An ultra-miniature, low cost single ended to differential filter for ISM band applications

We present a filter for the 2.4 GHz ISM band that has both filtering and balun functionality to deliver a single ended to differential (SE2DE) filter response and 4:1 impedance transformation. We integrate the balun and filter response into a single chip using two FBAR resonators stacked one on top of the other and separated by a thin, low acoustic impedance material. True SE2DE behavior is achieved by dividing the bottom FBAR into two 100 Ohm resonators and hooking them in parallel to present a 50 Ohm impedance to the endpoint. The two 100 Ohm resonators forming the top of the stack are connected in series to present a differential output impedance of 200 Ohms. Since the entire device is not much larger than a single 50 Ohm resonator, the total die size is extremely small (much smaller than a typical FBAR half ladder filter utilizing 4 to 8 resonators).