Repeatable Phase Spectrum Measurements of 75–110-GHz Modulated Signals Using a VNA-Based LO Phase Cancellation Technique

In this article, we present a vector network analyzer (VNA)-based measurement scheme and setup to obtain repeatable phase spectrum measurements of 75–110-GHz modulated radio frequency (RF) signals. In order to remove the random phase contribution of sweeping local oscillator (LO) in harmonic downconversion, the LO signal is coupled out into an auxiliary measurement channel where its random phase at the fundamental can be derived after additional modulation and downconversion treatments. Afterward, a user-defined math equation is used inside the VNA for LO phase cancellation, based on the traces of RF- and auxiliary-channel raw measurements. The key point of the proposed LO phase cancellation technique is measuring the 75–110-GHz test signal and the auxiliary-channel signal at the same GHz intermediate frequency so that the receivers of VNA can measure both of them simultaneously when working on the peculiar continuous-wave or zero-span mode. According to the method validation at 78, 93, and 108 GHz, a phase fluctuation of ±2° for single-run measurements is found reachable in practice.