Bandpass Filters With Localized Temperature Compensation Dielectrics in Low-Temperature Cofired Ceramic Packages

A series of SrTiO₃ (STO) based temperature compensation dielectrics that were cofireable with the commercial DuPont 951 low-temperature cofireable ceramic (LTCC) were developed. The STO30 dielectric with 30 wt% STO showed the highest positive temperature coefficient of resonant frequency (τ_f) that was opposite to the τ_f = -69 ppm/°C of the Dupont 951 LTCC, and was selected to design a temperature-compensated four-pole bandpass filter. The filter shows a near zero τ_f = 0.7 ppm/°C over a temperature range -20 °C to 80 °C. The variance of insertion loss of the filter over the same temperature span is 0.28 dB. The maximum-difference group delay of the filter is 37 pS. The insertion loss included two SMA connectors at 20 °C is greater than 2.45 dB. A general variational method with the transmission-line technique provided an analytical method to calculate the effective dielectric constant and the characteristic impedance of an arbitrary multilayer strip line structure. With this method, the thickness of STO30 compensation dielectric can be optimized to obtain a nearly full temperature compensation for the filter. The electromagnetic simulation results of the filter agreed well with measured data.