Synthesis and 3D LTCC Implementation of a Miniaturized Wide-Stopband Bandpass Filter Using Frequency-Dependent Coupling

This work presents an implementation-oriented synthesis methodology for a miniaturized wide-stopband bandpass filter in low-temperature co-fired ceramic (LTCC) technology. A fifth-order Chebyshev ladder prototype is first transformed to the bandpass domain, and frequency-dependent coupling (FDC) is introduced at the input and output resonators to generate near-band transmission zeros while preserving an inline topology. The inductive π networks are then converted into mutually coupled inductor equivalents that can be implemented by vertically overlapping metal traces in the multilayer LTCC stack, reducing the need for extreme lumped-element values. Auxiliary parallel LC resonators are further placed at the input/output ports as FDC pole units to create additional high-frequency transmission zeros and enlarge the far-stopband design margin. A 3–5 GHz prototype centered at 4 GHz was fabricated in a 4 mm × 2.8 mm × 1 mm package. Measurements confirm transmission zeros near 2 and 6 GHz and more than 30 dB rejection over DC–2 GHz and 6–18 GHz. The results demonstrate the feasibility of the proposed LTCC-oriented design methodology.