Variable Amplification Input Zero Delay Least Mean Squares Adaptive Filter for RFI Suppression in AERA Radio Detectors of Cosmic Rays

Extensive Air Showers (EAS) generate radio emission mainly in geomagnetic and charge excess processes. The Auger Engineering Radio Array (AERA) focuses on the coherent radio emission, generated in the frequency range of 30 - 80 MHz. However, this range is contaminated by human-made and narrow-band radio frequency interferences (RFI). At present, AERA uses a non-adaptive IIR filter tuned to the four strongest narrow-band carriers. The efficiency of the RFI suppression is very good, but for only well-known sources. In the near future, a 200 MW solar farm with a 250 kV power line is planned to be built in the Southern part of the Pierre Auger Observatory. The non-adaptive filter will probably not suppress the much more sophisticated RFI spectrum coming from high-power low-to-high voltage converters. The development of a very efficient adaptive filter seems to be a crucial issue.The Zero Delay Least Square Filter (ZDLMS) developed recently is a very promising solution. It allows extracting real cosmic ray signals totally hidden in the background. However, its efficiency is extremely high for very strong contaminations. For lower RFI levels its efficiency drops down. The paper presents an optimization of the ZDLMS filter introducing a dynamical signal amplification for using in a maximal way the efficiency of the ZDLMS. The additional FPGA segment controls the range of the detected signals and adjusts the 14-bit filter to leave only three MSBs for a safety margin of the potential cosmic rays events