The Design Of RF-IF Filter Of High Selectivity In The Front-end Of Ultra-short Wave Receiver

With the fast development of Ultra-short wave communication, it has widely used in civil and military. Ultra-short wave communication is gradually developing to automated, network intelligent, direction. Because of the anti-jamming ability, simple equipment and easy for temporary networking, Ultra-short wave radio has become the preferred method of emergency communication, and have great value in research. When communication between radios, then you need to use ultra-short wave receiver front-end filter to filter out interfering signals.In this subject, the author use variable capacitance diode by control the voltage of the diode to change the center frequency, it can make the filter have high selectivity. The design of Tunable filter is an important part of the research project. This Paper summarized the structure and the basic parameter of the receiver and the present situation of the RF front-end of the wireless receiver. It analyzed the key function to the indicator of the wireless receiver from the low noise amplifier and the filter. The theory and the engineering practice method of the low noise amplifier and the filter are studied thoroughly. It gives the design goals and the structures commonly used in the low noise amplifier and the filter. The detailed engineering practice plan of the filter and the low noise amplifier is the key content of the article. The primary design of tracking filter attune is based on the center frequency adjustable, high selectivity. The frequency are divided into four band (30MHz～39MHz,39MHz～51MHz,51MHz～67MHz,67MHz～88MHz), this can reduce the frequency cover coefficient, and to ensure the numerical control electric tuned tracking filter has the characteristics of good primary electricity tuned. The capacitance of the diode can change when the voltage of the diode is change, so we can change the voltage to control the capacitance of the diode. Using high-speed band selection switch to select frequency channel which is controlled by2-4decoder.D/A converter turn frequency channel into electricity voltage data, and then control the reverse voltage of the diode to realize the filter electric tuning. ADS simulation software to optimize the design of filter, and concludes that the filter element in the most optimal value. In order to improve the sensitivity of the system, employ a dual-gate FET3SK223whose noise figure is very low to design low noise amplifier.Each module is produced, installed and debugged, and then test the technical indicators of each module according to the given testing methods, and then test the various indicators of the system. All of the results of the short-wave receiver meet the subject requirements.