You can emulate a pstb analysis by combining the results of two pac analyses. This will require some work for setting up the simulations and equations, however. See http://www.designers-guide.org/Forum/?board=power-electronics;action=display;num... for details.

Like you said, a pstb analysis is the same as an stb analysis, just using pac instead of ac. You can either take the article mentioned in reply #48 as a guide or my implementation in LTspice mentioned in reply #16. If you follow the LTspice implementation, you might also want to take a look at the thread starting with message 2482 in the Yahoo group, which contains some explanations about the circuit.

With pac, you additionaly need to make sure that you are using the correct sideband. I would recommend you to use a node in your loop where you have a continuous signal and use sideband 0. The pstb analysis of SpectreRF is also using sideband 0.

What is generally known as Middlebrook's method is not the same as stb analysis because Middlebrook's method was developed for a circuit with no backward transmission whereas stb analysis takes both forward and backward transmission into account so that the orientation of the probe does not matter. Regarding Middlebrook's more recent method, please see my comment at http://www.designers-guide.org/Forum/YaBB.pl?num=1155668476/17#17.