DUAL BAND BANDPASS FILTER DESIGN

A PROJECT REPORT

Submitted in partial fulfilment of therequirement for the PROJECT BASED LEARNING ON THE SUBJECTMICROWAVE ENGINEERING[ ECE 402 ]

School of Electronics EngineeringVellore- 632014, Tamil NaduWinter SEM 2013

By

Rashi Garg(10BEC0043)Kamireddy Lakshmi Yasaswi(10BEC0428)Ramachandruni V Lakshmi Pravallika(10BEC0447)

Under the Guidance of

Prof. I.Srinivasa RaoSENSEVIT UNIVERSITY , Vellore

Objective:To design and optimize a Dual-band Coupled line band pass filter with operating frequencies around 2.4 GHz and 3.2GHz.

Softwares used:AWR Microwave Design Environment.Serenade Software.

Review of literature :Band pass filters operating in the microwave spectrum have a wide range of applications, including wireless handset and base stations, as well as satellite receivers and military applications. Recently published papers investigating Bandpass filter design reveal a need for ongoing development. In the present scenario continuous pressure is 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 to meet these demands. PCB materials with higher dielectric constants yield smaller filter structures. More importantly, filters using materials with high quality factor will have proportionally less midband signal loss. As a result, smaller, higher performance filters are becoming easier to design.

A review of some filter design basics were helpful. Pozar[5] provides a textbook overview of filter basics. Filter types include Butterworth, Chebychev and Linear filters. Chebychev filters are seen to have an advantage of fast transition between midband and out of band signal spectrum. Lumped element filters, i.e., filters constructed with discrete components such as capacitors and inductors are noted to be unsuitable for filter construction above certain frequencies, with 500MHz given as a guideline. For higher frequency filters, lumped element filter models can be converted to distributed structures.

Recent journal articles review the basic problems faced in design of the bandpass filters and provide the possible solutions. A simple approach for the design of dual band band pass filters is use of Reduced length parallel coupled lines developed by cascading the coupled lines[1].Another approach is based on multilayer ring resonators implemented with a number of squre loop resonators using forward and reverse coupling. Frequency is obtained by varying the looplengths and loop widths are stacked[2]. The approach using Stepped impedance resonator has sharp rejection band. To alter the frequency capacitors are introduced between resonator and input/output combining network[3]. Design using stub loaded resonators S queue loops are arranged adjacently so as to allow electromagnetic coupling. To achieve dualband we introduce a open ended microstrip stub. Frequencies are controlled by varying the length of the stub[4]. Microwave filter implemented using Microstrip loop resonator uses dual mode resonators which will work at a certain passband[6]. Coupled line structures are mostly preferred and are widely used in microwave circuits, serving as directional couplers or being key components in filters, baluns, matching networks or combiners. If realized in an inhomogeneous medium as encountered in microstrips, the effective permittivities of the coupled lines even and odd mode differ. Because of the above mentioned advantages we prefer designing the dualband band pass filter using the coupled lines and the same is implemented in this project.

Introduction:Coupled line theory :

A very common type of power divider used in Microwave circuits is the coupled line. In the structure the coupling between the ports is due to the interaction of the electromagnetic fields along transmission lines which have been placed in close proximity.

Coupled Line - Even and Odd Mode:

C11 and C22 represent the capacitor to ground for one strip (without the presence of the other). C12 is the capacitance between the two lines without any ground plane. If the strips are identical, then C11= C22.

In the even mode, a virtual open circuit occurs along the axis of symmetry, which removes the C12 from the circuit. The resulting capacitance to the ground is the even mode capacitance Ce= C11= C22. If the two strips are identical, the even mode impedance Zoe can be calculated as:

Coupled Line Odd Mode:

In the odd mode, a virtual ground forms along the axis of symmetry, which may be considered as a ground plane through the middle of C12, resulting in the capacitance as it shown above. The resulting capacitance for each conductor to the ground in the odd mode is Co: Co= C11+ 2C12. The Zoo can be calculated as:

Design of Coupled Line Bandpass Filter:

Equivalent Circuit of a coupled line section

The admittance inverter is quarter wavelength long of characteristic impedance, 1/J.Narrow Bandpass filters can be made with cascaded coupled line sections.

Figure1.General structure of parallel (edge)-coupled microstrip Bandpass filter. Bandpass filter prototype:

Design equations:1.2. where n=2,3,...,N3.,4.The even and odd mode impedances characteristic impedances for each section are found using ,

Figure2.Bandpass filter designes using AWR microwave office.

Figure3.Response of band pass filter

Obtaining a dual band bandpass filter from single band bandpass filter:

Some conventions are to be followed when designing a dual band bandpass filter from the single band design i.e, the filter designed should have same electrical performance at both the frequencies say f1 and mf1.The conventional following is the proposed dual band filter:

The conditions are:

where k is an integer such that .These conditions guarantee the exact same performance at both the frequencies f1 and mf1.These conditions are mainly obtained on the basis of equating the S-parameters at both the frequencies.The proposed dual-band coupled line is impractical since the branch lines must be placed in the narrow gap of the coupled line. However, from a filter standpoint, the proposed dual-band coupled line is indeed practical. That is, a dual-band bandpass filter can be developed by cascading the proposed dual-band coupled lines with the ports 2 and 3 grounded. This is similar to conventional bandpass filters that can be implemented by cascading conventional coupled lines with their ports 2 and 3 either open-ended or grounded. When the ports 2 and 3 are grounded it is obvious that the shunt stubs at these ports also gets grounded.Then the branch lines become grounded stubs, connected in shunt with ports 1 and 4, resulting in a practical structure .

Finally, the structure is simplified further by combining all the stubs at a node under the condition that the stub admittances at two frequencies, f1 and mf1 , are maintained before and after combining.Complete set of design equations that are derived for a dual-band filter with an arbitrary number of stages are as follows:

where Z0 is the system impedance and as defined above.Calculating for the frequencies f1=2.4GHz and mf1=3.2GHz, design is obatined as follows

Result:

To obtain the precise result the above design is further optimised and modified as follows:

From the Ze and Z0 obtained from the above equations the corresponding values of width(W),spacing (S),length(L) are calculated using serenade software.

By assigning the above obtained values to the dual band coupled line MCLIN and cascading them with their ports 2 and 3 open ended we obtain the response at desired frequencies.

Circuit Diagram:

Result:

EM simulation:

Results of EM Simulation:

Conclusions :This project demonstrates a dual-band band pass filter based on coupled lines.A coupled line bandpass filter was successfully designed by direct calculation and by using AWR design tool. Results obtained from the circuit simulation and the EM simulation were nearly identical and slight differences occurred due to the constraints in physical dimensions to be included in practical scenarios. These frequency responses were well within specified ranges.

References:[1]Seungku Lee and Yongshik Lee, A Planar Dual-Band Filter Based on Reduced-Length Parallel Coupled Lines, IEEE microwave and wireless components letters, vol. 20, no. 1, January 2010[2] SeyyedKamal Hashemi ,"Dual-Band Bandpass Filters Based on Multilayer Ring Resonators,"[3] Pankaj Sarkar , Rowdra Ghatak ,D. R. Poddar , "A Dual-Band Bandpass Filter Using SIR Suitable for WiMAX Band," 2011 International Conference on Information and Electronics EngineeringIPCSIT vol.6 (2011) [4] W. T. Koh, K. M. Lum," Dual-band Bandpass Filter Design Using Stub-loaded Resonators", Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 19, 2012[5] D.M.Pozar,Microwave engineering,John Wiley,3/e,2005[6] Sholeh Jahani Maleki, Samaneh Sadi, Kambiz Sadat Naja,Massoud Dousti," A Novel Passive Dual-band Bandpass Microwave Filter Using Microstrip Loop Resonators", Electromagnetics Research Symposium Proceedings, Kl, Malaysia, March 27-30, 2012