High-frequency and narrow-band STW filters for cellular power amplifiers

Feedforward power amplifiers for cellular base stations require a narrow-band filter in GHz range. For this application, surface transverse wave (STW) filters on Quartz have been developed. In consideration of frequency stability, the STW filters were assembled by a flip-chip technique. Flip-chip assembly can be suitable for high-frequency applications because of no bonding-wires and it can also provide high frequency-stability because of no adhesive. The insertion loss of less than 4 dB and and of more than 1500 are obtained for both of an 820 MHz filter and a 1970 MHz filter. In this paper, the frequency-stability of flip-chip STW filters is investigated. Also the center frequency shift in the flip-chip assembly process is presented.