Design And Implementation Of Tunable Filter In Receiver Of Spectrum Detection Equipment

Under the current rapid development of wireless communication technologies,military radar and civil communications have become increasingly demanding for microwave radio frequency technology.Superheterodyne receivers have features such as large dynamic range,wide bandwidth,and high pass band selectivity and receiving sensitivity,and are therefore used by more and more communication systems.The quality of the frequency selection components of the radio frequency front end of the spectrum detection equipment receiver directly affects the quality of the entire communication system,and thus plays a decisive role in the entire system.The main purpose of this project is to design a pre-selection filter to implement the RF front-end of the receiver of the spectrum detection equipment.The receiver needs a pre-selection filter to divide the received signal into frequency spectrum,which facilitates the following frequency conversion and digital processing.In the design index of this topic,The use of tunable filters to achieve filtering of 2.9-7GHz signal,bandwidth less than 0.9GHz,and strive to achieve circuit miniaturization and integration.Aiming at the design of radio frequency front-end frequency-selection component of spectrum detection equipment receiver,this paper proposes a tunable filter bank combining continuous tuning and discrete tuning.Firstly,an adjustable dual-mode filter is designed,and the center frequency deviation is achieved by the simultaneous proper change of odd and even modes.Then an adjustable open-loop filter is designed.The varactor is used as the center frequency tuning.The control device can quickly and sensitively shift with the center frequency,and the filter is improved based on the comb line structure,so that the filter occupies a smaller volume and the circuit is more compact.At the same time,in the study of maintaining the absolute bandwidth constant,this topic has studied two methods: one is based on odd-even mode theory,as long as the difference between the bandwidth of the odd and even modes and the center frequency is a constant value,the absolute value can be obtained.The bandwidth is constant,but this method requires two control signals to adjust the odd-mode and even-mode changes and is not easy to control;the other method is through continuous verification and analysis,resulting in the tunable filter,as long as the input and output coupling length is appropriate and the absolute bandwidth can also be kept constant within a certain range.Combining the design parameters of the subject and the adjustable limits of a single filter,the tunable filter is divided into three frequency bands: 2.9-4 GHz,4-5.5 GHz,and 5.5-7 GHz;the center frequency is continuously adjustable in each frequency band.The bandwidth is about 340 MHz.Since the varactor diode has equivalent resistance and equivalent inductance,the insertion loss is 12 dB,the test and simulation frequency deviation is 300 MHz,the band is flat,the out-of-band rejection is-35 dB,the waveform is stable,and the smaller size of the device group enables the miniaturization and integration of the circuit.