On the temperature coefficient of resonant frequency in microwave dielectrics

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Abstract
Developing a material with a temperature coefficient of the resonant frequency (TCF) of zero is probably the most difficult aspect of research into microwave dielectric ceramics. There are a host of high-quality-factor (Q) high-relative-permittivity (εr) materials whose TCF is far too high to be usable, notably TiO2 (εr = 104; Q = 14000 at 3GHz: TCF = +427 ppm K−1). The challenge is to find ways of tuning TCF while maintaining values of εr and Q suitable for use in microwave components. This paper explains, in terms of octahedral tilting and εr dilution, the tunability of TCF or, more precisely, the temperature coefficient of εr in Ca and Sr titanate-based ceramics. Examples of the structural changes that occur during tuning are also shown.