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).

Top View
Board
 Length (L) =  mm Width (W) =  mm
Trace
 Width (a) =  mm Length (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
 Digital Signal - Trapezoidal WaveformSwept Frequency - Constant Voltage Amplitude (A) = V Rise time (tr) = ns Fall time (tf) = ns Duty cycle (τ/T) = % Data rate = Mbps

Important Note: This algorithm was revised in January 2014. The new algorithm 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 new 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.