Analysis of Offset Spurs in Phase-Locked-Loops Employing Harmonic-Mixer-Based Feedback With Sample-and-Hold Operation

Fractional-N phase-locked loops using Harmonic-Mixer (HM) based feedback offer good phase noise and spur performance without relying on complex calibration schemes. The HMs in prior art are often realized using the Sample-and-Hold (S/H) operation. This, however, can result in unwanted tones due to intermodulation, which must be suppressed using properly-designed filters. This paper proposes a simple and accurate analysis of the location and magnitude of these tones. We also provide a design guideline for the HM based on this analysis to properly suppress these tones. The results are supported through behavioral and transistor-level simulations.