How to define the output impedance for LNA simulation ?

Dear All :
I design a LNA now , the band is 70M ~ 120M , I usae spectre do simulation.
I Run SP , PSS+ Pnoise. I have some question about the simulation. I use Vport or Vsource fot input / ouput. My question is input is 50 ohm. How to define the ouput impedance ? The Next stage is Mixer. I read some document , it apply ouput impedance is 50 ohm. Is it correct ? Thanks.

Due to mixers are very nonlinear components, input impedance of a mixer can not be defined precisely.Therefore we "can" assume the mixer input impedance "matched to 50 Ohm" ( in fact it's not true..) but there is no way out this..
For LNA, NF is not affected by output impedance but input one.Only Voltage gain can be seen different but when you consider this relatively, it's not a serious problem..

Dear BigBoss :

SO that in the output port ,the load is 50ohm for LNA design , But if the LNA is work 100MHz, Does it the same . Thanks.
Any another question, If I run P1dB , or IIP3 , Does it depend on output load ?
what simulation is depend on output load ? Thanks

if there is no image reject filter(off chip) between lna and mixer it isn't neccessary to match the output of lna and input of mixer to 50 ohm when those are integrated on the same chip.
no ip3 and p1db simulation aren't affected by the output impedance.sp parameters are affected by the output port impedance.