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Power is important. Measuring the power from an RF/microwave signal is a fundamental way to gain knowledge about signals and devices. Understandably, it is critical to accurately and consistently measure power. It also is essential to maintain a standard of power measurement that enables measured power to be communicated to customers, manufacturers, designers, and researchers. There are three main technologies involved in measuring RF/microwave power: the diode, thermistor, and thermocouple sensor. Each product type comes with its own benefits and uncertainties. Additionally, industry and manufacturer standards for power-measurement calibration need to be employed successfully to communicate absolute values of power (namely, the National Institute of Standards and Technology and other National Measurement Institutes). Having a clear understanding of the fundamentals of power measurement, including the technology and standards, will ensure that an engineer has the most reliable information for optimizing their designs.

Fig. 1

The goal of a power sensor, for example, is to absorb the incoming energy from an RF/microwave signal. By relying on energy conversion, it then attempts to transfer that energy into usable electrical energy that exactly replicates the absorbed energy. Of course, some energy is always lost in the impedance mismatch between the source and sensor. In addition, some energy is absorbed in parts of the sensor that do not render usable electrical energy. The meter itself also suffers from some conversion inefficiencies. Fortunately, however, there are calibrations provided by power-meter manufacturers that use a chart or built-in memory to help compensate for the sensor and meter inefficiencies.

An Old-Timey Thermistor

Of the three types of sensor technologies, the thermistor—a type of bolometer—has been around the longest. It is still used in many applications including science and research. The thermistor sensor converts RF/microwave energy received by a semiconductor-based material, which changes resistance as a function of temperature (Fig. 1). In addition, a feedback loop is used with a balanced-bridge typology. That typology employs a direct-current (DC) or alternating-current (AC) bias supply as a control to maintain a constant resistance for the sensor. As a result, the changes in bias power can be used as an indicator for the energy absorbed in the sensor.

Lower Error with Thermocouple Sensors

In many of today’s applications, thermocouple sensors have replaced thermistors. This trend is most likely due to the advantages offered by thermocouple sensors: higher dynamic range, sensitivity, increased durability, and lower measurement uncertainties associated with standing wave ratio (SWR). Thermocouple operation relies on two physical bases—the first of which uses the voltage generated from the Peltier effect at the junction of two electrically dissimilar materials. The other base leverages the Thomson effect at the junction of two different temperature sections of a conductor. Currently, metallic thin-film and semiconductor technology are used to create high-performing and linear thermocouples in very small footprints.

Fig. 2

A transmission method is often employed to enable use of the extremely low voltage—sometimes as low as 1 nV/mW—generated by the thermocouple. One method is to chop the DC signal into a series of square waves and transmit that over an AC-coupled path after amplification(Fig. 2). Because this is an open-loop approach, an outside calibration method must be used. Otherwise, various aspects of the thermocouple sensor can lead to drift in the DC response to RF/microwave power. Often, a highly accurate reference oscillator set to a specific known power is used between tests to calibrate the thermocouple-based sensor to match the reference source. Errors in the reference source, calibration mechanisms, or interconnect of the source and sensor can lead to measurement errors. It is therefore critical for the reference to have as little error as possible as well as a verifiable trace history to a credible NMI.

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