This model shows how a diode ring can be used to demodulate a frequency-modulated signal. The RF input has a fixed frequency of 9MHz, and the local oscillator has a fixed frequency of 11MHz. Hence the frequency-modulated signal is a sine wave of 2MHz. This 2MHz component is clearly visible in the IF response. The second component in the IF response is the sum of the RF and local oscillator frequencies i.e. 20MHz.
The spectrum shows that the diode ring mixer introduces some additional unwanted harmonics. The spectrum can be used to determine how to augment the behavioral ring mixer model with additional terms to produce an abstracted mixer model that can be used in a more complex circuit