  In FEL (Free-Electron Laser) accelerators, LLRF (Low-Level Radiofrequency)
systems usually deploy feedback or feedforward algorithms requiring precise
microwave measurement. The slow drift of the clock allocation network of LLRF
significantly impacts the measured microwave phase, thereby affecting the
stability of the closed-loop operation. The reference tracking algorithm is
used to eliminate the measurement drift. The conventional algorithm is to
perform phase and amplitude demodulation on the synchronous reference signal
from the main oscillator and subtract the reference phase in other measurement
channels. The demodulation is usually based on the CORDIC, which requires
approximately 16 clock cycles in FPGA (Field Programmable Gate Arrays). This
paper uses the multiplication of complex numbers, which only requires four
clock cycles of computational delay and achieves phase subtraction point by
point. However, experiments show that it causes irrelevant amplitude noise to
overlap and increase the amplitude measurement noise. Nevertheless, this
reference tracking algorithm is suitable for control algorithms with low-delay
requirements of microwave measurement.
