A Multiple-Frequency Phase Comparison Technique for the Determination of Remote Layer Thickness

A continuous-wave phase comparison technique which provides at least an order of magnitude improvement in accuracy over modulated carrier systems which operate in the same spectrum is described in this paper for the remote determination of the thickness of layered targets consisting of a known number of dielectric layers, each of a known maximum thickness. This system uses rationally related frequencies such as the fundamental and its harmonics to establish a multifrequency coherence relationship whereby a phase reference between frequencies can be conserved and information extracted from just the received and not the transmitted signals. Consequently, Doppler effects due to motion between the target layer and the apparatus, as well as local oscillator stability and drift limitations are avoided. Thus this system can measure the parameters of remote layered targets by interferometric techniques without the distance being a constraint.