[SI-LIST] Re: .NET Statements in HSPICE for S-Parameters

  • From: "Chien-Min Lin" <cmlin@xxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx, RJatou@xxxxxxxxxx
  • Date: Mon, 13 May 2002 10:40:09 -0700


Appreciate very much for the subtle comment of Ross.
Now, it did work as expected in the analytical one.
To extend the study in S-parameters, the .sp script
is provided for a typical transmission line network
with some stubs:

  IN---[Tx1]---n1---[Tx2]---n2---[Tx3]---n3---[Tx4]---OUT
   |           |            |            |             |
   |           |            |            |            [Rd]
[Vin]       [Sb1]        [Sb2]        [Sb3]           |
   |           |            |            |             0
   0           0            0            0


(00) S-Parameters / .NET
*********************************************************************
.OPTION Post
       + DCSTEP=1
*--[ Transmission Line (W element): XFX ]---------------------------*

.subckt nom_W4x1  1 2    3 4        Len_in=10
  $: Er=4.0 Zo =49.89 TD =2.03  num_sig=1  h1=4.65347 h2=4.9505 w=4
  W1 1 2    3 4        Umodel=umod N=1 L=Len_in
.model umod U Level=3 NL=1   Elev=2 Llev=0 Plev=1 Nlay=2
      + L11=8.453n
      +
      + Cr1=3.397p
      +
      + R1c=0.252
      +
      + Gr1=0
      +
      + R1s=2.90989e-05
.ends  nom_W4x1

*--[ Two-Port Network: Tx + Stub ]----------------------------------*
*
*     IN---[Tx1]---n1---[Tx2]---n2---[Tx3]---n3---[Tx4]---OUT
*      |           |            |            |             |
*      |           |            |            |            [Rd]
*    [Vin]       [Sb1]        [Sb2]        [Sb3]           |
*      |           |            |            |             0
*      0           0            0            0
*-------------------------------------------------------------------*

  Vin    IN  0   AC  1.0    $: exciation

  X_Tx1  IN  0   n1  0   nom_W4x1  Len_in=1.0   $: unit = inch
  X_Tx2  n1  0   n2  0   nom_W4x1  Len_in=1.0
  X_Tx3  n2  0   n3  0   nom_W4x1  Len_in=1.0
  X_Tx4  n3  0   OUT 0   nom_W4x1  Len_in=1.0

  X_Sb1  n1  0   0   0   nom_W4x1  Len_in=0.2
  X_Sb2  n2  0   0   0   nom_W4x1  Len_in=0.2
  X_Sb3  n3  0   0   0   nom_W4x1  Len_in=0.2

  Rd     OUT 0   1E14       $: open load

*-------------------------------------------------------------------*
.AC  dec  500    1  8500Meg

.NET V(OUT)  Vin  Rout=50.0  Rin=50.0

.probe  AC  S11  S11(dB) (-50,10)  S11(p) (-180, 180)
.probe  AC  S21  S21(dB) (-50,10)  S21(p) (-180, 180)
*-------------------------------------------------------------------*
.END
*********************************************************************


In brief, the S11 and S21 are referred to
                _________
           V1+ |         | V2+
           --> | 2-Port  | <--
   Vin o-------+         +-------o Vout
           <-- | Network | -->
           V1- |_________| V2-

that is,
          S11 = V1-/V1+  while V2+ = 0
          S21 = V2+/V1+  while V2+ = 0

Nonetheless, is there well defined for S11_ni and S21_ni
as depicted in the branched network?

  IN---[Tx1]---n1---[Tx2]---n2---[Tx3]---n3---[Tx4]---OUT
   |           |            |            |             |
   |           |            |            |            [Rd]
[Vin]       [Sb1]        [Sb2]        [Sb3]           |
   |           |            |            |             0
   0           0            0            0

That is, at Node n1: S11=?  S21=?
                 n2: S11=?  S21=?
                 n3: S11=?  S21=?

Can the .NET statements be applied in the .alt statements:

.alt (01) .NET V(n1)  Vin  Rout=50.0  Rin=50.0
           .NET V(n1)  Vin  Rout=50.0  Rin=50.0
           .probe  AC  S11  S21                  $: node n1

.alt (02) .NET V(n2)  Vin  Rout=50.0  Rin=50.0
           .NET V(n2)  Vin  Rout=50.0  Rin=50.0
           .probe  AC  S11  S21                  $: node n2

.alt (03) .NET V(n3)  Vin  Rout=50.0  Rin=50.0
           .NET V(n3)  Vin  Rout=50.0  Rin=50.0
           .probe  AC  S11  S21                  $: node n3


However, it confuses me with my understanding in the 2-port
network.  For instance, at the node n2:

  IN---[Tx1]---n1---[Tx2]---n2---[Tx3]---n3---[Tx4]---OUT
   |           |            |            |             |
   |           |            |   +---->   |            [Rd]
[Vin]       [Sb1]        [Sb2] |       [Sb3]           |
   |           |            |   |        |             0
   0           0            0   |        0
                                |
                                |
                           at n2: look into an open end

is it needed to break/remove [Tx3... to the right of n2] by
terminating at 500000ohms:

  IN---[Tx1]---n1---[Tx2]---n2---+[50000ohms]
   |           |            |
   |           |            |
[Vin]       [Sb1]        [Sb2]
   |           |            |
   0           0            0

such that
           .NET V(n2)  Vin  Rout=50.0  Rin=50.0

can be applied to calculate S11 and S21 at n2?

What is the difference in physics between these two models:
without removing [Tx3...] and 50000ohms terminated at n2?

Thanks for all your sophisticated advice, indeed.


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