LTCC Layer-to-Layer Misalignment-Tolerant Coupled Inductors and Their Application to Bandpass Filter and Helical Inductors

Mutual inductance between coupled inductors can provide large equivalent series inductance required by filter design. As mutual inductance is dependent on the distance between coupled metal lines within coupled inductors, mutual inductance in traditional straight line coupled inductors (SLCI) is susceptible to layer-to-layer misalignment during fabrication if two coupled lines are located at different substrate layers. Misalignment-tolerant coupled inductors (MaTCI) are proposed to alleviate the effect of misalignment on mutual inductance. Circuit model is also proposed to describe their mechanism. Their performance is verified and compared with traditional SLCI using coupled inductors based transmission zero circuits and bandpass filters. Measurement results of three samples of a transmission zero circuit based on proposed MaTCI show only 0.56% frequency variation at 1.9 GHz, while traditional SLCI give about 3.2% variation from samples at the same low temperature cofired ceramic substrate. Measured results of three samples of a bandpass filter using MaTCI also show more stable performance at transmission zero frequency, insertion loss and passband bandwidth as compared with the filter using traditional SLCI. Electromagnetic simulation also shows similar performance improvement for MaTCI under misalignment. The same concept is also applied to helical inductor and two misalignment tolerant helical inductors are proposed. Simulation results show that they are more stable than the traditional stacked helical inductor.