6J-4 Embedded Circuit Which Improves the Common Mode Response of a Balanced Output 2GHz SAW Filter

A two port balanced correction circuit which can be connected across the balanced output of a SAW filter is proposed. A theoretical analysis shows that this circuit can significantly attenuate a common mode portion of a signal emitted from the balanced output of a SAW filter without substantially effecting the differential mode component of the signal. A 3D structural layout of the balanced correction circuit is prepared and an EM simulation of the 3D structure is carried out. The 3D structure is embedded in multilayer LTCC, where the relative permittivity of the LTCC material is 72.3, and where the outer dimensions of the 3D structure are 1.6 times 1.0 times 0.6 mm³ . The s-parameters of a proprietary DCS RX SAW filter are measured. The balanced correction circuit is tuned to match a pure real termination to the balanced output of the SAW filter. The performance of the SAW filter for the case when the balanced correction circuit is connected across the SAW outputs and when the SAW filter is terminated into a pure real impedance is compared with performance of the SAW filter for the case when the SAW output is terminated into its optimum load. It is found that the balanced correction circuit is capable of reducing the common mode component of the signal by ~30 dB, with a penalty of ~0.4 dB increase in insertion loss