sentences:  For a signal consisting of a single component, the instantaneous frequency can be estimated from the partial derivatives of phase of any short-time Fourier transform channel that passes the component.If the signal is to be decomposed into many components,
  
  and the instantaneous frequency of each component is defined as the derivative of its phase with respect to time, that is,
  
  then the instantaneous frequency of each individual component can be computed from the phase of the response of a filter that passes that component, provided that no more than one component lies in the passband of the filter.This is the property, in the frequency domain, that Nelson called "separability" and is required of all signals so analyzed.