A high performance 3.0 mm ×3.0 mm ×1.1 mm FBAR full band Tx filter for U.S. PCS handsets

A stringent demand has been placed on the transmit and receive bands of the 1900 MHz U.S. PCS communication standard which requires that the bands be separated by a 20 MHz (1 %) guardband. This narrow guardband poses a challenge to RF filter designers who want to produce a transmit (Tx) filter with low insertion-loss and with sufficient receive band (Rx) rejection required to prevent Tx signal power from being splattered into the Rx signal path. The transition region (from pass to stop band) would have to be a fraction of the 20 MHz guardband. The unused portion of the guardband would have to be allotted to account for random and systematic variations in the center frequency of the transition due to filter manufacturing variations and operating temperature variations respectively. The difficulty in designing a Tx filter with a sufficiently narrow transition region has caused handset manufacturers to employ a dual filter scheme in which each filter operates over half of the Tx band and can be switched into or out of the transmit path. In this scheme the guardband for each filter is increased to 50 MHz, thus greatly relaxing the filter design constraints. The downside of this scheme is that each handset must employ a control line, two filters, two switches, and miscellaneous discrete circuit elements to select which filter is active. Thus the handset´s complexity and cost are increased. In this work a single standalone filter that meets the stringent handset requirements and which resides in a 3.0 mm ×3.0 mm ×1.1 mm ceramic package was fabricated using a suspended membrane thin film bulk acoustic resonator (FBAR) technology. The filter has a minimum insertion loss of ∼1 dB, is guaranteed to have a 60 MHz bandwidth (1850-1910 MHz), and typically exhibits 35 dB of rejection in the Rx band (1930-1990 MHz). Enabled by high-Q FBAR resonators, the filter has a 10 MHz rolloff so that the transition can occur within the 20 MHz guardband while accounting for both manufacturing and temperature drift variations (-30 C to +85 C). In addition to meeting these requirements, the filter topology and wire bond scheme are chosen to allow the filter to achieve 30 dB of rejection at the second harmonic (3860-3980 MHz) and typically 15 dB of rejection a- t the third harmonic (5790-5970 MHz). The filter requires neither external components nor a control line thus reducing the size, complexity, and cost of PCS handsets. This is the first manufacturable standalone transmit filter of which we are aware that can meet all of the major handset manufacturers´ technical requirements.