  We describe a simple method for measuring the residual fast phase noise of a
cavity-stabilized laser using the cavity as a reference. The method is based on
generating a beat note between the laser output and the strongly filtered light
transmitted through the cavity. The beat note can be directly analyzed without
requiring further calibration of system parameters. We apply the method to
measure the residual phase noise of an external-cavity diode laser (ECDL)
locked to a reference cavity and compare the results with an analysis of the
in-loop error signal of the feedback system.
