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Odd results and GAMMA and Z0

Question asked by wmlads on Oct 17, 2005
Latest reply on Oct 24, 2005 by ALEPEREZ
I'm getting some odd results when i simulate a simple microstrip. No matter what I do with the width of the microstrip the S11(and S22) result always shows very good match (-55 dB or lower).  I plotted Z0 but that wasn't close to 50 Ohms at all. My port impedance is set to 50 Ohm.  However, the plot of Z0 seemed to follow logic because as I increaed width Z0 went down.  I'm simulation in Microwave mode and from 1 GHz to 60 GHz.  Also, in most cases s11 remains below -50 dB right through the whole range of frequencies, which seems quite odd.

Secondly, how can I get the attenuation constant in Momentum? Is it just real(GAMMA)?  Also, is Z0 really the characteristic impedance or is it something else?  Since my results are so odd I'm doubting everything.

I want to mention that I'm using my own substrate stack up and I think I've defined it properly.  The process has seven metal layers.  I placed the conductor on top layer (metal 7) and the ground plane for microstrip on metal 1.  I mapped metal 7 as strip while ground plane was mapped as slot.  I placed my ports on metal 7 obviously. In between those metals is different dielectrics but they're not mapped as Vias or anything because I'm not connecting any metal.  Metal 2 thru 6 are not mapped since they're not used.  But, in my process I've some other layer on top of metal 7 for passivation purposes.  I want to include that in my substrate definition; however, when I do that Momentum doesn't calculate GAMMA and Z0 (it sets them to 0 and 50 respectively).  But when I remove those passivation dielectrics so that FreeSpace is right above Metal 7, Momentum solves for GAMMA and Z0 but I get odd results as I mentioned earlier.  Thanks