Are you asking for a simple method to estimate the gain in Simulation or make an estimate based on knowledge of the LNA and mixer.

If you're talking about by simulation then you're using the right method.

Otherwise its just the gain of the LNA in dB plus the gain of the mixer in dB. You have to take the loading into account too. If your mixer uses large device size it may be lowering the voltage gain of your LNA. Maybe you can use the XF (transfer function) simulation of the LNA+Mixer taking the output at the LNA output node to troubleshoot.

Aaron

It is hard to help you because you have given no specifics. For example, how did you set up the PXF and how do the results differ.

PXF and PSS naturally produce different results. PXF produces the gain and PSS calculates a voltage level. So you have to divide the PSS output voltage and input voltage to compare them. Furthermore, if your stimulus is being provided by a port, PXF is computing the gain assuming a perfect match at the input. In other words, it is computing the gain from the voltage source that is internal to the port, and dividing it by two.

If you are using a port as stimulus, you might want to use PAC rather than PXF. This way Spectre will compute both the input voltage and the output voltage, just like in PSS, and you can find the gain by dividing them.

-Ken