Characterization and calibration techniques for multi-channel phase-coherent systems

Electronic Warfare (EW) and Radio Detection and Ranging (Radar) are two of the many applications that rely on multi-channel and phase-coherent configurations for signal processing. We provide herein an overview of the complexities and requirements of a multi-channel phase-coherent measurement system. Multiple Input Multiple Output [MIMO] systems have to overcome key technical challenges related to phase, time and frequency synchronization in order to coherently receive and process the data acquired/generated from each input/output. In practical MIMO systems, the radio hardware should be capable of acquiring and/or generating such phase coherent signals across the multiple channels. Further, the systems need to be able to sustain the phase coherence over considerable duration of time, depending on the sensitivity of the system. However, drifts will occur owing to the effects like temperature, thermal expansion, mismatched cable lengths, uncorrelated phase noise, ADC sample clock phase noise and quantization noise. Thus, a calibration process is required to compensate for the drift, whenever it crosses a particular threshold value that defines the accuracy of the phase-coherent system. In this paper, an FPGA based software-defined calibration method is presented for synchronizing the phase and magnitude across multiple channels of a system. This method allows the phase/magnitude drift over time to be periodically monitored and calibrated, when there is a need. With the FPGA built into the system, the calibration can take place remotely without the need of connecting the system to an external calibration kit. Also, measurement results are provided for a state of the art super heterodyne receiver system to show that the phase drift is lesser than ±1 degree across a 500MHz - 26.5GHz frequency range at 23°C ± 5°C for 2 and 4 channels configurations. Such systems can find use in a variety of real-world MIMO implementations such as Direction Finding, Beam Steering, Passive Radars, MIMO and Phased-Array Radar Systems, where phase coherence, alignment and /or synchronization has added advantages to multi-channel systems.