Demonstration of a novel 2.0 mm × 1.6 mm × 0.9 mm PCS Tx filter with steep pass band rolloffs and high rejection

Improved process control of a thin film bulk acoustic (FBAR) resonator process has enabled the development of a low cost, high performance transmit filter capable of replacing the existing dual-filter "split-band" solution used in many PCS communication systems today. This transmit filter, centered at 1880 MHz, meets seven stringent specification requirements over a temperature range from -30C to +85C: (1) insertion loss < 3.5 dB over a 60 MHz (3%) bandwidth; (2) above band rejection > 35 dB over the 60 MHz (3%) Rx band; (3) below band rejection > 25 dB over the 60 MHz (3%) image band; (4) low-frequency rolloff < 9.0 MHz (from 3.5 dB to 25 dB); (5) high-frequency rolloff < 8.0 MHz (from 3.5 dB to 35 dB); (6) Bluetooth rejection (at 2.5 GHz) > 30 dB and 2^nd harmonic rejection (at 3.8 GHz) > 20 dB; and (7) an in-band return loss better than 10 dB. We present a novel filter design packaged in a 2.0 mm × 1.6 mm × 0.9 mm chip-on-board package, which meets these constraints. We discuss the process control requirements needed to produce this filter with high yields.