How can multi-stage filtering in an intelligent low-voltage monitoring unit avoid resonance?

Multi-stage resonant filters avoid parameter mismatch and damping design. The first stage filter cutoff frequency is set at 15kHz (CMZ3225A-501T+2.2μF), the second stage at 50kHz (CML4532A-101T+1μF), and the third stage at 150kHz (PBZ2012E600Z0T+100nF). Small series resistors (22Ω/0.25W) are used between each stage to provide damping. The transfer function is verified using SPICE simulation to ensure that the resonant peak is below -25dB. Actual testing uses a network analyzer to measure the S21 parameters. With this design, the gain at the resonant frequency is less than -35dB, and the filter network attenuates smoothly in the 10kHz-30MHz frequency band, meeting the IEC 61000-4-6 conducted immunity test requirements.