EMC Filter Design

An EMC (Electromagnetic Compatibility) filter is a passive low-pass filter placed on power or signal lines to suppress conducted electromagnetic interference (EMI). Electronic equipment generates high-frequency noise from switching power supplies, microcontrollers, and motor drives that can radiate or couple into other equipment. EMC filters block these high-frequency signals while passing the intended low-frequency power or signals. Most countries require electronic products to meet EMC standards (FCC, CE, CISPR) that limit conducted and radiated emissions.

An LC 2nd-order filter consists of one series inductor and one shunt capacitor (or vice versa), rolling off at 40 dB/decade. A Pi filter has two shunt capacitors with one series inductor between them, shaped like the Greek letter π, providing 60 dB/decade rolloff. A T filter has two series inductors with one shunt capacitor between them, shaped like the letter T, also providing 60 dB/decade. Pi filters work best when source and load impedances are high; T filters work best with low source and load impedances.

For a 2nd-order LC Butterworth low-pass filter: L = Z₀/ω_c and C = 1/(Z₀·ω_c), where Z₀ is the characteristic impedance and ω_c = 2π·f_c is the angular cutoff frequency. For the 3rd-order Pi filter: each capacitor C = 2/(Z₀·ω_c) and inductor L = Z₀/ω_c. For the T filter: each inductor L = Z₀/ω_c and capacitor C = 2/(Z₀·ω_c). These are normalized Butterworth prototype values scaled to the specified impedance and cutoff frequency.

The cutoff frequency should be well above your signal bandwidth but well below the interference frequency you need to attenuate. For switching power supplies operating at 50–100 kHz, a cutoff frequency of 10–20 kHz provides good filtering. For RF interference at MHz frequencies, a cutoff of 100 kHz to 1 MHz may be appropriate. Use this calculator to enter your required attenuation at the interference frequency and let it back-calculate the optimal cutoff frequency for you.

Insertion loss (IL) is the attenuation a filter provides at a specific frequency, measured in decibels (dB). It is defined as IL = 20·log₁₀(V_source / V_load) — the ratio of the voltage that would appear at the load without the filter to the voltage that appears with the filter inserted. For a 2nd-order filter, IL ≈ 40·log₁₀(f/f_c) above the cutoff frequency. For 3rd-order: IL ≈ 60·log₁₀(f/f_c). The higher the order and the further the frequency from cutoff, the greater the attenuation.

Required attenuation depends on the difference between your measured emission level and the regulatory limit. For FCC Class B (residential) conducted emissions limits, typical requirements are 20–40 dB of attenuation at the switching frequency fundamental and 40–60 dB at higher harmonics. For CISPR 22 Class B, limits are similar. This calculator lets you specify your required dB attenuation and find the filter configuration that achieves it.

EMC filter performance depends critically on the source and load impedances. The formulas assume a specific characteristic impedance Z₀ (typically 50Ω for RF circuits, or the actual line impedance for power filters). If the actual impedance differs significantly from the design impedance, the filter response will deviate from the calculated values. For power line EMC filters, a standard test impedance of 50Ω is defined by LISN (Line Impedance Stabilization Network) measurements in EMC standards.

Yes. For RF circuit design at 50Ω system impedance, the LC, Pi, and T filter topologies are standard building blocks for bandpass and low-pass filters in RF systems. Enter 50Ω as the impedance and specify your desired cutoff frequency in MHz or GHz. The calculated L and C values will correspond to standard RF filter designs. For power line EMC applications, use the actual source impedance or 50Ω (LISN impedance) as specified by your EMC test standard.