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MOM Warning: Unphysical Behavior at High Freq

Question asked by symotecads on Aug 30, 2007
Latest reply on Aug 30, 2007 by symotecads
I've been simulating some structures on a multilayer PCB that require the signal to travel from an embedded stripline to the microstrip structure on top and down again to an embedded stripline output. To make things clear a bit, my substrate stack is as follows:

FREESPACE
----      COND0 (MICROSTRIP)
SUB1      viaC2C0, viaC3C0
----      COND1 (GND1, configured as SLOT)
SUB2      viaC2C0, viaC3C0
----      COND2 (STRIPLINE)
SUB3      viaC3C0
----      COND3 (GND2)

Imagine a wilkinson combiner on COND0 with the inputs coming from 50 ohm striplines in COND2.  These inputs use viaC2C0 to cross SUB1 and SUB2 layers, while viaC3C0 (GND vias) are placed around each viaC2C0 to provide groundshields to viaC2C0.  Putting these viaC3C0 around a ViaC2C0 creates an effect that the vertical transition will have a 50 ohm characteristic impedance, as in a coax.  

The output of the combiner has to go back to COND2 again, and as such I used viaC2C0 again and the corresponding viaC3C0s.  

Everytime I do this kind of structure and simulate, however, my momentum simulation complains of unphysical behavior at high frequencies.  Specifically the warnings are:

S-parameters show unphysical behavior at high frequencies:
possible causes:
1. inaccurate (high frequency calibration)
2. higher order modes at the ports
3. slot mode at coplanar ports
4. grouped strip-slot ports
5. too coarse of a mesh

And indeed when the simulation completes, I observe some strange results.  I wish to address the warnings written above but it's only no.4 that appeals to my senses.  And even if I increase the meshing the problem is still there.

Appreciate any help you guys can provide.  

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