Multi-channel Phase-Coherent RF Measurement System Architectures and Performance Considerations

The paradigm for wireless communication is, shifting toward multiple-input multiple-output (MIMO) communications, enabled by multiple antennas at the transmitter and receiver. Instrument synchronization requirements for MIMO test are some of the most difficult in the test industry. In a MIMO test system, each channel of a multichannel RF instrument must achieve true channel-to-channel phase coherency. While MIMO test setups traditionally used multiple VSAs 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 multichannel VSAs which use independent LOs for each down converter. By contrast, ADC quantization noise dominates channel-to-channel phase uncertainty in multi-channel VSAs 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 down converter chain.