held an important historical position in RF/microwave power measurements. However, in recent years thermo- couple and diode technologies have captured the bulk of those applications because of their increased sensitivities, wider dynamic ranges, and higher power capabilities. Yet, thermistors are still the sensor of choice for power transfer standards because of their DC power substitution capability. The following material reviews the basic theory and operation of thermistor sensors and their associated dual-balanced bridge power meter instruments.

Bolometers are power sensors that operate by changing resistance due to a change in temperature. The change in temperature results from converting RF or microwave energy into heat within the bolometric element. There are two principal types of bolometers, barretters, and thermistors. A barretter is a thin wire that has a positive temperature coefficient of resistance. Thermistors are semiconductors with a negative temperature coefficient. Barretters are no longer used.

The thermistor sensor used for RF power measurement is a small bead of metallic oxides, typically 0.4 mm diameter with 0.03 mm diameter wire leads. Thus, the balanced-bridge technique always maintains the thermistor element at a constant resistance, R, by means of DC or low-frequency AC bias. As RF power is dissipated in the thermistor, tending to lower R, the bias power is withdrawn by an equal amount to balance the bridge and keep R at the same value. That decrease in bias power is then displayed on a meter to indicate RF power.


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