A Method for Attenuating the Spurious Responses of Aluminum Nitride Micromechanical Filters

We present a method for attenuating the spurious responses in aluminum nitride micromechanical filters and demonstrate the technique in a 4-pole self-coupled filter operating at 494 MHz. In the standard implementation of a 4-pole self-coupled filter, each filter pole is realized using physically identical resonators. The spur mitigation approach reported here realizes the four poles of the filter using two different physical implementations of the resonator. Both resonators are designed to have identical responses at the desired resonant frequency of 494 MHz, while many of the spurious responses of the two resonators appear at nonidentical frequencies and do not add constructively at the filter output. Using the reported method, the measured attenuation of the largest filter spur is increased by 47.5 dB when compared with a 4-pole filter realized using identical resonators (standard approach) to form each filter pole. The filter realized using the reported spur attenuation approach has >59.6 dBc of stopband and spurious response rejection over nearly a 2-GHz frequency span.