RESEARCH ARTICLE A dual‐mode tunable bandpass filter for GSM, UMTS, WiFi, and WiMAX standards applications Samaneh Sedighi Maragheh 1 | Massoud Dousti 1 | Mehdi Dolatshahi 2 | Behbod Ghalamkari 1 1 Department of Electrical and Computer Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran 2 Department of Electrical Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran Correspondence Massoud Dousti, Department of Electrical and Computer Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran. Email: m_dousti@srbiau.ac.ir Summary This paper presents a dual‐mode tunable bandpass filter (BPF) for global system for mobile communication, universal mobile telecommunications system, wireless fidelity, and worldwide interoperability for microwave access standard applications. The proposed filter consists of a stepped‐impedance resonator, single resonator, and coupled line, which are loaded with varactors. The center frequency and bandwidth of the proposed filter can be tuned with tuning varactors. Furthermore, the measurement results show that the BPF can be tuned over the frequency range of 1.8 to 2.5 GHz. Moreover, the bandwidth can be changed at each certain frequency. Furthermore, using PIN diodes, a bandstop filter is added to the tunable BPF to reduce the out‐ of‐band frequencies around the desired frequencies. The values of LC equiva- lent circuits are calculated, and the results are compared with those obtained from the layout of the proposed structure. Finally, the measurement results justify the simulation results. KEYWORDS LC model, microstrip, PIN diode, tunable bandpass filter, varactor 1 | INTRODUCTION Recently, the tunable radio‐frequency (RF) filters have played an important role in the development of telecommunica- tion systems as a part of RF front‐end systems. However, tunable filters are widely used due to their reduced size, lower fabrication costs, and less circuit complexity. Some other technologies, such as RF microelectromechanical systems, 1-3 yttrium‐iron‐garnet, 4 varactor diodes, 5-8 and PIN diodes, 9-11 have been introduced in the previously published literature for tuning filters, which can be categorized into either discrete or continuous types. Three different types of configurations that have been proposed for implementation of tunable filters are the follow- ing: (1) fixed bandwidth with tunable center frequency, 12-14 (2) fixed center frequency with tunable bandwidth, 15 and (3) simultaneous center frequency and bandwidth tunability. 8,16,17 Giving the above facts, this paper mainly focuses on the design of a bandpass filter (BPF) that its center frequency and bandwidth can be simultaneously tuned with tuning varactor capacitances. Therefore, the proposed structure supports some communications applications such as global system for mobile communication (GSM), universal mobile telecommunications system (UMTS), wireless fidelity (WiFi), and worldwide interoperability for microwave access (WiMAX) with center frequencies of 1.85, 2.1, 2.4, and 2.5 GHz, with bandwidth values of: 85, 60, 20, and 10 MHz, respectively. Received: 17 July 2018 Revised: 15 December 2018 Accepted: 20 January 2019 DOI: 10.1002/cta.2607 Int J Circ Theor Appl. 2019;1–11. © 2019 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/cta 1