While MIMO test setups traditionally used multiple VSA's with a shared 10 MHz reference clock, new modular RF instruments allow for a single LO to be shared between each channel of a multi-channel VSA. For both architectures, key contributors to channel-to-channel phase uncertainty are uncorrelated phase noise and ADC quantization noise. As the results from these experiments demonstrate, uncorrelated phase noise dominates channel-to-channel phase offsets in multi-channel VSA's which use independent LO's for each downconverter. By contrast, ADC quantization noise 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.