Multi-channel phase coherent measurements using COTS modular instruments

It is worth starting by expounding on the title a bit... In this paper we will look at the design challenges of building a measurement receiver that performs phase stable measurements between multiple phase coherent signals. The measurements range from narrow-band phase and gain channel ratios to frequency response functions (FRFs) requiring computation of Fast Fourier Transforms (FFTs). Applications utilizing multi-element antennas for beamforming or direction finding are typically where such a receiver is needed. The requirements for such a measurement receiver system can depend on several factors: - Modulation bandwidth of the signals under test - Number of phase coherent measurement channels - Signal power and noise levels of signals under test - Frequency range of signals under test (different than modulation BW, this is the center frequency range). In this paper we will review how the application requirements map to design specifications. We will look at the actual steps in developing a multi-channel coherent receiver from commercially available instruments. Additionally we will look at the software requirements to orchestrate a multi-instrument modular system for such a receiver. Finally, we will review some examples based on a real receiver including tests performed to validate the system meets requirements. The signals being measured by the test instrument configuration being developed are said to be “coherent”. Two signals are coherent if they have a constant relative phase at all instances in time. Because no two signals are perfectly coherent (coherence = 1) due to uncorrelated noise components and phase drift, we will take a realistic measurement centric view of the term coherent in this paper. Forthwith, coherent will refer to the ability of a measurement receiver to characterize the phase differences between two phase stable signals to a resolution of better than 1 degree during a given measurement interval or integration time.