1Power Measurement Basics
BLS 11/96
Welcome to Power Measurement Basics
2Power 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
3Power Measurement Basics
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
4Power Measurement Basics
BLS 11/96
Importance of Power in Microwave Applications
5Power Measurement Basics
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
6Power Measurement Basics
BLS 11/96
Power: P = (I)(V)
Amplitude
t
P
I
V
I
RV
+
-
DC component of power
AC component of power
7Power Measurement Basics
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
8Power 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
9Power Measurement Basics
BLS 11/96
Types of Power Measurements
Average Power
Pulse Power
Peak Envelope Power
CW RF signal
Pulsed RF signal
Gaussian pulse signal
10Power Measurement Basics
BLS 11/96
Average Power
time
Average over several modulation cycles
Average over many pulse repetitions
11Power Measurement Basics
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
10% amplitude points
90% amplitude points
12Power Measurement Basics
BLS 11/96
Peak Envelope Power
Rectangular pulse power using duty cycle methodRectangular pulse power using averaging method
50% amplitude points
For pulses that are not rectangular
13Power Measurement Basics
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
14Power 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
15Power Measurement Basics
BLS 11/96
Sources of Power Measurement Uncertainty
Sensor and source mismatch errors Power sensor errors Power meter errors
Mismatch
Sensor
Meter
16Power Measurement Basics
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
17Power Measurement Basics
BLS 11/96
Power Sensor Errors(Effective Efficiency)
Various sensor losses
DC signal
PowerSensor
PowerMeter
Pr
ElementPi Pg
l
Cal Factor: eKb=
PglPi
18Power Measurement Basics
BLS 11/96
Power Meter Errors
Power reference error
Instrumentation error
+/- 1 count
Zero Set
Zero Carryover
Noise
Drift
19Power Measurement Basics
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%
20Power Measurement Basics
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
21Power Measurement Basics
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
22Power 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
23Power Measurement Basics
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
24Power Measurement Basics
BLS 11/96
ThermistorsThermocouplesDiode Detectors
Characteristic curves of a typical thermistor element
25Power Measurement Basics
BLS 11/96
ThermistorsThermocouplesDiode Detectors
A self-balancing bridge containing a thermistor
Thermistor mount
26Power Measurement Basics
BLS 11/96
Power Meters for Thermistor Mounts
HP 432A Power Meter
Thermistor mounts are located in the sensor, not the meter.
27Power Measurement Basics
BLS 11/96
Physics of a thermocouple
ThermistorsThermocouplesDiode Detectors
Bound Ions
Diffused Electrons
E-field
28Power Measurement Basics
BLS 11/96
ThermistorsThermocouplesDiode Detectors
The principles behind the thermocouple
VhHot junction
Metal 1
Metal 2
- +V1
- +V2
Cold junction
-
+
1 2hV = V + V - V
0
29Power Measurement Basics
BLS 11/96
ThermistorsThermocouplesDiode Detectors
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
30Power Measurement Basics
BLS 11/96
ThermistorsThermocouplesDiode Detectors
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
31Power Measurement Basics
BLS 11/96
ThermistorsThermocouplesDiode Detectors
How does a diode detector work?
Vs
Vo
+
-
32Power Measurement Basics
BLS 11/96
The Basic Power Meter
Diode Sensor
Chopper
DiodeDetector
MeterSynchronousDetector LPF ADCRangingBPF
Square WaveGenerator
µProcessor
RF ACDC
220 Hz
DAC
AUTOZERO
33Power 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
34Power Measurement Basics
BLS 11/96
Considerations in Choosing Power Measurement Equipment
35Power Measurement Basics
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
36Power Measurement Basics
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
37Power Measurement Basics
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
38Power Measurement Basics
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
39Power Measurement Basics
BLS 11/96
Reflection Coefficient
Thermistors
Diode Detector
Thermocouple
40Power Measurement Basics
BLS 11/96
Any Questions?
Top Related