Radio Frequency (RF) & Wireless Design

When designing various circuits there becomes a need to filter out unwanted signals by using coupled magnetic inductance in a styles similar to a transformer, which is called a Common Mode Choke (CMC).

https://en.wikipedia.org/wiki/Choke_(electronics)

Wherein a transformer has a circuit topology that is for current isolation and can be used for voltage magnification. A Common Mode Choke is for voltage isolation and for current magnification and create a high impedance input, like below :

In addition by reversing the direction of the current on the return path (2-3) the transformer becomes a Differential Mode Choke (DMC) which is very useful for creating a low impedance input , but high differential noise mode rejection like below :

CMCs and DMCs are used for power filtering, high speed differential pair signaling, RF filters, and can be used for low frequency bidirectional couplers.

Some applications are listed here :

https://www.coilcraft.com/en-us/edu/series/a-guide-to-understanding-common-mode-chokes/

and a small video is shown here :

https://product.tdk.com/en/contact/faq/emc-components-0032.html

One other interesting concept is the DMCs can be used to reduce timing skew from cable effects :

https://article.murata.com/en-sg/article/basics-of-noise-countermeasures-lesson-13

However care must be taken into consideration in choosing a CMC or DMC as a low quality one can create more problems than they solve :

https://interferencetechnology.com/be-careful-with-low-cost-quality-common-mode-chokes/

This is a low or poor quality CMC :

and this is a high quality CMC :