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A simple scheme for measuring signal and noise powers of multiwavelength channels is realized by using double-pass filtering and a dithered reflector. The noise is passed through a tunable filter twice in order to obviate the need of an optical filter with very steep transfer function, while the signal power is measured at the output of a partial reflector that follows the filter. We demonstrate the optical signal-to-noise ratio (OSNR) measurement with a voltage-controlled optical filter and a dithered tunable reflector. It can provide polarization insensitive monitoring and high immunity to the influence of residual signals. The experiments show that it can measure up to 44-dB OSNR with an error<0.4 dB.