Investigation of Modified Microstrip Parallel-coupled Filters and Their Applications to Multifunctional Devices

改良的微帶平行耦合濾波器的研究及其在多功能器件中的應用

Student thesis: Doctoral Thesis

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Award date23 Oct 2018

Abstract

As an indispensable component of wireless communication systems, bandpass filters (BPFs) with good performance are highly desirable to meet the stringent requirements of wireless systems. In general, BPFs are widely used in two aspects. The first is confining the RF signal within the assigned spectrum ranges. Since the electromagnetic spectrum is finite and crowded, high selectivity of BPFs is highly desirable to confine the RF signal within the assigned bandwidth without interfering with adjacent channels. The other aspect is incorporating the filtering function into other devices to achieve compactness and multifunctional devices. BPFs are widely incorporated into power dividers, couplers, antennas, oscillators, among other devices. In these multifunctional devices applications, BPFs are used to manipulate either the magnitude response or phase response to achieve the desired performances. Therefore, this dissertation will concentrate on high-performance BPFs and their applications.

The first part will concentrate on BPFs with high selectivity. In this part, modified parallel-coupled filters with extra cross-coupling paths from the source/load to nonadjacent resonators are proposed. The extra cross-coupling paths from the source/load to nonadjacent resonators can be easily realized by implementing the first and last resonators as a combination of an open-ended stub and short-ended stub in the form of a microstrip line. Both electrical coupling and magnetic coupling can be achieved by this type of resonator. By building the cross-coupling path from the source/load to a nonadjacent resonator, a trisection or quadruplet coupling scheme can be easily obtained to generate the prescribed transmission zeros. By applying the extra cross-coupling paths from the source/load to nonadjacent resonators and the trisection or quadruplet coupling scheme to the design of the parallel-coupled filters, considerable space can be saved compared with traditional parallel-coupled filters with the same performance. A third-order filter with cascade trisection and a fourth-order filter with mixed trisection and quadruplet are fabricated to demonstrate the concept.

The second part will concentrate on multifunctional devices incorporating the filtering function. In this part, the modified parallel-coupled filters proposed above are used as the reference filter for incorporation into different devices.

First, a microstrip diplexer is designed based on the modified parallel-coupled filter. The diplexer not only possesses the merits of high selectivity of the filter but also achieves the required function.

Second, a microstrip filtering power divider with high selectivity is designed based on the modified parallel-coupled filter. In addition to the extra cross-coupling from the source/load to nonadjacent resonators, multiple isolation resistors, in contrast to the single isolation resistor found in conventional power dividers, are used for flexible optimization of isolation. A third-order filtering power divider is demonstrated to validate this idea.

Finally, a filtering antenna with high selectivity is designed using the modified parallel-coupled filter as the reference filter. Since the antenna and filter are resonator-based devices, replacing the last resonator in the filter with radiating elements in the antenna is a common design approach for filtering antennas. By using the extra cross-coupling scheme with the modified parallel-coupled filter, two independently controllable radiation nulls are generated in the filtering antenna. A third-order filtering stacked patch antenna is fabricated to validate the idea.

The designs mentioned above are fabricated on a printed circuit board (PCB) and measured by a vector network analyzer (VNA) and Satimo Starlab system. The measured results agree well with the simulated results. The results show that the modified microstrip parallel-coupled filters with extra cross-coupling paths from the source/load to nonadjacent resonators are good candidates for designing a filter with high selectivity and multifunctional devices incorporating the filtering function.

    Research areas

  • Bandpass Filters, Filtering Power Dividers, Filtering Antennas