Welcome to Power Measurement Basics BLS 1196 1
Welcome to Power Measurement Basics BLS 11/96 1
Agenda l l l 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 2
Importance of Proper Power Levels l Power too low – Signal buried in noise RL 0. 0 d. Bm ATTEN 10 d. B / DIV l Power too high – Nonlinear distortion can occur – START 150 MHz RB 3. 00 MHz VB 300 k. Hz Or even worse! STOP 1. 150 GHz ST 13. 89 msec Power Measurement Basics BLS 11/96 3
Importance of Power in Microwave Applications Power Measurement Basics BLS 11/96 4
Units and Definitions l Unit of power is the watt (W): 1 W = 1 joule/sec l The watt is a basic unit: 1 volt is defined as 1 W/ampere l Relative power measurements are expressed in d. B: l Absolute power measurements are expressed in d. Bm: P(d. Bm) = 10 log(P/1 m. W) P(d. B) = 10 log(P/Pref) Power Measurement Basics BLS 11/96 5
Power: P = (I)(V) I + V - P AC component of power R DC component of power Amplitude t I V Power Measurement Basics BLS 11/96 6
Power Measurements at Different Frequencies l DC + V Z ± S R L I l Low Frequency Z S R L V I l High Frequency Z S R L Z O V Inc VR Power Measurement Basics BLS 11/96 7
Agenda l l l 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 8
Types of Power Measurements l Average Power l Pulse Power l Peak Envelope Power CW RF signal Pulsed RF signal Gaussian pulse signal Power Measurement Basics BLS 11/96 9
Average Power Average over several modulation cycles Average over many pulse repetitions time Power Measurement Basics BLS 11/96 10
Pulse Power l Complete modulation envelope analysis Peak Power Pulse Top Amplitude Overshoot 90% amplitude points 50% amplitude points 10% amplitude points Average Power Pulse Base Amplitude Pulse Width t Risetime Offtime Falltime PRI Power Measurement Basics BLS 11/96 11
Peak Envelope Power For pulses that are not rectangular Rectangular pulse power using duty cycle method 50% amplitude points Rectangular pulse power using averaging method Power Measurement Basics BLS 11/96 12
Measurement Types Summary l l l 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 13
Agenda l l l 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 14
Sources of Power Measurement Uncertainty l l l Sensor and source mismatch errors Power sensor errors Power meter errors Sensor Mismatch Meter Power Measurement Basics BLS 11/96 15
Calculation of Mismatch Uncertainty Signal Source 10 GHz Power Meter Power Sensor HP 8481 A SWR = 2. 0 r = 0. 33 SOURCE SWR HP 437 B = 1. 18 = 0. 083 r. SENSOR Mismatch Uncertainty = ± 2 · r SOURCE · r · 100% SENSOR Mismatch Uncertainty =± 2 · 0. 33 · 0. 083 · 100% = ± 5. 5% Power Measurement Basics BLS 11/96 16
Power Sensor Errors (Effective Efficiency) Various sensor losses Pi Pgl Pr Element DC signal Power Meter Power Sensor Cal Factor: P gl K b = he Pi Power Measurement Basics BLS 11/96 17
Power Meter Errors Zero Carryover Pow +/- 1 count Drift Zero Set er r e fere nce erro r Noise Instrumen tation err or Power Measurement Basics BLS 11/96 18
Calculating Power Measurement Uncertainty Mismatch uncertainty: ± 5. 5% Cal factor uncertainty: ± 1. 9% Power reference uncertainty: ± 1. 2% Instrumentation uncertainty: ± 0. 5% Now that the uncertainties have been determined, how are they combined? Power Measurement Basics BLS 11/96 19
Worst-Case Uncertainty l In our example worst case uncertainty would be: = 5. 5% + 1. 9% + 1. 2% + 0. 5% = ± 9. 1% +9. 1% = 10 log (1 + 0. 091) = + 0. 38 d. B - 9. 1% = 10 log (1 - 0. 091) = - 0. 41 d. B Power Measurement Basics BLS 11/96 20
RSS Uncertainty l In our example RSS uncertainty would be: = (5. 5%) 2 + (1. 9%) 2+ (1. 2%) 2+ (0. 5%) 2 = ± 6. 0% + 6. 0% = 10 log (1 + 0. 060) = +0. 25 d. B - 6. 0% = 10 log (1 - 0. 060) = -0. 27 d. B Power Measurement Basics BLS 11/96 21
Agenda l l l 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 22
Methods of Sensing Power Thermistors Thermocouples Net RF power absorbed by Diode Detectors sensor Power Sensor Substituted DC or low frequency equivalent Power Meter Display Power Measurement Basics BLS 11/96 23
Thermistors Thermocouples Diode Detectors Characteristic curves of a typical thermistor element Power Measurement Basics BLS 11/96 24
Thermistors Thermocouples Diode Detectors l A self-balancing bridge containing a thermistor Thermistor mount Power Measurement Basics BLS 11/96 25
Power Meters for Thermistor Mounts l HP 432 A Power Meter Thermistor mounts are located in the sensor, not the meter. Power Measurement Basics BLS 11/96 26
Thermistors Thermocouples Diode Detectors l Physics of a thermocouple Bound Ions E-field Diffused Electrons Power Measurement Basics BLS 11/96 27
Thermistors Thermocouples Diode Detectors l The principles behind thermocouple Hot junction V 1 + Metal 1 - Metal 2 Vh - + Cold junction + V 2 V 0 = V +1 V - Vh 2 Power Measurement Basics BLS 11/96 28
Thermistors Thermocouples Diode Detectors RF power gold leads l Thermocouple implementation cold junction Cc Thin-Film Resistor hot RF Input hot junction n - Type Silicon Thin-Film Resistor cold Thermocouples To dc Voltmeter Cb Power Measurement Basics BLS 11/96 29
Thermistors Thermocouples Diode Detectors Linear Region Square Law Region of Diode Sensor Vo µ PIN VO (log) Noise Floor 0. 1 n. W -70 d. Bm 0. 01 m. W -20 d. Bm PIN [watts] Power Measurement Basics BLS 11/96 30
Thermistors Thermocouples Diode Detectors l How does a diode detector work? + V s V o - Power Measurement Basics BLS 11/96 31
The Basic Power Meter Diode Detector RF DC Diode Sensor BPF Ranging Synchronous Detector Meter LPF ADC Chopper AC AUTOZERO 220 Hz µProcessor Square Wave Generator DAC Power Measurement Basics BLS 11/96 32
Agenda l l l 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 33
Considerations in Choosing Power Measurement Equipment Power Measurement Basics BLS 11/96 34
Thermistors as Transfer Standards Rising Costs / Better Accuracy Microcalorimeter National Reference Standard NIST Working Standards NIST Commercial Measurement Reference Standards Laboratory Standard Transfer Standard General Test Equipment Manufacturing Facility User Power Measurement Basics BLS 11/96 35
Power Ranges of the Various Sensor Types Thermistors Thermocouple square-law region Extended range using an attenuator Diode detector square-law region -70 --60 -50 -40 -30 -20 -10 0 +10 +20 +30 +40 +50 Power Measurement Basics BLS 11/96 [d. Bm] 36
Susceptibility to Overload 8478 B Thermistor Mount Max Average Power Max Energy Per Pulse Max Envelope Power 30 m. W 8481 D PDB Diode Mount 100 m. W 10 W×ms 200 W 100 m. W 8481 A Thermocouple Mount 8481 H Thermocouple Mount 300 m. W 3. 5 W 30 W×ms 100 W×ms 15 W 100 W Power Measurement Basics BLS 11/96 37
Frequency Ranges V POWER B Series 0 to +44 d. Bm 8481 B 8482 B H Series -10 to +35 d. Bm 8481 H 8482 H 8487 A Q 8486 A W 8486 A R 8486 A A Series -30 to + 20 d. Bm 8485 A OPT 33 8481 A 8482 A 8483 A 8487 D Q 8486 D D Series -70 to -20 d. Bm R 8486 D OPT 33 8485 D | 100 k. Hz 10 MHz 50 MHz 2 GHz 4. 2 GHz 18 GHz 26. 5 GHz 33 GHz 40 GHz | | 50 GHz | | 75 GHz | | | 110 GHz V 8481 D | | | FREQUENCY Power Measurement Basics BLS 11/96 38
Reflection Coefficient Thermistors Diode Detector Thermocouple Power Measurement Basics BLS 11/96 39
Any Questions? Power Measurement Basics BLS 11/96 40
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