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A portable orthogonal phase-locked amplifier for weak signal detection has been developed based on FPGA. First,the signal processing module receives the signal,does variable gain amplification on the signal and puts it through the power frequency noise filtering. The signal is put into the FPGA through A/D conversion module conversion. By the digital phase-locked loop ,the amplifier completes the measured signal phase lock, extracts the feedback signal to calculate the measured signal amplitude, and completes weak signal detection. The lock-in amplifier amplitude measurement range is 100 nV~100 μV, dynamic range can reach up to 60 dB, phase accuracy can attain 0.001°. The system is designed as a portable device, low cost, easy to maintain, can meet the measurement requirements of the special environment such as fieldwork or etc, and has broad application prospects.