Reduction of noise induced by power supply lines using phase-locked loop

An experimental implementation for the reduction of power-line noise in delicate signal detection is presented. This implementation improves the signal-to-noise ratio without limiting the bandwidth of the measurement. A sinusoidal wave and its harmonics, both synchronized with the frequency of the power line, are used to cancel out the power supply noise induced in the measurement signal. The wave and the harmonics are generated via a phase-locked loop implementation. Their amplitude and phase are adjusted, and then they are added to the measurement signal using a series of operational amplifiers to compensate for the noise. Although we applied this method to the particular case of scanning tunneling microscopy measurements, considerably improving the image quality, our implementation can be applied to other measurement systems for which noise from the power lines can compromise the signal detection.