Progress In Electromagnetics Research Symposium Proceedings, Xi’an, China, March 22–26, 2010 493 Novel Rectangular Coupled Line Bandpass Filter Souren Shamsinejad 1 , Shila Shamsadini 2 , and Mohamad Soleimani 1 1 Electrical Engineering Department, Iran University of Science and Technology (IUST), Tehran, Iran 2 Electrical Engineering Department, South Tehran Branch, Azad University, Tehran, Iran Abstract— This paper reports a novel band pass filter without any periodic response. The novel filter has been named Rectangular Parallel Coupled Line filter (RPCL) and derived from reforming the conventional parallel coupled line filters. The novel band-pass filter has narrowband response. RPCL filter with center frequency of 475 MHz, 1 dB pass-band of 10 MHz and 25 dB stop-band of 50 MHz has been reported here. The area which has been occupied by this RPCL filter is approximately about 52 cm 2 . The RPCL can be tuned for various applications at desired center frequency and bandwidth. The occupied area by the filter reduced related to the center frequency of RPCL. High power transmission capability is another distinguished feature of the RPCL filter. Furthermore, at high frequency applications the filter can be designed due to use in high power MMIC packages. The periodic frequency responses related to approximately all types of Microstrip passive filters has been suppressed in the novel RPCL filter. The novel RPCL filter can be designed for dual band applications such as 3G/GSM in a 3 cm 2 MMIC package. 1. INTRODUCTION Filters play an important role in the design of microwave circuits and their applications are various. So far, quite a lot of Filter structures with various types such as LPF, BPF and HPF have been proposed. Parallel coupled line filters are one of the most practical and common structures in mi- crowave transceivers. In almost all of the parallel coupled line filter structures, quarter wavelength long transmission lines are used as the basic building section resulting in a significant circuit size. Consequently, continuous efforts were carried on to reduce the device length while increasing the original performances [1, 2]. Periodic response is another feature which is existed in almost all of the microstrip filter struc- tures. The novel filter has suppressed the periodic response. This suppression has been estimated about -25 dB. Some of the narrowband transceivers require narrowband band-pass filters with high rejection at near frequency offsets. One of the vital requirements of the microwave transmitters is high power elements include filters. The microstrip passive elements can transmit high power signals and be- cause of this feature, microstrip filters [4] are indispensable components of microwave transmitters. The novel proposed filter is one of the modified and enhanced microstrip filters and has a new shape with smaller area on board. However, conventional parallel coupled line filters are quite long especially below C-Band where the quarter-wave transmission lines can be several centimeters long. The design of proposed filter has been based on theoretical parallel coupled line relationships for a Butterworth filter. The filter employs one stub for suppressing the periodic response. To fully illustrate this approach, the characteristics and the shape of novel Rectangular Coupled Line filter together with related equations are presented and its dimensions and simulation results are compared to the conventional one. The characteristics include 1 dB bandwidth, 3 dB bandwidth, 25 dB rejection, and insertion loss. The new enhanced filter is designed at frequency of 475 MHz which is the operational frequency of a specific telecommunication system. 2. NOVEL BAND PASS FILTER DESIGN Indeed, the novel filter is a modified version of the conventional parallel coupled line filter. There- fore, at the beginning, the conventional one should be designed at desired center frequency. This one consists of some λ/4 transmission lines which have been coupled with adjacent lines, as shown in Fig. 1. 2.1. Conventional Filter Design It is a Butterworth filter and the amplitude-squared transfer function for Butterworth filters with insertion loss L Ar =3.01 dB at the cutoff frequency Ω C = 1 is given by [4]: |S 21 (j Ω)| 2 = 1 1+Ω 2n (1)