Pnoise simulation for a down-conversion mixer

Hi all,
I'm simulating the NF af a down-conversion mixer using pss and pnoise.
All the cadence tutorails say you should set the RF port to DC,
and then, when you choose the reference sideband in the pnoise form, you should choose + or -1 so your input frequency is Flo.
I don't understand this.
Isn't the input frequency the Frf frequency? and why isn't it being taken into account?

Thanks