P1 dB and iip3 for homodyne receivers

hi all
i am simulating zero -IF receiver for wimax.
i want to calculate 1 dB compression point but in HB it is not allowing 0 Hz as output frequency.
and for 3 mixer tones what is the iip3 equation
pls help
thanks

Zero IF is net IF=0Hz, maybe it's 10KHz.