Miniaturized ceramic filters

RECENT DEVELOPMENTS have resulted in ferroelectric ceramics suitable for use in bandpass piezoelectric filters. The properties of a piezoelectric ceramic are specified by the dielectric constant, the frequency of mechanical resonance, and the coefficient of electromechanical coupling. Of these, resonate frequency and coupling coefficient are the most critical for filter applications. These parameters change with temperature and with time after the constant which may be as large as several years. Materials are now becoming available with resonant frequency stabilities better than 0.1% over the temperature range from -40 ° C to + 85 ° C, and aging variation of 0.2% for a ten-year period.