RFI filtering in AERA radio-detection of cosmic rays

Radio detectors in Auger Engineering Radio Array (AERA) can observe radio signals caused by coherent emissions due to a geomagnetic radiation and charge excess processes mainly in the frequency band from 30 to 80 MHz. This range is contaminated by human-made RFI. In order to improve the signal to noise ratio, RFI filters have to be used to suppress this contamination. Several filters were used in AERA to suppress the RFI: 1. based on the FFT and median filter in the frequency domain 2. IIR notch filter: (a) a non-adaptive filter with fixed coefficients (b) an adaptive filter with coefficients calculated in the NIOS® processor implemented inside the FPGA FIR filter based on the linear prediction. FIR coefficients calculated: (a) in an external ARM processor (b) in the NIOS
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processor (c) directly in the FPGA fabric. 32 linear equations are solved using 64-bit floating-point FPGA Mega-functions. The paper presents a comparison of characteristics of the filters used in the standard AERA (2a) and proposed filters (2b, 3a, 3b and 3c): the refreshment time and the RFI suppression efficiency.