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

OK, the details of this have had me pondering for some time.... (my 
transmission line theory
class was waaaayyyyy to long a go). When we say "renormalize
the S-parameters", what is really going on?  and when do I need to 
renormalize?
If I have a 50 ohm system under test (SUT), and a 50 ohm test system 
(VNA), and the results
I want are for 50 ohms, I assume no renormalization is necessary.  If, 
however, as
suggested below, I have a 75 Ohm SUT and a 50 Ohm VNA I get reflections 
at the
connection between the two, and my S11 / S21 measurements get 
discombobulated
due to the mismatch.  Now, if I really want a 75 Ohms system, do I 
renormalize the
results (since the VNA was 50 Ohms)?  To which - 50 Ohms  or 75 Ohms - 
in this
case, which value is the "base" (non-normalized) and which would I 
normalize to?
Can I use the results in a 75 Ohm system simulation?

Thanks,
Ralph Wilson
Alcatel-Lucent

Brad Brim wrote:
> 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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