Harmonic Analysis And Application Research On Harmonic Rejection Mechanism Of N-Path Filter In Receiver Front-End

In recent years,the development of radio communication technology has entered a new stage with the rapid progress of integrated circuit technology.N-path filter is widely used in multi-mode and multi-frequency radio receiver system because of its advantages of easy integration,programmable adjustment to center frequency,high frequency selectivity and good linearity.However,due to the sampling characteristics of N-path filter,unwanted harmonic noise will appear at the multiple of switching frequency,which affects the performance and communication quality of receiver.Therefore,it is necessary to research in the harmonic rejection of N-path filter,which is a hot topic in recent years.The traditional method of harmonic rejection is to add a pre-filter in front of N-path filter.However,for the harmonics which move dynamically in frequency spectrum with the change of center frequency,the pass band of the prefilter is relatively fixed,so it is difficult to achieve good harmonic suppression effect.Therefore,it is necessary to develop a specific harmonic rejection method to dynamically reject the varying harmonics in the frequency spectrum.The differential N-path filter can eliminate even harmonics,so that the key to the design of harmonic rejection Npath filter is to reject odd harmonics.In this paper,the harmonic theory and harmonic rejection mechanism of N-path filter are studied.Based on the theoretical research,two harmonic rejection N-path filters are designed and verified by simulation.The main work and research results are as follows:1.In theory:(1)Based on the harmonic transfer functions of N-path band-pass filter derived by predecessors,the complete harmonic transfer functions of N-path band-stop filter is derived,which provides theoretical basis for the research of harmonic rejection and anti-aliasing of N-path filter.An equivalent model of adjoint network is established for N-path band-stop filter.The original analysis of linear periodic time varying system is simplified to that of linear time invariant system.And the original solution process of the capacitance sampling output response in the frequency domain is simplified from the perspective of the time domain.(2)Based on the derived harmonic transfer functions,the influence of the main parameters of the filter on the effect of harmonic aliasing is analyzed by simulation.The theoretical analysis and the simulation results show that the harmonic aliasing can be effectively rejected by appropriately increasing the number of paths or the capacitance.According to the functions,the the pass band range of the pre-selected frequency device is determined to achieve great anti harmonic aliasing effect.2.In circuit structure:(1)The 3-branch N-path filter based on polyphase signal synthesis technology and its harmonic rejection mechanism are studied.Theoretically,the filter can completely eliminate the 3rd and 5th harmonic when the gain amplitude ratio of the 3 branches is the ideal value.However,the ratio is irrational,it cannot be realized in the actual circuit but only be approximated.The Npath filter with the original one-stage gain structure is improved to two-stage gain structure,and the gain amplitude ratio of the filter is optimized by the two-stage gain structure,which is closer to the theoretical value compared with the one-stage structure,so it has better harmonic rejection effect.The 3rd and 5th harmonic rejection is 31 dB and 37 dB respectively,which is better than the one-order structure.(2)In order to solve the problem that the pass band of the pre-filter cannot be dynamically adjusted with the center frequency of the N-path filter in traditional harmonic rejection methods,the original pre-filter with relatively fixed pass band is improved to the N-path notch filter,whose center frequency is set to 3 times of the N-path band-pass filter,then the third harmonic can be dynamically suppressed.The path number of the N-path bandpass filter is 5 so that the cascaded filter can reject 3rd and 5th harmonics simultaneously.The 3rd and 5th harmonic rejection of the cascaded filter are 26 dB and 31 dB respectively.Compared with the designs that achieves the same function in recent years,the cascaded filter does not have low noise amplifier or transconductance amplifier,and the circuit structure is simple and is easy to realize.