Designing and fabricating a printed circuit board (PCB) at millimeter-wave frequencies starts with the circuit material, although the choice of transmission-line technology can play quite a part in how much performance can be delivered at those high frequencies. With the ongoing consumption of RF/microwave frequencies for so many cellular and wireless communications, interest continues to grow in millimeter-wave frequencies for many short range, lower power systems, such as automotive radars and Fifth Generation (5G) wireless networks, to add enough bandwidth for the global masses. Circuit designers might first think of microstrip, grounded coplanar waveguide (GCPW), or even rectangular waveguide as their transmission-line technology of choice at millimeter-wave frequencies, but what about stripline? It can perform quite well at 24 GHz (the higher frequencies at which many 5G base stations will be operating) and above in densely packed circuits. There are just a few things to pay attention to when designing and constructing stripline circuits at millimeter-wave frequencies.
Stripline’s structure is unique, often compared to a shielded coaxial cable run over by a truck. It features layers: a conductor surrounded by top and bottom dielectric layers (circuit materials), with the dielectric layers surrounded by top and bottom shield layers. The layers add to complexity but also provide outstanding isolation of the conductors and transmission lines, making it possible to realize extremely small circuits at RF, microwave, and--depending upon the qualities of the PCB material--millimeter-wave frequencies.
The complexity of stripline adds to manufacturing time and cost but provides some excellent benefits. In addition to high isolation and miniaturization, the top and bottom ground planes of stripline circuits contribute to low radiation loss, especially at millimeter-wave frequencies where the much higher radiation losses of microstrip circuits can sometimes turn them into unwanted antennas. Stripline may lack the manufacturing simplicity of microstrip or GCPW, but it may be the best transmission-line option for some millimeter-wave circuit designs, especially where high performance (with no interference) is needed from densely packed circuits or in applications sensitive to unwanted circuit radiation and electromagnetic interference (EMI).
Fortunately, by following a few well-tried design and manufacturing tricks, outstanding performance to 77 GHz and beyond can be consistently “coaxed” from stripline PCBs. And for those in need of a quick refresher on microstrip and GCPW, join host John Coonrod on the Coonrod’s CORNER video appearing on YouTube: “Comparisons of Microstrip and Grounded Coplanar Waveguide at Millimeter-Wave Frequencies”
Along with other transmission-line formats, stripline circuits will shrink with increasing frequencies, to accommodate the smaller wavelengths of millimeter-wave circuits, but they will maintain high isolation between circuit traces due to the multiple circuit layers. Stripline circuits are also capable of wide bandwidths, so that a single millimeter-wave circuit design might handle multiple applications. Proper precautions must be practiced when designing and implementing stripline circuits at millimeter-wave frequencies to avoid unwanted behavior, such as spurious signal modes related to wideband coverage, and to extract the highest performance levels possible. The choice of PCB material will play a key role in how well stripline circuits do at millimeter-wave frequencies.
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Thin laminates are typically used due to the short wavelength of millimeter-wave circuits. But even with very thin dielectric materials, stripline circuits and their multiple layers will typically be thicker at a given frequency than microstrip or GCPW circuits. The consistency of the dielectric material is critical for consistent (and computer-predictable) signal propagation across the PCB at those higher frequencies. Multiple layers of dielectric material in stripline circuits contribute to higher dielectric loss and insertion loss at millimeter-wave frequencies than microstrip and GCPW circuits. But by starting with circuit materials exhibiting low dielectric loss or low dissipation factor (Df), stripline insertion loss can be controlled and minimized even through millimeter-wave frequencies.
The surface roughness of a copper conductor can be a concern for stripline circuits at millimeter-wave frequencies, which are typically fabricated on thinner dielectric materials because of the small wavelengths. Rougher copper conductor surfaces will slow the propagation of EM waves across the conductor compared to smoother copper conductor surfaces. In addition, inconsistencies in the amount of surface roughness across the conductor and the PCB can result in changes in EM propagation behavior across the PCB, especially evident as phase variations at millimeter-wave frequencies.
Variations in copper surface roughness can lead to variations in a PCB material’s dispersion behavior. A PCB’s dispersion is a function of both the conductor and the dielectric material. Inconsistent dispersion may not cause problems for circuits at RF or even microwave frequencies but can result in variations in phase response at millimeter-wave frequencies, to which some circuit applications are sensitive.
Achieving an effective signal launch from, for example, a coaxial connector, to the PCB requires proper preparation with stripline circuits compared to the simplicity of launching signals from a coaxial connector to a microstrip or GCPW circuit. In a microstrip circuit, assuming the same impedance (such as 50 Ω) for the connector center conductor and the circuit transmission line with its single ground plane, a direct connection is usually made to transfer signal energy from connector to circuit.
A signal launch from a coaxial connector to a stripline circuit requires a bit more effort since the circuit signal plane is not at the surface. While the center conductor of the connector may reach to the top layer of a stripline circuit, the signal plane is beneath that shield layer and the dielectric layer beneath it. The signal launch or transition from the connector center conductor to the stripline signal plane is usually by means of a plated through hole (PTH) with extremely small diameter due to the wavelength of the operating frequency. To form an electrically uniform ground plane in a stripline circuit, similar PTHs are typically used to connect the circuit’s top and bottom ground planes. This minimizes the potential for current-density differences in the separate ground planes. Of course, it important to minimize the length of the transitional PTHs that carry signal energy from the connector to the circuit, especially at millimeter-wave frequencies. Any unnecessary length in the signal path can result in reflections and degraded return loss or even generation of spurious or harmonic signals with the stripline circuit.
What type of laminate serves as a suitable starting point for stripline circuits at millimeter-wave frequencies? One example is RO3003™ laminates from Rogers Corp., ceramic-filled polytetrafluoroethylene (PTFE) composite materials. With a dielectric constant that is maintained within ±0.04 of 3.00 across the material, it has shown the consistency needed for millimeter-wave circuits through 77 GHz (automotive radar). RO3003 laminates have a low Df of 0.0010 at 10 GHz and they feature the temperature stability, measured as consistent coefficient of thermal expansion (CTE) in all three axes of the material. The consistent CTE behavior over temperature ensures that PTHs formed in stripline circuits will remain sound and reliable, even at millimeter-wave frequencies.
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