Improving the Performance of Direct-Conversion SDRs for Radiated Pre-Compliance Measurements

Abstract

In this paper, a highly linear frontend to improve the performance of (software-defined radios) SDRs for radiated precompliance measurements is presented. In CISPR band C/D, the test receiver needs to fulfill stringent requirements for using the quasi-peak detector. Usually, an expensive preselection filter bank is necessary to make fully compliant measurements of broadband transients. Direct-conversion based SDRs show a limited out-ofthe-box performance for pre-compliance measurements caused by, e.g., harmonic mixing and saturation effects. With the use of a triple-balanced mixer, a highly linear up-conversion stage is built, eliminating the need for a filter bank. The dynamic range requirements for the SDR are strongly reduced by a narrowband intermediate frequency filter, making CISPR 16-1-1 compliant measurements possible. The sensitivity of the frontend is comparable to professional receivers on the market, although no low-noise amplifier is implemented. The performance is verified by continuous wave and transient signals according to CISPR norms. The broadband measurement results are compared with traditional characterizations of the dynamic range using the compression level and the noise figure. It shows that assumptions on the RF-link budget for a compliant design can be made with continuous wave measurements. Based on our results, the requirements for the SDR frontend are derived.