Research And Design Of N-path Filter For SAW-less Receiver

The traditional surface acoustic wave(SAW)filter cannot meet the needs of RF frontend integration due to its bulky,which can only be placed outside the chip and the frequency band is fixed.Because of its tunable center frequency,high Q value,and excellent linearity,the N-path filter can be used to implement band-pass and band-stop filters for RF front ends.The purpose of the article is aims to study and design the N-path filter for SAW-less receivers,the design and simulation verification of two N-path filters are completed.(1)The Walsh function has ideal cross-correlation characteristics,and the Walsh sequence orthogonality is not affected by the overlap between itself.Applying Walsh orthogonal sequence to N-path filter,Such sequence-driven N-path filters can potentially reject or select signals based on their properties in the spectral,spatial,and code domains.An N-path notch filter based on Walsh function(WF)sequence is designed,which proves the feasibility of parallel band-stop filtering using WF sequences.(2)According to the problem that the current N-path filter has a limited stop-band rejection capability and cannot reject the signal at the LO frequency harmonics,A high-order active N-path band-pass filter with harmonic rejection was designed using a three-stage second-order N-path filter unit.By inserting a negative resistance and a gyrator between the second and third stages,the Q value,bandwidth and linearity of the filter are improved;The clock signal with phase difference is used to control the second-order n-path filter unit of the final stage,which has a relatively high roll-off slope.By rejecting the third harmonic,the adjacent frequency selectivity of the RF front end is improved,thereby reducing the design pressure on the linearity of the post-stage mixer.Based on 0.18-μm CMOS process simulation,the results show that the maximum gain of the filter is 20.12 d B,the center frequency can be adjusted from 0.1 to 1GHz,and at fs = 700 MHz,the out-of-band rejection is up to 50.2d B,and the noise figure is 4.71-6.9d B,and the frequency deviation is 50 MHz,the out-of-band IIP3(input third-order intermodulation point)is 16.3d Bm.