Nyquist

Nyquist was a Swedish-born American electrical engineer who spent much of his career at Bell Telephone Laboratories. His work contributed to foundational theory in telecommunications, signal processing, and control systems. In the late 1920s and 1930s he studied how sampling and feedback affect the transmission and stability of systems, laying groundwork that would become central to modern digital engineering. The Nyquist–Shannon sampling theorem, often associated with him, states that a continuous-time, band-limited signal can be precisely reconstructed from its samples if the sampling rate exceeds twice the signal's highest frequency. The corresponding Nyquist frequency is half the sampling rate, and the concept figures prominently in anti-aliasing and digital conversion.

In control engineering, Nyquist developed a stability criterion for linear feedback systems. The Nyquist stability criterion determines whether a closed-loop system is stable by examining the open-loop transfer function's frequency response, typically visualized as a Nyquist plot, a polar plot of the complex gain as frequency varies. If the plot does not encircle a critical point, stability is inferred under certain conditions. The Nyquist plot also provides insight into gain and phase margins.

Numerous concepts and analysis tools bear his name, reflecting his influence on digital signal processing, sampling theory, and control theory. His work continues to underpin practical methods for designing and analyzing communication systems, audio processing, and feedback-controlled systems.