PRINCIPLES OF POWER MEASUREMENT A Reference Guide on RF & Microwave Power Measurement REVISED EDITION Principles of Power Measurement A Primer on RF & Microwave Power Measurement As we constantly strive to improve our products, the information in this document gives only a general indication of the product capacity, performance and suitability, none of which shall form part of any contract. We reserve the right to make design changes without notice. This material has been complied from a number of sources and Wireless Telecom Group accepts no responsibility for errors or omissions. Parent company Wireless Telecom Group © Wireless Telecom Group, 2015. All trademarks are acknowledged . www.WirelessTelecomGroup.com About Us Wireless Telecom Group is a global designer and manufacturer of radio frequency (“RF”) and micro- wave-based products for wireless and advanced communications industries. We market our prod- ucts and services worldwide under the Boonton Electronics (“Boonton”), Microlab/FXR (“Microlab”) and Noisecom brands. Our Brands and products have maintained a reputation for their accuracy and performance as they support our customers’ technological advancements within communications. We offer our customers a complementary suite of high performance instruments and components meeting a variety of standards including peak power meters, signal analyzers, noise sources, power splitters, combiners, diplexers, noise modules and precision noise generators. We serve commercial and government markets within the satellite, cable, radar, avionics, medical, and computing applica- tions. We are headquartered in Parsippany, New Jersey, in the New York City metropolitan area and we maintain a global network of Sales offices dedicated to providing excellent product support. 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Our technological capabilities along with our customer service strategies remain essential competencies for our success. iii Principles of Power Measurement A Primer on RF & Microwave Power Measurement Table of Contents Section 1 RF and Microwave Power Measurement Fundamentals 1 Chapter 1: Power Measurement Basics 3 1.1 What is Power? 3 1.2 Why Measure Power? 5 1.3 Power Measurement History 7 Chapter 2: Power Measurement Technologies 11 2.1 Thermal RF Power Sensors 11 2.2 Detector (diode) RF Power Sensors 14 2.3 Receiver-based Amplitude Measurement 18 2.4 Monolithic Amplitude Measurement 19 2.5 Direct RF Sampling Amplitude Measurement 19 2.6 What is an RF Power Meter? 20 Chapter 3: CW, Average and Peak Power 23 3.1 CW Power Meter Limitations 23 3.2 The “Peak Power” Solution 24 3.3 It’s all about Bandwidth 25 3.4 Understanding the Importance of Dynamic Range 27 Section 2 Making Power Measurements 31 Chapter 4: Equipment Selection 33 4.1 Choosing the Right Power Meter 33 4.2 Choosing an RF Power Sensor 35 4.3 Selecting a Measurement Mode 37 4.4 Power Meters versus Spectrum Analyzers 40 4.5 Oscilloscopes and Detectors 48 Chapter 5: Calibration Issues 52 5.1 Factory Open-loop Calibration 52 5.2 Single, Double, and Multipoint Linearity Calibration 53 5.3 Field Linearity Calibration Methods 56 5.4 Frequency Response Correction 59 iv Chapter 6: RF Power Analysis 62 6.1 Continuous Measurements 63 6.2 Triggered and Pulse Analysis 64 6.3 Statistical Power Analysis 70 Chapter 7: Power Measurement Applications 76 7.1 Low Duty-Cycle Pulse Measurements 76 7.2 Statistical Analysis of Modern Communication Signals 81 7.3 Using Power Meters for EMC Testing 87 Chapter 8: Performance Tips 93 8.1 Reducing Measurement Noise 93 8.2 Optimizing ATE Performance 96 8.3 Communication Amplifier Testing 106 Chapter 9: Measurement Accuracy 112 9.1 Introduction to Uncertainty 112 9.2 Power Measurement Uncertainty Contributions 114 9.3 Sample Uncertainty Calculations 118 Section 3 Power Measurement Reference 127 Chapter 10: Reference Tables 128 10.1 Amplitude Measurement Conversions 128 10.2 Return Loss / Reflection Coefficient / VSWR Conversions 129 10.3 Wireless and Radar/Microwave Bands 131 10.4 Sensor Cable Length Effects 131 Chapter 11: Boonton Solutions 133 11.1 55 Series USB Peak Power Sensor 133 11.2 4500B RF Peak Power Analyzer 134 11.3 4540 Series RF Power Meter 135 11.4 4530 Series RF Power Meter 136 11.5 4240 Series RF Power Meter 137 11.6 Peak Power Sensors 138 11.7 CW Sensors 140 11.8 Most Popular Peak Sensors 142 Table of Contents v Section 1 RF & Microwave Power Measurement Fundamentals RF Power Measurement is a broad topic that has been of importance to designers and operators since the earliest days of wire line and wireless communication and infor- mation transmission. With today’s complex modulation schemes, increased popular- ity of wireless transmission and pulsed communication modes, the need to accurately and efficiently measure RF power has become crucial to obtaining optimum perfor- mance from communication systems and components. 1 What is Power? A discussion of the physical definition of power, and the electrical con- cepts of volts, amps, and watts. This leads to how the measurement of AC and RF power is complicated by complex impedance and phase shift. Why do we want to measure Power? There are many reasons to measure RF power, span- ning a wide range of industries and technologies. This subsection discusses the common uses of power measurement instruments, and the rationale. A brief history of RF power measurements. Power measurement has evolved consider- ably since the earliest days of wireless. Some of this history can be traced to well-known radio pioneers, and much of the innovation took place among companies still involved in the measurement industry. A considerable amount of history took place in the northern New Jersey region that is still home to Boonton Electronics. Power Measurement Technologies. A discussion of the key methods in use today for measuring RF power, including Thermal, Diode, Receiver-based, Direct RF Sampling, and Monolithic (IC) solutions. CW versus Peak Power. Power measurement has come a long way since early methods, which only produced meaningful measurements for unmodulated signals. This section focuses on the limitations of various types of power meters when measuring modulated signals, and how modern solutions have improved the situation. Bandwidth and Dynamic Range Issues. Not every signal aligns neatly with the capabili- ties of power measurement instruments. By understanding the bandwidth and dynamic range characteristics of your signal, it becomes easier to select the best measurement tech- nology. Section 1: RF & Microwave Power Measurement Fundamentals 2 Chapter 1: Power Measurement Basics 1.1 What is Power? In physics terms, power is the transfer rate of energy per unit time. Just as energy has many different forms (kinetic, potential, heat, electrical, chemical), so does power. One mechanical definition of energy is force multiplied by distance – the force moving an object multiplied by the distance it is moved. Energy = Force x Distance To get the power, or transfer rate of that energy, we divide that energy by the length of time to perform the move. Since distance per unit time is velocity, mechanical power is often computed as force times velocity. Power = Force x Distance / Time = Force x Velocity In electrical terms, force equates to voltage, also known as Electromotive Force (EMF). This describes how much “pressure” the electrons are under to move. The velocity is analogous to electrical current, which is the charge (number of electrons) per unit time. Power(electrical) = EMF x Current EMF is typically measured in volts, and current is typically measured in amperes, or amps. One ampere is one coulomb (a unit of charge, equal to 6.2 x 1018 electrons) per second. Multiplying current and voltage together yields the power in watts. Watts = Volts x Amps In the case of steady voltage or steady current flow, computing the average power is simple – just multiply average volts by average amps. However, if both values fluctuate, as will be the case with alternating current, or AC, the average power can only be computed by performing a mathematical average of the instantaneous power over one or more full sig- nal periods. Limiting our discussion only to sinusoidal, AC waveforms, we can see that the power will fluctuate in synchronization with the voltage and current. For resistive loads, the current and voltage will be in-phase. That is both will be positive at the same time, and both nega- tive. Analyzing graphically, one can see that either case produces positive power, since multiplying two negative numbers yields a positive result. See Fig. 1.1.1 3 If there is a phase shift between current and voltage, there will be times that the voltage and current are of opposite polarities, resulting in a negative power flow. This has the ef- fect of reducing the average power, even though the magnitude of the voltage and current has not changed. For this reason, it is not generally sufficient to simply measure the voltage or current to characterize a signal’s power. A direct power measurement is best, in which the signal is applied to a precision termination (load), which keeps the voltage and current very close to in-phase. If this is done properly, a voltage measurement across the load can be performed to yield a meaningful power value, or the dissipated power can be computed directly by measuring the heating effect of the signal upon the load. This is discussed in great detail in the next section. 50 ohm Resistive