Phase Compensation for Dynamic Doppler Frequency Shifts

Schematic for compensating dynamic Doppler frequency shifts
Schematic for compensating dynamic Doppler frequency shifts

The goal of this project is to compensate the phase delay introduced by the filters in real time. When using heterodyne interferometry, filtering is routinely performed to process and determine the measured phase change, which is proportional to the displacement from one target location to another. The filtering in the signal processing introduces a phase delay dependent on the detection frequency, which leads to displacement errors when target velocity is non-constant as is the case in dynamic calibrations. The displacement error could be up to hundreds nanometers depending on the non-constant phase response of filters and the Doppler frequency shift. Positioning calibration under dynamic conditions is becoming increasingly of interest for high precision fields such as additive manufacturing and semiconductor lithography. The phase delay could cause problem when using heterodyne interferometry dynamically calibrate a stage’s position.

This project illustrates a phase delay compensation method by measuring instantaneous detection frequency and solving for the corresponding phase delay in an FPGA in real time. The FPGA hardware-in-the-loop (HIL) simulation shows this method can significantly decrease the displacement error that is ±100’s to ±3 nm in dynamic cases and it will still keep sub-nanometer resolution for quasi-static calibrations.