An apparatus and method for analyzing phase noise in a signal. A plurality of signal samples, each signal sample representing a value of phase noise in a signal-under-test at a corresponding offset frequency, and filter data representing filter characteristics on a first side of a spectrum boundary, are used to derive filtered signal samples. A measure of noise is derived from the filtered signal samples. This abstract is not to be considered limiting, since other embodiments may deviate from the features described in this abstract.