power measurement basics bls 11/96 1 welcome to power measurement basics

1Power Measurement Basics

BLS 11/96

Welcome to Power Measurement Basics


BLS 11/96

Agenda

Importance and definitions of

power measurements Types of power

measurements Measurement uncertainty Sensor types and power

meters Considerations in choosing

power measurement equipment


BLS 11/96

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 dBmATTEN 10 dB10 dB / DIV

START 150 MHz STOP 1.150 GHzRB 3.00 MHz VB 300 kHz ST 13.89 msec


BLS 11/96

Importance of Power in Microwave Applications


BLS 11/96

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


BLS 11/96

Power: P = (I)(V)

Amplitude

t

P

I

V

I

RV

+

-

DC component of power

AC component of power


BLS 11/96

Power Measurements at Different Frequencies

DC

Low Frequency

High Frequency

VInc

VR

ZS

ZO

RL

VRL

V RL

-

+

±

ZS

ZS

I

I


BLS 11/96

Agenda

Importance and definitions of power measurements

Types of power measurements

Measurement uncertainty Sensor types and power




BLS 11/96

Types of Power Measurements

Average Power

Pulse Power

Peak Envelope Power

CW RF signal

Pulsed RF signal

Gaussian pulse signal


BLS 11/96

Average Power

time

Average over several modulation cycles

Average over many pulse repetitions


BLS 11/96

Pulse Power

Complete modulation envelope analysis

Pulse Top Amplitude

Risetime Falltime

Average PowerPulse Base Amplitude

PRI

Offtime

PulseWidth

PeakPower

50% amplitude points

Overshoot




BLS 11/96

Peak Envelope Power

Rectangular pulse power using duty cycle methodRectangular pulse power using averaging method


For pulses that are not rectangular


BLS 11/96

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


BLS 11/96

Agenda

Importance and definitions of power measurements

Types of power measurements

Measurement uncertainty Sensor types and power




BLS 11/96

Sources of Power Measurement Uncertainty

Sensor and source mismatch errors Power sensor errors Power meter errors

Mismatch

Sensor

Meter


BLS 11/96

Calculation of Mismatch Uncertainty

Signal Source10 GHz

PowerSensor

PowerMeter

Mismatch Uncertainty =±2 0.33 0.083 100% = ± 5.5%

Mismatch Uncertainty = ±2 100%SOURCE SENSOR

SOURCE

SWR = 2.0

SENSOR

SWR = 1.18 = 0.33 = 0.083

HP 8481A HP 437B


BLS 11/96

Power Sensor Errors(Effective Efficiency)

Various sensor losses

DC signal

PowerSensor

PowerMeter

Pr

ElementPi Pg

l

Cal Factor: eKb=

PglPi


BLS 11/96

Power Meter Errors

Power reference error

Instrumentation error

+/- 1 count

Zero Set

Zero Carryover

Noise

Drift


BLS 11/96

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%


BLS 11/96

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 (1 + 0.091) = + 0.38 dB

- 9.1% = 10 log (1 - 0.091) = - 0.41 dB


BLS 11/96

RSS Uncertainty

In our example RSS uncertainty would be:

= (5.5%) + (1.9%) + (1.2%) + (0.5%)

= ± 6.0%

+ 6.0% = 10 log (1 + 0.060) = +0.25 dB

6.0% = 10 log (1 0.060) = -0.27 dB

2 2 2 2


BLS 11/96

Agenda







BLS 11/96

Methods of Sensing Power

Substituted DC or low frequency equivalent

Net RF power absorbed by sensor Power

Sensor

PowerMeter

Display

Thermistors

Diode Detectors

Thermocouples


BLS 11/96

ThermistorsThermocouplesDiode Detectors

Characteristic curves of a typical thermistor element


BLS 11/96


A self-balancing bridge containing a thermistor

Thermistor mount


BLS 11/96

Power Meters for Thermistor Mounts

HP 432A Power Meter

Thermistor mounts are located in the sensor, not the meter.


BLS 11/96

Physics of a thermocouple


Bound Ions

Diffused Electrons

E-field


BLS 11/96


The principles behind the thermocouple

VhHot junction

Metal 1

Metal 2

- +V1

- +V2

Cold junction

-

+

1 2hV = V + V - V

0


BLS 11/96


Thermocouple implementation

RF Input

Thin-FilmResistor

n - TypeSilicon

hot junction

hot

cold

cold junction

Thin-FilmResistor

To dc Voltmeter

Cc

Cb

n - TypeSilicon

gold leads

gold leads

RF power

Thermocouples


BLS 11/96


Square Law Region of Diode Sensor

0.01 mW-70 dBm

VO (log)

Linear Region

[watts]0.1 nW

-20 dBm

Vo PIN

PIN

Noise Floor


BLS 11/96


How does a diode detector work?

Vs

Vo

+

-


BLS 11/96

The Basic Power Meter

Diode Sensor

Chopper

DiodeDetector

MeterSynchronousDetector LPF ADCRangingBPF

Square WaveGenerator

µProcessor

RF ACDC

220 Hz

DAC

AUTOZERO


BLS 11/96

Agenda







BLS 11/96

Considerations in Choosing Power Measurement Equipment


BLS 11/96

Thermistors as Transfer Standards

NIST

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


BLS 11/96

Power Ranges of the Various Sensor Types

-70 --60 -50 -40 -30 -20 -10 0 +10 +20 +30 +40 +50[dBm]

Thermistors

Thermocouple square-law

region

Extended range using an

attenuator

Diode detector square-law

region


BLS 11/96

Susceptibility to Overload

8478B Thermistor

Mount

8481DPDB Diode

Mount

8481A Thermocoupl

e Mount

8481H Thermocoupl

e Mount

Max Average Power

Max Energy Per Pulse

Max Envelope

Power

30 mW 100 mW 300 mW 3.5 W

10 Ws 30 Ws 100 Ws

200 W 100 mW 15 W 100 W


BLS 11/96

Frequency Ranges

| | | | | || | | | |

POWER

FREQUENCY

B Series0 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 kHz 10 MHz 50 MHz 2 GHz 4.2 GHz 18 GHz 26.5 GHz 33 GHz 40 GHz 50 GHz 75 GHz 110 GHz

OPT 33

OPT 33

V

V


BLS 11/96

Reflection Coefficient

Thermistors

Diode Detector

Thermocouple


BLS 11/96

Any Questions?

power measurement basics bls 11/96 1 welcome to power measurement basics

Documents