Millimeter-wave frequencies, ranging from 30 GHz to 300 GHz, present enormous bandwidth potential that cannot be overlooked. As technologies evolve, particularly in 5G wireless networks and Advanced Driver-Assistance Systems (ADAS) in vehicles, engineers face the challenge of designing and manufacturing circuits that work efficiently at these high frequencies. In the first part of this two-part blog series, we discussed how printed circuit boards (PCBs) can be adapted for millimeter-wave frequencies by considering the material properties and adapting circuit designs from the microwave range. This second part will delve deeper into the specific circuit technologies and materials used at both microwave and millimeter-wave frequencies.
Transmission Line Technologies for Millimeter-Wave Circuits
Signals in the millimeter-wave frequency range (30 GHz to 300 GHz) require careful consideration of the transmission lines used to guide the electromagnetic waves. At these frequencies, traditional technologies such as microstrip, stripline, substrate integrated waveguide (SIW), and grounded coplanar waveguide (GCPW) are employed. These technologies are not only common in microwave circuits but are increasingly being utilized for millimeter-wave applications, though some are more suitable than others depending on the specific needs of the design.
Microstrip Technology
Microstrip is one of the simplest and most common transmission-line technologies used in microwave circuits due to its ease of fabrication and good yield. However, when transitioning to millimeter-wave frequencies, microstrip faces significant challenges.
One key issue is radiation losses. At higher frequencies, microstrip circuits tend to behave like antennas, radiating energy into the air surrounding the circuit. This results in unwanted signal loss that increases as the frequency rises. Additionally, microstrip circuits require precise fabrication, with strict tolerances needed for conductor widths and copper thicknesses. As frequency increases, the tolerance requirements become even tighter, and small variations in fabrication can cause significant performance issues.
Another challenge is the behavior of electromagnetic (EM) waves in microstrip circuits. The waves propagate not only through the circuit material but also through the surrounding air, which has a lower dielectric constant (Dk). This low Dk of air affects the effective Dk of the entire circuit and must be accounted for when modeling the circuit. At millimeter-wave frequencies, circuit materials with a lower Dk are preferred to minimize signal loss, but this can result in slower wave propagation and phase shifts.
Stripline Technology
Stripline is another reliable circuit technology capable of operating at millimeter-wave frequencies, offering superior isolation since the conductors are fully enclosed by dielectric material and ground planes. This design ensures that the EM waves propagate entirely within the circuit material, without interaction with the surrounding air. However, the challenge with stripline is that it can be difficult to launch signals into the circuit due to the enclosed structure.
Creating connectors for signal input and output becomes more challenging, especially at millimeter-wave frequencies. Additionally, the technology is sensitive to fabrication process variations, and achieving the necessary tolerances can be difficult. For these reasons, stripline is less commonly used for millimeter-wave circuits, except for specific applications such as automotive radar systems.
Substrate Integrated Waveguide (SIW)
Substrate Integrated Waveguide (SIW) technology is gaining popularity for millimeter-wave applications, particularly in automotive radar and other communication systems. SIW combines the best features of waveguide technology and printed circuit board (PCB) fabrication. It uses a top metal layer and a bottom ground plane with rows of plated through-holes (PTHs) to form a compact rectangular waveguide. This design allows for low-loss signal propagation even at high frequencies.
However, the precision required in manufacturing SIW circuits is significant. The PTHs must be placed within very tight tolerances, especially for higher frequencies, making the fabrication process challenging. Additionally, SIW requires materials with minimal variations in dielectric constant (Dk), further complicating the manufacturing process.
Grounded Coplanar Waveguide (GCPW)
Grounded Coplanar Waveguide (GCPW) is another promising transmission-line technology for millimeter-wave circuits. GCPW structures combine both dielectric and copper conductors to achieve low-loss signal propagation. They are particularly well-suited for broadband RF, microwave, and millimeter-wave applications, such as test and measurement systems. GCPW is also used in integrated designs where both millimeter-wave and lower-frequency circuits are required on the same PCB.
However, GCPW circuits are sensitive to fabrication process variations, such as variations in dielectric material Dk, substrate thickness, and copper surface roughness. These factors can cause phase distortion, which becomes more critical at millimeter-wave frequencies. To ensure optimal performance, tight control over fabrication processes is necessary, including maintaining precise conductor widths and thicknesses.
Key Considerations for Millimeter-Wave Circuit Design
As millimeter-wave circuit applications such as automotive radar and 5G wireless networks continue to grow, designers must consider several key factors when selecting circuit materials and transmission-line technologies. These include:
- Fabrication Tolerances: Millimeter-wave circuits require extremely tight tolerances for conductor widths, dielectric thickness, and copper surface quality.
- Signal Integrity: Factors like radiation losses, phase distortion, and material Dk variations must be minimized to ensure reliable performance at high frequencies.
- Material Selection: The choice of PCB material plays a crucial role in millimeter-wave circuit performance. Low-Dk materials are preferred to reduce signal loss, but the material’s properties must remain stable at high frequencies.
Conclusion
Designing circuits for millimeter-wave frequencies presents unique challenges but also offers tremendous opportunities in emerging applications like 5G networks and ADAS. Understanding the strengths and limitations of different transmission-line technologies such as microstrip, stripline, SIW, and GCPW is essential for making informed decisions when transitioning from microwave to millimeter-wave designs.