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

Hi Ralph,
My comments on this are as follows:

1) 50 Ohm and 75 Ohm s-parameters should be identical from a simulation
standpoint, as long
as the simulator respects the reference impedance (which should be stated
in any touchstone file
representing the s-parameters).  Some software (like some of my own)
ignores the Z0 statement
and assumes 50 Ohms.  It's probably safest to use 50 Ohm s-parameters.

2) the difference will come in a viewing and interpretation standpoint.  If
I plot the values of any
s-parameter over frequency, I am looking at the behavior of my system under
a very special
condition.  For example if I plot the magnitude of s[3][1] and my reference
impedance Z0 is 50
Ohms, then I am looking at the wave that comes out of port 3 due to an
incident wave on port 1
when all ports are terminated in that reference impedance and the driven
port 1 is driven with
a source whose impedance is the reference impedance.

So, when you say, you have a 75 Ohm system, I think what you mean is that
you would want to
see the behavior of the system when driven by a 75 Ohm source and all other
ports are terminated
in 75 Ohms.  Looking at the 50 Ohm s-parameters would be an unfruitful
exercise (but again, they
should work in simulation).

To transform the s-parameters, between impedances, you can use the
following function which I
think is self-explanatory.

(Embedded image moved to file: pic21724.jpg)

So, the new s-parameters, "normalized to a new reference impedance" will be
functionally equivalent to the old
ones, but will be more suitable for viewing and interpreting in the desired
reference impedance.

I don't know if there are any measurement issues in using a 50 Ohm VNA for
measurement of 75 Ohm s-parameters.

Pete Pupalaikis
LeCroy Corporation



                                                                           
             Ralph Wilson                                                  
             <wilson@alcatel-l                                             
             ucent.com>                                                 To 
             Sent by:                  bradb@xxxxxxxxxxx,                  
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             eelists.org                                                cc 
                                                                           
                                                                   Subject 
             01/15/2009 07:44          [SI-LIST] Re: S11 and S21 for PCB   
             AM                        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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