Common-Mode EMI Calculator

The fields that couple from a circuit board trace directly to cables attached to the circuit board can induce common-mode currents on these cables resulting in radiated emissions. Electric-field coupling is proportional to the magnitude of the voltage on the trace. Magnetic field coupling is proportional to the current flowing in the trace. For a given board geometry, a closed-form expression for the maximum emissions due to these coupling mechanisms is described in [1, 2].

Assumptions:
  • The board is not within a well shielded enclosure. (There's a different calculator for this case.)
  • There is at least one cable attached to the board and the cable length is much greater than the board dimensions.
  • The board is located approximately one meter above a ground plane (as in a CISPR or FCC radiated emissions test).
Side View
circuit board trace geometry
Top View
Board
Length (L) =  mmWidth (W) =  mm
Trace
Width (a) =  mmLength (lt) =  mm
Height (t) =  mm 
Source location (red square):x1 =  mm  y1 =  mm
Load location (blue square):x2 =  mm  y2 =  mm
Load resistance (RL) =  Ohm
Load capacitance (CL) =  pF
Voltage Source
Amplitude (A) =  V
Rise time (tr) =  ns
Fall time (tf) =  ns
Duty cycle (τ/T) =  %
Data rate =  Mbps

Important Note: This calculator does not ask for the source impedance. The voltage input by the user is the signal voltage, not the open-circuit source voltage. The signal current is the signal voltage divided by the load impedance. The algorithm is described in [3].

[1] C. Su and T. Hubing, "Improvements to a Method for Estimating the Maximum Radiated Emissions From PCBs With Cables," IEEE Trans. on Electromagnetic Compatibility, vol. 53, no. 4, Nov. 2011, pp. 1087-1091.

[2] C. Su and T. Hubing, "Imbalance Difference Model for Common-Mode Radiation from Printed Circuit Boards," IEEE Trans. on Electromagnetic Compatibility, vol. 53, no. 1, Feb. 2011, pp. 150-156.

[3] C. Zhu and T. Hubing, "Maximum Radiated Emission Calculator: Common-Mode EMI Algorithm," CVEL-13-051, Dec. 23, 2013.

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