Crystal and mechanical filters

As the design of crystal and mechanical filters requires a knowledge of techniques not generally known to the designer of LC filters, these techniques are emphasized in this paper. The paper discusses the commonality of mechanical resonance possessed by these filters and how this leads to design restrictions. The basics of resonator theory are discussed and also the coupling techniques used to make multi-resonator filters. A listing is supplied of the types of filters that are generally made with mechanical or crystal resonators. A flow chart shows the normal design process, and the electrical transformations that are commonly used to develop the LC designs into useful form are also described. The subjects of tuning techniques and manufacturing methods, in general, are discussed as they can have an important bearing on these types of filters. The properties of the materials conmmonly used for the resonators and the transducers are described, as well as the restrictions that they in turn place on the design process. The paper concludes with a discussion of some topics where more work can and should be done. The aim of the paper is to impart to the nonspecialist some idea of how this important class of filters is designed and manufactured, as well as giving an idea of the performance that can reasonably be expected from them.