Patent ID: 8364430

Claim:
An apparatus for measuring a phase shift in a physical system excited by an excitation signal, comprising: a frequency generator configured to produce the excitation signal having a excitation frequency (f x ), a local oscillator signal having a local oscillator frequency (f lo ), and a reference signal, where the reference signal has a difference frequency that is f x −f lo ; a mixer configured to detect an output of the physical system in response to the excitation signal and mix the output with the local oscillator signal; a filter configured to select a difference frequency component of output of the mixer; and a computational element configured to: cause f x , or the f lo , or both, to vary such that |f x −f lo | is constant, but a relationship between f x and f lo changes from f lo <f x to f lo >f x , measure a phase shift of the difference frequency component with respect to the reference signal while f lo <f x to produce a first measurement, and while f lo >f x to produce a second measurement, and subtract the second measurement from the first measurement, and divide in half, to produce a phase shift result.