Noise (electronics)

In the field of electronics, noise refers to an unwanted disturbance within an electrical signal. It acts as a random, unpredictable disruption to the useful information being transmitted. Noise is not just a technical nuisance; it is an inherent part of physics. It is central to the study of thermodynamics. Any conductor that possesses electrical resistance will naturally generate thermal noise. To completely eliminate thermal noise, a device would need to be cooled cryogenically. Even under those extreme conditions, quantum noise would still remain present.

Noise is often categorized by its specific mechanism of origin. Thermal noise, or Johnson–Nyquist noise, is one of the most fundamental types. It is caused by the random thermal motion of charge carriers, such as electrons, inside a conductor. This movement happens regardless of whether a voltage is applied to the material. Thermal noise is described as being approximately white. This means its power spectral density is nearly equal across the entire frequency spectrum. In communication systems, this is often modeled as additive white Gaussian noise (AWGN).

Another distinct type is shot noise, which arises from the discrete nature of electricity. Shot noise occurs when charge carriers, like electrons, traverse a gap or a potential barrier. Because electrons arrive at discrete, random intervals, the current exhibits statistical fluctuations. An analogy for this is the sound of rain hitting a tin roof. While the rain may seem like a steady stream, the individual drops arrive one by one. In vacuum tubes, shot noise occurs as electrons randomly leave the cathode to reach the anode. This effect is also seen in certain mesoscopic resistors where the element is extremely short.

Electronic components also produce specialized forms of noise based on their internal structure. Partition noise occurs when an electric current divides between two or more different paths. For instance, a transistor typically produces more noise than the combined shot noise of its two PN junctions. Flicker noise, also known as 1/f noise, is a process where the frequency spectrum falls off steadily as frequencies increase. This results in what is called a pink spectrum. There is also burst noise, frequently called popcorn noise in audio circuits. This noise consists of sudden, unpredictable jumps between voltage or current levels that can last from milliseconds to seconds.

Beyond the internal components, noise can be introduced by the external environment. This is often referred to as coupled noise. It can enter a circuit through inductive coupling, capacitive coupling, or even through a radio antenna. Atmospheric noise, or static, is caused by natural events like lightning discharges during thunderstorms. Industrial noise comes from man-made sources like automobiles, aircraft, and high-voltage power lines. Even space provides significant noise sources. Solar noise originates from the Sun, and solar storms can increase electrical disturbances. Cosmic noise is generated by distant stars. This cosmic noise is the strongest component in the 20 to 120 MHz frequency range.

Engineers use several precise methods to mitigate or reduce these unwanted disturbances. A Faraday cage is a metal enclosure used to isolate a circuit from external electromagnetic noise. For wiring, engineers often use shielded cables, which act like a Faraday cage for the wire itself. Another technique is using twisted pair wiring. By twisting two wires together, the magnetic fields induce currents in opposite directions in alternate loops, which cancels out the net noise current. If a specific frequency is causing trouble, such as the 50 or 60 Hz from power lines, a notch filter can be used to reject that specific band. For extremely high-accuracy applications like radio telescopes, thermal noise is reduced by cooling the circuits.

While noise is usually a problem, it can occasionally be used for a beneficial purpose. One such application is dither. Dither is the intentional introduction of additional noise into a signal. This technique can be used to reduce quantization error, which is a type of noise in digital systems. By adding dither, it becomes possible to retrieve signals that are actually below the normal detection threshold of an instrument. This phenomenon is known as stochastic resonance. In these specific cases, the presence of noise actually helps reveal the underlying information more clearly.