Considerations when Selecting a Power Sensor

RF Power Sensor Types

Most power sensors now available fall into one of three design categories: thermistor, thermocouple, and diode detector. In a thermistor design, RF power is measured through DC substitution of RF energy heating the thermistor. In a thermocouple design, absorbed microwave power causes heating of a thermocouple junction, resulting a voltage proportional to power.

Diode based detectors produce a voltage proportional to the input power. Because diode designs offer quicker response times and more efficiently convert RF energy to a measureable voltage they are often preferred over both thermistor and thermocouple designs. The improved conversion efficiency of the diode design enables the ability to measure significantly lower power levels. Multipath designs coupled with this ability to measure very low power levels provide diode based designs an extremely wide dynamic range. In this article we will focus our attention on diode based sensors.

LADYBUG POWER SENSORS


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Classically, RF power sensors were used in conjunction with a separate power meter. They are slower, cost more, require more rack space and require a complex zero and measurement calibration procedure before use. Further they were often limited to measuring only true average RF power. The introduction of the USB RF power sensor (see Figure 1) has brought several advantages to making RF power measurements. These sensors are fast, low cost, compact, sophisticated and easy to use. Simply connect to a computer and you are up and running. Available USB sensors are fast, accurate, and have a full set of measurement capability. Measurements range from simple CW to modulation analysis to full featured pulse profiling and time domain gated analysis. Such features are all available with the LB480A USB sensor from Lady Bug Technologies.

USB power sensor and pulse profiling software
Figure 1 - USB power sensor and pulse profiling software

Applications for Power Sensors

RF power measurement is fundamental to the development and verification of any wireless communication system whether it is terrestrial or satellite based. Radar or electronic warfare systems and their components in aerospace and defense applications have always had the need for accurate power measurement. Power Sensors are used during all phases of the product life cycle including design, integration, test, calibration and repair. Different use models include bench usage, portable situations, remote sensing, remote access and large embedded systems.

Generally, RF power sensors provide a terminating measurement. There are applications where in-line measurements are required such as continuous monitoring of a signal or scalar reflection and transmission measurements. A coupler or bridge with an attached sensor can serve this purpose.

USB Sensors versus Classic Sensors & Meters

Temperature-sensitivity
Figure 2 - Temperature sensitivity of a Lady Bug USB sensor versus classic diode sensors

Accuracy and Measurement Uncertainty

Several different error terms go to make up the accuracy or measurement uncertainty specification of a power sensor. The list below indicates the most significant ones. Figure 3 shows errors that are unique to the classic power meter & sensor combination not present with USB sensors.

Accuracy
Figure 3 - Accuracy errors unique to classic power meters and sensors that USB sensors do not have

Dynamic Range

The total dynamic power range of a sensor is directly related to the linearity of the sensor used. Sensors are available that can cover a range up to 90 dB. There are several methods used to extend the dynamic range of diode power sensors beyond the linear portion of their square-law region. One popular method involves using a multiple diode path architecture that incorporates two or more diodes with different square law characteristics to extend the range. Another method, sometimes used in conjunction with multiple paths, is to use switched attenuators to maintain the signal level within the diode’s linear region. And, of course to some degree, calibration factors are utilized. These calibration factors must account for not only the input power level input but also the input frequency and temperature of the sensor at the time of the measurement. This results in a three dimensional table of calibration factors to cover the power, frequency and temperature ranges specified for the sensor

Frequency Range

Power sensors are available in a variety of frequency ranges that cover bands from 9 kHz to 110 GHz. The most common bands are from 10 MHz to 40 GHz. Variations in the frequency response of the sensor are accounted for in the calibration table stored within the sensor.

Zero & Cal

The zero & cal process for traditional power sensors involves multiple disconnections from the measurement point and connections to an external calibration source. By themselves zero & cal do not provide an accurate power measurement even when executed after each change in temperature. Newer sensors that have an internal zero & cal capability don’t require an external calibration source but also exhibit sensitivity to temperature. In this case, the internal zero & cal is automatically performed periodically but not for each measurement. The sensitivity of a USB sensor with internal zero is shown in Figure 4. Notice that for the lower power levels, the measurement error climbs higher than 1.5 dB and represents a measurement error of over 40%.

Temperature-sensitivity
Figure 4 - Temperature sensitivity of a typical USB sensor with internal zero versus the temperature compensated USB sensor from LadyBug Technologies

Sensors from Ladybug Technologies undergo a rigorous calibration process and are fully temperature compensated so there is no need to cal or zero the sensor.

Measurement Capabilities and Features

Trigger-InOut-ports
Figure 5 - Trigger In/Out ports on the Lady Bug Technologies USB sensor

PC-power-meter-display
Figure 6 - PC power meter display for a USB sensor

Conclusion

When selecting a power sensor, much depends on the application. But beyond the needed dynamic range and frequency coverage there are many aspects to consider when making a choice. The accuracy and uncertainty that can affect the measurements need to be clearly understood in terms of the application. Will there be wide temperature variations? Will the sensor be connected and re-connected often? Will many different devices be measured such that mismatch may be an issue?

Another decision to be made is whether the convenience of an USB sensor is needed or a traditional sensor/meter configuration is appropriate. If portability is important, or the sensor is to be embedded in a system, or remote measurements are needed, then a USB sensor may be your best choice. The PowerSensor+ USB sensors from LadyBug Technologies deliver high performance in terms of accuracy, settling time and stability over temperature at the lowest price in class.

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