Concerning PCBs and the Transition from Microwaves to Millimeter Waves
Millimeter-wave frequencies offer too much bandwidth to ignore. As a result, circuit developers for many emerging applications, such as 5G wireless cellular networks and ADAS vehicles, are faced with designing and producing practical circuit solutions from 30 to 300 GHz. As readers learned in the first part of this two-part ROG blog,1 printed circuit boards (PCBs) can be made for millimeter-wave frequencies by considering circuit-material characteristics when making the transition upward from microwave to millimeter-wave circuits. This second part of the blog explores how different circuit technologies often used at microwave frequencies and different circuit materials handle higher-frequency, millimeter-wave circuits.
Millimeter-wave signals from 30 to 300 GHz and even “down” to 24 GHz in automotive radars propagate through different circuit functions via different transmission-line technologies, such as microstrip, stripline, substrate integrated waveguide (SIW), and grounded coplanar waveguide (GCPW). These transmission-line technologies (Fig. 1) are used at microwave frequencies and at times at millimeter-wave frequencies, some more than less, with circuit laminates specified for desired performance at such high frequencies.2 Microstrip, the simplest and most common microwave circuit technology, typically provides high circuit yields with standard circuit fabrication processes. But it may not be the best circuit technology at millimeter-wave frequencies. Each circuit technology has positive and negative aspects and may sacrifice one goal to achieve another. Microstrip, for example, for its ease of fabrication, must overcome typically high radiation losses when used at millimeter-wave frequencies.
Figure 1. Microwave circuit designers are faced with a choice of at least four transmission-line technologies used at microwave frequencies when they make the transition to millimeter-wave frequencies.
Microstrip’s open structure provides easy physical access to the circuitry but also can cause problems at higher frequencies. In microstrip transmission lines, electromagnetic (EM) waves propagate through the circuit material’s conductors and dielectric substrate but also in part through the air around them. The low Dk of the air contributes to an effective Dk for the circuit that is lower than the value of the circuit material and must be accounted for when modeling circuits. Circuits fabricated on materials with high Dk values tend to oppose the propagation of EM waves through them compared to the low Dk of the air around them, so circuit materials with low Dk values are typically used for millimeter-wave circuits, where signal energy is typically limited.
Since some of the EM energy is in the air, microstrip circuits radiate outward into the air, with a tendency to behave like antennas. This results in unwanted radiation losses for microstrip circuits, with losses increasing as a function of increasing frequency and challenging millimeter-wave circuit designers working with microstrip to limit the radiation losses, among other circuit losses. The radiation losses can be reduced by fabricating microstrip on circuit materials with higher Dk values, but the rise in Dk will slow the EM wave propagation (relative to air) and result in signal phase shifts. Radiation losses can also be reduced by fabricating microstrip on thinner circuit materials, but thinner circuit materials are more sensitive than thicker materials to the effects of copper surface roughness, including signal phase shifts at millimeter-wave frequencies.
Despite its straightforward configuration, microstrip circuits require exacting tolerances, such as tightly controlled conductor widths, with tolerances growing tighter with increasing frequency. As a result, microstrip is sensitive to fabrication process variations, including the thicknesses of the dielectric and copper in the circuit material, with the tolerance requirements extremely tight for the circuit dimensions required for millimeter-wave circuits.
Stripline is a reliable circuit technology and stripline is capable of good performance through millimeter-wave frequencies but, in contrast to microstrip, the enclosed conductors do not make it easy to attach connectors or other input/output ports to stripline circuits for signal launches. Stripline can be thought of like a flattened coaxial cable, with a conductor surrounded by dielectric layers which are then covered by ground planes. The configuration provides excellent circuit isolation, with propagation that remains within the circuit materials (and not the air around them). The EM waves propagate consistently through the circuit materials and stripline circuits can be modeled according to the characteristics of the circuit materials without having to account for the effects of the EM waves in air. But the enclosed circuit configuration is sensitive to fabrication process variations and the challenge of creating signal launches can make stripline difficult to work with, especially at the smaller connector dimensions at millimeter-wave frequencies. As a result, stripline is not often used for millimeter-wave circuits except for some automotive radar applications.3
Components based on SIW circuit technology have been designed and fabricated for active and passive circuit functions, including resonators and filters, and have been used in automotive radars and other millimeter-wave applications. The circuit approach supports low-loss signal propagation even at the higher frequencies but, as with the other circuit technologies, provides a balance of benefits and challenges at millimeter-wave frequencies.
In SIW, a circuit signal path is formed with a top metal layer and bottom ground plane with rows of plated through-holes (PTHs) between them. In effect, it forms a compact rectangular waveguide filled by the circuit material’s dielectric material. It is capable of low loss at millimeter-wave frequencies but the PTHs must be located within extremely tight tolerances, especially at the higher frequencies, so SIW is sensitive to circuit fabrication process variations. SIW circuits can be difficult to realize at millimeter-wave frequencies since it requires circuit materials with minimal Dk variations and precisely drilled holes with tight drilling tolerances during circuit fabrication.
In contrast, circuits fabricated with GCPW structures on low-Dk circuit materials are gaining popularity at millimeter-wave frequencies and for broadband RF/microwave/millimeter-wave frequency coverage, such as in test/measurement applications. The symmetrical blend of dielectric and copper conductors provides low-loss signal propagation at higher frequencies. Millimeter-wave circuits based on GCPW are often combined with lower-frequency microstrip circuits on the same circuit material, such as a receiver with lower-frequency intermediate-frequency (IF) section, requiring materials that meet the requirements for both circuit technologies.
GCPW circuits are capable of repeatable, consistent performance at millimeter-wave frequencies, but circuit fabrication variables must be well controlled and combined with low-loss circuit materials for best results. GCPW conductors are assumed to have rectangular shapes although the conductors are more often trapezoidal in shape with variations in that shape. Variations in conductor shape and thickness can result in signal phase distortion at millimeter-wave frequencies (Fig. 2). As with microstrip, variations in conductor width and conductor thickness must be minimized. As with SIW, GCPW PTHs must be precisely located to minimize variations in impedance and loss and form a consistent, continuous propagation path.
Figure 2. Conductors for GCPW circuits are often assumed to be ideally rectangular in shape (top) although they may be fabricated with trapezoidal shapes (bottom) with different effects at millimeter-wave frequencies
For many emerging millimeter-wave circuit applications, such as automotive radars, that are sensitive to signal phase responses, any causes for inconsistent phase should be minimized. GCPW circuits at millimeter-wave frequencies are sensitive to material and fabrication process variations, including variations in material Dk and substrate thickness, and performance can be impacted by copper conductor thickness, which should be maintained within a tight tolerance, and copper surface roughness, which should be minimized. The choice of plated finish on a GCPW circuit can also affect its millimeter-wave performance; for example, nickel has more loss than copper and a plated nickel finish will increase signal loss on GCPW or microstrip (Fig. 3). In addition, because of the small wavelengths, variations in the thickness of the finish cause variations in the phase response, although more with GCPW than for microstrip.
Figure 3. When microstrip and GCPW circuits were fabricated on the same circuit material (8-mil-thick RO4003C™ laminates from Rogers Corp.) and tested, the impact of the ENIG plated finish on the GCPW circuit is seen as much greater than on microstrip through millimeter-wave frequencies.
For additional information on selecting circuit materials and transmission-line technologies for millimeter-wave circuits, this blog and its first part are based on a MicroApps presentation made by the author at the 2021 IEEE International Microwave Symposium (IMS) virtual event, “Designers Guide to the Transition from Microwave to Millimeter-Wave, when using PCB Technology,” 2021 IEEE IMS, virtual event, June 20-25, 2021.
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1. John Coonrod, “Concerning PCBs and the Transition from Microwaves to Millimeter Waves,” ROG Blog, Part 1, Microwave Journal, June 2021
2. John Coonrod, “Design Considerations and Tradeoffs for Microstrip, Coplanar, and Stripline Structures at Millimeter-Wave Frequencies,” Microwave Journal webinar, 2019.
3. John Coonrod, “Stripline Circuitry for Millimeter-Wave and Very High Speed Digital,” IEEE International Microwave Symposium (IMS), MicroApps sessions, June 9, 2021, Atlanta, GA.