On the temperature coefficient of resonant frequency in microwave dielectrics

Developing a material with a temperature coefficient of the resonant freque ncy (TCF) of zero is probably the most difficult aspect of research into mi crowave dielectric ceramics. There are a host of high-quality-factor (Q) hi gh-relative-permittivity (epsilon (r)) materials whose TCF is far too high to be usable, notably TiO2 (epsilon (r) = 104; Q = 14 000 at 3 GHz; TCF = 427 ppm K-1). The challenge is to find ways of tuning TCF while maintaining values of epsilon (r) and Q suitable for use in microwave components. This paper explains, in terms of octahedral tilting and epsilon (r) dilution, t he tunability of TCF or, more precisely, the temperature coefficient of eps ilon (r) in Ca and Sr titanate-based ceramics. Examples of the structural c hanges that occur during tuning are also shown.