Line Widths for Various Characteristic Impedance of Center Stripline Devices in RT duroid Laminates
In designing stripline circuits for high-frequency applications, maintaining the desired characteristic impedance (Z0Z_0Z0) is crucial for signal integrity. The geometry of the trace, including line width, separation from ground planes, and material properties, plays a significant role in determining Z0Z_0Z0. RT-duroid® laminates, with their excellent dielectric properties, are commonly used for such applications.
Key Parameters Affecting Impedance
- Dielectric Constant (DkD_kDk):
- RT-duroid 6000 series laminates typically have DkD_kDk values between 2.94 and 10.2, depending on the specific material.
- Trace Dimensions:
- Line width (WWW) and thickness (TTT).
- Substrate Thickness (HHH):
- Distance between the trace and ground planes.
- Ground Plane Spacing:
- Contributes to the effective dielectric constant.
Impedance Formula for Center Stripline
The characteristic impedance for a center stripline can be approximated using the following formula:Z0=30πDk⋅ln(4H0.67W+T)Z_0 = \frac{30 \pi}{\sqrt{D_k}} \cdot \ln\left(\frac{4H}{0.67W + T}\right)Z0=Dk30π⋅ln(0.67W+T4H)
Where:
- Z0Z_0Z0: Characteristic impedance (Ohms)
- DkD_kDk: Dielectric constant of the laminate
- HHH: Substrate thickness (distance from trace to ground planes, in mm or mils)
- WWW: Trace width (in mm or mils)
- TTT: Trace thickness (in mm or mils)
Design Considerations
Line Width for Impedance Control
- For higher impedance (Z0Z_0Z0), reduce the line width (WWW).
- For lower impedance, increase WWW or decrease HHH.
Laminate Selection
- Use laminates with a lower DkD_kDk (e.g., RT-duroid 6002) for wider traces at the same Z0Z_0Z0.
- Higher DkD_kDk materials (e.g., RT-duroid 6010) are suitable for compact designs.
Tolerances
- Maintain tight control over WWW, HHH, and copper thickness during fabrication.
Sample Line Widths for Common Impedances
| Material | Dielectric Constant (DkD_kDk) | Substrate Thickness (HHH) | Characteristic Impedance (Z0Z_0Z0) | Line Width (WWW) |
|---|---|---|---|---|
| RT-duroid 6002 | 2.94 | 0.020″ (20 mils) | 50 Ohms | ~10.4 mils |
| RT-duroid 6006 | 6.15 | 0.020″ (20 mils) | 50 Ohms | ~6.3 mils |
| RT-duroid 6010 | 10.2 | 0.020″ (20 mils) | 50 Ohms | ~4.5 mils |
| RT-duroid 6002 | 2.94 | 0.020″ (20 mils) | 75 Ohms | ~6.8 mils |
| RT-duroid 6006 | 6.15 | 0.020″ (20 mils) | 75 Ohms | ~4.1 mils |
Simulation and Validation
- Use EM simulation tools (e.g., HFSS, CST) to validate the design.
- Account for manufacturing tolerances and variations in DkD_kDk at operating frequencies.
Accurate calculation of line width is essential for achieving the desired impedance in center stripline designs using RT-duroid® laminates. By following these guidelines and validating with simulations, designers can optimize circuit performance for high-frequency applications.
