Least Mean Squares Filters Suppressing the Radio-Frequency Interference in AERA Cosmic Ray Radio Detection

Abstract

The production of radio waves from extensive air showers (EASs), initiated by ultra-high-energy cosmic rays, has been attributed to geomagnetic emission and charge excess processes. These days, the radio detection technique is used in many experiments aimed at studying EAS. One of them is the Auger Engineering Radio Array (AERA), located within the Pierre Auger Observatory. The frequency band observed by AERA radio stations is 30–80 MHz. The investigated frequency range is often very contaminated by human-made and narrowband radio-frequency interference (RFI). Suppression of these contaminations is crucial to lowering the rate of spurious triggers. An adaptive filter based on the least mean squares (LMS) algorithm can be an alternative one for currently used infinite impulse response (IIR)-notch nonadaptive filters. Measurements show that the LMS filter is very efficient in suppressing RFI and only slightly distorts radio signals. This article presents the 32-stage filters based on a transposed finite impulse response (FIR) filter implemented into cost-effective CycloneIV and CycloneV Altera field-programmable gate arrays (FPGAs) with a sufficient safety margin of the timing performance for the global clock above 200 MHz to obey the Nyquist requirement.