Fig. 4. The near-field measurement system and FPGA-based control module for the time-varying metasurface.

(A) The schematic of near-field measurement platform, where the probe is scanning in the xy plane, whereas the 2D scanner is placed directly in front of the metasurface to test its surface electric fields. The metasurface is positioned in the xy plane perpendicular to the ground, and FPGA and its control cables are set behind the metasurface for time modulations. The two port of the scanner can selectively connect two test systems respectively for linear (VNA) and harmonic testing (spectrum analyzer and signal generator), respectively. In the test system, PC is used as the control terminal to complete the test data collection, 2D scanner, and FPGA control. (B) Experimental scenario of near-field measurement. Behind the metasurface is the FPGA-based high-speed signal control module, consisting of a daughter board and a mother board. The mother board generates time-varying control signals from FPGA and sends them to the daughter board. The driver array of the daughter board helps achieve stable signal output and monitor the signal waveform. An oscilloscope is connected to the output end of the daughter board through coaxial cables. The 2D scanner in front of the metasurface measures the near-field electric field distribution.