  This letter presents principles and applications of a virtual multi-channel
lock-in amplifier that is a simple but effective method to recover small ac
signal from noise with high presison. The fundamentals of this method are based
on calculation of cross-correlation function. Via this method, we successfully
built up a magnetoelectric measurement system which can perform precise and
versatile measurements without any analog lock-in amplifier. Using the virtual
multi-channel lock-in amplifier, the output of the magnetoelectric measurement
system is extensively rich in magnetoelectric coupling behaviors, including
coupling strength and phase lag, under various dc bias magnetic field and ac
magnetic field.
