Research And Design Of Tunable Band Pass Filter For Cognitive Radio Receiver

Cognitive radio technology attracts lots of research interests around the world in recent years.That,the utilization of spectrum in licensed frequency band is low and the spectrum resource in unlicensed band is insufficient will limit the development of wireless communication.Cognitive radio is proposed to solve the problem.Therefore,it is of great significance to research and design receiver chip for cognitive radio.Low-IF receiver is one of the most popular wireless receiver structures with high integration.The complex band pass filter plays an important role in the Low-IF receiver,bearing double function as image rejection and channel selection.The purlpose of this paper is to research and design a proper complex filter for cognitive radio receiver.We introduced the basic theory of filter,such as technical indicators,classification and synthesis method.Then the principle and implementation method of complex band pass filter were analyzed deeply.Finally,according to the requirement of the cognitive radio Low-IF receiver,a method of cascading biquad was adopted to realize an eight-order Butterworth Gm-C complex filter.To achieve specific circuit design,first,we used the linearization technique of the source negative feedback to design an OTA,which was suitable for high frequency applications.An eight-order Butterworth low pass filter was realized on the basis of two-order Butterworth low pass filter.Then,the frequency response of capacitance in the low pass filter was linearly shifted to translate the frequency response of the low filter to complex filter response.In order to reduce chip area,a technology of floating capacitor was used to halve the number and value of capacitor.At last,considering the great deviation in the frequency parameter of the Gm-C filter resulting from external factors such as temperature change,process bias and device aging,we employed an automatic frequency tuning circuit based on PLL to stabilize the frequency parameter of the filter.The VCO of PLL system was constituted of the transconductance amplifiers and capacitors.Meanwhile,a novel simple tansconductance amplifier as a nonlinear negative resistance was designed to limit the amplitude of VCO output signal in linear input range of tansconductance amplifier.The complex Gm-C filter is designed in SMIC 0.18μm CMOS process.The whole layout area of the filter is 416μm × 418μm.Simulation results show that the filter is centered at 50MHz with the bandwidth of 8MHz.The out-band rejection at 40MHz and 60MHz is higher than 40dB,and the image rejection reaches 170dB.The group delay of the filter is less than 0.08 μs and the tuning error is less than ±3%.When the power supply voltage is 1.8V,the current consumption of the filter is less than 7.5mA.