Delayed LMS filters suppressing the RFI in cosmic rays radio detection

Abstract

The emission of radio waves from Extensive Air Showers (EAS), initiated by ultrahigh-energy cosmic rays, has been attributed to geomagnetic emission and charge excess processes. At frequencies from 10 to 100 MHz this process leads to coherent radiation. Nowadays, the radio detection technique is used in many experiments consisting in studying EAS. One of them is the Auger Engineering Radio Array (AERA), located at the Pierre Auger Observatory. The frequency band observed by the AERA radio stations is 30-80 MHz. This investigated frequency range is often highly contaminated by human-made and narrow-band radio frequency interferences (RFI). The suppression of this contamination is crucial to lower the rate of spurious triggers. An adaptive filter based on the Least Mean Squares (LMS) algorithm can be an alternative to the currently used IIR-notch non-adaptive filter. The paper presents 32/64-stage filters based on a non-canonical FIR filter implemented into cost-effective CycloneIV and CycloneV Altera FPGAs with a sufficient safety margin of the registered performance for a global clock above 200 MHz to satisfy the Nyquist criterion.