Isotropic near field measurement system for new generation communication signals: Preliminary design and USRP calibration

In the present article we propose a solution based on Software Defined Radio for isotropic, real-time, and simultaneous measurement of six channels corresponding to six field probes (electric and magnetic). The measurement system is much needed for exposure evaluation campaigns conducted in the near field of emitting devices (mobile phone, WLAN router, etc.). However, the use of an SDR pose other problems that need careful and precise solving, to report accurate field levels. To this extent, in this work we present the calibration of an instrument model USRP N310 in the n78 5G (3.5-3.7 GHz) frequency band. The instrument response is investigated for a 5G-like signal, namely a 20 MHz wide, LTE-A, OFDM signal. The instrument sensitivity is analysed in function of the used channel bandwidth and compared to a mid-range swept spectrum analyser sensitivity. The frequency and amplitude response of the instrument are commented on. Results suggest suitability of the instrument for the proposed measurement system. To increase the accuracy of the measured field levels a quick frequency response calibration is indicated before each measurement campaign.