Mitigation of cosite interference in nonlinear receivers with MEMS filters

It is common practice in modern receiver front ends to utilize a nonlinear low-noise amplifier (LNA) to provide the initial signal power gain. While nonlinear amplifiers have been shown to provide power savings over linear amplifiers in cases where the received signal is constant envelope, there are drawbacks to their use in the presence of nearby interferers. In the case where nearby interferers are strong, nonlinear amplifiers can spawn significant intermodulation (IM) products which can interfere with the demodulation process. One way to mitigate this is to pre-filter the strong interferers so as to attenuate their IM effect. The measure of effectiveness of this method is the receiver's IP3 point, the point at which the 3rd order intermodulation product's power matches the power of the desired signal. A more general approach is to determine the point at which the output of the front-end has a given signal-to-interference ratio (SIR) which takes into account the effect of all the interfering IM products, not just the third order. The goal of this paper is to introduce high Q MEMS filters to the nonlinear amplifier architecture with the goal of mitigating the IM product interference and to detail the analysis framework that can be used to understand IM distortion. We determine the filter Q required to achieve a given SIR in the presence of the aforementioned IM interference.