Both FRD and IDFRD are used to store Freqyency Response Data, that is, the complex frequency response vector (Mag.*exp(i*Phase)) measured over a given frequency grid. FRD belongs to Control System Toolbox. It has properties like ResponseData, and Frequency to store information on the frequency response.
IDFRD belongs to System Identification Toolbox. It extends FRD of Control System Toolbox to store additional information that is relevant for identification such as response covariance and spectrum information.
- Either FRD or IDFRD can be used data source for identification of parametric models (transfer functions, state-space etc). For example, sys = tfest(FRD, n) creates an n-pole transfer function that fitsthe frequency response in FRD over its frequency range.
- IDFRD can be an output of a non-paramteric estimation too. For example, the command SPA performs spectral analysis on a given time-domain data (that is, input and output signals) and returns the results as an IDFRD object. It contains the estimated frequency response in the property ResponseData, its covariance in the property CovarianceData, the estimated noise spectrum in the property SpectrumData, and its covariance in the property NoiseCovariance. You can "convert" this IDFRD into parametric form using commands such as TFEST, SSEST, PROCEST.
FRDs are non-parametric representation of linear dynamics. There is no underlying differential equation or difference equations stored in them. That is why they cannot be used directly in Simulink.