NETWORK FUNCTIONS OF TIME-VARYING CIRCUITS - Computer Methods for Analysis of Mixed-Mode Switching Circuits - page 45

The complete spectrum of the response is given in Fig.3.2. The 2nd-order
and 3rd-order harmonic distortions in the base band are computed from

provided that

If the input of a nonlinear time-varying system contains two different
frequencies

with the spectrum of the response can also be obtained in a
similar manner. It can be shown that the 3rd-order intermodulation
distortion, denoted by is computed from

If the system is further nonlinear periodically time-varying, then it can
be shown that the spectrum of the response is given in Fig.3.3. The
3rd-order intermodulation distortion at

in the base band is

computed from

where has been assumed.