Compact Microstrip Quasi-Elliptic Bandpass Filter With Ultrabroad Reflectionless Range and High Selectivity

In this letter, a method of designing quasi-elliptic bandpass filter with ultrabroad reflectionless bandwidth and high selectivity is proposed. It is implemented on the basis of a quasi-elliptic absorptive lowpass filter (ALPF) prototype that features both a low-reflection response over all normalized frequencies and two controllable transmission zeros near the passband. A distributed-element absorptive bandpass filter (ABPF) scheme using coupled-line units is derived from this quasi-elliptic ALPF prototype. With this distributed-element ABPF scheme, both ultrabroad reflectionless bandwidth and compact size can be realized. Equations for closed-form design are derived. A microstrip ABPF operating at 2.0 GHz (i.e., f₀) is designed for validation. The measured roll-off rates of the upper and lower transition bands are 116.44 and 157.41 dBc/GHz, respectively. The return loss is better than 10 dB from direct current to 6.25f₀. The size of the overall circuit is 0.12λ_g².