Make Practical Use of GCPW at mmWaves
More wireless applications are moving higher in frequency, to millimeter-wave (mmWave) frequencies, as bandwidths become congested at lower RF and microwave frequencies. Bandwidths are available at 24 GHz and beyond for such applications as Fifth Generation (5G) wireless cellular telephones and advanced driver assistance systems (ADAS). But signal power tends to decrease with increasing frequency, and mmWave circuit technologies must strive to minimize signal losses while making best use of available signal power. Conserving signal power within mmWave circuits can rely not only upon the printed circuit board (PCB) material but on the choice of transmission line technology. At those frequencies, grounded coplanar waveguide (GCPW) transmission lines are capable of excellent performance when teamed with low loss PCB materials, if it is possible to anticipate any performance variations stemming from circuit design and fabrication processes.
GCPW circuit technology has benefits compared to other high frequency transmission line technologies, such as stripline and microstrip, especially at mmWave frequencies. GCPW, has a straightforward configuration, with single dielectric plane sandwiched by top-side ground-signal-ground (GSG) transmission line and bottom-side ground plane and with the ground planes interconnected by plated through holes (PTHs). While it does not match the simple configuration of microstrip, with a top-side conductor, dielectric layer, and bottom ground plane, GCPW is simpler than stripline’s conductive transmission lines with top and bottom dielectric layers. The tradeoff for microstrip’s simplicity is increased loss at mmWave frequencies compared to GCPW. Microstrip circuits are also more likely to radiate signal energy than GCPW circuits at mmWave frequencies, for potential interference and electromagnetic-compatibility (EMC) issues especially in tightly spaced circuits and enclosures.
Still, successful application of GCPW technology involves understanding how the performance levels from fabricated PCBs can differ from the near-ideal performance levels predicted for GCPW circuit designs by commercial computer-aided engineering (CAE) software tools. A few factors can lead to differences between what the software predicts and what is achieved by a GCPW circuit, especially for mmWave circuit designs at high volumes.
Even before circuits are fabricated, small variations in the PCB material can impact GCPW circuit performance, especially at the small wavelengths of mmWave frequencies which are so sensitive to those variations. For example, variations in the thickness of the dielectric material and in the thickness of the conductive material can result in GCPW performance variations at mmWave frequencies. The amount of surface roughness in the copper conductor can also impact GCPW performance, as can variations in any additional plating, such as for the PTHs, used to fabricate the GCPW circuits.
Assuming that GCPW transmission line technology is well suited for producing consistent PCBs at mmWave frequencies, it must still be teamed with a circuit material with tightly controlled characteristics, such as dielectric constant (Dk) and dissipation factor (Df). Also, the mmWave circuits must be fabricated by a repeatable process for good consistency at high volumes. Process variations can be causes of different forms of PCB performance variations. As an example, the locations of PTHs used to connect the two ground planes in a GCPW circuit can vary from circuit to circuit as one cause of performance variations.
The shapes of the GCPW conductors can vary from circuit to circuit, leading to performance differences between fabricated GCPW circuits. CAE simulation software programs typically assume ideal conductors that are rectangular in shape (from a cross-sectional view) for the performance levels that they predict for a given circuit design. But most GCPW circuits are fabricated with trapezoidal-shaped conductors with some amount of variation in the conductors from circuit to circuit. These conductor variations result in electrical performance variations in the GCPW circuits, specifically in insertion loss and signal phase angle, with the impact of the variations increasing with increasing frequencies.
The differences in conductor shapes, from actual to ideal, contribute to differences between performance levels achieved by circuits fabricated with trapezoidal conductors and those predicted by computer simulation programs based on ideal rectangular conductor shapes. With the enhanced sensitivities at the small wavelengths of mmWave frequencies, those ideal GCPW conductor shapes project near-theoretical minimal limits of circuit effective dielectric constant (Dk) and the associated phase response compared to the typically different (and more realistic) effective Dk and phase angle resulting from the types of trapezoidal-shaped conductors produced by GCPW transmission line fabrication processes. Standard PCB fabrication processes suffer normal variations in circuit features which can yield performance variations from circuit to circuit.
Depending upon how closely spaced the sidewalls in a GSG configuration, a GCPW circuit can have different amounts of coupling, with closer line spacing yielding tighter coupling. Tightly coupled GCPW circuits have greater current density on the sidewalls of the coplanar conductors compared to loosely coupled GCPW transmission lines. GCPW circuits with very loose coupling are not as sensitive to circuit fabrication process variations as tightly coupled GCPW circuits. However, unlike tightly coupled GCPW circuits, they do not gain the benefit of additional grounding and behave very much like microstrip circuits.
For any circuit material selected for fabricating GCPW circuits for mmWave applications, such as RO3003™ laminate from Rogers Corp. (with a Dk of 3.00 ± 0.04 in the z-axis and Df of 0.0010 at 10 GHz), the amount of roughness of the copper surface at the copper dielectric interface will impact the performance of circuits fabricated on that material, especially at higher frequencies such as mmWave frequencies and for thinner circuits. Rougher copper surfaces will result in increased insertion loss and slower signal phase velocity for those circuits. The conductor insertion loss will also be impacted by the relative width of the copper conductor along with the thickness of the conductor copper. Wider conductors will exhibit less loss while thicker copper will result in GCPW transmission lines using more air (with its low Dk value of unity) for propagation with lower loss. The slower phase velocity will also be caused in a circuit material with higher Dk value.
Plan on Plating
Part of fabricating a GCPW circuit of any kind involves plating the PCB material, such as with a copper layer on top of the PCB’s copper layer and plating holes drilled through the material to form the conductive paths between the top and bottom ground planes. Plating is also performed on GCPW circuits to form a final plated finish and protect the conductive copper, with most metals being used having less conductivity than copper, but this will increase the conductor loss and result in an increase in the insertion loss which must be anticipated. The final plated finish can impact the phase response as well.
Differences should be expected between simulated performance levels and measurements of fabricated mmWave GCPW circuits. One key for successful large volume production of mmWave GCPW circuits is to minimize the variations as much as possible by expecting certain material traits and certain circuit features. By understanding how a well-established circuit material such as RO3003 laminate is impacted by different GCPW fabrication processes, it is possible to establish meaningful production performance tolerances so that high yields can be achieved even for mmWave ADAS circuits at 77 GHz.
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