An N-Path Enhanced-Q Tunable Filter With Reduced Harmonic Fold Back Effects

A high-Q, tunable, bandpass filtered amplifier structure is proposed which is based on a novel Q enhancement technique in N-path filters. Using Fourier series analysis, frequency response of an N-path filter as well as the aliasing effects which are present in it are derived. Frequency translation of unwanted contents at higher frequencies to the center frequency of the bandpass filter is called harmonic fold back (HFB). It is shown that using an additional N-path filter with the same clock frequency but different clock phases can reduce the HFB. The required conditions for fold back elimination are derived from Fourier series expansion analysis. In order to achieve HFB reduction as well as increasing stop-band rejection and Q factor, an LNA containing an N-path filter is added to each of the two filters. The LNA in the proposed structure is designed for high linearity and low NF, utilizing noise and distortion cancellation methods. The filter is designed and simulated in 0.18 um CMOS with an ideal balun. The voltage gain is 17.5 dB across the 0.25-1 GHz frequency band, while achieving a NF between 2.2 to 2.4 dB, an IIP3 between -2 to +2 dBm, HFB attenuation of between 25 to 45 dB and power consumption of 4.5 mW for each LNA. Finally, the proposed filter (with 4-path sub-filters) is simulated and compared to filters with 4-path and 8-path sub-filters without the proposed built-in HFB reduction.