How to avoid resonance in a multi-stage filter for a motion controller (MC)

To avoid resonance in the multi-stage filter circuit of the motion controller MC, it is necessary to rationally plan the cutoff frequencies of each stage and increase damping. During the design, the cutoff frequencies of adjacent filter stages should be staggered by at least one octave, for example, the first stage cutoff frequency is 10kHz, and the second stage is 100kHz. In LC filters, the parasitic capacitance of the inductor and the parasitic inductance of the capacitor can create high-frequency resonance. Therefore, components with good high-frequency characteristics should be selected, such as ferrite beads instead of inductors, and low-ESL capacitors instead of electrolytic capacitors.

Connecting a damping resistor in parallel across the inductor or a small resistor in series with the capacitor can reduce the Q value at the resonance point. For active filters, stability analysis is required to ensure sufficient phase margin. Frequency domain analysis should be performed using simulation software to check for spikes in the transfer function. In actual testing, the transmission characteristics of the filter are measured using a network analyzer, and the smoothness of the amplitude-frequency curve is observed. Optimizing the layout and reducing the parasitic inductance and capacitance of the inter-stage connections can also suppress resonance. The final design should exhibit a monotonically decreasing attenuation characteristic within the target suppression frequency band.