Study Of Ultra-narrow Band Pass Filter

Microwave filter is a two-port and indispensable device in modern communication systems. Signals in the pass band can get though the filter without any attenuation. However, there is attenuation in stop band. Thus the signals’ response within a range of frequencies can be controled by the filter. To guarantee each system work normally, a serious of demands are put out on devices’ frequency characteristics due to the limitation of the frequency. In order to inhibit the crosstalk among the signals, good selectivity and highly rejection out of the pass band are basic requirements. Microwave filters are widely used in various communication systems, radar systems and electronic warfare systems. The ultra-narrow band filter has already become a hot issue for a better selectivity. And waveguide filter is irreplaceable because of the high power capacity and the high quality factor.This thesis facuses on the filter starting with the actual project requirements, and this thesis was written based on some test results. During the process of design, there are some requirments. Firstly, the requirements of narrow band filter’s engineering application were put in consideration. Secondly, the designed model should be repeatable. Lastly, the temperature’s influences on a narrow band filter is analyzedFirstly, this thesis was started with the present research status of the filters, especially the narrow band filter. And the characteristic and application of these filters were analyzed.Secondly, the theory of the filter design is explained and emphasis the rectangular resonator’s characteristics and the iris coupling.Based on these theories mentioned above, some filter models are designed. The first filter is a rectangular waveguide filter with symmetrical inductive irises. Technical indexes are as follows: center frequency is 10 GHz, bandwidth is 100 MHz, and insert loss is less than 2dB, and the inhibition is at less 30 dB where the frequency is 100 MHz away from the pass band. Lastly, the VSWR should be less than 1.5. In order to conveniently connect with external device, the standard waveguides are applied to input and output ports. With the external fixation splint, there is a more stable model for practical engineering application. Importantly, a new method to tune coupling screws was put out. The model was processed and assembled. After tuning the screws, it’s turned out to be good results. Thus the engineering application was verified. And another two types of narrow band filters are designed. They are different in coupling structure. The coupling structures are cutoff-mode waveguide and the asymmetry inductive irises. Both give out a good simulation results.