A method measures a characteristic of a SUT using a signal measurement device having multiple input channels. The method includes digitizing first and second copies of the SUT in first and second input channels to obtain first and second digitized waveforms; repeatedly determining measurement values of the SUT characteristic in the first and second digitized waveforms to obtain first and second measurement values, respectively, each second measurement value being paired with a first measurement value to obtain measurement value pairs; multiplying the first and second measurement values in each of the measurement value pairs to obtain measurement products; determining a mean-squared value (MSV) of the SUT characteristic measurement; and determining a square root of the MSV to obtain a root-mean-squared (RMS) value of the measured SUT characteristic, which substantially omits variations not in the SUT, which are introduced by only one of the first or second input channel.