FPGA-Based Digital Lock-in Amplifier With High-Precision Automatic Frequency Tracking

Aiming to overcome the problem of the frequency error between the reference signal and the measured signal in an analog lock-in amplifier, this paper presented a digital lock-in amplifier (DLIA) with accurate frequency automatic tracking to improve its performance. In this paper, a modular design approach based on DSP Builder was employed to develop a quadrature vector type lock-in amplifier with automatic frequency tracking. A reference signal generator with high-frequency resolution and a digital filter with good performance were implemented. The performance of the design was tested by determining the linearity, the <inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> value, and the noise immunity of the DLIA. The experimental results showed that the proposed DLIA has good linearity and the <inline-formula> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> value can reach up to 82. The relative errors of the signals with amplitudes greater than 10 mV were less than 1% when the equivalent input was a sine wave signal with a frequency of 1 kHz and an amplitude of 500 mV. When the superimposed noise was less than or equal to 400 mV, the relative error was less than 2% in the same condition. The proposed DLIA has higher precision and efficient noise immunity.