EMI Filter Insertion Loss Required

Formula

Description

When a product exceeds EMC emission limits, an EMI filter must provide enough insertion loss to bring emissions below the limit at each failing frequency. The required insertion loss is simply the difference between the measured emission level and the regulatory limit. This calculation is the starting point for filter design and component selection. Always add a safety margin of 6-10 dB to account for production variation, component aging, and measurement uncertainty.

Variables

• IL — Required filter insertion loss (dB)
• V_measured — Measured emission level without filtering (dBµV or dBm)
• V_limit — Regulatory emission limit at that frequency (dBµV or dBm)

Practical Notes

EMC limits like CISPR 32 Class B are specified as quasi-peak and average values across the frequency range. A filter that provides 20 dB insertion loss at 1 MHz may only provide 6 dB at 30 MHz due to parasitic resonances. Always verify filter performance across the entire failing frequency range, not just the worst-case frequency.