Microwave power meter
Encyclopedia
A microwave power meter is an instrument which measures the electrical power
at microwave
frequencies
.
Usually a microwave power meter will consist of a measuring head which contains the actual power sensing element, connected via a cable to the meter proper, which displays the power reading. The head may be referred to as a power sensor or mount.
Different power sensors can be used for different frequencies or power levels. Historically the means of operation in most power sensor and meter combinations was that the sensor would convert the microwave power into an analogue voltage
which would be read by the meter and converted into a power reading. Several modern power sensor heads contain electronics to create a digital output and can be plugged via USB into a PC which acts as the power meter.
Microwave power meters have a wide bandwidth—they are not frequency-selective. To measure the power of a specific frequency component in the presence of other signals at different frequencies a spectrum analyzer
or measuring receiver
is needed.
Thermocouple power sensors make up the majority of the thermal power sensors sold at present. They are generally reasonably linear
and have a reasonably fast response time and dynamic range
. The microwave power is absorbed in a load
whose temperature rise is measured by the thermocouple. Thermocouple sensors often require a reference DC
or microwave power source for calibration
before measuring; this can be built into the power meter.
Thermistor
-based power sensors such as the Agilent 8478B are generally only used in situations where their excellent linearity is important, as they are both much slower and have a smaller dynamic range than either thermocouple or diode-based sensors.Thermistor-based power sensors are still the sensor of choice for power transfer standards because of their DC power substitution capability (Agilent 2006).
Other thermal sensing technologies include microwave calorimeter
s and bolometer
s,and quasi-optic pulsed microwave sensors.
(s) to rectify
the incident microwave power, and have extremely fast response. The diode would generally be used in its square-law region and hence give an output voltage proportional to the incident RF power. In order to extend their dynamic range beyond the square-law region, linearity correction circuits or multiple diode stacks are used. With advancement in comprehensive data compensation algorithm and diode stacks topology, diode sensors like the Agilent E9300A is able to respond properly to complex modulated signals over a wide dynamic range. (Agilent, 2006)
Like thermocouple sensors, they often require a reference source.
-vane, electron
-beam, MEMS
, Hall effect
and atomic fountain
based sensors.
(Agilent 2006)
(decibel
s relative to 1 milliwatt
), dBW (decibels relative to 1 watt) or watts.
Manufacturers of microwave power meters include: Rohde and Schwarz, Aeroflex
, Agilent, Anritsu
, Amitec, Atten Electronics, Boonton Electronics
,and TEGAM Inc..
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
at microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
.
Usually a microwave power meter will consist of a measuring head which contains the actual power sensing element, connected via a cable to the meter proper, which displays the power reading. The head may be referred to as a power sensor or mount.
Different power sensors can be used for different frequencies or power levels. Historically the means of operation in most power sensor and meter combinations was that the sensor would convert the microwave power into an analogue voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
which would be read by the meter and converted into a power reading. Several modern power sensor heads contain electronics to create a digital output and can be plugged via USB into a PC which acts as the power meter.
Microwave power meters have a wide bandwidth—they are not frequency-selective. To measure the power of a specific frequency component in the presence of other signals at different frequencies a spectrum analyzer
Spectrum analyzer
A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals...
or measuring receiver
Measuring Receiver
In telecommunication, a measuring receiver or measurement receiver is a calibrated laboratory-grade radio receiver designed to measure the characteristics of radio signals. The parameters of such receivers can usually be adjusted over a much wider range of values than is the case with other radio...
is needed.
Sensor technologies
There are a variety of different technologies which have been used as the power sensing element. Each has advantages and disadvantages.Thermal
Thermal sensors can generally be divided into two main categories, thermocouple power sensors and thermistor-based power sensors. Thermal sensors depend on the process of absortbing the rF and microwave signal energy, and sense the resulting heat rise. Therefore they respond to true average power of the signal, whether it is pulsed, CW, AM/FM or any complex modulation. (Agilent 2008).Thermocouple power sensors make up the majority of the thermal power sensors sold at present. They are generally reasonably linear
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
and have a reasonably fast response time and dynamic range
Dynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
. The microwave power is absorbed in a load
Dummy load
A dummy load is a device used to simulate an electrical load, usually for testing purposes.-Radio:In radio this device is also known as a dummy antenna or a radio frequency termination. It is a device used in place of an antenna to aid in testing a radio transmitter...
whose temperature rise is measured by the thermocouple. Thermocouple sensors often require a reference DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
or microwave power source for calibration
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
before measuring; this can be built into the power meter.
Thermistor
Thermistor
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...
-based power sensors such as the Agilent 8478B are generally only used in situations where their excellent linearity is important, as they are both much slower and have a smaller dynamic range than either thermocouple or diode-based sensors.Thermistor-based power sensors are still the sensor of choice for power transfer standards because of their DC power substitution capability (Agilent 2006).
Other thermal sensing technologies include microwave calorimeter
Calorimeter
A calorimeter is a device used for calorimetry, the science of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal microcalorimeters, titration calorimeters and accelerated rate calorimeters are among the most common...
s and bolometer
Bolometer
A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley...
s,and quasi-optic pulsed microwave sensors.
Diode
Many microwave power heads use one or more diodeDiode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
(s) to rectify
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
the incident microwave power, and have extremely fast response. The diode would generally be used in its square-law region and hence give an output voltage proportional to the incident RF power. In order to extend their dynamic range beyond the square-law region, linearity correction circuits or multiple diode stacks are used. With advancement in comprehensive data compensation algorithm and diode stacks topology, diode sensors like the Agilent E9300A is able to respond properly to complex modulated signals over a wide dynamic range. (Agilent, 2006)
Like thermocouple sensors, they often require a reference source.
Field Strength
Other technologies have been investigated or implemented for use as power sensors but are not widely used today; these include torqueTorque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
-vane, electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
-beam, MEMS
Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
, Hall effect
Hall effect
The Hall effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current...
and atomic fountain
Atomic fountain
An atomic fountain is a cloud of atoms that are tossed upwards by lasers in the Earth's gravitational field. If visible, it would resemble the water in a fountain. While weightless in the toss, the atoms are measured to set the frequency of an atomic clock....
based sensors.
Type of Microwave Power Meters
The three main types of microwave power meters are- Average power meter – measures true average power of the signal
- Peak and average power meter - display profile or envelope of the signal and measures peak, average and peak to average ratio power
- USB power meter or sensor – a compact, small form factor hardware that combines the functionality of a meter and a sensor. It can also be divided into average USB power meter or peak and average USB power meter
(Agilent 2006)
Display
Power meters generally report the power in dBmDBm
dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form...
(decibel
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
s relative to 1 milliwatt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
), dBW (decibels relative to 1 watt) or watts.
Manufacturers of microwave power meters include: Rohde and Schwarz, Aeroflex
Aeroflex
Aeroflex Inc. is an American company which produces and markets RF and microwave integrated circuits, components, and systems used for wireless communications. Aeroflex consists of Aeroflex Microelectronic Solutions , a fabless manufacturer of rad-hard and hi-rel semiconductor devices and...
, Agilent, Anritsu
Anritsu
is a Japanese corporation that is in the test and measurement market. Products include microwave, RF, and optical signal generators , spectrum analyzers, and network analyzers. It was formed with the merger of two companies, the Annaka Corporation and Kyoritsu Electric in Japan in 1931. In 1990...
, Amitec, Atten Electronics, Boonton Electronics
Boonton Electronics
Boonton Electronics is an American manufacturer of electronic test equipment. Originally founded in 1947 in Morris Plains, New Jersey, they are now located nearby in Parsippany...
,and TEGAM Inc..