signal integrity

Stripline Impedance

Calculate the characteristic impedance, capacitance, propagation delay, and inductance of a PCB stripline.

PCB stripline refers to a type of trace/transmission line routed in the inner layers of a PCB. It is enclosed by a single material — usually the PCB substrate — and is most commonly found in multilayer PCB designs where the signal trace has ground planes both above and below it. This arrangement keeps high-frequency signals inside the PCB, leading to fewer emissions and shielding against incoming signals.

This calculator determines the characteristic impedance of a symmetric stripline in ohms by entering the relative permittivity, trace height, trace thickness, and trace width. It also provides associated capacitance, propagation delay, and inductance.

Understanding Symmetric Stripline Impedance

Symmetric Stripline Configuration

A flat conductor is positioned symmetrically between two ground planes — with equal spacing between the conductor and each ground plane. This symmetrical arrangement ensures uniform propagation of signals to the reference planes.

Key Design Factors

Factor	Effect
Dielectric Constant ( $e$ )	Influences both the characteristic impedance and signal propagation speed
Conductor Width ( $w$ )	Wider traces generally result in lower characteristic impedance
Trace Thickness ( $t$ )	Affects the impedance calculation alongside trace width
Distance to Ground Planes ( $h$ )	Greater spacing from the ground planes increases the characteristic impedance

Applications

Symmetric striplines are frequently used in:

High-Speed Digital Circuits — where signal integrity is critical
High-Frequency Analog Circuits — requiring balanced transmission with low radiation
Microwave Circuits — where controlled impedance and low crosstalk are essential

Design Considerations

When designing a symmetric stripline, it is crucial to account for:

The dielectric constant of the substrate material
The conductor thickness and trace width
The separation between the conductor and the ground planes

These elements directly impact the characteristic impedance and overall circuit performance.

Calculators and Tools

A variety of calculators and tools are available to help designers determine the characteristic impedance of a symmetric stripline. These tools use mathematical formulas and models based on user-provided input parameters — such as Altium Designer's integrated layer stack editor, which supports both coplanar and standard stripline configurations.

Conclusion

Symmetric stripline impedance is a critical concept in the design of high-speed digital, high-frequency analog, and microwave circuits. Understanding the factors that determine characteristic impedance — combined with the right tools and calculators — helps designers create dependable, high-performing circuits.

Formulas

$Z_0 = \frac{60}{\sqrt{e}} \times \ln\left(\frac{1.92 \times (2h + t)}{0.8w + t}\right)$

$C = \frac{5.55 \times 10^{-11} \times e}{\ln\left(\frac{3.81h}{0.8w + t}\right)}$

$P = 3.34 \times 10^{-9} \times \sqrt{e}$

$L = C \times Z_0^2$

where:

$Z_0$ = Characteristic Impedance (Ω)
$e$ = Relative Permittivity of the Dielectric (Substrate Dielectric)
$h$ = Height of Trace (mil or mm)
$w$ = Width of the Trace (mil or mm)
$t$ = Trace Thickness (mil or mm)
$C$ = Capacitance per Unit Length (F/m)
$P$ = Effective Propagation Delay (s/m)
$L$ = Inductance per Unit Length (H/m)

Inputs

Relative Permittivity of the Dielectric

Relative permittivity (dielectric constant) of the PCB substrate — FR4 ≈ 4.5

Height of Trace(mm)

Distance from trace centre to the nearest ground plane in millimetres

Trace Thickness(mm)

Thickness of the copper trace in millimetres

Width of the Trace(mm)

Width of the copper trace in millimetres

Results

⚠Relative permittivity must be greater than zero

Characteristic Impedance—ΩCharacteristic impedance of the stripline in ohms

Capacitance Per Unit Length—F/mDistributed capacitance per unit length in farads per metre

Effective Propagation Delay—s/mPropagation delay per unit length in seconds per metre

Inductance Per Unit Length—H/mDistributed inductance per unit length in henries per metre