[SI-LIST] Re: Common-mode return loss in Hspice

TK,
There are several issues with your netlist. First of all, as far as spice is
concerned your differential circuit topology is a 4-port structure not two
port. You should get the 4-port (so called single-ended) s-parameters first
and then convert them to mixed-mode s-parameters to extract Scc and Sdd
sub-matrices. If you post-process the spice output you need the data in real
and imaginary format to do that not in dB as you suspected. Remember
addition in the logarithmic scale is equivalent to multiplication in the
linear scale. Finally, .net parameter only applies to 2-port structures in
hspice. As far as I know hspice doesn't have any built-in function to
extract the s-parameters of a multiport structure above two ports. For your
diff. pair you need to write your own routines to get the 4-port s-parameter
matrix.
Regards

Ihsan


On 5/26/06, Jeon, Tae-Kwang <Tae-Kwang.Jeon@xxxxxxxx> wrote:
>
> Hello all,
>
>
> I'm trying to simulate common-mode return loss in hspice, and want to
> double check my simulation with you because the result showed me was
> somewhat different than what I expected over 100MHz ~ 10GHz.
>
> First of all, below is my simple diagram implemented in the spice deck.
>
>
>
>          Port1                                         Port2
>
>
>             + -------[[[[[[[[[[[[[[[[[[[[[[[[[[[[[-------+
>
>       +----+                 dif'l traces             +-------+
>
>        |    +--------[[[[[[[[[[[[[[[[[[[[[[[[[[[[[-------+        |
>
>     ((V))  common-mode                                /////
>
>        |     input voltage source
>
>      /////
>
>
>
> Since I assume that both source impedance and the load impedance are
> 50ohms, I put 25ohms of Zcm to Rin and Rout variable as shown in the
> input file below. So, here are my questions to ask.
>
>
>
> 1. Is my diagram is reasonable for scc11 measurement?
>
> 2. What impedance value should both Rin and Rout have in the .NET
> statement. Is it Zo, Zeven or Zcm? If I set it to Zo(=50ohms), I have
> 6dB less loss than I expected. My W-elements model was based on dif'l
> traces having roughly 70ohms of even mode impedance, which is Zcm =
> Zeven/2 = 35ohms, although the odd mode impedance is ~53ohms.
> Interestingly, If I set both Rin and Rout in .NET statement to
> 35ohms(=Zcm of trace), then it resulted in exactly what I expected. So I
> want to clarify what is the right setting.
>
> 3. Is it right to use Sij(db) rather than either Sij(m) or Sij(r) for
> the simple math like scc11=(s11+s21+s12+s22)/2?
>
>
>
> *My spice deck
>
> Vin vcm_in 0  AC=1
>
>
>
> Xline1  vcm_in  vcm_in  vcm_out  vcm_out  0 trace1
>
>
>
> Rdummy  vcm_out  0  1meg
>
>
>
> .OPTIONS  accurate post=2  probe
>
> .NET    v(vcm_out) Vin Rin=25  Rout=25
>
> .AC     lin  1k  10meg  10000meg
>
> .PROBE  scc11=par('(s11(db)+s21(db)+s12(db)+s22(db))/2')
>
>
>
> .SUBCKT trace1  in1     in2     out1    out2    Vss
>
> Wline1          in1     in2     Vss     out1    out2     Vss
>
> +RLGCFILE= 'my_trace.rlc'    N=2 L=.049
>
> .ENDS
>
> .END
>
>
>
> Thanks,
>
> TK
>
>
>
>
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