Reconfigurable multiband SAW filters for LTE applications

Reconfigurable surface acoustic wave (SAW) filters are presented for the 700 MHz frequency band currently allocated to Long Term Evolution (LTE). Unlike traditional SAW filters with a fixed center frequency, the proposed filters have a reconfigurable center frequency. First, a novel modular ladder type topology is proposed incorporating SAW resonators and GaAs Single Pole Single Throw (SPST) switches within the basic reconfigurable filter. The measured center frequency in the low band state is 696 MHz and the BW is 3.4%. In the high band state the measured center frequency is 718 MHz and the BW is 2.6%. The center frequency shift is 3.2%. Spurious responses caused by switches are an important design consideration and the root causes of the spurious responses in high band and low band are identified. Then a novel filter topology using Single Pole Double Throw (SPDT) absorptive switches is proposed to remove the spurious responses in the high band state. Finally the root cause of the remaining spurious response is analyzed and a solution is proposed.