Chapter One - Introduction & Literature Survey
Filters play important roles in many microwave applications. They are used to separate or combine different frequencies. The electromagnetic spectrum is limited and has to be shared; filters are used to select or confine the microwave signals within assigned spectral limits. Emerging applications such as wireless communications continue to challenge microwave filters with ever more stringent requirements higher performance, smaller size, lighter weight, and lower cost. Depending on the requirements and specifications, microwave filters may be designed as lumped element or distributed element circuits; they may be realized in various transmission line structures, such as waveguide, coaxial line, and microstrip.
The recent advance of novel materials and fabrication technologies, including monolithic microwave integrated circuit (MMIC), micro electro mechanic system (MEMS), micromachining, high-temperature superconductor (HTS), and low-temperature cofired ceramics (LTCC), has stimulated the rapid development of new microstrip and other filters. In the meantime, advances in computer-aided design (CAD) tools such as full-wave electromagnetic (EM) simulators have revolutionized filter design. Many microstrip filters with advanced filtering characteristics have been demonstrated .
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This project topic is ultra-wideband filter Design for wireless application, specifically looks in to filter design of ultra wideband filter using finite difference time domain (FDTD) methods.
Ring type filters are broadly used as band pass filters in modern microwave and millimeter-wave subsystems due to their compactness, excellent stop band and selectivity performance, and ease of integration. If they are built on microstrip substrate, very low cost and small size can be achieved making them very attractive for mobile communication and wideband radar system. On the other hand, they need of flexibility in commercial and military radio frequency applications .
The ring type filters have the following attractive features :
- Compact size
- Strong stop-bands and the stop-bands above the primary pass band can be made to be strong.
- Adequate coupling can be maintained between resonators elements with sizeable spacing between resonator lines. (This feature means that the proper coupling can be maintained in manufactured filters without unreasonable production tolerance requirements.)
- If desired, they can be designed to have an unusually sleep rate of cutoff on the high side of the pass band.
1.2 UWB technology
Ultra-wideband (UWB) technology is based on transmission of very narrow electromagnetic pulses at a low repetition rate. The result is a ratio spectrum that is spread over a very wide bandwidth much wider than the bandwidth used in the spread spectrum systems .Ultra-wideband transmissions are virtually undetectable by ordinary radio receivers and therefore can concurrently with existing wireless communications without demanding additional spectrum or exclusive frequency bands.
There are some of the advantages cited for ultra-wideband technology,
- Very low spectral density - very low probability of interference with other radio signals over its wide bandwidth.
- High immunity to interference from other radio systems
- Low probability of interception or detection by other than the desired communication link terminal.
- High multipath immunity.
- Many high data rate ultra-wideband channels can operate concurrently.
- Fine range-resolution capability
- Relatively simple, Low cost construction , based on nearly all digital architectures [ch1-P3]
1.3 Literature Survey
In microwave circuit design it is often observed that the physical circuit performance differs significantly from that theoretically calculated. This is due to factors such as attenuation, dispersion, and discontinuity effects, which again are functions of physical parameters and the actual circuit layout. The conclusion which may be drawn from this observation is that optimum microstrip design should be based directly upon physical dimensions.
Many techniques for design Microstrip filters had been used before, these techniques may be classified as non-adaptive and adaptive types.
The following articles present the work done by previous work researchers:-
Vincze, A. D.,  in 1974 introduced a general circuit model resembling a comb-line type structure developed using the concept of self and mutual node admittances rather than self and mutual capacitances per unit length. The merit of this approach lies in the relative simplicity of design formulas readily applicable to filters utilizing microstrip or strip-line construction. It also offers easy adaptability to computer aided design. Equations are derived relating even-odd mode impedances and transmission line lengths to prototype elements and other design parameters.
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Edward G. Cristal,  in 1975 proposed exact, general, open-wire-line equivalent circuits for tapped-line, comb-line and interdigital arrays are derived using a combination of graph transformations and induction.
A significant feature of the equivalent circuits is that they do not require commensurate length sections.
The equivalent circuit for tapped-line interdigital arrays is utilized to develop design equations for tapped-line interdigital filters.
Isam Hasan Zabalawi,  in 1982 proposed a multi wire approach to develop design equations for linear phase selective comb-line filter. The filter under consideration consists of two rows of inductive resonators
separated by a slotted coupling surface. Through the development process a multi path prototype network was considered.
George L. Matthaei, in 1997 proposed Hairpin-comb filters of attractive properties for the design of compact, narrow-band filters often desired for high-temperature-superconductivity (HTS) and other applications.
Luigi Greco,  in 1998 designed a 1900MHz PCS microstrip hairpin filter using Serenade Wireless Design Suite to demonstrate the Capabilities of Serenade in the design of distributed element passive devices for microwave frequency.
A Casanueva,  in 2000 studied hairpin-line filters in microstrip and suspended microstrip structures in the UHF band. The hairpin-line filter is another kind of filter that is very much like the parallel coupled tune filter.
Hsieh,  in 2000 used a transmission-line model to extract the equivalent lumped-element circuits for the closed- and open-loop ring resonators. The unloaded Q's of the ring resonators can be calculated from the equivalent lumped elements G, L, C and Four different configurations of microstrip ring resonators fabricated on low and high dielectric-constant substrates are used to investigate the lumped elements and unloaded Q's.
German Torregrosa-Penalva,  in 2001 discussed a new systematic approach for designing wideband tunable comb-line filters.
They presented new results on tunable comb-line filter theory and proposed explicit design formulas, to obtain the filter design parameters from specifications, which were included.
E. Semouchkina,  in 2002 used the FDTD simulations of electromagnetic fields in the time domain to study the wave-propagation process and the frequency-domain analysis of the fields to visualize standing
waves in edge- and side-coupled ring resonators. The simulation of standing-wave patterns is very important for ring resonator design modification.
H. Ishida,  in 2003 realized a band pass filter (BPF) in ultra-wide band (UWB) communications; a device using a ring resonator with a stub is investigated. A new proposal is made to control the frequency of the attenuation poles using both the ring and the stub impedance.
1.4 Aim of the work and Objectives
Aim of the dissertation is to design and analysis a UWB (Ultra Wide Band ) Microstip ring type band pass filter and Objectives are as under :
- Gather background information on UWB Microstrip Bandpass filter & FDTD techniques.
- Simulate a simple UWB Microstrip Bandpass ring type filter.
- Investigate the properties and to study the performance of UWB Microstrip ring type Bandpass filter.
- Optimized the design of UWB Microstrip ring type Bandpass filter.
- Comparisons with measurement results.
1.5 Background of Project
Several rigorous methods are available for analysis of microstrip, such as the spectral domain method, equivalent waveguides model method, mode- matching method, finite difference time domain (FDTD) method, finite element method, one of the most popular methods for analysis microstrip including multilayer microstrip is (FDTD) method .The main terminologies that are used in this project of UWB bandpass filter design are FDTD, UWB, short description of which is as follow:-
1.5.1 Finite Difference Time Domain (FDTD)
The FDTD method was primarily used to model the scattering of electromagnetic waves from objects. Later, after techniques were developed for including sources within the computation lattice, the technique was used to model radiation structures. The first filter analyzed with the FDTD method was simple structures.
The excellent agreement between the FDTD results and measured data for these simple filters show the potential of the method for modeling filter having realistic complexity, the most important features of FDTD method are mathematically preprocessing is minimal and can be applied to a wide range of structures but in the same time its have some disadvantages such as numerically inefficient, precautions must be taken when the method is applied to an open-region problem also need layer computer storage for accurate equation .[ ch1—P7 ]
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1.5.2 Why UWB
UWB is a technology that interconnects diverse devices supporting and providing each apparatus with information voice communication signal as well as multimedia signals all at the identical instance, at a high transmission pace transfer . however the power use for both infrared and Bluetooth are fairly low , but cannot be compared with that which would be achieve from the new competent technology over a great deal more longer distance with superior transmission speed
1.6 Tools used for Simulation
The major software that will be utilized for the simulation of this assignment will be MATLAB 7.0. It is a very influential software that is extensively used for different type of simulation not just telecommunication but other type of engineering as well.
1.6.1 Classical FDTD Software
This software as based on the technique described before in section 1 and developed at Brunel University by Dr.R. Nilavalan in 2006. The software can be executed using CL_FDTD.exe and the software simulates the problem defined in two input files namely,
a) xxx.geo: Specifies the geometry of the problem space. Such as metal blocks, dielectric blocks etc
b) xxx.inp: Specifies the mesh(the grid) and other parametrs such as the probe parameters, Excitation parameters, Problem boundary parameters etc.
xxx.geo and xx.inp can be generated or edited using the FDTD_view software described in section 3.
The window in Figure 2. 1 appears once the CL_FDTD.exe is executed. The xxx.geo file can be loaded using the Load .geo File button and the xxx.inp file can be loaded using Load .inp File button.
Run FDTD button starts the FDTD simulations and the simulated results will be stored in a data file called probe.dat. This data file can be accessed using any standard software such as MATLAB or Excel to analyses the plot the simulation results.[ ch1---P 10-12]
1.6.2. FDTD View Software
Electromagnetic analysis techniques can model structures of considerable complexity.
Specifying and editing these structures in three dimensions without any graphical feedback is difficult. In order to overcome these shortcomings Brunel University has developed this Graphical User Interface using Delphi.
The FDTD_view is a graphical front-end for the CL_FDTD programme described in section 2. This programme was written by Dr. R. Nilavalan using Delphi in 2006.
This graphical interface software can be started using FDTD_View.exe file.
The following functions are available from this software,
a) Control: This option is not useful with the current version of the
b) Space: The problem space can be defined and edited using this.
c) Mesh: This option is useful when editing the mesh. Re-alignment of the mesh, mesh deletion, creating new mesh are possible. The
Select, Zoom, Move and Change tools are useful when realigning the mesh. The delete and new options are available from the Edit menu.
d) Boundaries: This option can be used to specify the type of boundaries for the problem space. Metal boundary and Mur's 1st order boundaries are currently available.
e) Excitation: The excitation parameters can be specified and edited using this button.
f) Probes: Probes are useful to monitor/record filed values in the problem space. These probes can be placed anywhere in the problem space and the parameters can be specified.
g) Objects: Metal and dielectric objects can be specified using this button.
h) Tools bar: Select, Zoom, Move and Change tolls will work with most of the above buttons. They are useful for editing the above features graphically.[ ch1---P 10-12]
1.7 Dissertation layout and structure
This dissertation is divided and consists of five chapters including the present chapter organized as follows:
- This chapter includes an introduction or general background information of the project, aim of the work and main objectives of the project as well as a literature survey and the techniques used in the dissertation is covered , finally the project's organization and common outlines
- Chapter two presents and provides full description of microstrip line and filter theory are clearly phased with detailed
- Chapter three offers and contains an overview of microstrip ring band pass filter analysis.
- Chapter four introduces the methodology of project and experimental description which starts with the flow chart of the project as well as illustrates in details overview of the complete design and software simulation of the microstrip Ring type band pass filter and about techniques like FDTD and UWB & its rival it also includes the short introduction about MATLAB and how MATLAB is implemented in this project, the comparison between theory and practice is emphasized
- Chapter five contains conclusion and Recommendations for future work., the conclusion is given based on the analysis of discussions for assessment results that derived from the previous chapters and expressed and some ideas about how filters could be improved in order to meet special requirements .