Why is it necessary to control the parasitic capacitance of common-mode inductance in communication lines to be below 10pF?

Parasitic capacitance (primarily caused by interlayer coupling of windings and pin distribution) significantly harms the integrity of high-frequency signals:

 High-frequency resonance risk: Parasitic capacitance Cp and inductance L form an LC circuit, with a self-resonant frequency formula. If \(Cp > 10pF\), it may cause impedance valleys within the suppression band (such as 1MHz~100MHz), weakening the filtering effect and leading to signal reflection or oscillation.

 Bandwidth degradation: Parasitic capacitance acts as a high-frequency bypass connected in parallel across the inductance, leading to an early decrease in common-mode impedance at high frequencies and an inability to effectively suppress communication frequencies (such as GHz-level noise in USB 3.0/Ethernet).

 Radiation interference intensifies: insufficiently attenuated common-mode current may radiate through cables, violating FCC/CISPR radiation limit standards.

 Timing degradation: High-speed signals (such as HDMI/PCIe) have extremely high edge rates, and parasitic RC delay can lead to increased inter-symbol interference (ISI) or bit error rate.