408 P. S. TOMAR, M. S. PARIHAR, A MINIATURIZED LOW PASS FILTER WITH EXTENDED STOPBAND AND HIGH PASSBAND … DOI: 10.13164/re.2023.0408 A Miniaturized Low Pass Filter with Extended Stopband and High Passband Selectivity Pankaj Singh TOMAR, Manoj Singh PARIHAR Dept. of Electronics and Communication Engineering, Indian Institute of Information Technology Design and Manufacturing (IIITDM) Jabalpur, India 1822606@iiitdmj.ac.in, mparihar@iiitdmj.ac.in Submitted January 9, 2023 / Accepted May 24, 2023 / Online first July 31, 2023 Abstract. In this work, an ultra-wide stopband low pass filter (LPF) with high selectivity is proposed using coupled stepped impedance resonators (SIRs), open shunt stubs and circular slots in the ground plane. The proposed LPF has been modeled using a lumped equivalent circuit which is extracted from the EM model. The design has been vali- dated through the simulation and experimental results. The fabricated prototype has a 3-dB cutoff frequency (fc ) of 2.44 GHz and an ultra-wide stopband extended up to 20.5 GHz (8.4 f c ) with an attenuation level > 20 dB. The transition bandwidth (from 3 dB to 20 dB) is 0.09 GHz and the roll-off rate is 225 dB / GHz (reference to 30 dB). The passband insertion loss is 0.35 dB at 1.22 GHz and the normalized circuit size of the filter is 0.045. Keywords Ground slot with via, open shunt stubs, roll-off rate, step impedance resonator, ultra-wide stopband 1. Introduction A compact and low-profile low-pass filter (LPF) is crucial for realizing an efficient wireless communication system by eliminating spurious and undesired higher-order frequency components. In order to achieve seamless and reliable communication, the LPF must possess several key features, including high passband selectivity, a wide stop- band rejection capability, low passband insertion loss, and strong attenuation in the stopband. There have been nu- merous design methodologies and techniques proposed so far to achieve the desired characteristics of LPF. The use of planar resonator circuits, particularly based on microstrip technology is widely popular in designing low-pass filters due to their compact size, ease of fabrication, and ability to integrate with other high-frequency devices and circuits. The recent articles have reported various circuit topologies of the LPFs, which have been analyzed in detail based on several parameters such as roll-off rate (ROR), stopband bandwidth (SB), out-of-band rejection, and passband inser- tion loss (IL) [1–13]. In [1], a number of microstrip filters are reported to miniaturize the size. The LPF proposed in [2] has employed a coupled line hairpin unit to achieve a low pass response with a wide stopband, although the roll-off rate did not meet the required specifications. A radial resonator is used in the LPF circuit described in [3] to achieve a sharper roll-off rate, albeit with a limited stopband bandwidth. In [4], T-shaped resonator is utilized for achieving a wide stopband and low insertion loss simul- taneously. In [5] multiple transmission zeros are created using SIR stubs in the stopband. A filter based on mi- crostrip stepped-impedance polygonal patch resonators is reported in [6] to achieve sharp transition band but with narrow stopband. In [7], symmetrically patches loaded LPF is proposed to for ultra-wide stopband. In [8], a defected ground c-shaped structure is employed to achieve a wide stop bandwidth, but exhibits high radiation losses in the stopband. The wide stopband can be achieved in LPF by using a lattice-shaped resonator, as proposed in [9]. A very high selectivity/roll-off rate LPF is realized in [10], alt- hough the stopband is insubstantial and in-band impedance matching is poor. In [11], a radial open stub and a butterfly open stub resonators are used to improve the stop band- width of conventional LPF, but it has a complex design. A LPF with improved roll-off rate is achieved using SIRs and multiple rectangular stubs at low cutoff frequency in [12]. (It is relatively more challenging to obtain a high roll- off rate when the operating frequency is higher). In [13], a LPF with an exceptionally wide stopband is proposed through a pair of SIR, open stubs and ground slots although it shows a poor roll-off rate which needs to be improved further for better passband selectivity. Besides none of the reported LPFs can achieve size compactness, high roll-off rate, and wide stopband features simultaneously and alto- gether. In this article a highly miniaturized LPF for RF frontend application is proposed which offers improved the roll-off rate (ξ) using the technique of coupled SIRs and has very compact size realized due to meandering of the inductive lines. The stopband band bandwidth has been enhanced through a pair of quarter-wave open stubs and two circular ground slots which act as stopband series res- onators and as stopband circular cavity parallel resonators, respectively. The manuscript is structured in the following manner: Section 2 describes the design of a microstrip low pass filter, followed by an analysis of simulated results. Section 3