Widely tunable filters for software-defined radio applications.

摘要

This dissertation presents several new ideas for implementing widely tunable bandpass filters for software defined radios and especially for the Joint Tactical Radio System (JTRS) transceivers with a frequency coverage from a few megahertz to a few gigahertz. The tunable filters are designed for reducing the form factor. maximizing frequency coverage and minimizing the losses. Three different types of filters are presented. The first class of filters investigated is based on the conventional combline topology. By allowing switching and tuning in both inductive and capacitive parts of the filter, wide tuning ranges have been achieved. A design with non-uniform transmission lines is proposed which offers fixed fractional bandwidth filtering in the UHF hand. Another design implemented in coplanar waveguide topology achieves a. frequency tuning range of 6.5:1. The second class of filters which is considered is based on overlapped designs. Overlapped filters are end coupled resonator filters that share the same host transmission line for realizing the resonators. By selectively switching the coupling elements, the filter is tuned to different states. Different states of an overlapped filter topology can possess different number of degrees of freedom. This topology is very good for the cases that the bands to be selected have different characteristics. The third class of filters presented is based on hi-modal periodical filter stages cascaded to form multi-resolution filters covering from DC up to a few gigahertz in a multitude of equi-bandwidth channels. The details of the design are presented and a 16-channel prototype is demonstrated that covers from DC to 2.5 GHz. Also methods for miniaturizing transmission lines in wideband applications are investigated, and a new low-dispersion implementation is proposed that can achieve large miniaturization factors. The feasibility of accommodating the miniaturized lines in the multi-resolution filters is also studied. and it is shown that with a proper design it can be utilized in the multi-resolution filter topology to reduce its form factor. Finally, the nonlinear behavior due to the tuning elements is studied, and methods are proposed to predict the nonlinearity at the presence of loss and over both pass-band and stop-band.