Using Ansys SIwave 2020R2.

This post introduces how to extract the s-parameter of a pair of PCB traces using SIwave 2020R2.

Resource

From SIwave documentation:

Sample Board

The schematic: Focus on the RX pair with a pair of AC-coupling capacitors.

The PCB layout:

  • Trace length around 4100 mils
  • Trace width/spacing=4.2mil/6.3mil

The 8-layer stackup in Allegro.

Importing to SIwave

Stackup

Dielectric material: TU-768 - Taiwan Union Technology Corporation

Use select all on the bottom-left corner to select and edit all conductor/dielectric layers.

For example,

  • Medal: copper, (also, dielectric fill: FR-4)
  • Dielectric: FR-4

Assign Capacitor Model

Tools-S-param Model Assignment, use filter to narrow down the model list.

Adding Ports

Tools-Generated Port:

For the RX pair in the sample board, J1---cap---J2, for each trace, 2 ports are needed at J1 and J2:

Check the generated port and make sure that the negative net -Net is GND and is near the positive net +Net.

Compute SYZ Parameters

Simulation-Compute SYZ Parameters:

  • Set the frequency range
  • Export Touchstone file, this will generate an .snp file, where n is the total number of ports. In this case, an .s4p file.

Display Results

Select the SYZ simulation-right click-Compute Differential S-parameters:

For example, insertion loss:

  • +Port and -Port pair: RX_P and RX_N ports at J1 (or J2)
  • Plot: from diff. port DIFFP0:Diff to DIFFP1:Diff

Also, the .snp file could be used in other software such as ADS: like in this youtube video

SnP File

In an .s4p, at each frequency, there are 1 stimulus and 16 S-parameters, each S-parameter consists of real and imaginary parts, so 33 parameters at each frequency point.

Such as:

! S-parameters
# Hz S MA R 50.000000
5.000000000000e+06 7.415729611535e-03 -9.049652032862e+00 2.905053518013e-03 7.206460994824e+01 9.921850136786e-01 -1.314852190849e+00 1.330022519835e-03 -1.285113705145e+02
                   2.905053518013e-03 7.206460994824e+01 7.414583662428e-03 -9.086051281811e+00 1.330024196348e-03 -1.285117281406e+02 9.921868920609e-01 -1.314594245442e+00
                   9.921850136786e-01 -1.314852190849e+00 1.330024196348e-03 -1.285117281406e+02 7.547298314186e-03 -8.975382166206e+00 2.901011285899e-03 7.215890285145e+01
                   1.330022519835e-03 -1.285113705145e+02 9.921868920609e-01 -1.314594245442e+00 2.901011285899e-03 7.215890285145e+01 7.545921799068e-03 -9.010637065165e+00

According to SnP File Format

*.s1p Files
Each record contains 1 stimulus value and 1 S-parameter (total of 3 values).

Stim  Real (Sxx)  Imag(Sxx)

*.s2p Files
Each record contains 1 stimulus value and 4 S-parameters (total of 9 values)

Stim  Real (S11)  Imag(S11)  Real(S21)  Imag(S21)  Real(S12)  Imag(S12)  Real(S22)  Imag(S22)

*.s3p Files
Each record contains 1 stimulus value and 9 S-parameters (total of 19 values)

Stim   Real (S11)  Imag(S11)  Real(S12)  Imag(S12)  Real(S13)  Imag(S13)

          Real (S21)  Imag(S21)  Real(S22)  Imag(S22)  Real(S23)  Imag(S23)

          Real (S31)  Imag(S31)  Real(S32)  Imag(S32)  Real(S33)  Imag(S33)

*.s4p Files
Each record contains 1 stimulus value and 16 S-parameters (total of 33 values)

Stim   Real (S11)  Imag(S11)  Real(S12)  Imag(S12)  Real(S13)  Imag(S13)  Real(S14)  Imag(S14)

          Real (S21)  Imag(S21)  Real(S22)  Imag(S22)  Real(S23)  Imag(S23)  Real(S24)  Imag(S24)

          Real (S31)  Imag(S31)  Real(S32)  Imag(S32)  Real(S33)  Imag(S33)  Real(S34)  Imag(S34)

          Real (S41)  Imag(S41)  Real(S42)  Imag(S42)  Real(S43)  Imag(S43)  Real(S44)  Imag(S44)

This pattern continues for N ports involved in the measurement.