Understanding the effect of uncorrelated phase noise on the phase coherency of multi-channel RF vector signal analyzers

While MIMO test setups historically used multiple vector signal analyzers (VSA´s) with a shared 10 MHz reference clock, the rise of modular RF instruments allow for a common local oscillator (LO) to be shared between each channel of a multi-channel VSA. In multi-channel VSA systems, a broad range of factors can introduce measured channel-to-channel phase variation including: uncorrelated LO phase noise, uncorrelated ADC sample clock phase noise, signal to noise ratio (SNR), and ADC quantization noise. As the results from these experiments demonstrate, uncorrelated phase noise dominates channel-to-channel phase uncertainty in multi-channel VSA´s which use independent LO´s for each downconverter. By contrast, SNR dominates channel-to-channel phase uncertainty in multi-channel VSA´s using a common synthesized LO for each channel. Moreover, the absolute channel-to-channel phase uncertainty is significantly better in the multi-channel VSA which uses a common LO for each downconverter chain.