RF PCB Design-Part 3: Transmission Line Design

Transmission line design is fundamental in RF PCB design for maintaining signal integrity and minimizing power loss at high frequencies. A transmission line is a structure that carries RF signals efficiently from one point to another, typically between a source and load, such as an antenna, amplifier, or other RF components.

---

1. Key Types of Transmission Lines

a. Microstrip Line

• A trace on the top layer of the PCB, with a ground plane beneath it.
• Characteristics:
  • Easy to fabricate and widely used.
  • Impedance depends on the trace width, substrate thickness, and dielectric constant.
  • Less shielding than other types, susceptible to external noise.
• Applications:
  • Low-cost and standard RF designs up to several GHz.

b. Stripline

• A trace sandwiched between two ground planes in a multilayer PCB.
• Characteristics:
  • Superior shielding compared to microstrip.
  • More complex to manufacture and costly.
  • Symmetric electric fields around the trace lead to lower radiation losses.
• Applications:
  • High-frequency designs requiring minimal noise and interference.

c. Coplanar Waveguide (CPW)

• A trace on the same layer as ground planes, separated by gaps.
• Types:
  • With ground beneath (grounded CPW).
  • Without ground beneath.
• Characteristics:
  • Allows better impedance control than microstrip.
  • Offers reduced crosstalk between adjacent traces.
• Applications:
  • High-frequency and mixed-signal designs.

d. Coaxial Lines

• Used primarily for off-board RF connections.
• Characteristics:
  • Excellent shielding and impedance control.
  • Not commonly implemented on PCBs but interfaces with them.
• Applications:
  • Antenna feeds and RF test connections.

---

2. Parameters That Affect Transmission Line Design

Characteristic Impedance (Z0)

• Determines how the transmission line matches the source and load.
• Calculated based on the physical dimensions of the trace and substrate properties.
• Typical values are 50Ω (most common in RF) or 75 Ω (used in video systems).

Propagation Delay

• The time it takes for the signal to travel through the transmission line.
• Depends on the dielectric constant of the substrate and trace length.

Signal Attenuation

• Caused by resistive, dielectric, and radiation losses.
• Minimized by choosing low-loss materials and optimal trace dimensions.

Return Loss

• A measure of reflection caused by impedance mismatches.
• Higher return loss values (in dB) indicate better matching.

---

3. Microstrip Transmission Line Design

The characteristic impedance (Z0) of a microstrip is calculated using the following approximation:

$$Z_0 = \frac{87}{\sqrt{D_k + 1.41}} \ln \left( \frac{5.98h}{0.8W + T} \right)$$

Where:

• $Z_0$
  = Impedance in ohms.
• $D_k$
  = Dielectric constant of the substrate.
• $h$
  = Height of the substrate (distance from trace to ground plane).
• $W$
  = Width of the trace.
• $T$
  = Thickness of the copper trace.

---

4. Stripline Transmission Line Design

For a stripline, the characteristic impedance is given by:

$$Z_0 = \frac{60}{\sqrt{D_k}} \ln \left( \frac{4H}{0.67 \pi (W + T)} \right)$$

Where:

• $\mathrm{<li><span\; style="font-family:\; "Times\; New\; Roman";\; font-size:\; medium;\; text-align:\; justify;\; text-transform:\; none;">H=Distance\; between\; the\; two\; ground\; planes.</span></li><li><span\; style="font-family:\; "Times\; New\; Roman";\; font-size:\; medium;\; text-align:\; justify;\; text-transform:\; none;">Other\; parameters\; as\; defined\; above.</span></li>}$

---

5. Coplanar Waveguide Design

For a grounded coplanar waveguide, the impedance is approximated as:

$$Z_0 = \frac{30 \pi}{\sqrt{D_k}} \ln \left( \frac{1 + \frac{4h}{\pi (W + 2G)}}{1 - \frac{4h}{\pi (W + 2G)}} \right)$$

Where:

• $G$
  = Gap between the trace and the coplanar ground.
• Other parameters as defined above.

---

6. Design Steps for Transmission Lines

1. Determine Impedance Requirements:
   • Decide on Z0 based on the RF system (usually 50Ω or 75Ω).
2. Choose Substrate Material:
   • Consider Dk, tanδ and thickness.
3. Calculate Trace Dimensions:
   • Use equations or PCB design software to determine trace width, spacing, and thickness.
4. Minimize Losses:
   • Use low-loss materials and avoid sharp bends or via transitions.
5. Simulate the Design:
   • Tools like ANSYS HFSS, CST Studio, or ADS to verify impedance and performance.
6. Fabrication Considerations:
   • Account for manufacturing tolerances in trace width and thickness.
7. Testing and Tuning:
   • Measure with a vector network analyzer (VNA) and adjust for mismatches.

---

7. Common Challenges in Transmission Line Design

• Impedance Variations:
  • Ensure consistent trace geometry and dielectric properties.
• Crosstalk:
  • Maintain sufficient spacing between adjacent transmission lines.
• Losses:
  • Minimize resistive and dielectric losses by choosing suitable materials.
• Via Discontinuities:
  • Avoid or carefully design vias in RF paths.

---

8. Practical Tips

• Trace Bends:
  • Use smooth, curved bends or mitered corners to reduce impedance discontinuities.
• Via Usage:
  • Minimize vias; if unavoidable, use via stitching around the trace.
• Ground Planes:
  • Ensure a continuous ground plane below microstrip lines or adjacent to coplanar waveguides.
• Simulation:
  • Simulate all high-frequency traces for impedance and loss verification.

---