Tunable infrared filters are important for various optical and optoelectronic systems. Ideally, such filters should span wide spectral ranges while retaining constant performance. Here, as a fundamental approach, we theoretically treat tunable resonant filters and realize favorable spectral profiles. Implementing a chirped zero-contrast grating on wedged sublayers, we optimize the resonant tunable filter for operation in the ∼5-14µm band. To clarify the root causes of the physical processes enabling the observed performance, attendant resonance modal processes and background reflection behavior are analyzed in detail by equivalent models as well as by rigorous electromagnetic models. The key innovative contribution of this research is that it enables efficient filters with simultaneously tuned operational wavelengths and sidebands.