frequencyselective

Frequency selectivity is a property of a system where its response varies with frequency. A frequency-selective system does not have a flat transfer function; instead it emphasizes or attenuates signals in certain frequency bands. This behavior can arise from physical media, resonant structures, or deliberate signal processing design.

In communications and signal processing, frequency selectivity often refers to channels whose impulse response extends over multiple symbol periods, causing different frequency components to experience different gains and phases. This leads to frequency-selective fading and intersymbol interference. The coherence bandwidth is a measure of how wide a frequency range remains relatively undistorted; narrower coherence bandwidth means more selectivity. Techniques such as equalization and multicarrier modulation (e.g., OFDM) mitigate these effects.

Frequency-selective filtering refers to filters that pass some frequency ranges while attenuating others. Common types include low-pass, high-pass, band-pass, and notch filters. The design targets include passband ripple, stopband attenuation, and transition width. Digital filters (FIR, IIR) realize frequency selectivity by shaping the transfer function in the frequency domain.

Frequency-selective surfaces are periodic arrays of resonant elements that reflect or transmit electromagnetic waves differently across frequency bands. They are used in antennas, radar, and electromagnetic shielding, and can be implemented with tunable elements to switch behavior in real time.

The concept also appears in optics and acoustics, where materials or structures exhibit wavelength- or frequency-dependent transmission or reflection. Understanding frequency selectivity is essential for designing communication systems, sensors, and metamaterial devices.