Research And Design Of N-path Filter Based On Harmonic Suppression And Anti-harmonic Folding Back

The N-path filter was first proposed in the 1960s.Due to the limited radio frequency technology at that time,its research was limited.With the improvement of CMOS technology and the rapid development of wireless communication,the N-path filter is widely applied to the front end of a radio frequency receiver because it has many advantages such as adjustable center frequency,easy integration on chip and it occupies small area and so on.However,the switch sampling characteristic makes it face the challenges of small harmonic suppression,harmonics folding effect(HFE),and small out-of-band suppression.This paper focuses on these challenges and starts research.The main research results are as follows:(1)By applying pre-switch LC band-stop pre-filtering technology,which is used to attenuate the harmonics at(k N±1)f _LO(k is a positive integer,N=8)in the filter,thereby increasing the gain difference between the desired signal at the center frequency and the maximum aliased harmonics.Under the TSMC40nm COMS process,the Cadence Spectre RF simulation tool is used for simulation verification.The simulation results show that the harmonic aliasing suppression at 7f_sand 9f_sare both greater than 90d B,and there is a good anti-harmonic folding effect.This technology is applied to a single-ended to double-ended differential 8-path filter,the designed filter even harmonics can be suppressed up to 120d B.(2)Based on the phase difference principle of the local oscillator clock waveform of the up-conversion mixer in each path,a 6-path filter that suppresses 3k(k is a positive integer)harmonics is proposed.The method is to add outputs with a phase difference of 60°to suppress odd harmonics in3k harmonics,and a differential structure with a phase difference of 180°to suppress even harmonics in 3k harmonics.Under the TSMC40nm CMOS process,the Cadence Spectre RF simulation tool is used for simulation verification,and the 3rd harmonic suppression is up to 42d B,and the out-of-band suppression is up to 32d B,achieving better harmonic suppression.A transconductance amplifier composed of two self-biased feedback resistors and an AC coupling capacitor is used to achieve voltage-current conversion to increase the gain,and the filter voltage gain can reach 10d B.This paper studied two filters which can achieve better harmonic suppression and anti-harmonic folding back characteristics,good impedance matching characteristics,and low return loss.The results of the pre-simulation and the post-simulation are basically the same,and they have good application prospects in the design of the front-end of radio frequency superheterodyne receivers.