A non-intrusive channel-impairment analyzer is provided for measuring at least one quasi-static impairment characteristic of a band-limited data communications channel. The analyzer comprises a data receiver section and a measurement section. The data receiver section is arranged to receive data modulated onto quadrature phases of a carrier signal in two quadrature forward processing paths. The receiver section includes a data recovery circuit for effecting a decision as to the identity of the original data on the basis of the outputs from said processing paths, and decision-directed compensation means disposed in said paths and arranged to compensate for channel-impairment effects on the received signal. The measurement section is responsive to signals generated in the receiver section during the receipt of random data, to derive a measurement of at least one said channel impairment, such as phase or amplitude jitter.