in SimRF you have two modelling technologies: Equivalent Baseband and Circuit Envelope. Both technologies are suitable for modelling super-etherodyne systems.
With Equivalent Baseband the LO signal is "embedded" in the model of the mixer that performs a down- or an up- conversion. The model assumes the LO to be a sinusoidal tone. It is not possible to specify arbitrary LO signals but it is possible to specify the phase noise spectral mask of the LO tone.
With Circuit Envelope you can specify arbitrary LO signals, these can be generated with Circuit Envelope signal sources (such as sinusoidal sources), or can be generated in Simulink and then connected to the LO port of the Circuit Envelope Mixer using an input port as a gateway between the two simulation domains. The Simulink signal will represent the envelope (modulation) of the LO signal centred around the carrier frequency specified by the input port.
The webinar "Design and Verify RF Transceivers for wireless communications systems" provides some more insights and examples:
I hope that this helps, with best regards, Giorgia