Wattmeter for electrical engineering for polytechnic

Wattmeter for electrical engineering for polytechnic

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

2. 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

3. 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 dBm
ATTEN 10 dB
10 dB / DIV
START 150 MHz STOP 1.150 GHz
RB 3.00 MHz VB 300 kHz ST 13.89 msec

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

5. 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

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

7. 7Power Measurement Basics
BLS 11/96
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

8. 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

9. 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

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

11. 11Power Measurement Basics
BLS 11/96
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

12. 12Power Measurement Basics
BLS 11/96
Peak Envelope Power
Rectangular pulse
power using duty
cycle method
Rectangular pulse
power using
averaging method50%
amplitude
points
For pulses that are not
rectangular

13. 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

14. 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

15. 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

16. 16Power Measurement Basics
BLS 11/96
Calculation of Mismatch
Uncertainty
Signal Source
10 GHz
Power
Sensor
Power
Meter
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

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

18. 18Power Measurement Basics
BLS 11/96
Power Meter Errors
P
o
w
e
r

r
e
f
e
r
e
n
c
e

e
r
r
o
r
Instrumentation error
+/- 1 count
Zero Set
Zero Carryover
Noise
D
rift

19. 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%

20. 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

21. 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

22. 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

23. 23Power Measurement Basics
BLS 11/96
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

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

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

26. 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.

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

28. 28Power Measurement Basics
BLS 11/96
Thermistors
Thermocouples
Diode Detectors

The principles behind the thermocouple
Vh
Hot
junction
Metal 1
Metal 2
- +
V1
- +V2
Cold
junction
-
+
1 2h
V = V + V - V
0

29. 29Power Measurement Basics
BLS 11/96
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
gold
leads
RF
power
Thermocouples

30. 30Power Measurement Basics
BLS 11/96
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
I
N
Noise Floor

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

32. 32Power Measurement Basics
BLS 11/96
The Basic Power Meter
Diode Sensor
Chopper
Diode
Detector
MeterSynchronous
Detector
LPF ADCRangingBPF
Square Wave
Generator
µProcessor
RF ACDC
220 Hz
DAC
AUTOZERO

33. 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

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

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

36. 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

37. 37Power Measurement Basics
BLS 11/96
Susceptibility to Overload
8478B
Thermistor
Mount
8481D
PDB 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 Ws 30 Ws 100
Ws
200 W 100 mW 15 W 100 W

38. 38Power Measurement Basics
BLS 11/96
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 kHz 10 MHz 50 MHz2 GHz4.2 GHz 18 GHz 26.5 GHz 33 GHz40 GHz 50 GHz 75 GHz 110 GHz
OPT 33
OPT 33
V
V

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

40. 40Power Measurement Basics
BLS 11/96
Any Questions?