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A microstrip bandpass filter with a three type of capacitive coupled resonator which a Parallel-Coupler Filter, Hairpin Filter and Zigzag Filter was designed in this project. The first goal of the design was to create a microstrip bandpass filter which operates at center frequency 5.597 GHz and with other characteristics. The second goal of the design was to make the overall filter design a miniature as possible. The first step of the design was to create a lumped element filter configuration with corresponding element values which exhibit the desired filter response characteristics. The lumped element filter configuration was used mainly as a draft prototype to determine the parameters of corresponding microstrip transmission lines. The concept of the configuration is simple and it could be used to design bandpass filters with other critical frequencies by just varying the lumped elements values.
The lumped element filter configuration is then transformed into its microstrip equivalent circuit using specified formulas or expressions and tools. The parameters of the microstrip lines were optimized and tuned in order to achieve the desired response of the filter. Extra microstrip lines were such as discontinuity elements were added in the filter configuration in order to enhance the accuracy of the simulation results. Two approaches were done using the microstrip equivalent circuit. First, a non-symmetrical approach was applied on the configuration in which the parameters of each microstrip line were different from corresponding microstrip lines in other resonators. The optimization of each parameter was done independently, thus each value of the parameters would differ from each other. The resonators in this case were not identical as they were in the lumped element configuration. Although the schematic simulation results were satisfactory, however its auto-generated layout momentum simulations results were dissatisfactory due to certain factors which were discussed in Chapter 4. Next, a symmetrical approach was applied in which the parameters of corresponding microstrip lines in each resonator were assigned to variables and these variables were optimized later in order achieve the desired filter response. The same symmetrical filter configuration can be used to obtain filters with other critical frequencies by just optimizing and tuning the parameter values due to its simple configuration.
Although the configuration provides satisfactory filter response, the constraint of the design is that it does not meet the second goal of the project which is overall filter size minimization. Two from three designs achieve the goal to design the miniature design and the other last design not achieves their design specification. In the previous layout configuration, the filter does not maximize the utilization of space provided. The concept of capacitive coupling between resonators was applied in this layout design. Thus, a smaller layout design with symmetrical dimensions 6.3 mm x 2.3 mm of Parallel-Coupler Filter was able to be achieved through filter resonators optimization or reshaping concept. The problem that we can see is the bigger size of size Zigzag Filter.
As a conclusion, three bandpass filter layout were managed to be designed and realized based on the same schematic configuration. Through the design and fabrication of these filter layout, the projectâ€™s design goals which are to compare the performance of bandpass filter with center frequency in S-band and other specified characteristics and overall filter size minimization were managed to be achieved. Both designs have their own advantages and disadvantages. The advantages of the filter design without resonator reshaping (auto-generated schematic configuration) are listed.
The filter has a reasonable size to be fabricated according to the original size using the laboratory facilities
The filter response is excellent provided the hardware realization is done with a lot of attention and precision using best available tools and facilities. The design concept is simple and has a low complexity. The parameters of microstrip lines in the filter schematic configuration can be optimized and tube easily and accordingly to produce symmetrical bandpass filters with other frequency bands apart from S-band.
This filter configuration can be used for educational purposes or in microwave systems where filter size is not emphasized. The sized of the realized filter is large in S-band frequency range. However, if the same configuration is used to design filter in higher frequency bands, the overall filter would be much smaller and would be suitable for integration with typical microwave systems.
The disadvantages of this filter design are listed.
The realized filter has on overall filter size which is reasonably large for center frequency 2.5GHz although it meets the desired filter response characteristics.
Grounding on the board after fabrication need to be done with extra precision. This is because the size of the ground holes is very small and a miniature sized drill rod needs to be used. The performance improves accordingly to the amount of grounding connection done.
Although the hardware results were not so satisfactory, theoretically these designs meet the first goal of the project which could be proved through schematic and momentum simulation results in Chapter 4. Although the size is not miniature for Zigzag Filter, the overall size for Hairpin Filter and Parallel-Coupled Filter can be considered small for standard WiMax application. In the context, the filter size is considered small. The distortion and deviation in hardware results were caused by losses and errors which were explained in Chapter 4.
The advantages of the designed bandpass filter with resonator reshaping are listed:
The size of the realized filter is miniature and suitable for integration with various microwave systems. The filter has a compact size and utilizes available board space efficiency. For higher center frequencies, the overall filter size would be smaller accordingly.
Grounding needed not be done on the board after fabrication. This is because the design in made in such a way using capacitive microstrip patches which act as capacitance to ground. Thus, the chances for distortion in the measured results would be very low.
The filter response characteristics are excellent. Measurement results should be excellent provided the filter is realized as its original size without the need of undesired enlargement.
The disadvantages of this filter designed are listed.
Although the goal of producing miniature sized filter has been achieved, the miniature size of the filter is too small to be fabricated perfectly using available laboratory facilities. The picture of the filter which was fabricated as its original size is shown in Chapter 4 and it proves that the perfect fabrication could not be done because of its miniature size. The least value of line width which could not be fabricated in laboratory is 0.3mm.The minimum line width in the filter design is 0.1147 mm, which could not be done in the laboratory. Prediction of line widths could not be done before designing because the adjustment of line widths and lengths were done based on trial and error method.
The layout design is quite tedious and need to be done very precisely since a slight change in its line dimensions would affects the filter response on a whole. A lot of time and effort need to be spent in order to produce perfect filter with excellent response.
The hardware results were not satisfactory due filter size enlargement. Theoretically this design meets both goals of the project which could be proved through momentum simulation results in Chapter 4. This filter size is considered very much small compared to various filter configurations for the same center frequency and characteristics. The distortion and deviation in measured results were caused by undesired filter enlargement and losses which were explained Chapter 4.
5.3 Future Prospects and Suggestions
The filter configuration can be used to design filters with center frequencies in other frequency bands by optimization of its parameters. If considered of size is not emphasized, the layout of the filter can be auto-generated from the schematic configuration. However, if size is emphasized then the filter layout with resonator reshaping need to be done manually based on the schematic configuration. Thus, the designer has two choices in designing filter layout using the schematic configuration with center frequencies in other bands apart from S-band. Theoretically, this configuration can be used to design bandpass filter with center frequency values up to Ku-band frequency range. Thus, it is beneficial for usage in a wide frequency range. Besides that, a different approach of filter resonator reshaping may be tried and experimented during the manual layout design to produce filters which are even smaller than the one designed. Microstrip patches which represent capacitance to ground can be replaced with microstrip lines with direct connection to ground of the board and this might make the overall layout smaller. The value of Zoo and Zoe is extremely important to determine the value of width, length and space of each coupled line in order to full-fill our project. Target is too sure that design has a good agreement with the design specification. To achieve the miniature design, value of space need to be smaller but the other factor that influence such as limitation of fabrication must be considers. For the Zigzag filter design. The best result can be gain by using the EM tools to obtain the minimum parameter of filter which a space, width and length.