Efficient FPGA-based phase measurement for heterodyne interferometry

INTRODUCTION: Phase measurement plays an important role in high-precision applications such as optical interferometry, metrology, and scientific instrumentation, where accurate and low-latency phase information is required. OBJECTIVES: This paper presents a real-time phase difference measurement system implemented on a low-cost Kintex-7 (XC7K325T) FPGA. METHODS: The proposed system directly estimates the instantaneous phase difference between two heterodyne signals using a feed-forward quadrature processing approach. Each input signal is processed with filtering and the Hilbert transform to obtain the in-phase and quadrature components, followed by amplitude normalization to reduce the influence of variations in signal magnitude. The phase difference is then computed by quadrature mixing to generate sine and cosine components, which are converted to phase values using a CORDIC-based arctangent module. The entire signal processing chain is implemented in a fully pipelined FPGA architecture, enabling deterministic, low-latency real-time operation. RESULTS: Experimental results using controlled heterodyne signals generated by a Moku: Lab instrument demonstrate that the proposed system provides accurate phase measurements with a small, consistent offset (approximately 1.2°) and an end-to-end processing latency of approximately 420 ns. CONCLUSION: The proposed fully pipelined FPGA architecture is suitable for deterministic, low-latency real-time phase measurement in interferometric systems.