Novel MMS SAW filter structure with a new type of chirping for High Load Impedance applications

Abstract

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 Multi- Mode SAW (MMS) RF filters are designed to have standard output impedance (for example, differential 100, 150 or 200 Ohm). This device is then matched to the LNA, having much higher input impedance, by 3 element pi-matching networks (2 series Capacitors and 1 parallel Inductor) on the PCB or Module laminate. An alternative will be to design the filter to have output impedance such 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 (in some cases 300-600 Ohms on the differential output port). Traditional DMS filter structures and known design approaches may not be sufficient to design these filters. We propose new MMS filter structures with multiple IDTs (3, 4, 5 or even more) connected electrically in series between output ports (such a structure doesn't have any internal pads if number of IDTs in series chain is even). 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 IDTs) with multiple IDTs in series causes passband narrowing and (or) fangs inside the passband of the filter. To eliminate this problem we have found a new chirping configuration in which edges of the inner IDTs in the series chain remain regular but their central areas are modulated. We show simulation and measurement results for an EGSM SAW filter employing the above mentioned MMS SAW filter structure with 4 IDTs in out chain and compare it to different architectures. The device has four output IDTs connected in series with the novel chirping. Devices fabricated on 42-LiTa03 having passband insertion loss of better than 2.25 dB over full temperature range (-30 to +85 degree C) along with the necessary transition band width, amplitude and phase balance and return loss are presented.