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Novel Tunable Filters and Directional Filters for Modern Multiplexing Applications


As the varieties of wireless services increase nowadays, selecting one of the service the user is targeting for becomes important. To do so, there are two popular methods, to use a tunable filter to dynamically select the service wanted, or to build a multiplexer capable of separating signals from different services to different channel for processing. Each method has its difficulties.

For tunable filters, the tuning frequency range seldom exceeds 2:1. This value is the ratio of the highest frequency the filter can be tuned to to the lowest. This means it can only select services within 2:1 frequency range. Also, as the filter is tuned to different frequency, its bandwidth usually varies as well.

Thus in the first two chapters, we first investigate a low-profile tunable filter structure that can be stacked vertically. By doing so, we can create a compact tunable filter bank. By switching to different tunable filters in the filter bank, we can cover the service in different frequency range. In Chapter 2, we describe a tunable filter, of which the bandwidth variation can be maintained small over the tunable filter tuning frequency range. The scheme to achieve this is simple and can be iteratively optimized numerically, which is very computationally efficient.

Multiplexer usually cannot be tuned. However, signal of each service may occupy different bandwidth at different frequency. Multiplexer can cater its response to each service it targets to, since a filter is dedicated to each service.

Directional filter is one of the multiplexing methods. Its main advantage is to enable a complex multiplexer system to be designed module by module. However, the directional filter structures existing in the literature are either too simple to support a good filtering response or has a manufacturing tolerance too tight to be practical.

Thus, the latter half of this dissertation is dedicated to the investigation of a new method to directional filter design so that it can realize high performance filtering and has more relaxed manufacturing tolerance. The theory of this method, the effectiveness and limitations are investigated and discussed in detail in Chapter 3 through Chapter 6.

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