Radiofrequency Interference Filters Design Based on Complementary Split Rings Resonators

Low frequency analog and digital electronic circuits are susceptible to radiofrequency interference (RFI). This disturbance is produced when the coupled RF signal is rectified by the nonlinear behavior of the semiconductors used in the small signal analog input stages of the electronic system. These circuits present an AM demodulation effect produced by nonlinearity of internal transistors, generating parasitic signals in the low-frequency range and undesired offset voltage. In this paper, an alternative to the current standard EMI filters is presented by combining the conventional printed circuit board layout with complementary split ring resonators (CSRRs), in order to reduce the output offset impact due to RFI. An operational amplifier circuit has been designed with a 4-stage CSRR filter, electromagnetically simulated and experimentally tested. Two prototypes have been implemented, with and without CSRRs, in order to compare the filter properties in standard FR4 substrate. The resonance frequency of the CSRRs has been designed in the vicinity of 2.4 GHz in order to prevent susceptibility in the ISM band. Electromagnetic and electrical equivalent circuit model simulations are also provided and compared with experimental results. Measurement data show an effective rejection of the undesired RF demodulation without affecting the signal integrity out of the filter band, and therefore a significant reduction concerning output offset voltage impact in terms of RFI amplitude, with no-extra cost in terms of the device area or manufacturing process.