Spurious Bypass Method for Increasing Passband Width
A major restriction on the width of passbands, for highpass and bandstop filters in particular, is presented by the unavoidable spurious resonances of the components used to implement the filters. Some of these resonances are intrinsic, such as the periodic or higher-order mode responses of distributed elements, while others are due to parasitic reactive properties of lumped elements. The method proposed herein will essentially prevent energy at the spurious resonant frequencies from impinging on the circuit elements causing the spurious resonance, using a lowpass filter with cutoff below the first spurious resonance, and bypassing the energy at those frequencies through a secondary path. This secondary path is high pass in nature, with a cutoff almost identical to the lowpass. The two paths are combined at the common output, using a lowpass matching network.