Electrical feedthrough governs symmetry of phononic frequency combs in MEMS resonators

Understanding parasitic electrical feedthrough in microelectromechanical (MEMS) resonators is essential for controlling nonlinear spectral phenomena. Here, we demonstrate that electrical feedthrough fundamentally governs the symmetry of phononic frequency combs in a capacitively transduced MEMS resonator. Rather than acting as a mere measurement artifact, feedthrough forms a coherent electromechanical feedback pathway through direct AC coupling between drive and sense ports. This additional electrical channel modifies the effective electrostatic stiffness and damping, reshapes the intrinsic Duffing nonlinearity, and redistributes modal energy within the comb cavity. As a consequence, feedthrough breaks spectral symmetry, elevates the comb-generation threshold, compresses the spectral span, and reduces coherence across comb lines. To quantitatively evaluate spectral symmetry and repeatability, an Overlap Ratio (OR) metric is introduced to measure comb-line coincidence in both frequency and amplitude domains. By implementing real-time differential cancellation to null the feedthrough-induced feedback, the intrinsic nonlinear dynamics are restored. After suppression, the resonance frequency shifts upward, the effective quality factor increases, the comb threshold decreases, and the spectral bandwidth expands. The OR improves dramatically from 0.125 to 0.895, confirming recovery of highly symmetric and stable comb structures. These results establish a unified electromechanical framework linking electrical feedthrough to nonlinear comb formation and symmetry breaking, providing a practical route toward symmetric, low-threshold, and broadband phononic frequency comb generation for precision sensing and energy-efficient signal processing.