Today, microstrip filters are widely used in the communication systems. One of the most important filters which are used in the microwave devices is microstrip wideband bandpass filters (MWBBPF). Due to the importance of these types of filters, various design methods have been presented to them. Coupled stepped impedance resonator is one of the most practical resonators for designing wideband bandpass filters [1-17]. Simple structure and wide bandwidth are one of the advantages of this type of resonator but the absence of transition zeros at the edge of the passband decreases the response sharpness. Another benefit of this resonator is existence of the interface formulas between the values of inductors and capacitors, and the dimensions of the stepped impedance stubs. Using the modified multi-mode resonators (MMRs) is a way to increase the efficiency of MWBBPFs [1] to[11]. In [2] a U-shaped MMR is used to increase the sharpness of MWBBPF response. The open loop resonators have been added to the stepped impedance stubs and increased its efficiency [3]. A triple stepped impedance stubs structure in [9-10] has been used to increase harmonic elimination, but the response of this structure is not sharp. In [11] to [14], the structure of the MWBBPFs provides two different paths for the input signal, which creates the transmission zeros in the edges of the passband and increase the sharpness but the dimensions of the such filters are large and inappropriate. Using the defected microstrip structure (DMS) and defected ground structure (DGS) have been considered as other methods for designing MWBBPFs [15] to [17] but using these structures makes it difficult to integrate with other devices. In this article, we try to describe the well-known polynomials in the LC filters design, such as Elliptic, Butterworth, Chebyshev, Hourglass, Bessel, and Gaussian, and then we provide microstrip equivalent structures for each part of these circuits. Now, by combining these microstrip parts
