Striplines are essential in microwave and high-frequency circuit design due to their ability to provide controlled impedance and low loss. Broadside and edgewise configurations offer different advantages in terms of compactness, coupling, and isolation. This guide provides key design equations for both types.

1. Broadside Stripline

A broadside stripline consists of two conductors placed on parallel planes and sandwiched between two ground planes. The conductors are typically aligned to maximize coupling for differential or tightly coupled lines.

Key Parameters:

• h: Distance between the ground planes.
• s: Spacing between the two conductors.
• t: Thickness of the conductors.
• ε_r: Relative permittivity of the substrate.

1.1 Impedance (Single-Ended):

For a single-ended broadside stripline:Z0=60ϵrln⁡(4hw+t)Z_0 = \frac{60}{\sqrt{\epsilon_r}} \ln\left(\frac{4h}{w+t}\right)Z0​=ϵr​​60​ln(w+t4h​)

Where www is the width of the stripline.

1.2 Differential Impedance:

The differential impedance is influenced by the spacing sss between the strips. An approximate formula is:Zdiff=2Z0(1−k)Z_{diff} = 2Z_0 \left(1 – k\right)Zdiff​=2Z0​(1−k)

Where kkk is the coupling coefficient, given by:k=Zodd−ZevenZodd+Zevenk = \frac{Z_{odd} – Z_{even}}{Z_{odd} + Z_{even}}k=Zodd​+Zeven​Zodd​−Zeven​​

1.3 Even/Odd Mode Impedances:

• Even Mode Impedance: Zeven≈Z0×(1+k)Z_{even} \approx Z_0 \times \left(1 + k\right)Zeven​≈Z0​×(1+k)
• Odd Mode Impedance: Zodd≈Z0×(1−k)Z_{odd} \approx Z_0 \times \left(1 – k\right)Zodd​≈Z0​×(1−k)

2. Edgewise Stripline

In edgewise striplines, the conductors are oriented vertically, aligned edge-to-edge between the ground planes. This arrangement minimizes the footprint while providing effective coupling.

Key Parameters:

• h: Distance between ground planes.
• d: Edge-to-edge spacing between conductors.
• t: Thickness of the conductors.
• ε_r: Relative permittivity of the substrate.

2.1 Impedance (Single-Ended):

For single-ended edgewise striplines, an approximate formula is:Z0=87ϵrln⁡(4hd)Z_0 = \frac{87}{\sqrt{\epsilon_r}} \ln\left(\frac{4h}{d}\right)Z0​=ϵr​​87​ln(d4h​)

2.2 Differential Impedance:

For differential operation:Zdiff=2Z0(1−k)Z_{diff} = 2Z_0 \left(1 – k\right)Zdiff​=2Z0​(1−k)

Where kkk is the coupling coefficient, similar to broadside striplines.

2.3 Coupling Coefficient:

For edgewise configurations, kkk depends more strongly on the edge spacing ddd:k=ln⁡(2hd)ln⁡(2ht)k = \frac{\ln\left(\frac{2h}{d}\right)}{\ln\left(\frac{2h}{t}\right)}k=ln(t2h​)ln(d2h​)​

3. Design Notes and Considerations

1. Substrate Effects: Higher relative permittivity (εrε_rεr​) reduces impedance, requiring wider strips for the same target Z0Z_0Z0​.
2. Conductor Thickness: For high-frequency designs, consider skin effect and current crowding due to finite thickness.
3. Edge Coupling: Edgewise striplines have inherently stronger coupling for differential pairs due to their vertical alignment.
4. Fabrication Tolerances: Broadside striplines are more sensitive to alignment errors compared to edgewise designs.
5. Loss Minimization: Choose substrates with low loss tangent (tanδtan\deltatanδ) and optimize conductor dimensions to minimize resistive and dielectric losses.

4. Application Examples

• Broadside Striplines: Used in high-speed differential signaling due to balanced impedance and reduced crosstalk.
• Edgewise Striplines: Preferred in compact designs where PCB real estate is limited and coupling needs to be tightly controlled.