Design and Fabrication of Centre Frequency and Bandwidth Tunable Cavity Filters

Abstract

To facilitate efficient utilization of limited and expensive frequency spectrum in communication system, fully tunable filters are highly demanded. Realizing an absolute constant bandwidth, a reasonable return loss, and a high Q value over a relatively wide tuning range are some of the challenges in developing tunable filters for real system applications. This thesis deals with the design and implementation of combline cavity filters with bandwidth and centre frequency tunability. Cavity filters offer high Q and good power handling capability.

This dissertation is organized in three parts. In the first part, we study the design of constant absolute bandwidth tunable filters. We first design and implement a 5-pole C-band Chebyshev tunable filter with 40 MHz bandwidth and frequency tuning range of 4.9-6 GHz. The inter-resonator coupling elements are realized using horizontal irises. The fabricated filter exhibits a stable response over the tuning range with a bandwidth variation of ±5.3%. We then exploit semi-vertical irises to realize adjacent coupling values in a 7-pole X-band Chebyshev tunable filter with 80MHz bandwidth and frequency tuning range of 7-7.9 GHz. The fabricated filter reveals a stable response over the tuning range with a bandwidth variation of ±6.7% . We also show that using vertical irises, the normalized coupling values remain almost independent of the cavity resonant frequency. This is in contrast to horizontal or semi-vertical irises where a tedious optimization is required to achieve the same goal.

In the second part, the objective is to design a fully tunable filter with wide tuning range. To achieve bandwidth tuning, two tunable filters are cascaded. Due to the wide tuning range and large bandwidth requirements, each individual filter can not maintain a constant absolute bandwidth over the tuning range. Since only the cascaded response matters, the concept of constant return loss design is introduced where the return loss remains constant while the bandwidth could change over the tuning range. All the inter-resonator coupling elements as well as normalized input and output impedances should scale with the same factor in a constant return loss design. We show that this requirement can be achieved by careful design of filter geometry for a 6-pole tunable Chebyshev filter with tuning range of 1.8-2.6 GHz. Despite the large bandwidth variation ±12.8% , the return loss of the fabricated filter is better than 15 dB over the tuning range. Two of such filters are then cascaded by an isolator to provide the bandwidth tunability. The cascaded response exhibits a tunable bandwidth of 20-100MHz.

In the third part, the focus is on the design of a fully tunable two-port triplexer in which three fully tunable bandpass filters are connected together through input and output manifolds. After designing three fully tunable filters, manifold dimensions are optimized in ADS circuit simulator. Simulation results show that the bandwidth and centre frequency of each band can be independently tuned.