Striplines are transmission lines embedded within a dielectric medium and are widely used in high-frequency circuits for their excellent signal integrity and predictable impedance. Broadside and edgewise striplines are two common configurations, differing in how the signal and ground planes are arranged. Understanding the design equations is crucial for impedance control and circuit optimization.
1. Broadside Stripline
Configuration
- Consists of two signal conductors placed parallel and vertically stacked (broadside) between two ground planes.
- Used for differential signaling due to its symmetry and reduced crosstalk.
Key Parameters
- WWW: Width of the signal conductor.
- HHH: Distance between the ground planes.
- SSS: Separation between the two signal conductors.
- ttt: Thickness of the signal conductors.
- ϵr\epsilon_rϵr: Relative dielectric constant of the medium.
Impedance (Differential and Single-Ended)
Differential Impedance (ZdiffZ_{diff}Zdiff)
Zdiff=2⋅Z01−(2CmC0)2Z_{diff} = \frac{2 \cdot Z_0}{\sqrt{1 – \left(\frac{2C_m}{C_0}\right)^2}}Zdiff=1−(C02Cm)22⋅Z0
Where:
- Z0Z_0Z0: Impedance of a single stripline.
- CmC_mCm: Mutual capacitance between the two signal conductors.
- C0C_0C0: Capacitance of one signal conductor to ground.
Single-Ended Impedance (Z0Z_0Z0)
For a single conductor:Z0=60ϵrln(4Ht)Z_0 = \frac{60}{\sqrt{\epsilon_r}} \ln\left(\frac{4H}{t}\right)Z0=ϵr60ln(t4H)
Capacitance
- Capacitance per unit length (CCC) can be derived using:
C=ϵrϵ0WHC = \frac{\epsilon_r \epsilon_0 W}{H}C=Hϵrϵ0W
2. Edgewise Stripline
Configuration
- Signal conductors are aligned horizontally side by side (edgewise) between two ground planes.
- Commonly used for multi-conductor systems requiring tightly controlled spacing.
Key Parameters
- WWW: Width of the signal conductor.
- HHH: Height between the ground planes.
- SSS: Edge-to-edge spacing between signal conductors.
- ttt: Thickness of the conductors.
- ϵr\epsilon_rϵr: Relative dielectric constant of the medium.
Impedance (Differential and Single-Ended)
Differential Impedance (ZdiffZ_{diff}Zdiff)
Zdiff=120⋅ϵrln(2HS)Z_{diff} = \frac{120 \cdot \sqrt{\epsilon_r}}{\ln\left(\frac{2H}{S}\right)}Zdiff=ln(S2H)120⋅ϵr
Single-Ended Impedance (Z0Z_0Z0)
For a single conductor:Z0=87ϵr+1.41ln(5.98H0.8W+t)Z_0 = \frac{87}{\sqrt{\epsilon_r + 1.41}} \ln\left(\frac{5.98H}{0.8W + t}\right)Z0=ϵr+1.4187ln(0.8W+t5.98H)
Capacitance
- Per unit length:
C=ϵrϵ0Z0C = \frac{\epsilon_r \epsilon_0}{Z_0}C=Z0ϵrϵ0
3. Comparison: Broadside vs. Edgewise
| Aspect | Broadside Stripline | Edgewise Stripline |
|---|---|---|
| Signal Alignment | Vertical, stacked conductors | Horizontal, side-by-side conductors |
| Crosstalk | Lower due to symmetry | Higher, requires tighter spacing control |
| Differential Use | Ideal for differential signals with minimal skew | Can be used, but less optimal than broadside |
| Impedance Control | More challenging due to mutual coupling between layers | Easier, with simpler edge spacing |
4. Practical Considerations
a. Dielectric Material Selection
- Use low-loss dielectric materials like RT-duroid® 5870 for high-frequency applications to maintain signal integrity.
b. Manufacturing Tolerances
- Ensure tight control over WWW, HHH, SSS, and ttt to achieve the desired impedance.
c. Simulation and Validation
- Use electromagnetic simulation tools (e.g., HFSS, CST) to validate design equations against real-world effects like fringing fields and conductor losses.
