txlineCPW

Create coplanar waveguide transmission line

Description

Use the txlineCPW object to create a coplanar waveguide (CPW) transmission line. You can also use the txlineCPW object to create an infinite-bottom conductor or ground plane CPW transmission line. The cross-section of a CPW transmission line is shown in this figure. The physical characteristics of the CPW transmission line include conductor width (w), conductor thickness (t), slot width (s), substrate height (d), and permittivity constant (ε).

Creation

Syntax

txline = txlineCPW

txline = txlineCPW(Name,Value)

Description

txline = txlineCPW creates a default CPW transmission line object.

txline = txlineCPW(Name,Value) sets Properties using one or more name-value pairs. For example, txline = txlineCPW('SlotWidth',0.0046) creates a CPW transmission line with a slot width of 0.0046 meters.

Properties

Name — Name of CPW transmission line

'cpw' (default) | string scalar | character vector

Name of the CPW transmission line, specified as a string scalar or a character vector.

Example: 'Name','cpw1'

Example: txline.Name = 'cpw1'

Data Types: char | string

ConductorWidth — Physical width

0.0006 (default) | positive scalar

Physical width of the conductor, specified as a positive scalar in meters.

Example: 'ConductorWidth',0.0200

Example: txline.ConductorWidth = 0.0200

Data Types: double

SlotWidth — Physical width of slot

0.0002 (default) | positive scalar

Physical width of the slot, specified as a positive scalar in meters.

Example: 'SlotWidth',0.0008

Example: txline.SlotWidth = 0.0008

Data Types: double

Height — Physical height of conductor or dielectric thickness

0.000635 (default) | positive scalar

Physical height of the conductor or dielectric thickness, specified as a positive scalar in meters.

Example: 'Height',0.000835

Example: txline.Height = 0.000835

Data Types: double

Thickness — Physical thickness

0.000005 (default) | positive scalar

Physical thickness of the CPW transmission line, specified as a positive scalar in meters.

Example: 'Thickness',0.000008

Example: txline Thickness = 0.000008

Data Types: double

EpsilonR — Relative permittivity of dielectric

9.8 (default) | positive scalar

Relative permittivity of the dielectric, specified as a positive scalar.

Example: 'EpsilonR',8.8

Example: txline.EpsilonR = 8.8

Data Types: double

LossTangent — Loss angle tangent of dielectric

0 (default) | nonnegative scalar

Loss angle tangent of the dielectric, specified as a nonnegative scalar.

Example: 'LossTangent',1

Example: txline.LossTangent = 1

Data Types: double

SigmaCond — Conductivity of conductor

Inf (default) | nonnegative scalar

Conductivity of the conductor, specified as a nonnegative scalar in Siemens per meter (S/m).

Example: 'SigmaCond',2

Example: txline.SigmaCond = 2

Data Types: double

LineLength — Physical length

0.0100 (default) | positive scalar

Physical length of the CPW transmission line, specified as a positive scalar in meters.

Example: 'LineLength',0.0200

Example: txline.LineLength = 0.0200

Data Types: double

ConductorBacked — Infinite bottom conductor or ground plane

0 (false) (default) | 1 (true)

Infinite-bottom conductor or ground plane, specified as a numeric or logical. When you specify 0, you create a transmission line without any conductor backing.

Example: 'ConductorBacked',1

Example: txline.ConductorBacked = 1

Data Types: double

Termination — Stub transmission line termination

'NotApplicable' (default) | 'Open' | 'Short'

Stub transmission line termination, specified as 'NotApplicable', 'Open', or 'Short'.

Example: 'Termination','Short'

Example: txline.Termination = 'Short'

Data Types: char

StubMode — Type of stub

'NotAStub' (default) | 'Series' | 'Shunt'

Type of stub, specified as 'NotAStub', 'Series', or 'Shunt'.

Example: 'StubMode','Series'

Example: txline.StubMode = 'Series'

Data Types: char

NumPorts — Number of input and output ports

2 (default) | positive scalar

This property is read-only.

Number of input and output ports, returned as positive scalar.

Data Types: double

Terminals — Terminals of coplanar waveguide transmission line

{'p1+' p2+' 'p1-' 'p2-'} (default) | cell array of strings

This property is read-only.

Terminals of the CPW transmission line, returned as a cell array of strings.

Data Types: char | string

Object Functions

sparameters	Calculate S-parameters for RF data, network, circuit, and matching network objects
groupdelay	Group delay of S-parameter object or RF filter object or RF Toolbox circuit object
noisefigure	Calculate noise figure of transmission lines, series RLC, and shunt RLC circuits
getZ0	Calculate characteristic impedance with and without dispersion for transmission line
circuit	Circuit object
clone	Create copy of existing circuit element or circuit object

Examples

S-Parameters of Coplanar Waveguide Transmission Line

Create a coplanar waveguide transmission line using these specifications:

• Conductor width : 45 mu m

• Slot width : 50 mu m

• Height of substrate : 525 mu m

• Thickness : 1 mu m

• GaAS permittivity or Epsilon R : 2.5 F/m

• Conductivity : 3.33e7 S/m

cpwtxline = txlineCPW('ConductorWidth',45e-6,'SlotWidth',50e-6,'Height',525e-6,...
    'Thickness',1e-6,'EpsilonR',2.5,'SigmaCond',3.33e7)

cpwtxline = 
  txlineCPW: CPW element

              Name: 'CPW'
    ConductorWidth: 4.5000e-05
         SlotWidth: 5.0000e-05
            Height: 5.2500e-04
         Thickness: 1.0000e-06
          EpsilonR: 2.5000
       LossTangent: 0
         SigmaCond: 33300000
        LineLength: 0.0100
       Termination: 'NotApplicable'
          StubMode: 'NotAStub'
          NumPorts: 2
         Terminals: {'p1+'  'p2+'  'p1-'  'p2-'}

Calculate the S-parameters of the transmission line at 20 GHz.

sparam = sparameters(cpwtxline,20e9);

Behavioral vs EM Solver Model of CPW Transmission Line

This example uses RF PCB Toolbox to calculate an electromagnetic (EM) solver S-parameter model of a conductor-backed coplanar waveguide (CPW) transmission line.

Behavioral Modeling

Select the FR4 dielectric and silver metal the metal and dielectric libraries, respectively, of RF PCB Toolbox.

dFR4 = dielectric('FR4');
mSilver = metal('Silver');

Design an infinite-bottom conductor-backed CPW transmission line at 6 GHz and ${\mathit{Z}}_0$ of 75$\Omega$.

tx = txlineCPW('ConductorBacked',true, ...
    'LineLength',0.0145,'ConductorWidth',2.6482e-4, ...
    'SlotWidth',2e-4,'Height',6.35e-4, ...
    "EpsilonR",dFR4.EpsilonR,"LossTangent",dFR4.LossTangent, ...
    "SigmaCond",mSilver.Conductivity,'Thickness',2e-7, ...
    'StubMode',"NotAStub","Termination","NotApplicable");

Calculate the S-parameters using a behavioral model from RF Toolbox with the reference impedance of 50$\Omega$. The behavioral model is also called an analytical or closed-form model.

freq = (1:5:66)*100e6;
Srf = sparameters(tx,freq,50);

EM Modeling

Create an RF PCB Toolbox coplanarWaveguide object.

tx_em = coplanarWaveguide(tx);

View the conductor-backed CPW transmission line.

show(tx_em)

Calculate the S-parameters using the EM solver from RF PCB Toolbox.

Sem = sparameters(tx_em,freq,50);

Compare S-parameters

Compare the S-parameters from the behavioral and EM solver models.

rfplot(Srf,'-s','db')
hold on
rfplot(Sem,'-x','db')
title('Behavioral vs. EM solver modeled S-parameters')

References

[1] Garg, Ramesh, I. J. Bahl, and Maurizio Bozzi. Microstrip Lines and Slotlines . 3rd ed. Artech House Microwave Library. Boston: Artech House, 2013.

Version History

Introduced in R2020b

R2021b: Coplanar waveguide transmission line object changed

Behavior changed in R2021b

Starting in R2021b, the txlineCPW circuit object has a new property, ConductorBacked. Use this property to design a CPW transmission line with an infinite-bottom conductor or ground plane.

When you open a model created before R2021b containing a txlineCPW circuit object, the software replaces the object with the R2021b version of the object and sets the ConductorBacked property to 0.