An FPGA based trigger and RFI filter for radio detection of cosmic rays

For the observation of UHECRs by the detection of their coherent radio emission an FPGA based trigger and RFI filter was developed. Using radio detection, the electromagnetic part of an air shower in the atmosphere may be studied in detail, thus providing information complementary to that obtained by water Cherenkov detectors which are predominantly sensitive to the muonic content of an air shower at ground. For an extensive radio detector array, due to the limited communication data rate, a sophisticated self trigger is necessary. However, radio signals in the frequency range of 30-80MHz are significantly contaminated by radio frequency interferences (RFI) and human made distortions. The digitized signals are converted from the time to frequency domain by a FFT procedure, then deconvolution and RFI-filters are applied to correct the frequency response and to suppress the RFI. Finally the filtered data is transformed back into the time domain by an iFFT, also generating an envelope as a base for the final self-trigger. To avoid leakage effects and to create an overlap of successive data blocks, trapezoidal windowing is applied with internal overclocking. The algorithms for two polarization channels have been successfully implemented in a single Altera^® Cyclone III FPGA chip EP3C80F780C6 and tested in a prototype board with an 180 MHz sampling rate, 16 bit dynamic range, and 12-bit resolution.