| Background, Motivation and Objective Single ended – Balanced (Se-Bal) SAW filters are widely used in RF Front End stages of mobile devices with differential Low Noise Amplifiers (LNA). Typically SAW filters are designed to have standard output impedance (for example 50 Ohms Single Ended input and 150 Ohms differential output). These devices are matched then to the LNA by 3 element pi-matching networks (2 Capacitors and 1 Inductor) on the PCB or laminate. An alternative will be to design the filter to have such load impedance that it can be matched to the LNA by only a single parallel inductor. Such a custom design can provide smaller module size, simpler module layout, lower cost (components, materials and assembling) and in some cases even better performance. However, the Trade Off is loss of flexibility. The load seen by the filter is a strong function of LNA input impedance and layout of the laminate/PCB traces. Any change of the laminate layout will require a redesign of the filter. Filters to be used in this new configuration may need to have non-standard port impedance (for example 300-400 Ohms on the differential output port). Traditional DMS filter structures and known design approaches may not be sufficient to design these filters. Statement of Contribution/Methods In this work we propose new multi-mode (MMS) LSAW filter structures with multiple IDTs (3, 4, 5 or even more) connected electrically in series between output ports. This structure has naturally high output impedance which can be matched to high impedance LNA with a single matching inductor. But traditional chirping (chirping the junctions between adjoining transducers) with multiple IDTs in series causes fangs inside the passband of the filter. To eliminate this problem we propose a new chirping configuration in which edges of the inner IDTs in the series chain remain regular and central their areas become chirped (modulated). Results We show simulation and measurement results of EGSM SAW filter employing the above mentioned MMS SAW filter structure. The device has four output IDTs connected in series with the novel chirping. Devices fabricated on 42-LiTaO3 having passband insertion loss of better than 2.0 dB over full temperature range (-30 to +85 degree C) along with the necessary transition band steepness, amplitude and phase balance and return loss. Discussion and Conclusions New simple and very small size MMS LSAW filter structure has been proposed. New type of chirping for some transducers has been demonstrated and analyzed. Finally, a comparison between traditional and new chirping is provided and efficiency on the new chirping for proposed structure was demonstrated. |