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.

I. Introduction

A high pass filter network is required to provide attenuation below and low loss above, a frequency called the cut-off frequency (f_c). Extending the low loss region as far as possible above the cut-off frequency is a critical design problem. The problem is primarily avoidance of spurious resonances or modes of resonance due to internal geometry. The problem is illustrated in Fig. 1, where f_s is the first spurious resonance frequency. This frequency is between the cut off and the desired maximum passband frequency, f_m. In the case of waveguide high pass networks, higher order eigenmodes of the particular waveguide cross section are encountered. For distributed TEM structures, periodicity limits the extent of the upper passband area. For example, short-circuited quarter wave length lines will again resonate at a three-quarter wavelength frequency. For lumped element structures, parasitic properties, such as capacitance between inductor turns or inductance of the wires or lines connecting capacitors, cause self-resonance of the lumped elements at frequencies above the cut-off frequency. Similar limitations of passband extent are found in the design of bandstop filters, particularly in the passband above the bandstop region. This paper will focus on the lumped element designs. In these networks, reduction of parasitic elements is the usual solution to maximizing the extent of the passband. As an alternative, we will propose a method for avoiding the parasitic resonances using a secondary path to bypass the frequency region subject to the spurious resonances, along with a lowpass network preventing the incidence of that frequency region into the spurious-producing network (Fig. 2). The example used in this paper is implementation of a high pass filter with a cut off at 30 MHz, passband above cut off extending to 6 GHz. As a state-of-the-art lumped element high pass design typically displays a maximum ratio between the first spurious frequency and the cut off frequency of no more than 40 to 1, the new approach will be shown to significantly increase the ratio, reaching 200:1 in the design example discussed herein. A defined figure of merit r will be used in this paper (referencing Fig. 1):

$$\begin{equation*}{\text{r}} = {{\text{f}}_{\text{s}}}/{{\text{f}}_{\text{c}}}\tag{1}\end{equation*}$$