Tonal Interference

Introduction Tonal interference appears in many engineering measurement systems. Switching regulators introduce narrowband spurs, rotating machines produce harmonic vibration components, and electromagnetic coupling injects periodic interference into sensor signals. These narrowband spectral components are often referred to as tones. Even when their amplitude is small, they can significantly degrade measurement accuracy or corrupt downstream signal processing pipelines. Detecting these tones reliably is therefore a fundamental step in many DSP workflows. ...

Introduction Notch filters are commonly used to remove narrowband interference from signals. Typical applications include removing mains hum, switching noise, or mechanical vibration tones. In theory, designing a notch filter is straightforward. Once the interference frequency is known, a filter can be placed precisely at that frequency. However, in real engineering systems notch filters often perform poorly. The filter may fail to remove the interference, distort nearby signals, or even introduce numerical instability. ...

Introduction Narrowband tonal interference appears in many real-world DSP systems. Common sources include: switching power supply spurs rotating machinery harmonics clock leakage in mixed-signal electronics EMI coupling in sensor pipelines These tones contaminate measurements and often degrade downstream signal processing. A typical engineering response is simple: compute a PSD find the largest spectral peak insert a notch filter While this method works for clean signals, it often fails in realistic environments where noise, drift, and spectral variance dominate. ...

Introduction Notch filters are highly effective at suppressing narrowband interference — when the interference stays exactly where it is expected. In real systems, it rarely does. Engineers frequently encounter interference that: drifts with temperature shifts with load or aging wanders slowly over time appears intermittently across a frequency band Designing a narrow notch at a single center frequency often works in the lab and fails in the field. This article explains why frequency drift breaks traditional notch designs and how STFT-based drift tracking enables robust suppression in real-world DSP systems. ...