see also:

• Victorian radio
• CB 2-way radio
• Ham amateur radio
• radio receivers
• software defined radio (SDR) receivers
• transceivers - software defined radio (SDR)
• radio antennas
• satellites and their radio signals
• radio basics

• 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
  • common mode
• some devices intentionally transmit EM energy such as transceivers - software defined radio (SDR)

• 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

• 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
  • component placement
  • trace routing
  • etc