Influence of PCB material on microstrip line and grounded coplanar waveguide circuit
When choosing the optimal PCB material for a circuit design, the high frequency circuit designer usually needs to consider the circuit's performance change, physical size and power level. The choice of different transmission line technologies can affect the final performance of the circuit design, such as the use of microstrip lines or grounded coplanar waveguides (GCPW). Most designers understand the obvious difference between high-frequency microstrip lines and strip lines, but grounded coplanar waveguides are much different from traditional microstrip lines. Grounded coplanar waveguides offer many benefits and conveniences for the design of high frequency circuit designers. When choosing different circuits, understanding the effects of different PCB materials on the microstrip line and grounded coplanar waveguide circuits is very helpful. The different structures of the two circuits can be seen in the figure below.
Figure 1: Comparison of the same structure
We can see that the structure of the microstrip line circuit is that the signal conductor line is processed at 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, at the top of the dielectric layer, two additional ground planes are added and the signal conductors are placed in the two ground planes and spaced apart from each other. A metal-filled via connects the ground planes at the top and bottom to achieve consistent grounding performance. In addition, in order to ensure uniformity of discontinuities in circuits such as joints, many grounded coplanar waveguide circuits implement electrical connections between the two top ground conductors via ground straps.
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. The spacing between the ground conductor and the signal conductor increases, and the impedance also increases. When the pitch of the top ground conductor and the signal conductor of the grounded coplanar waveguide increases, the influence of the ground conductor on the circuit is reduced. When the spacing is large enough, the grounded coplanar waveguide circuit is similar to a microstrip line circuit.
Why is a certain transmission line superior to other transmission line technologies? It is clear that the microstrip line structure is simpler than the grounded coplanar waveguide, which is more convenient for processing and computer modeling. Microstrip and stripline are the most commonly used transmission line technologies in the microwave band, but in the millimeter wave band, the losses of the microstrip and stripline circuits will increase. This makes the two transmission line technologies work less efficiently in the 30 GHz and above bands. However, the grounded coplanar waveguide has a solid ground structure and lower losses in the high frequency band. This provides potential advantages and stability for designs in the millimeter wave band and even in the 100 GHz and above bands.
What role does PCB material play in choosing a microstrip line or grounded coplanar waveguide transmission line technology? Material parameters such as dielectric constant (Dk) and dielectric constant consistency affect the electrical performance of the transmission line. Since the electromagnetic field can propagate inside and outside the material of the dielectric constant Dk, its propagation mode in the circuit structure is different to affect the effective dielectric constant of the circuit material. For the microstrip line 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 is concentrated on the edge of the signal conductor. Therefore, the effective dielectric constant of the microstrip line circuit is closely related to the dielectric constant value of the PCB material, such as Rogers' RO4350B hydrocarbon ceramic PCB material, and the dielectric constant process standard value in the z (thickness) direction at 10 GHz. For 3.48, the dielectric constant deviation across the material is maintained at ±0.05.
The effective dielectric constant of the PCB material will determine the size of the circuit structure, such as a 50 ohm characteristic impedance. For example, based on the microstrip transmission line of the RO4350B hydrocarbon ceramic circuit material, the circuit width at 50 ohms characteristic impedance will be based on the material's dielectric constant value of 3.48. However, for a grounded coplanar waveguide using this material, the effective dielectric constant will be reduced. Since the electromagnetic field will be more distributed in the air above the circuit than in the PCB dielectric material, the effective dielectric constant of the grounded coplanar waveguide will be reduced compared to the microstrip line. The difference in effective dielectric constant between the grounded coplanar waveguide and the microstrip line also depends on the thickness of the grounded coplanar waveguide medium and the spacing between the signal line and ground of the top layer.
The PCB processing factor has less impact on the microstrip line circuit than on the grounded coplanar waveguide circuit. For example, the difference in PCB copper plating thickness has little effect on the performance of the microstrip line circuit, but it will affect the performance of the grounded coplanar waveguide circuit. For microstrip line circuits, thicker PCB copper layer thickness only slightly reduces insertion loss and reduces the effective dielectric constant of the circuit. For a grounded coplanar waveguide circuit, a thicker PCB copper layer thickness will result in an increase in the top-ground-signal-ground electromagnetic field, 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 results in a significant reduction in the circuit losses 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 band and the millimeter wave band and it is difficult to achieve high-order mode suppression, the microstrip line can be applied to a circuit with a relatively narrow bandwidth in the microwave band. And the microstrip line circuit is less sensitive to PCB processing technology and copper layer thickness and thickness difference. In contrast, grounded coplanar waveguides have relatively low radiated losses in the millimeter wave band and can achieve good high-order mode rejection, making grounded coplanar waveguides a candidate transmission line technology for the 30 GHz and above bands. In addition, the grounded coplanar waveguide circuit is relatively less demanding on PCB processing and deviation requirements, which makes the grounded coplanar waveguide suitable for mass production and application in high frequency bands.