Design Of Microstrip Bandpass Filters

Rapid development of wireless communication technology is pushing the completion of a global information system. High speed, wide bandwidth and large storage are demanded due to the extensive large requirement of information. New communication products are developed in order to satisfy the demand of national defense and consumers. How to retrieve the specific signal among signals is under intensive study. Filters are playing important role in these researches, and achieved much success. Filters with mall size and high performance are essential to the development of microwave devices.As an important type of microwave component, bandpass filters are being concerned in recent years; most researches are focusing on its miniaturization and high performance. This paper started with two-port network analysis, give us general picture of different types of filters, mapping from low-pass to bandpass, analyzed microstrip bandpass filters with planar structure. This paper based on former designs, using HFSS software, Ansoft Inc., did simulation and analysis on classical types of structures, hence introduced a relatively new design. This design made the structure be more miniaturized and in a higher performance. A few designs with different parameters were made and tested, the experimental results showed highly similarity to theoretical results.Starting from the results which have already had, we introduced many classic designs which invented by former researchers. For instance, the paralleled coupled line, interdigital, and hirepin line structure that has been discussed in Chap. 1. In these designs, the author can improve the bandwidth, insertion loss and stability of the filters through control the order of the filters. Then we discussed the basic concept of the bandpass filter, beginning with the simplest structure, discussed the characters of microstrip bandpass filters. Introduced singled and coupled line microstrip line structures and their effective permittivity and characteristic impedance. In coupled line structure we brought the expression of odd and even mode. Based on these, we exemplified some equivalent LC circuit of some classic structures and introduced some expression with high precision.In addition, we began with transmission coefficients and gave some concept such as transmission function and refraction index, etc. We discussed Butterworth and Chebyshev response, then analyzed pass band character and poles. We also discussed the normalized low pass prototype based on these two types of response, then did the component mapping and frequency shifting, hence gave the design steps of bandpass filters and drew the equivalent LC circuit. Combining with the real microstrip circuit and designed the bandpass filters with certain character.In our design, because of the dimensional limit, the author improved the former design and miniaturized the structure. The design will be discussed in Chap. 4. The measurement result indicated there is a very small permeability even less than 1 for this type of YIG material. This can well explain the reason why the central frequency is the highest when there is no bias magnetic field. Different from the simulated result, when magnetic field applied below 500Gauss, the material showed FMR character, when field below 300G was applied, the energy was absorbed and failed to act as filters. However, there was an about 1GHz shift between the maxima and minima, the range is close to the simulated result. One thing need to mention that there was a very low insertion loss then magnetic field was applied.In general, we can see our design combined with some advantages such as small structure and high performance, and this is the trends of the modern research. In future, devices with small structure and high performance are likely to be the main research area.