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Evaluating RF, microwave power sensors and meters

Posted: 28 Feb 2012     Print Version  Bookmark and Share

Keywords:RF  microwave  power meters  ATE 

Power measurements are integral to the development of any RF or microwave product, whether it's a mobile phone or a sophisticated radar system. The choice of an RF or microwave power measurement system is more complex than ever with the recent availability of new functions in power meters previously reserved for higher-end analysers. With the large variance in product offerings and specifications on manufacturer's data sheets, it's helpful to have an understanding of the most important factors when evaluating USB power sensors/meters.

Basic factors
Choosing a USB power sensor involves many of the same criteria as traditional power meters and sensors. Factors like frequency range, dynamic range, accuracy, zero and calibration, speed of measurements, and triggering continue to be critical to the selection process.

Power sensors cover frequencies from several kHz to 110GHz. The most commonly used ranges are through 6GHz to 20GHz. Since power sensors are broadband detectors, they detect all RF power at their input across the entire frequency range. Variations in the frequency response of the sensor are accounted for in the calibration table stored within the sensor.

Dynamic range depends on the type of sensor technology used. Diode based sensors have the widest dynamic range usually ranging from -60 dBm to +20 dBm or more. Their wide dynamic range coupled with their quick response time make diodes the preferred solution in most applications. A diode sensor achieves a wide dynamic range by extending the useful range of the diodes beyond their square law region through the use of correction factors, and the use of multi-multiple diode paths.

When using multi-multiple paths, the method used to switch between these paths can have an effect on linearity. Most sensors measure one path at a time and switch at some threshold. The transition point can be a point of discontinuity or hysteresis leading to non-linearity or measurement delays.

The latest power sensors continuously digitise both paths simultaneously and use a weighted average over the transition point.

Compared to diode based sensors, thermistor-based sensors have a limited dynamic range from -20 dBm to +10 dBm, whereas thermocouple sensors typically have a dynamic range from -35 dBm to +20 dBm. The typical maximum input power value for most power sensors is +20 to +23 dBm. Power attenuators and couplers can be used to reduce the maximum power at the input of a power sensor, but their use introduces added reflections between sensor and attenuator. These reflections decrease measurement accuracy and require proper matching and more set-up time to calibrate out VSWR mismatches.

Overall accuracy is a combination of several error sources and is typically calculated by combining the errors in a standardised way. These error sources include: sensor to DUT mismatch, calibration factors, linearity, noise, temperature, and zero-offset. Most manufacturers follow the ISO Guide (ISO/IEC Guide 98) to the Expression of Uncertainty in Measurement which explains in detail how uncertainty factors combine. Overall accuracies for power sensors range from 2 to 5 per cent.

Calibrating a power sensor requires connecting it to an external reference source. Zeroing a sensor usually requires disconnecting it from the device under test. Zero and calibration requirements can increase test times and cost, especially in automated test systems. If a power sensor requires periodic zeroing or calibration, the ATE system must be designed to accommodate these procedures. This usually requires some combination of costly switches, manual setup procedures, or dedicated software. Some newer sensors have eliminated the need for zero and calibration.

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