ques related to sps simulation of downconversion mixer in virtuoso

can any1 tell how to simulate sps analysis of downconversion mixer in virtuoso. actually when i m giving sweep type relative it is showing 8dB gain( that is similar what i m getting through transient). but rf sweep is vaying. when i m giving sweep type absolute. IF harmonic is cmng negative and gain is also negetive. i m not able to understand why.