A Common-Mode-Choke is a passive circuit-level protection device designed to suppress common-mode-noise. It does this by generating a magnetic field that opposes the magnetic field of the common-mode-noise. These are used in both high-power and signaling circuits. Filters also have the similar purpose yet they differ in their primary function.

Both common mode chokes and filters can help reduce electromagnetic interference in electronic circuits, but they do so using different techniques. While a CMC suppresses only common mode noise, a filter suppresses noise, isolates different parts of circuits or improves the overall quality of a signal. Based on their characteristics filters are classified into low-pass filters, high-pass filters, band-pass filters, and band-stop filters. So the CMC may be defined as a specific type of filter.

Common mode noise is a common problem in high-speed digital circuits. It can be caused by a variety of factors, such as electromagnetic interference (EMI) or ground loops. Differential pairs are a very effective way to suppress common mode noise. Differential pairs are a pair(s) of conductors that are used to transmit a signal. The signal is encoded as the difference in voltage between the two conductors. This means that the signal is unaffected by common mode noise, which is noise that is present on both conductors of the pair.

Common-mode Operation

Common-mode noise, upon entering a differential pair, passes through the common mode choke (CMC), resulting in the induction of magnetic flux within both windings of the CMC. The windings are arranged in such a way that the noise-induced magnetic flux in one line combines with the magnetic flux generated in the other line, creating a rotating magnetic field within the CMC core. In this state, the CMC behaves as an inductor, with an impedance that is proportional to the noise frequency. This mode of operation which allows lower frequencies to pass while blocking the high-frequency noise is known the common mode operation

However, if the current of the differential signal flowing through the signal pair moves in opposite directions, each line generates a magnetic flux that is equal in magnitude but with the opposite polarity of the other line. As a result, the two induced magnetic fluxes cancel each other out, allowing the differential signal to pass through with minimal attenuation.

Types
Common-mode chokes come in two main types:

1. RF (radio frequency)
2. AF (audio frequency)

RF chokes utilize powdered iron or ferrous bead cores, while AF chokes incorporate solid magnetic iron cores. The solid ferrite cores have higher saturation flux density and current ratings compared to powdered iron cores. The RF chokes block frequencies above 30 kHz whereas the AF chokes can block up to 30 kHz.