How to avoid resonance in multi-stage filtering for industrial servos?

If the parameters of a servo multi-stage filter (such as an input LC + output common-mode inductor) are not properly matched, the cascaded impedance may form resonance peaks at certain frequencies, amplifying noise. Avoiding resonance requires system impedance analysis and design. Etymotics suggests: First, use simulation software to build a filter model, including the measured S-parameters of the CMZ common-mode inductor and PBZ ferrite bead, as well as the parasitic parameters of the X/Y capacitors and cables. Identify potential resonant frequencies through simulation (usually between 100kHz and 5MHz). During design, stagger the resonant frequencies of the preceding and following filter stages; for example, set the resonant point of the first-stage LC filter at 50kHz and the resonant point of the second-stage common-mode filter at 500kHz. Simultaneously, connect a damping resistor in series in the filter network, such as a 10Ω resistor in parallel with the X capacitor, or a small-value resistor (such as 2.2Ω) in series after the ferrite bead, to reduce the Q value to below 1. Experimental verification shows that the optimized multi-stage filter can suppress resonance peaks by more than 15dB, and the overall conducted interference curve is smooth, meeting the requirements of the EN55032 standard.