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Fundamentals of RF and Microwave Power Measurements (Part 4)

Notas de Aplicação

Table of Contents

  • Introduction
  • A Review of Various Power Measuring Instrumentation
    • Instrument alternatives for measuring RF/microwave power
    • Types of superheterodyne for measuring power
    • Power measurement considerations for superheterodyne instruments
    • Power measurement considerations for test-set-type instruments
  • Power Sensor/Meter Methods and Comparisons
    • Accuracy vs. power level
    • Frequency range and SWR (reflection coefficient)
    • Waveguide sensor calibration
    • Speed of response at low signal levels
    • Automated power measurement
    • Susceptibility to overload
    • Signal waveform effects
    • Computed data and analyzer software package
  • Capabilities Overview of Keysight Sensors and Power Meters
    • An applications overview of Keysight sensors
    • A capabilities overview of Keysight power meters

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 the 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 Keysight sensors and meters. It includes a functionality chart for the compatibility of sensors with meters. Some early sensor technologies like thermocouples work with all Keysight 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 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.

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.

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