I think the problem comes from the fact that you have chosen a Bandpass filter, and not a “low pass filter”, contrary to what you have mentioned in your reply #25834. The filter is centered at 18GHz with a bandwidth of 2GHz. Please see the output signal from PIN, in which a harmony is seen at 1.5GHz (Otherwise, I don’t know why you have mentioned “LPF” in your comment).
Regards