[SI-LIST] Re: S11 and S21 for PCB trace with different dielectic think

hi John,

You'll see the same thing if you use a 50 ohm network analyzer to measure a
75 ohm system :-)

Yes, you are seeing impedance mismatch from your ports/probes to the system
you are simulating. Simply renormalize the S-parameters to the impedance of
the nets. You can simulate a single net in isolation to extract its
impedance or simply get the impedance of the lines from the field solver,
available in some simulators for certain port setups.

cheers,
 -Brad
 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx 
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of John Kwan
> Sent: Wednesday, January 14, 2009 9:15 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] S11 and S21 for PCB trace with different 
> dielectic think 
> 
> Hi,
>  
> I am trying to extract the S11 and S21of some PCB traces 
> using one of the field solver tools. The traces are running 
> on top of a solid ground plane, and I run the simulation for 
> two different dielectric thickness settings. In one case, the 
> Zo for the trace is around 50-ohm, and the other case the Zo 
> is around 80-ohm. The S11 and S21 for the two case is quite 
> different. The insertion loss S21 is much less and smoother 
> through out the frequency range in the case of 50-ohm. The 
> reflection coefficient S11 is also much closer to zero in the 
> case of 50-ohm. 
> 
> Is it because the port reference is set to 50-ohm for the 
> simulation, so the result for the 50-ohm case is much better? 
> If so, how can we use the result if the system is not 50-ohm 
> in the real case?
> 
> Regards,
> John 

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