Application of digital lock-in amplifier in nonlinear ultrasonic inspection

In view of the difficulty in obtaining the amplitude of the signal within a specific bandwidth near the set frequency by the current digital signal processing method, this paper proposes to use the digital lock-in amplifier, use discrete equipment to obtain the digital waveform signal, adopt the signal processing idea of the lock-in amplifier in RAM 5000 SNAP, and use the digital phase sensitive detector, two sinusoidal / cosine modulated signals with specific frequency are obtained, the frequency is shifted to DC and the signals in the nearby frequency band are shifted to the low frequency band near DC; the two output signals of the phase sensitive detector will go through the digital low-pass filter, the out of band noise and the signal shifted to higher frequency band will be filtered out. The experimental results show that this method can obtain the accurate sub harmonic, fundamental frequency and second harmonic signal amplitude in a more quick manner.