Computing the NF of the three blocks combined is straight forward, however computing P1dB or IP3 is not possible because SpectreRF currently does not allow an autonomous circuit, the VCO, to be driven by a large tone, the input signal. However, this is generally not a problem. One does not normally include the VCO when using simulation to predict P1db, iIP3, and NF of the LNA & mixer because the VCO does not substantially affect these performance metrics.
However, having said that, these metrics are affected by the phase noise of the VCO when a large blocker in present. In this case, one
would want to co-simulate the LNA, mixer, and VCO. In this situation, the best approach is to simulate the VCO alone, determine its phase noise, and then replace the VCO with a behavioral model that produces the same phase noise but is not autonomous. This procedure is described in
Introduction to RF simulation and its application, which can be found at
http://www.designers-guide.com/Analysis. Look in 6.3.3 on page 38.
-Ken