The Impact of LO Phase Noise in N-Path Filters

As passive mixers (N-path filters) are being used more frequently and in more applications, it becomes increasingly important to understand how various non-idealities affect the performance of passive mixer circuits. One such non-ideality is perturbations in the local oscillator (LO), including phase noise. Here, we consider a basic N-path filter and develop a preliminary model to analyze the effects of LO phase perturbations in shunting N-path filters. We do so by developing a transfer function from phase perturbations in the LO to voltage perturbations on a strong RF signal at the mixer's RF port. We find that LO phase noise is suppressed where it is strongest (i.e., for small offset frequencies). As a result the RF spectrum's noise peak does not appear around the signal tone, but rather around the LO frequency, due to the bandpass characteristic generated by a typical capacitive baseband load. These analytical results are verified using numerical and schematic simulation in custom software and Cadence, and they are further confirmed by measurement of a frequency-scaled, board-level implementation of an N-path filter.