Problem with EM/CKT co-simulation with ADS

Hi,

I have designed a circuit and designed a layout for it. I am using ADS to do the simulation. And I have also run S-parameter simulation in ADS momentum for my layout. Then I use two different way to do EM/CKT co-simulation.

1. I use ADS momentum to generate a layout component and put it into CKT simulation.
2. I extract the momentum result into a touchstone file, which is a 37-port S-parameter file. And import this S37P file into CKT simulation.

What I find is the two simulation give significant difference. I don't understand why there is difference?
Does anyone have idea on how this difference comes about?

Thanks!