Microwave power meter

A microwave power meter is an instrument which measures the electrical power occurring 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, which reports the power reading. The head may also 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 / meter combinations was that the sensor would convert the microwave power into an analogue voltage which would then 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.

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Sensor Technologies

There are a variety of different technologies which have been used as the power sensing element. Each have pros and cons.

Thermal

Thermocouple power sensors make up the majority of the thermal power sensors sold at present, these are generally reasonably linear and have a reasonably fast response time and dynamic range. The microwave power is absorbed in a load and the temperature rise of this is measured by the thermocouple. Thermocouple sensors often require a reference DC or microwave power source for calibration before measuring and these can be built into the power meters. 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. Other thermal sensing technologies include microwave calorimeters and bolometers and quasi-optic pulsed microwave sensors.

Diode

Many microwave power heads use one or several diode(s) in order to rectify the incident microwave power, consequently these power sensors often have extremely fast response times. 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 to beyond the square-law region linearity correction or multiple diode stacks are used. Despite this diode sensors generally have poor linearity and can be inaccurate when measuring modulated signals and like thermocouple sensors would 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 torque-vane, electron-beam, MEMS, Hall effect and atomic fountain based sensors.

Display

Power meters generally report the power in dBm (decibels relative to 1 milliwatt), dBW (decibels relative to 1 watt) or watts.

Manufacturers of microwave power meters include: Aeroflex, Agilent, Anritsu, Amitec, and Boonton Electronics.

References