1. Signal processing apparatus for processing first and second input signals differing in phase and having common frequency variations to remove the frequency variations for subsequent improved phase measurements comprising: a first mixing device having the first input signal as its first input; signal generating means providing a signal as the first output of the signal processing apparatus and the second input to the first mixing device; and a second mixing device having its first input the difference output from the first mixer and its second input the second input signal to provide a second difference output of the signal processing device in which the frequency variations are removed and which differs from the first output only by the initial phase difference.