  In this paper we present several techniques for characterizing longitudinal
impedances based on transient measurements of the growth rates and tune shifts
of unstable coupled-bunch modes. These techniques are applicable to measurement
of both fundamental and higher-order mode impedances and allow characterization
of shunt impedances and quality factors of the HOMs. Methods presented here are
complementary to lab bench measurements of RF cavities, in that the beam based
measurements directly sense the physical impedance in the installed
configuration. In contrast to a single-bunch integrated impedance measurement
these techniques resolve the impedances in the frequency domain. These methods
allow determination of the impedance's unaliased frequency by analyzing
synchronous phase transients. Experimental results from ALS and BESSY-II are
presented showing the use of these techniques to measure complex impedances.
