The Research And Design Of Tunable N-Path Filter

The demand for multi-mode multi-frequency wireless communication devices is driving the rapid development of wireless transceivers,and fully integrated,small-sized,low-power and low-cost wireless communication devices have become research hotspots.However,total integration places high demands on the designer's theory,experience,and hardware.In the early 1960 s,an N-Path Filter was proposed.The structure is a combination of N single-path filter units with the same characteristics.Each channel is composed of a switched capacitor and driven by an external clock generator.At the time,due to limitations in RF technology,research on N-path filters was limited and has not been promoted in the field of RF wireless.In recent years,with the development of RF technology and multi-mode multi-frequency communication systems,N-path filters have once again attracted people's attention,and a large number of theoretical analysis proves that it is very good to use them in the front end of multi-mode multi-frequency transceivers in the future Choice.This thesis mainly studies the following work:(1)By studying the theoretical basis of the traditional N-path notch filter,this thesis studies a Band-Stop Filter with two second-order N-path notch filters.The device and 2N transconductance amplifier units are configured to shift the frequency of the two channels upward and downward relative to the total output frequency by the transconductance unit,so that the stop-band bandwidth of the filter can be tuned.Under the supply voltage of 1.8V,the Cadence Spectre RF simulation shows that the gain of the filter is greater than-2dB,the notch frequency can be adjusted from 0.3GHz to 1.2GHz,and the bandwidth can be adjusted from 20 MHz to 48 MHz.The band rejection is 12 dB,the IIP3 is 9.8dBm at a frequency offset of 150 MHz,and the NF is 3.5 to 6.5dB.The band-stop filter studied not only realizes the bandwidth tunable function,but also improves the overall performance of the circuit,which provides a useful solution for multi-standard,multi-frequency radio applications.(2)The conventional N-path filter itself has two advantages: first,it has an accurate and freely adjustable center frequency;second,a high quality factor and a wide dynamic range.However,there is no gain,and the out-of-band rejection is small,and the frequency selection characteristics are poor.This thesis designs a gain-enhanced N-path band-pass filter based on the traditional N-path band-pass filter,which is used in two N-path switching capacitors.A transconductance amplifier is connected as a forward path to increase the loop gain.The input inductance between the input and output introduces two transmission zeros on both sides of the pass-band to improve the frequency selection characteristics of the band-pass filter.Using the TSMC 180 nm CMOS process,the Cadence Spectre RF simulation at 1.2V supply voltage shows that the frequency of the filter can be tuned from 1.4GHz to 6GHz,the gain is greater than 10 dB,the noise figure is less than 4.5dB,and the frequency is the out-of-band IIP3 is 8.05 dBm at 150 MHz.The N-path band-pass filter circuit studied has a simple structure and is easy to be fully integrated.(3)In order to solve the problems that the center frequency of the conventional filter is not easy to adjust,the Q value is low,the out-of-band suppression is small,and the gain is small.Firstly,this thesis designs a tunable high-Q gain-increasing differential N-path band-pass filter,which is composed of two N-channel differential structures and two transconductance amplifiers.The differential structure eliminates even harmonics,the transconductance amplifier increases the circuit gain,and the off-chip transformer acts as a balun,improving the filter Q and achieving impedance matching.Secondly,in order to derive the transfer function of the differential N-path band-pass filter for more efficient calculation,this thesis uses the adjoint network method to analyze the differential structure of the circuit.By analyzing the core structure of the differential N-path filter,since each core is Equivalently,the transmission characteristics of a differential N-path filter are studied by analyzing a core.Finally,under the 1.2V supply voltage,the TSMC 180 nm CMOS process is used,and N=4 is used to form a differential 4-path filter.The Cadence Spectre RF simulation results show that the gain of the filter is greater than 8.5dB,the center frequency can be adjusted from 0.1GHz to 1GHz,the in-band insertion loss S11 is greater than 10 dB,the out-of-band IIP3 is greater than 10 dBm,and the noise figure is less than 2.2dB,at 1)=300MHz.At the out-of-band,the out-of-band rejection is 28 dB.The filter's high Q,high tunability and high performance make it widely used in cognitive wireless.