Removing the Random Contributions of LO Phases from Multi-Tone RF Phase Measurements Based on Down-Conversion
This paper proposes a measurement strategy to remove the random contributions of local oscillator (LO) phases from multi-tone radio frequency (RF) phase measurements based on down-conversion, so that a stable phase spectrum result can be obtained for each run of tone-by-tone sweeping measurement. To achieve this, the LO used for down-conversion is coupled out to an extra path, and modulated to become a wideband multisine signal. For each run of LO frequency sweep, the baseband modulating signal is unaltered, hence for each LO frequency the multisine phases are only changed by the random LO phase. By monitoring the multisine phase change at a specific and fixed frequency point, the random LO phase offsets can be derived, and further compensated in the down-converted phase measurements. In this paper, two approaches of compensation are given as alternative choices. According to the experimental results based on a VNA-based test bench, a phase stability of ± (0.2~0.4) deg., and a relative phase deviation of ± (0.4~0.6) deg. over 10 hours are reachable for multi-tone measurements around 1.8 GHz.