I don't sell any of these nor do I receive any remuneration if you buy them, and I have not personally reviewed all of them, they are listed here to give you perspective
Introduction
EMI is the interference of the functioning of a system caused by electromagnetic effects generated by another system
this could be via either:
radiated - through electromagnetic fields
wires carrying current generated radiating EM fields
conductive - through wires connecting the systems
eg. sharing a common ground plane or power supply
there are two main modes:
differential mode
eg. the unwanted AC portion of a DC signal such as the ripple voltage from a switch mode power supply
each country has regulations which limit the amount of EMI a device is allowed to create to protect the functioning of critical devices such as medical devices
in the US, these are detailed in FCC part 15
devices are usually tested in a certified EMI an-echoic chamber by:
an antenna connected to a spectrum analyzer while the device is rotated 360deg usually across 30MHz-1GHz for radiated EMI tests
a Line Impedance Stabilisation Network (LISN) device is used to test for conductive EMI but only needed if the device needs to plug into AC power
this tests in the range 150kHz to 30MHz
Reducing EMI from a device
device design planning
allows more options and is less costly
system level
adding ferrite to cables, adding shielding to cables
circuit level
allows dealing with differential and common mode noise separately which is more effective an flexible than system level measures
filters
RC snubber circuit
ferrite beads
PCB level
these changes usually add negligible cost to design