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A simple technique for improving the sensitivity of measuring devices based on angle modulation of a continuous-wave carrier is presented. The proposed approach involves the addition of a second carrier signal separated by twice the modulation frequency. In the Fourier domain, the upper sideband of the first carrier and the lower sideband of the second carrier intermix. By sweeping the relative phase between the two carriers, the magnitude of the resultant sideband can grow up to double the original value. In noisy environments, this effect can lead to an improvement of the signal-to-noise ratio. As a proof of concept, the technique is applied to a Doppler ultrasonic vibration sensor, with encouraging results. © 2003 American Institute of Physics.