Tunable Evanescent Mode Filters

摘要

A single pole evanescent resonator has been designed and built to test feasibility. The results are shown in Figure 9 and indicate that the filter structure resonates at 3 GHz and has a Q of 460. Due to the lumped element character of the evanescent mode micromachined structure, parasitic resonances do not exist as shown by the measured and theoretically calculated data. As shown by Figure 9 only one resonance from 0 to 20 exists. Higher order periodic resonance are suppressed by the cutoff of the guide and the presence of the post. Using a similar synthesis method, a 2-pole 3 bandpass filter has been designed and simulated at X band. The volume of the designed filter is 10.5 cu mm cu compared to a 400 cu mm 2-pole micromachined resonant cavity filter with comparable performance. To add extra poles to an evanescent filter, additional posts separated by evanescent cavities are needed. This makes multiple pole filters easy to fabricate since only two silicon wafers are needed to create the cavity. The capability to fabricate these filters using IC fabrication technique allows for unique designs that may reduce size even further.