Radio Frequency (RF) filters operating in the microwave frequency range are needed for applications including wireless and satellite communications as well as military applications. These applications demand high performance filters that will contribute as little as possible to a system’s size and cost. Advances in materials used to construct these filters have played a significant part in meeting these demands. The bandpass coupled line filter presented here is specified to have a midband at 2.317 GHz and bandwidth of 68.34%. Passband insertion and return loss is specified to be 10dB respectively. The design was derived from standard filter design theory and formula available in the literature. An optimized computer aided (CAD) design was also generated for comparison. The ‘Microwave Office’ design software was provided by Applied Wave Research Inc., operating with an educational license. Both formula and simulation based designs had nearly identical physical structure and performance under simulation

REVIEW OF LITERATURE

RF filters operating in the microwave spectrum have a range of applications, including wireless handset and base stations, as well as satellite receivers and military applications. Recently published papers investigating RF filter design reveal a need for ongoing development. These filters operate in an increasingly crowded signal spectrum. They may operate in harsh environments subject to shock, vibration, and extreme temperatures. The industry experiences continuous pressure to improve performance while reducing the size and cost of the filters and their associated systems. Improvements in the electrical properties of available materials are helping meet these demands. PCB materials with higher dielectric constants yield smaller filter structures. The problem that comes while designing ultra wideband filters is that it becomes difficult to obtain a flat passband using the regular design equations. Hence design equations published in the papers especially for wideband filters were used for greater accuracy.
And to improve the stop band loss and to eliminate spurious response outside the passband , resonators could be placed with in the planar structure or transmission poles or zeros could be created using inductive or capacitive elements between the lines. Or a technique a defected ground structure or DGS, where the ground plane metal of a microstrip (or stripline, or coplanar waveguide) circuit is intentionally modified to enhance performance. The name for this technique simply means that a “defect” has been placed in the ground plane, which is typically considered to be an approximation of an infinite, perfectly conducting current sink. DGS allows the designer to place a notch (zero in the transfer function) almost anywhere. When placed just outside a bandpass filter’s passband, the steepness of the roll off and the close-in stopband are both improved. Simple microstrip filters have asymmetrical stopbands, and the need for a more complex design can be avoided if DGS elements are used to improve stopband performance

Design

The design equations for the coupled line are as follows
The bandpass filter was designed as a maximally flat third order filter. Now, we get the low pass prototype values from the standard Butterworth table: g_0=1, g_1=1 g_2=2 g_3=1 g_4=1 Now, we use the following design equations to get the inverter constants for a coupled line filter with N+ 1 sections: