PCB Tech - High frequency circuit board: comparison of microstrip line and grounded coplanar waveguide

When selecting the optimal PCB high-frequency circuit board material for a certain circuit design, the high-frequency circuit board designer usually needs to consider the performance changes, physical size, and power level of the circuit. The choice of different transmission line technology will affect the final performance of the circuit design of the high-frequency circuit board, such as the use of microstrip line or grounded coplanar waveguide (GCPW). Most designers know the obvious difference between the high-frequency microstrip line and the strip line of the high-frequency circuit board, but the coplanar waveguide in the grounded high-frequency circuit board design is quite different from the traditional microstrip line.

The grounded coplanar waveguide can bring many benefits and conveniences to the design of the circuit designer of the high frequency microwave radio frequency board. When choosing different circuits, it is very helpful to understand the influence of different PCB high-frequency circuit board (microwave radio frequency board) materials on the microstrip line and grounded coplanar waveguide circuit. The different structures of the two circuits can be seen in the figure below.

We can see that the structure of the microstrip circuit designed by the high-frequency circuit board is that the signal conductor line is processed on the top of the dielectric layer, and the ground conductor surface is at the bottom of the dielectric layer. In the grounded coplanar waveguide structure, in addition to the ground plane at the bottom of the dielectric layer, two additional ground planes are added on the top of the dielectric layer and the signal conductors are in these two ground planes and are spaced apart from each other. The top and bottom ground planes are connected through metal-filled vias to achieve consistent grounding performance. In addition, to ensure the consistency of circuit discontinuities such as joints, many grounded coplanar waveguide circuits use grounded bus bars to achieve electrical connection between the two top-level ground conductors.

The difference between the two transmission line technologies is that in the grounded coplanar waveguide, the small spacing between the top ground conductor and the signal conductor can achieve low impedance of the circuit, and the impedance of the circuit can be changed by adjusting the spacing. As the distance between the ground conductor and the signal conductor increases, the impedance will also increase. When the distance between the top ground conductor and the signal conductor of the grounded coplanar waveguide increases, the influence of the ground conductor on the circuit will be reduced. When the spacing is large enough, the grounded coplanar waveguide circuit is similar to a microstrip circuit.

Why do certain transmission lines have advantages over other transmission line technologies? Obviously, compared to grounded coplanar waveguide, the microstrip line has a simple structure, which is more convenient for processing and computer modeling. The microstrip line and strip line of high frequency circuit boards are the most commonly used transmission line technology in the microwave band, but in the millimeter wave frequency band, the loss of the microstrip line and strip line circuit will increase. This reduces the efficiency of these two transmission line technologies in the frequency bands of 30 GHz and above. However, the grounded coplanar waveguide has a solid grounding structure and lower loss in the high frequency band. This provides potential advantages and stable performance for the design of millimeter wave frequency bands and even 100GHz and above frequency bands.

The effective dielectric constant of the PCB high-frequency circuit board material will determine the size of the circuit structure, such as 50 ohm characteristic impedance. For example, based on the Rogers high frequency board RO4350B hydrocarbon ceramic circuit material microstrip transmission line, the circuit width under the 50 ohm characteristic impedance condition of the Rogers high frequency board will be based on the dielectric constant value of the material 3.48. But for grounded coplanar waveguides using this material, the effective dielectric constant will decrease. Because the electromagnetic field will be more distributed in the air above the circuit rather than in the dielectric material of the PCB high-frequency circuit board, the effective dielectric constant of the grounded coplanar waveguide will be reduced compared to the microstrip line. The difference between the effective dielectric constant of the grounded coplanar waveguide and the microstrip line also depends on the dielectric thickness of the grounded coplanar waveguide and the spacing between the signal line and the ground of the top layer.

When choosing to use high-frequency microstrip line or grounded coplanar waveguide transmission line technology, what role does PCB high-frequency circuit board material play? Material parameters such as permittivity (Dk) and permittivity consistency will affect the electrical performance of the transmission line. Because the electromagnetic field can propagate both inside and outside the material of the dielectric constant Dk, its propagation mode in the circuit structure is different, which affects the effective dielectric constant of the circuit material. For the microstrip circuit structure of the top transmission line and the bottom ground plane, its electromagnetic field is mainly distributed inside the dielectric material between the two metal planes, and concentrated on the edge of the signal conductor. Therefore, the effective dielectric constant of the microstrip circuit is closely related to the dielectric constant value of the PCB material. For example, RO4350B hydrocarbon ceramic PCB material of Rogers Corporation, the process standard value of the dielectric constant in the z (thickness) direction at 10GHz It is 3.48, and the dielectric constant deviation of the whole material is kept at ±0.05.

PCB high-frequency circuit board processing factors have less influence on microstrip circuit than on grounded coplanar waveguide circuit. For example, the PCB copper thickness difference has little effect on the performance of the microstrip circuit, but it will affect the performance of the grounded coplanar waveguide circuit. For microstrip circuits, the thicker PCB copper layer thickness only slightly reduces the insertion loss and reduces the effective dielectric constant of the circuit. As for the grounded coplanar waveguide circuit, a thicker PCB copper layer thickness will increase the electromagnetic field between the top ground-signal line and the ground, which increases the electromagnetic field distribution in the air above the grounded coplanar waveguide circuit. The increase in the electromagnetic field distribution in the air leads to a significant decrease in the circuit loss of the grounded coplanar waveguide circuit using a thicker PCB copper layer thickness and the effective dielectric constant of the PCB.

It can be found that although the microstrip line has high radiation loss in the high frequency and millimeter wave frequency bands and it is difficult to achieve high-order mode suppression, the microstrip line can still be suitable for circuits with relatively narrow microwave band bandwidth. And the microstrip circuit is relatively insensitive to PCB high-frequency circuit board processing technology and copper layer thickness and thickness differences. In contrast, grounded coplanar waveguides have relatively low radiation loss in the millimeter wave band and can achieve good high-order mode suppression, which makes grounded coplanar waveguides a candidate transmission line technology for 30GHz and above. In addition, the grounded coplanar waveguide circuit has relatively less stringent requirements on the processing technology and deviation of the PCB high-frequency circuit board, which makes the grounded coplanar waveguide suitable for mass production and application in the high frequency band.