Agilent Fundamentals of RF and Microwave Power Measurements Application Note 64-1C Classic application note on power measurements Newly revised and updated Table of Contents I. Introduction The importance of power.................................................................................................................................5 A brief history of power measurement .........................................................................................................6 A history of peak power measurements........................................................................................................7 II. Power Measurement Units and definitions........................................................................................................................................9 IEEE video pulse standards adapted for microwave pulses......................................................................13 Peak power waveform definitions ..................................................................................................................14 New power sensors and meters for pulsed and complex modulation…...................................................15 Three methods of sensing power ...................................................................................................................15 Key power sensor parameters .......................................................................................................................16 The hierarchy of power measurement, national standards and traceability .........................................17 III. Thermistor Sensors and Instrumentation Thermistor sensors ..........................................................................................................................................19 Coaxial thermistor sensors . ...........................................................................................................................20 Waveguide thermistor sensors........................................................................................................................20 Bridges, from Wheatstone to dual-compensated DC types .......................................................................20 Thermistors as power transfer standards.....................................................................................................22 Other DC-substitution meters.........................................................................................................................22 Some measurement considerations for power sensor comparisons.........................................................23 Typical sensor comparison system ................................................................................................................23 Network analyzer source system ....................................................................................................................24 NIST 6-port calibration system.......................................................................................................................25 IV. Thermocouple Sensors and Instrumentation Principles of thermocouples ..........................................................................................................................26 The thermocouple sensor ................................................................................................................................27 Power meters for thermocouple sensors.......................................................................................................31 Reference oscillator ..........................................................................................................................................33 EPM series power meters ................................................................................................................................33 V. Diode Sensors and Instrumentation Diode detector principles ................................................................................................................................35 Using diodes for sensing power......................................................................................................................37 Wide-dynamic-range CW-only power sensors ..............................................................................................39 Wide-dynamic-range average power sensors................................................................................................40 A new versatile power meter to exploit 90 dB range sensors...................................................................42 Traceable power reference ..............................................................................................................................44 Signal waveform effects on the measurement uncertainty of diode sensors ........................................45 2 VI. Peak and Average Diode Sensors and Instrumentation Pulsed formats ................................................................................................................................................48 Complex modulation wireless signals.........................................................................................................49 Other formats..................................................................................................................................................50 Peak and average power sensing .................................................................................................................51 EPM-P series power meters ..........................................................................................................................53 Computation power........................................................................................................................................55 Bandwidth considerations............................................................................................................................56 Versatile user interface..................................................................................................................................57 VII. Measurement Uncertainty An international guide to the expression of uncertainties .....................................................................58 Power transfer, generators and loads ........................................................................................................59 RF circuit descriptions ..................................................................................................................................59 Reflection coefficient ....................................................................................................................................61 Signal flowgraph visualization.....................................................................................................................62 Mismatch uncertainty....................................................................................................................................66 Mismatch loss and mismatch gain...............................................................................................................67 Other sensor uncertainties ...........................................................................................................................67 Calibration factor ..........................................................................................................................................68 Power meter instrumentation uncertainties ............................................................................................69 Calculating total uncertainty........................................................................................................................71 Power measurement equation .....................................................................................................................72 Worst case uncertainty..................................................................................................................................74 RSS uncertainty..............................................................................................................................................75 New method of combining power meter uncertainties ...........................................................................76 Power measurement model for ISO process ..............................................................................................77 Standard uncertainty of mismatch model.................................................................................................79 Example of calculation of uncertainty using ISO model .........................................................................80 VIII. Power Measurement Instrumentation Compared Accuracy vs. power level ...............................................................................................................................84 Frequency range and SWR (reflection coefficient) ..................................................................................86 Speed of response ..........................................................................................................................................87 Automated power measurement .................................................................................................................88 Susceptibility to overload..............................................................................................................................89