10E-4 Modeling and Design of a Wire Bonded Low Loss Single Ended DMS Filter Having Broadband Rejection of -45 dB

Size and cost considerations often require engineers and RF module designers to explore various possible implementations. While some of the alternatives have clear advantages in terms of size and performance they could be expensive. Often there are alternatives which could be very attractive (cost and size wise) provided the required performance can be achieved. Having a flexible design tool/framework which can help in this exploration process is essential to answer some of these difficult questions with some level of confidence during early stages of design. We explored the possibility of designing a wire bonded single ended 881.5 MHz inter-stage SAW (surface acoustic wave) RF filter for use in the WCDMA (wideband code division multiple access) path of a dual mode (GSM/WCDMA) receiver section. This filter not only had to meet the requirements of low insertion loss and high rejection in the Tx band (824 MHz to 849 MHz) but also had to meet a -40 dB broadband rejection up to 6 GHz. Meeting these stringent broadband requirements using wire bonds is not easy. (High frequency resonances created by transducer capacitance and the increased ground inductance due to wire bonds degrades broadband rejection.) We first describe some of the basic elements of our flexible design framework (which has been implemented as a design kit in Agilent's ADS, advanced design system). Secondly, we show some of the SAW structures (Pure DMS, Hybrids (DMS + Ladder Elements)) that we explored to meet this filter requirement using our design framework. The modeling and analysis of wire bond effects using lumped element models and wire bond models (in ADS) in conjunction with the Full die EM (Electromagnetic) simulation (using ADS Momentum) was key to understanding the dominant parasitic effects. We summarize some of our observations based on this analysis. This helped us arrive at an interesting implementation of a 4-track DMS SAW Filter structure to achieve -45 dB rejection up to 7 GHz (~5 dB margin w.r.t spec) and low loss of -2.5 dB over -30 C to +85 C temperature range. We show simulation and measurement results of this filter fabricated on 42-LiTaO3 substrate.