Electronic phase comparison apparatus for the remote measurement of layer thickness
First Claim
1. Apparatus for the remote measurement of the thickness of n layers in a dielectric target where n ≧
- 1, comprising;
transmitter means having n + 1 single frequency channels for directing n + 1 related frequency signals to the target; and
receiver means for receiving the n + 1 signals from the target, said receiver means including first means for converting the signals to a common frequency and second means for comparing the phase of pairs of the converted signals for determining the thicknesses l of the n layers as a function of the phase differences where l <
λ
/4, λ
being the shortest wavelength of the difference frequency between the related n + 1 signals.
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Abstract
A continuous wave, phase measurement system which provides at least an order of magnitude improvement in accuracy over modulated carrier systems is disclosed for the determination of the thicknesses of layered targets consisting of a known number of dielectric layers, each of a known maximum thickness. This system uses related frequencies such as the fundamental and its harmonics to establish a multi-harmonic coherence relationship whereby a homodyne phase reference between harmonics can be conserved and information extracted from just the received and not the transmitted signals. Consequently doppler effects due to motion between the target and the apparatus, as well as severe local oscillator stability and drift limitations are avoided, and thus this system can measure remote target parameters by interferometric techniques without the distance being a constraint.
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Citations
8 Claims
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1. Apparatus for the remote measurement of the thickness of n layers in a dielectric target where n ≧
- 1, comprising;
transmitter means having n + 1 single frequency channels for directing n + 1 related frequency signals to the target; and receiver means for receiving the n + 1 signals from the target, said receiver means including first means for converting the signals to a common frequency and second means for comparing the phase of pairs of the converted signals for determining the thicknesses l of the n layers as a function of the phase differences where l <
λ
/4, λ
being the shortest wavelength of the difference frequency between the related n + 1 signals. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
- 1, comprising;
Specification