2.15: Bandpass Filter Topologies

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Figure \(\PageIndex{1}\): Microstrip layout of a third-order combline filter based on coupled lines. Each of the vertical microstrip line forms a resonator with the top gap capacitors. The input and output gap capacitors provide impedance matching.

The essential structure of a bandpass filter comprises resonators that are coupled to each other. A large number of filter architectures based on this concept have been deployed. A parallel coupled line (PCL) filter called a combline filter is shown in Figure \(\PageIndex{1}\). Here the microstrip lines form the resonator and the coupling of parallel lines provides the interresonator coupling. A variation on a filter with PCL resonators is shown in Figure \(\PageIndex{2}\) [22]. This filter uses edge-coupled microstrip resonators that are \(\lambda /2\) long (at the center frequency). Figure \(\PageIndex{3}\) shows another distributed bandpass filter topology utilizing end-coupled microstrip resonators. That is, the gaps provide the interresonator coupling. All transmission line bandpass filters have spurious passbands [23, 24, 25]. The root cause of the spurious responses derives from the transformation of the parallel \(LC\) resonators into their transmission line form.

Figure \(\PageIndex{2}\): Microstrip layout of a parallel coupled bandpass filter.

Figure \(\PageIndex{3}\): General microstrip layout for an end-coupled bandpass filter (series coupling gaps between cascaded straight resonator elements).

Figure \(\PageIndex{4}\): A microstrip coupled dielectric resonator bandpass filter configuration.

A dielectric-resonator-based bandpass filter is shown in Figure \(\PageIndex{4}\) [26]. Here the pucks are the bandpass resonators and they are coupled and the evanescent fields outside the pucks provide the interresonator coupling. The pucks shown are cylinders of high-permittivity material (typically having a relative permittivity of \(500–85000\)) with an approximate magnetic wall at the cylindrical surface of the puck. Thus the puck resonates when its diameter is approximately \(\lambda /2\).\(^{1}\)

With all these filters the interresonator coupling functions as an inverter. Thus the basic functional unit of the bandpass filters is the coupled resonator structure shown in Figure 2.14.1(b).

Footnotes

[1] A better estimate is developed from the zeros of Bessel functions, as the fields inside the pucks have a Bessel function dependence (this is the form of the solution of the wave equation in cylindrical coordinates).