Characteristic impedance of a transmission line

Signals propagate on a transmission line as electromagnetic waves with a propagation speed and at any point on the line, there exists a relation between the instantaneous AC voltage and instantaneous AC current are related as follows:

This impedance is referred to as the characteristic impedance of the transmission line. If the transmission line has uniform cross-section along its length, it is called a uniform transmission line.

On a transmission line, whenever a propagating signal encounters a change in characteristics at any point, part of the signal will be reflected, and signal distortion will occur. Therefore, to achieve good signal integrity, it is important to maintain a uniform characteristic impedance.

Controlled impedance of a PCB transmission line

A PCB transmission line comprises a signal trace and its return path — usually the nearest reference plane(s). If the cross-sectional geometry, the placement of the trace, its return path, and the material between them is unchanged throughout the length of the transmission line, then we have a uniform transmission line. A uniform transmission line will have the same characteristic impedance along its length. Due to practical manufacturing considerations, achieving a reasonably uniform transmission line is termed a controlled impedance. The characteristic impedance is also referred to as controlled impedance.

When high-frequency signals propagate on transmission lines, uniform impedance is critical to achieve high-quality signal transmission without distortion. Typically, you will need controlled impedance lines for clocks, DDR signals, USB-HS, SATA, PCIe, HDMI, Thunderbolt, and high-frequency RF/microwave applications.

Parameters required to calculate trace impedance

The impedance of circuit board traces is determined by:

• Trace width and thickness
• Height of the dielectric layer between the signal trace and the reference planes
• Dielectric constant(s) of the dielectric material used in the board
• Spacing between differential pair traces

Types of impedance models

PCBs typically use two types of transmission line structures: microstrips and striplines. There are also variations within each type – uncoated, coated, and embedded. Each of the transmission lines consists of a signal trace and one or more reference planes. For each model, the following combinations are available:

Our impedance calculator features two main types of impedance models, single-ended and differential, within each trace structure.

• There are 3 types of single-ended models: single-ended non-coplanar, coplanar single-ended, and coplanar single-ended without ground.
• There are also 3 types of differential models – differential, coplanar differential, and coplanar differential without ground.

List of our impedance calculators

Following is the table of 82 impedance calculators that are designed and categorized based on the geometry of the trace structure.

Single-ended

Differential pair

Coplanar single-ended

Coplanar differential pair

Coplanar single-ended without ground plane

Coplanar differential pair without ground plane

Features of Impedance Calculator

How to use the tool

The Impedance Calculator evaluates single-ended and differential impedance for various impedance models. The tool also calculates signal losses of any given trace.

Calculation of impedance and trace width

After selecting the relevant impedance model, you’ll have to key in the input parameters. For instance, for a single-ended model, the input parameters are:

Dielectric information:

• Dielectric height (H1)
• Dielectric constant (Er1)

Trace information:

• Trace width (W)/Targeted impedance
• Δw (W-W1)

The table below helps you choose the right Δw value for different copper weights. It also provides guidelines for minimum trace widths for inner and outer layers.

• Trace thickness (T)

Once all the input parameters are entered, hit calculate to view the results.

The output parameters are:

• Target SE impedance/Trace width
• Calculated SE impedance
• Propagation delay
• Inductance
• Capacitance
• Effective dielectric constant

Please note that the input parameters might change depending on the impedance model you choose.

If you require assistance with the Dk and Df value of the material, click on Show PCB Dielectric Material Construction table. This table shows the Dk and Df of the selected material for different thicknesses and resin contents.

If you’d like to update the dielectric height and the corresponding dielectric constant, click on the update icon next to the dielectric height field.

The units (mils, inch, mm, cm, μm) can be changed as per your requirement.

Computing signal loss of an impedance model

To calculate the signal loss for the chosen impedance model, you need to click on the Signal Loss Calculator tab. Please note that signal loss calculations use the dielectric and trace information entered in the impedance calculator. You won’t be able to compute signal loss without performing an impedance calculation.

The input parameters are:

• Dissipation factor
• Frequency of the signal
• Surface roughness of copper foil
• Length of the trace

As mentioned earlier, the input parameters are subject to change depending on the chosen impedance model.

Once all the input parameters are entered, click on Calculate Loss.

Now, let’s enter the dissipation factor as 0.025, the frequency as 10 GHz, the surface roughness as 6 μm, and the length of the trace as 1 inch.

The values of conductor loss, dielectric loss, insertion loss, and total insertion loss will be displayed as shown in the image above. If you require additional information on any of these input/output parameters, click on the corresponding parameter name.

How to calculate near-end and far-end crosstalk

Based on the measurement zone, crosstalk can be classified into:

• Near-end crosstalk (NEXT): Noise on the driver end of the victim line
• Far-end crosstalk (FEXT): Noise on the receiver side of the victim line

To calculate them for your design, you need to provide the values of:

• Coupled trace length
• Signal rise time
• Signal voltage

Let’s enter the values as 1 inch, 100 picoseconds, and 2 volts for these parameters, respectively, and hit calculate crosstalk.

The tool displays the values of:

• Near and far-end crosstalk
• Near and far-end voltage
• Saturation length in inches
• FEXT co-efficient

Designing an appropriate impedance model to route your high-speed traces is essential to avoid signal integrity issues. Our impedance calculator will give you the right impedance values based on your dielectric and conductor attributes.