Lock-in Amplifier Input Voltage
A lock in amplifier requires a phase reference for the (ac) signal it is trying to recover and it is, by definition, an AC amplifier and many devices implement a balanced demodulator approach. 

Whether the reference signal is provided internally or externally is application and device dependent.

It is useful to see a test circuit to understand the operation of such a set-up:



Here we can see the input, original signal and output:



In this application, a signal of known frequency (which may carrry information by being modulated although the basic application is that of a frequency selective voltmeter) has been recovered from noise 100dB greater than the signal of interest.

This amplifier configuration is specifically used to recover a signal at a specific frequency from within a wideband (noisy) signal.

To recover said signal of some specific frequency, we need to generate a signal at that known frequency and therefore two input signals are required.