interdigitalCapacitor

Create basic interdigital capacitor

Since R2021b

Description

Use the interdigitalCapacitor object to create an interdigital planar capacitor (IDC). IDCs are used in high frequency applications such as:

• Receiver circuits where antenna radiators are connected to RF

• Wireless data communications with RFID

• Humidity and solution concentration measurements

• Lab-on-chip devices (LOCs)

Note

This PCB object supports behavioral modeling. For more information, see Behavioral Models. To analyze the behavioral model for an interdigital capacitor, set the Behavioral property in the sparameters function to true or 1

A two-port series IDC with microstrip form feeder lines supports single and multiple dielectrics. It is a coplanar structure consisting of multiple comb electrodes or intersecting fingers with spaces between the fingers. An IDC can have identical port line lengths and widths on either sides.

Creation

Syntax

capacitor = interdigitalCapacitor

capacitor = interdigitalCapacitor(Name=Value)

Description

capacitor = interdigitalCapacitor creates a basic interdigital capacitor with default property values for an operating bandwidth of 3.6-4 GHz.

capacitor = interdigitalCapacitor(Name=Value) sets Properties using one or more name-value arguments. For example, interdigitalCapacitor(NumFingers=10) creates an interdigital capacitor with 10 fingers. Properties not specified retain their default values.

Properties

NumFingers — Number of fingers on capacitor

4 (default) | positive scalar

Number of fingers on the capacitor, specified as a positive scalar.

Example: capacitor = interdigitalCapacitor(NumFingers=10)

Data Types: double

FingerLength — Length of overlapping fingers

0.0137 (default) | positive scalar

Length of the overlapping fingers in meters, specified as a positive scalar.

Example: capacitor = interdigitalCapacitor(FingerLength=0.0217)

Data Types: double

FingerWidth — Width of overlapping fingers

3.1600e-04 (default) | positive scalar

Width of the overlapping fingers in meters, specified as a positive scalar.

Example: capacitor = interdigitalCapacitor(FingerWidth=4.8000e-04)

Data Types: double

FingerSpacing — Distance between fingers

3.0000e-04 (default) | positive scalar

Distance between the fingers in meters, specified as a positive scalar.

Example: capacitor = interdigitalCapacitor(FingerSpacing=2.9000e-04)

Data Types: double

FingerEdgeGap — Gap between edges of fingers

3.4100e-04 (default) | positive scalar

Gap between the edges of the fingers in meters, specified as a positive scalar.

Example: capacitor = interdigitalCapacitor(FingerEdgeGap=2.05000e-04)

Data Types: double

TerminalStripWidth — Width of terminals

5.0000e-04 (default) | positive scalar

Width of the terminals in meters, specified as a positive scalar.

Example: capacitor = interdigitalCapacitor(TerminalStripWidth=4.9000e-04)

Data Types: double

PortLineWidth — Width of ports

0.0019 (default) | positive scalar

Width of the ports in meters, specified as a positive scalar.

Example: capacitor = interdigitalCapacitor(PortLineWidth=0.0020)

Data Types: double

PortLineLength — Length of ports

0.0030 (default) | positive scalar

Length of the ports in meters, specified as a positive scalar.

Example: capacitor = interdigitalCapacitor(PortLineLength=0.0040)

Data Types: double

Height — Height from ground plane to capacitor

7.8700e-04 (default) | positive scalar

Height from the capacitor to the ground plane in meters, specified as a positive scalar.

Example: capacitor = interdigitalCapacitor(Height=6.9000e-04)

Data Types: double

GroundPlaneWidth — Width of ground plane

0.0030 (default) | positive scalar

Width of the ground plane in meters, specified as a positive scalar.

Example: capacitor = interdigitalCapacitor(GroundPlaneWidth=0.0040)

Example: double

Substrate — Type of dielectric material

dielectric object

Type of dielectric material used as a substrate, specified as a dielectric object. The dielectric material in an interdigitalCapacitor with default properties have the following properties:

• Name—{'Roger'}

• EpsilonR—3.2

• LossTangent—0.0002

• Thickness—0.000787

Example: d = dielectric("FR4"); capacitor = interdigitalCapacitor(Substrate=d)

Data Types: string | char

Conductor — Type of metal used in conducting layers

metal object

Type of metal used in the conducting layers, specified as a metal object. he type of metal in a couplerBranchlineWideband object with default properties is Copper.

Example: m = metal("PEC"); capacitor =interdigitalCapacitor(Conductor=m)

Data Types: string | char

IsShielded — Flag to add metal shielding

false or 0 (default) | true or 1

Flag to add a metal shielding to the PCB component, specified as a logical 0 or logical 1. The default value is logical 0.

Example: IsShielded = true or 1 add a metal shield.

Note

To enable FEM solver required for the metal shield property, download the Integro-Differential Modeling Framework for MATLAB. To download this add-on:

1. In the Home tab Environment section, click on Add-Ons. This opens the add-on explorer. You need an active internet connection to download the add-on.

2. Search for Integro-Differential Modeling Framework for MATLAB and click Install.

3. To verify if the download is successful, run

   matlab.addons.installedAddons

   in your MATLAB^® session command line.

4. On Windows, to run the IDMF add-on, you must install the Windows Subsystem for Linux (WSL). To install WSL, see Install Linux on Windows with WSL.

   The Windows Defender Firewall can block the PostgreSQL server when using the IDMF add-on. To resolve this issue, you can allow the server to communicate on desired networks if the firewall prompts. Alternatively, you can manually add the executable file of the PostgreSQL server located in <matlabroot>\sys\postgresql\win64\PostgreSQL\bin\postgres.exe. For more information, see https://support.microsoft.com/en-us/windows/risks-of-allowing-apps-through-windows-defender-firewall-654559af-3f54-3dcf-349f-71ccd90bcc5c.

Data Types: logical

Shielding — Metal shielding

shape.Box object

This property is read-only.

Metal shield for the PCB component, specified as a shape.Box object. The length and width of the box must be equal to the length and width of the ground plane. The center of the box is at [0 0 Shielding.Height].

Dependencies

To enable the Shielding property, set the IsShielded property to true or 1.

Connector — Type of RF connector

RFConnector object

Type of RF connector assembled at the feed locations of the PCB component, specified as a RFConnector object.

Example: Create connector from RFConnector object like this: coaxial = RFConnector adds a coaxial connector.

Dependencies

To enable the Connector property, set the IsShielded property to true or 1.

Object Functions

capacitance	Calculate capacitance
charge	Calculate and plot charge distribution
current	Calculate and plot current distribution
feedCurrent	Calculate current at feed port
getZ0	Calculate characteristic impedance of transmission line
layout	Plot all metal layers and board shape
mesh	Change and view mesh properties of metal or dielectric in PCB component
shapes	Extract all metal layer shapes of PCB component
show	Display PCB component structure or PCB shape
sparameters	Calculate S-parameters for RF PCB objects
RFConnector	Create RF connector

Examples

Create Default Interdigital Capacitor

Create and view a default interdigital capacitor.

idcapacitor = interdigitalCapacitor

idcapacitor = 
  interdigitalCapacitor with properties:

            NumFingers: 4
          FingerLength: 0.0137
           FingerWidth: 3.1600e-04
         FingerSpacing: 3.0000e-04
         FingerEdgeGap: 3.4100e-04
    TerminalStripWidth: 5.0000e-04
         PortLineWidth: 0.0019
        PortLineLength: 0.0030
                Height: 7.8700e-04
      GroundPlaneWidth: 0.0030
             Substrate: [1x1 dielectric]
             Conductor: [1x1 metal]
            IsShielded: 0

show(idcapacitor)

Multilayer Interdigital Capacitor

Create and view a multilayer interdigital capacitor with two different dielectrics.

idcapacitor = interdigitalCapacitor;
sub = dielectric("FR4","Teflon"); 
sub.Thickness =[0.00003 0.00003];
idcapacitor.Substrate = sub;
idcapacitor.Height = 0.00003;
show(idcapacitor);

Analyze Interdigital Capacitor Using Behavioral S-parameters

Create an interdigital capacitor using gold as the conductor.

capacitor = interdigitalCapacitor;
capacitor.Conductor = metal("Gold");
show(capacitor)

Compute and plot the behavioral S-parameters of the capacitor at 3.8 GHz.

spar = sparameters(capacitor,3.8e9,Behavioral=true);
rfplot(spar)

More About

Parametric Analysis Guidelines

• The capacitance of an interdigital capacitor is directly proportional to its physical parameters such as NumFingers, FingerLength, FingerWidth, and FingerSpacing.

• Increasing the height of the dielectric decreases the parasitic capacitance.

To design the capacitor at high frequency consider the following assumptions:

• Increasing the number of fingers creates a periodic and smooth structure.

• Capacitor dimensions should be much smaller than the quarter wavelength.

References

[1] Pozar, David M. Microwave Engineering. 4th ed. Hoboken, NJ: Wiley, 2012.

[2] Jungreuthmayer, Christian, Gerald M. Birnbaumer, Peter Ertl, and Jürgen Zanghellini. “Improving the Measurement Sensitivity of Interdigital Dielectric Capacitors (IDC) by Optimizing the Dielectric Property of the Homogeneous Passivation Layer.” Sensors and Actuators B: Chemical 162, no. 1 (February 2012): 418–24. https://doi.org/10.1016/j.snb.2011.12.009.

[3] Ruppin, R. “Surface Polaritons of a Left-Handed Material Slab.” Journal of Physics: Condensed Matter 13, no. 9 (March 5, 2001): 1811–18. https://doi.org/10.1088/0953-8984/13/9/304.

[4] Caloz, Christophe, and Tatsuo Itoh. Electromagnetic Metamaterials: Transmission Line Theory and Microwave Applications: The Engineering Approach. Hoboken, NJ, USA: John Wiley & Sons, Inc., 2005. https://doi.org/10.1002/0471754323.

Version History

Introduced in R2021b

R2023b: Metal shielding for interdigital capacitor

Use the IsShielded, Shielding, and Connector properties to add a metal shield to the interDigitalCapacitor object.