Power Measurement Basics BLS 11/96 1 Welcome to Power Measurement Basics

Power Measurement Basics BLS 11/96 2 Agenda Importance and definitions of power measurements Types of power measurements Measurement uncertainty Sensor types and power meters Considerations in choosing power measurement equipment

Power Measurement Basics BLS 11/96 3 Power too low – Signal buried in noise Importance of Proper Power Levels Power too high – Nonlinear distortion can occur – Or even worse! RL 0.0 dBm ATTEN 10 dB 10 dB / DIV START 150 MHz STOP GHz RB 3.00 MHz VB 300 kHz ST msec

Power Measurement Basics BLS 11/96 4 Importance of Power in Microwave Applications

Power Measurement Basics BLS 11/96 5 Unit of power is the watt (W): 1W = 1 joule/sec The watt is a basic unit: 1 volt is defined as 1 W/ampere Relative power measurements are expressed in dB: P(dB) = 10 log(P/Pref) Absolute power measurements are expressed in dBm: P(dBm) = 10 log(P/1 mW) Units and Definitions

Power Measurement Basics BLS 11/96 6 Power: P = (I)(V) Amplitude t P I V I R V + - DC component of power AC component of power

Power Measurement Basics BLS 11/96 7 Power Measurements at Different Frequencies DC Low Frequency High Frequency V Inc V R Z S Z O R L V R L V R L - + ± Z S Z S I I

Power Measurement Basics BLS 11/96 8 Agenda Importance and definitions of power measurements Types of power measurements Measurement uncertainty Sensor types and power meters Considerations in choosing power measurement equipment

Power Measurement Basics BLS 11/96 9 Types of Power Measurements Average Power Pulse Power Peak Envelope Power CW RF signal Pulsed RF signal Gaussian pulse signal

Power Measurement Basics BLS 11/96 10 Average Power time Average over several modulation cycles Average over many pulse repetitions

Power Measurement Basics BLS 11/96 11 Pulse Power Complete modulation envelope analysis Pulse Top Amplitude Risetime Falltime Average Power Pulse Base Amplitude PRI Offtime Pulse Width Peak Power  50% amplitude points Overshoot 10% amplitude points 90% amplitude points

Power Measurement Basics BLS 11/96 12 Peak Envelope Power Rectangular pulse power using duty cycle method Rectangular pulse power using averaging method 50% amplitude points For pulses that are not rectangular

Power Measurement Basics BLS 11/96 13 Measurement Types Summary For a CW signal, average, pulse, and peak envelope power give the same results Average power is more frequently measured because of easy-to-use measurement equipment and highly accurate and traceable specifications Pulse and peak envelope power can often be calculated from average power

Power Measurement Basics BLS 11/96 14 Agenda Importance and definitions of power measurements Types of power measurements Measurement uncertainty Sensor types and power meters Considerations in choosing power measurement equipment

Power Measurement Basics BLS 11/96 15 Sources of Power Measurement Uncertainty Sensor and source mismatch errors Power sensor errors Power meter errors Mismatch Sensor Meter

Power Measurement Basics BLS 11/96 16 Calculation of Mismatch Uncertainty Signal Source 10 GHz Power Sensor Power Meter Mismatch Uncertainty =±2  0.33   100% = ± 5.5% Mismatch Uncertainty = ±2      100% SOURCE SENSOR SOURCE SWR = 2.0 SENSOR SWR= 1.18  = 0.33  = HP 8481A HP 437B

Power Measurement Basics BLS 11/96 17 Power Sensor Errors (Effective Efficiency) Various sensor losses DC signal Power Sensor Power Meter P r Element P i P gl Cal Factor:  e K b = P gl P i

Power Measurement Basics BLS 11/96 18 Power Meter Errors Power reference error Instrumentation error +/- 1 count Zero Set Zero Carryover Noise Drift

Power Measurement Basics BLS 11/96 19 Calculating Power Measurement Uncertainty Mismatch uncertainty: Cal factor uncertainty: Power reference uncertainty: Instrumentation uncertainty: Now that the uncertainties have been determined, how are they combined? ± 5.5% ± 1.9% ± 1.2% ± 0.5%

Power Measurement Basics BLS 11/96 20 Worst-Case Uncertainty In our example worst case uncertainty would be: = 5.5% + 1.9% + 1.2% + 0.5% = ± 9.1% +9.1% = 10 log ( ) = dB - 9.1% = 10 log ( ) = dB

Power Measurement Basics BLS 11/96 21 RSS Uncertainty In our example RSS uncertainty would be: = (5.5%) + (1.9%) + (1.2%) + (0.5%) = ± 6.0% + 6.0% = 10 log ( ) = dB  6.0% = 10 log (1  0.060) = dB

Power Measurement Basics BLS 11/96 22 Agenda Importance and definitions of power measurements Types of power measurements Measurement uncertainty Sensor types and power meters Considerations in choosing power measurement equipment

Power Measurement Basics BLS 11/96 23 Methods of Sensing Power Substituted DC or low frequency equivalent Net RF power absorbed by sensor Power Sensor Power Meter Display Thermistors Diode Detectors Thermocouples

Power Measurement Basics BLS 11/96 24 Thermistors Thermocouples Diode Detectors Characteristic curves of a typical thermistor element

Power Measurement Basics BLS 11/96 25 Thermistors Thermocouples Diode Detectors A self-balancing bridge containing a thermistor Thermistor mount

Power Measurement Basics BLS 11/96 26 Power Meters for Thermistor Mounts HP 432A Power Meter Thermistor mounts are located in the sensor, not the meter.

Power Measurement Basics BLS 11/96 27 Physics of a thermocouple Thermistors Thermocouples Diode Detectors Bound Ions Diffused Electrons E-field

Power Measurement Basics BLS 11/96 28 Thermistors Thermocouples Diode Detectors The principles behind the thermocouple V h Hot junction Metal 1 Metal V V 2 Cold junction h V = V + V - V 0

Power Measurement Basics BLS 11/96 29 Thermistors Thermocouples Diode Detectors Thermocouple implementation RF Input Thin-Film Resistor n - Type Silicon hot junction hot cold cold junction Thin-Film Resistor To dc Voltmeter C c C b n - Type Silicon gold leads RF power Thermocouples

Power Measurement Basics BLS 11/96 30 Thermistors Thermocouples Diode Detectors Square Law Region of Diode Sensor 0.01 mW -70 dBm V O (log) Linear Region [watts] 0.1 nW -20 dBm V o  P IN P Noise Floor

Power Measurement Basics BLS 11/96 31 Thermistors Thermocouples Diode Detectors How does a diode detector work? V s V o + -

Power Measurement Basics BLS 11/96 32 The Basic Power Meter Diode Sensor Chopper Diode Detector Meter Synchronous Detector LPF ADC Ranging BPF Square Wave Generator µProcessor RF AC DC 220 Hz DAC AUTOZERO

Power Measurement Basics BLS 11/96 33 Agenda Importance and definitions of power measurements Types of power measurements Measurement uncertainty Sensor types and power meters Considerations in choosing power measurement equipment

Power Measurement Basics BLS 11/96 34 Considerations in Choosing Power Measurement Equipment

Power Measurement Basics BLS 11/96 35 Thermistors as Transfer Standards NIST Commercial Standards Laboratory Manufacturing Facility User Rising Costs / Better Accuracy Microcalorimeter National Reference Standard Measurement Reference Standard Working Standards General Test Equipment Transfer Standard

Power Measurement Basics BLS 11/96 36 Power Ranges of the Various Sensor Types [dBm] Thermistors Thermocouple square-law region Extended range using an attenuator Diode detector square-law region

Power Measurement Basics BLS 11/96 37 Susceptibility to Overload 8478B Thermistor Mount 8481D PDB Diode Mount 8481A Thermocouple Mount 8481H Thermocouple Mount Max Average Power Max Energy Per Pulse Max Envelope Power 30 mW100 mW300 mW3.5 W 10 W  s30 W  s 100 W  s 200 W100 mW15 W100 W

Power Measurement Basics BLS 11/96 38 Frequency Ranges ||||||||||| POWER FREQUENCY B Series 0 to +44 dBm H Series -10 to +35 dBm A Series -30 to + 20 dBm D Series -70 to -20 dBm 8481B 8482B 8481H 8482H 8487A Q8486A W8486A R8486A 8485A 8481A 8482A 8483A 8487D Q8486D R8486D 8485D 8481D 100 kHz10 MHz50 MHz 2 GHz4.2 GHz 18 GHz26.5 GHz33 GHz40 GHz50 GHz75 GHz110 GHz OPT 33 V V

Power Measurement Basics BLS 11/96 39 Reflection Coefficient Thermistors Diode Detector Thermocouple

Power Measurement Basics BLS 11/96 40 Any Questions?