| With the rapid development of radar technology and wireless communications, widespread attention has been given to microwave/RF circuits and systems. Among these circuits and system the microwave passive filter takes an important role, i.e. the performance of these circuits and systems mainly depend on the one of the filter. In addition, as the more and more wireless systems have been used, the frequency bands of different systems will become closer and closer, which gives some specific requirements to the filter. Among various kinds of filters, the microstrip filter has become the most commonly used filter type in recent years. The reason mainly focuses on the follows:firstly it has many advantages such as small size, low cost, easy to be fabricated or integrated and with higher performance etc. At the other hand as the the development of more high-performance microstrip substrate materials, this make it possible to further increase the application frequency and reduce the size. In a word the microstrip filter should be the good one to be chosen to satificate the specific requirements of different wireless systems. Based on above the microstrip filter will be chosen to study the low pass filter in this thesis.Lowpass filter is an important part of the filter family. It mainly used in the transmitter or receiver to filter out the unwanted high frequency signals. Among the parameters of the low pass filter the band rejection is an important one. Because there are frequently many parasite passbands in lots of present lowpass filters, which will cause the interference signals to get into the receiver system to decrease the performance of the system. Nowdays the frequency spectrum of being most commonly used in wireless communication has been almost fully developed, if the receiver wants to obtain purer signal frequency, a lowpass filter with a high degree of suppression and wide band rejection should become necessary.In this thesis, firstly the technologies about broading stopband of lowpass filter have been summarized, which is based on the analysis of the relevant literatures and the theory of microstrip filter. Then a detail study on the high-low impedance lowpass filter has been carried out. After these, a lowpass filter with 2GHz cutoff frequency was designed, simulated and validated by use of the 3-D EM simulators—ADS and HFSS. the results showed good agreements between the main performances of the design and simulation. But unfortunately there appeared the spurious passband in the stopband near the passband. then the defected ground structure was studied and used to suppress the hammonics of this structure, at last a defected ground structure with a asymmetrical dumbbell-shaped was designed and inserted into the filter to reject the spurious passband. The results of simulation and measurement showed that there is no change on the pass band characteristics.but the inhibition is greater than 30dB between 3.2-18GHz.It showed that the defected ground structure can broarden the stopband.Finally, a high-performance compact wide stopband low-pass filter based on the high-low impedance structure was simulated and fabricated, which applied the following step/technologies:changing the structure; embedding a simple lowpass filter to suppress the harmonics; adding quasi-fractal structure to improve the in-band characteristics and reducing the filter area by use of the transmission line T-equivalent. The results of simulation and experiment showed that the maximum insertion loss, the fluctuations and the maximum S11 is 0.3dB,0.25dB and-13dB respectively in the frequency range of 0-2.0GHz and the degree of suppression is greater than 33dB in the frequency range of 2.8-18GHz; the area of the filter is changed from the initial 61.8×12.4mm2 to 32.6×26.9 mm2 and the maximum size of this filter is effictively reduced. |
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