Post-silicon validation of RF/mixed-signal circuits is challenging due to the need to excite all possible operational modes of the DUT in order to establish equivalence between its specified and observed behaviors and to ensure that the DUT does not produce any unexpected behaviors that can lead to system failure. In this research, we first develop a methodology for determining if the DUT contains behaviors that are not explicitly included in its behavioral model. A complex (optimized) test waveform is applied to the DUT and its test response signature is captured. It is seen that in the presence of unexpected DUT behaviors, the residual error in the test response signature from that defined by the model, cannot be minimized below a certain threshold by manipulating the model parameters in any way. If however, it is determined that the model is adequate but the signature is different from the expected, then a procedure is developed for determining which sub module is responsible for the difference (i.e. causes the system level specifications to be different from that specified by the behavioral model). Experiments on an RF transceiver are performed to demonstrate the effectiveness of the proposed validation and diagnosis approach.