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

Ralph,

The important thing to keep in mind is that the normalization impedance 
for scattering (S) parameters
DOES NOT change the behavior of your network; you can consider it as 
changing the parameters
(like scaling, offset) on your display when you plot data.  The way how 
you plot it, will not change
the data itself.  So within reason, any normalization impedance works as 
long as it is carried through
consistently. 

There are a couple of important reasons why in practice renormalization 
may be a good idea. First,
when you look at the S parameters of a uniform transmission line, the 
curves look the 'smoothest'
if the normalization impedance is close to the characteristic 
impedance.  (note: the characteristic
impedance of a real-life non-ideal transmission line is always complex 
and frequency dependent, so
we can never get the reflection parameters down to zero at all 
frequencies with a single real
normalization impedance, like 50 Ohm).  As we gradually deviate the 
normalization impedance
from the characteristic impedance, there is an increasing 'ripple' 
showing up both in the main
diagonal elements (reflection terms) and in the off-diagonal elements 
(transmission terms). The
periodicity of the 'ripple' corresponds to the multiples of 
half-wavelength resonance on the line.
The 'ripple' in the transmission terms rides on top of the attenuation 
caused by losses and/or
radiation, so to judge visually the losses/radiation, the easiest is to 
eliminate the 'ripple' by using
a normalization impedance that is close to the characteristic 
impedance.  The second reason is
about validation and measurement and goes back to the first point: if 
the measuring impedance
has about the same characteristic impedance as your uniform transmission 
line, it will give the
smoothest possible curves, easy for quick visual interpretation.

As a summary, you dont HAVE TO use renormalization (unless the impedance 
mismatch is
orders of magnitude, which would create numerical problems); you may 
want to do it as a
matter of convenience in data interpretation.

Regards,

Istvan Novak
SUN Microsystems

Ralph Wilson wrote:
> 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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