A PLL based adaptive power line interference filtering from ECG signals

One of the challenges for electrocardiogram (ECG) signals extraction is additive power line interference (PLI) which contains 50Hz or 60Hz base frequency and several harmonics. Variety of fixed-frequency and adaptive notch filter structures are proposed for power line interference suppression. Because of frequency varying behavior of power line interference, fixed frequency filters suffer from the problem of low noise attenuation rate when the interference frequencies drift from their nominal values, and also signal amplitude and phase destruction around interference frequencies due to relatively high notching bandwidth. On the other hand adaptive filters are given high calculation costs. In this paper we propose a real-time, low calculation cost adaptive notch filter, using phase-locked loop (PLL). Experimental results show that in extremely noisy environments where input SNR is around - 20dB, average SNR enhancement of proposed method is approximately 40dB. This value is also as good as 35dB for -10dB input SNR.