signal

A signal is a time-varying voltage, current, or electromagnetic field that encodes information for transmission or processing. In electrical engineering, it is the medium through which data, audio, video, or control instructions move from one point to another. Unlike a static voltage that remains constant, a signal changes in amplitude, frequency, or phase; these changes constitute the message being conveyed.

Signals exist in two fundamental forms: analog and digital. An analog signal is continuous, with voltage that smoothly varies across a range; a microphone produces an analog signal proportional to air pressure fluctuations, for instance. A digital signal is discrete, typically switching between two voltage levels (such as 0 V and 5 V) to represent binary data. Most modern systems convert analog signals to digital form using an analog-to-digital converter (ADC) to permit processing, storage, and noise immunity.

Signal characteristics and measurement

Engineers describe signals by amplitude (peak voltage or power level), frequency (measured in hertz), phase (relative timing), and bandwidth (the range of frequencies the signal occupies). A 1 kHz sine wave and a 10 kHz sine wave both have amplitude but differ in frequency. The signal-to-noise ratio (SNR) compares the power of the desired signal to the power of unwanted noise; poor SNR degrades fidelity in audio or bit-error rate in data transmission. Oscilloscopes and spectrum analyzers are standard tools for observing and characterizing signals.

Signal integrity becomes critical in high-speed circuits and long-distance transmission. Attenuation causes signal amplitude to decrease with distance; impedance mismatches create reflections that distort the signal. In RF and microwave work, signals are often transmitted along controlled-impedance transmission lines (typically 50 ohms in modern systems) and terminated with matching loads to prevent reflections. Crosstalk, where one signal induces unwanted coupling into a nearby conductor, is a common failure mode in densely packed electronics.

The term 'signal' derives from the Latin 'signum' and has been used in electrical contexts since the telegraph era. A telegraph operator's key produced on-off voltage pulses; these were signals, though crude by modern standards. Today, signal processing (filtering, amplification, modulation, demodulation) is a discipline unto itself, central to communications, instrumentation, and control systems across all industries.