Auto-Tuned 50 Hz Notch Filter Using a PLL-Based Frequency Tracker
Ahmed Ouenzar
Abstract: Mains hum at 50 Hz is a persistent source of interference for low frequency electrical measurements, audio systems, and sensor-based instrumentation. Fixed Twin-T notch filters can mitigate this disturbance, however, they become ineffective where the grid frequency drifts, or where component tolerances shift the filter’s centre frequency. To address this issue, this project develops an auto-tuning 50 Hz active notch filter, as the centre frequency is adjusted continuously with a lightweight PLL-based zero-crossing tracker. The system was designed and simulated in MATLAB/Simulink, where the instantaneous mains frequency is estimated in real time and used to update the digital Twin-T notch coefficients. The mains frequency was estimated to be stable in real time around 49.8 – 50.2 Hz, while the adaptive notch filter achieved a maximum of 40 dB of attenuation at the tracked frequency. The results illustrate a clear reduction of the 50 Hz harmonic in the frequency-domain output spectrum. In the time-domain, the results show that the useful signal is preserved, with the 50Hz component being the only signal component removed. Overall, the proposed adaptive filtering system illustrates that combining classical passive notch filtering theory with PLL online frequency estimation significantly improves performance with respect to mains-frequency drift.
Keywords
Adaptive Notch Filter, PLL, 50 Hz Mains Hum, Twin-T Filter, Frequency Tracking, Simulink Modeling, Signal Processing
Published online 4/25/2026, 9 pages
Copyright © 2026 by the author(s)
Published under license by Materials Research Forum LLC., Millersville PA, USA
Citation: Ahmed Ouenzar, Auto-Tuned 50 Hz Notch Filter Using a PLL-Based Frequency Tracker, Materials Research Proceedings, Vol. 64, pp 493-501, 2026
DOI: https://doi.org/10.21741/9781644904091-62
The article was published as article 62 of the book Energy Futures
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 license. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
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