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

Abstract

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 0.12λg2.