Research And Design Of A Narrow-band Frequency Hopping Filter Worked At 225~400MHz

With the rapid development of wireless communication technology, the scarcity of radio spectrum resources has become an outstanding problem. Frequency hopping communication can switch between multiple channels within a frequency band, which improves the spectrum efficiency, and it also shows good performance in anti-jamming and security. As an indispensable part of the frequency hopping system, frequency hopping filter determines whether the system can work stably and efficiently. The center frequency of narrow band frequency hopping filter is easily affected by external factors leading to drift, and the useful signal cannot fall in the frequency band or the system performance worse. Therefore, it is necessary to study the temperature stability of the narrow band frequency hopping filter.In this paper, equivalent circuit parameters of the frequency hopping filter are gained by circuit simulation based on the tapped comb-line resonator theory firstly. Then the connection between group delay with external Q and the coupling coefficient is derived, which is used to guide the 3D electromagnetic simulation and obtain the filter's structure parameters. Subsequently, the filter is fabricated and debugged according to the simulated parameters.As the designed frequency hopping filter consists of both metal cavity structure and lumped circuit, there are many factors that affect the temperature stability of the filter. In this paper, we mainly focus on the frequency drift caused by the expansion and contraction of metal cavity. And the method that using PCB with negative temperature coefficient of dielectric constant to compensate frequency drift is proposed. The frequency drift caused by various structure and material parameters when temperature changes is simulated, utilizing 3D electromagnetic simulation software. Then, a fabricated frequency hopping filter with temperature compensation is compared with the filter without temperature compensation. It is proved that the temperature compensation scheme is effective and feasible.Finally, a narrow-band frequency hopping filter worked at 225~400MHz is designed and fabricated in this paper. Of which the fractional bandwidth is less than 2%, insertion loss is less than5 d B, VSWR is less than 1.5, 30 d B/3d B inhibition is less than 7 in the whole band. More important, the frequency temperature coefficient is less than ±20ppm/? in the range of-40?~75?.