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3.6: Summary

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    Filters using parallel coupled-line (PCL) segments are an important class of microwave filters. There are various ways a pair of coupled lines, a fourport, can be configured using shorts and opens to realize a two-port network. Many of these PCL configurations have desirable frequency selectivity characteristics. That is, they inherently have the bandpass, lowpass, highpass, or bandstop characteristics desired of a filter. In particular, the various configurations often have very sharp responses. One drawback, however, is that they have spurious passbands that derive from a line and its counterpart that is one-half wavelength longer having the same input reflection coefficient. These spurious passbands can often be pushed up in frequency by setting, in the case of a bandpass filter, the resonant frequency of the transmission line resonators above the center frequency of the filter being developed.

    The synthesis of PCL filters follows the general microwave design philosophy of identifying a transmission line structure that inherently has the desired response. The combline filter, a type of PCL filter, considered in this chapter, for example, exploits the bandpass properties of coupled microstrip transmission lines that are all shorted at the same end. Once a filter with appropriate topology has been designed, the physical realization is optimized in a circuit simulator to account for parasitics and higher-order EM effects.

    While this chapter specifically addressed the design of PCL filters, the principles can be used with all distributed filter design. Bandpass filters are the most important microwave filter type and it was seen that a bandpass filter comprises coupled resonators. So other physical structures that present coupled resonators can also become components of a bandpass filter. The synthesis approach provides design insight and exploitation of all parameters of a transfer function. This leads to optimum network topologies. Synthesis can be time consuming and specialized, but it is the only way to develop filters with optimum performance.

    3.6: Summary is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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