Novel Sensor Model Calibration Method for Resistively Loaded Diode Detectors

Romain Meyer, Sven Kühn, Katja Pokovic, Fin Bomholt, and Niels Kuster, IEEE Transactions on Electromagnetic Compatibility, Volume 57, Early View, online July 24, 2015 

Miniaturized radio-frequency electromagnetic near-field broadband probes based on resistively loaded diode detectors have a limited linear dynamic range of ca. 30 dB. Generic linearization schemes have been proposed and applied for continuous-wave-like or pulse-modulated signals to extend the dynamic range to over 50 dB. Modulation-specific linearization has been proposed to also enable precise measurements of current wireless systems with complex modulation schemes. However, calibrations that require an experimental amplitude sweep for each signal have become impractical due to the growing number of wireless communication systems. In this paper, the development and validation of a novel sensor model calibration method is described. The method is based on calibration of the optimized sensor equivalent circuit model that was derived from the sensor response as a function of bandwidth, duty cycle, modulation schemes, data rate, and statistical distribution. It is shown that all the elements of the equivalent circuit model can be sufficiently accurately determined by the dynamic response to a set of ten generic signals. This model is then used to numerically determine the linearization parameters for any digitized communication signal. The method, which was tested on various probes for over 200 modulations, resulted in a linearity uncertainty of less than <0.4 dB (k = 2) for a dynamic range of >50 dB. The proposed method will improve the precision of measurements, reduce calibration costs, increase the flexibility for application of diode-loaded sensors, and enable the use of real-time information for automated probe linearization during or after measurements.

The scientific and technical impact of the study can be summarized as:

• A novel, cost-effective, and flexible calibration procedure for testing modern communication protocols has been developed. 
• The method, based on characterization of the physical properties of the sensor for a sufficient but minimal set of signals, overcomes the limitations of previous inflexible and time-consuming calibration protocols. 
• The new procedure is independent of specific communication signals and enables numerical linearization of the response for any modulation during or after measurements without recalibration.
• The uncertainty of the calibration procedure is in compliance with requirements of current SAR test protocols for arbitrary communication signals. 

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