1. Because the radio frequency (RF) PCB operates as a distributed parameter circuit, it readily experiences skin effect and coupling, presenting challenges in controlling interference during circuit operation. Common issues include mutual interference between digital and analog circuits, power supply noise interference, and grounding-related disruptions. Balancing these factors to achieve a suitable compromise during PCB design is crucial for minimizing interference and ensuring the success of RF circuit PCB design. This article focuses on PCB layout techniques aimed at enhancing the anti-interference capabilities of radio frequency circuits.
2. This section discusses the arrangement of components on multilayer boards. An essential aspect of component layout is securing them along the RF path. By optimizing their orientation, the length of the RF path is minimized. Moreover, positioning the input far from the output and maintaining a considerable separation between high-power and low-power circuits are important. Additionally, sensitive analog signals should be isolated from high-speed digital signals and RF signals.
3. Various techniques are commonly employed in component layout:
4. **Linear Layout for RF Main Signal Components:** Whenever feasible, RF main signal components should be arranged linearly, as depicted in Figure 1. However, due to constraints posed by the PCB board or cavity space, achieving a straight layout may not always be possible. In such cases, an L-shaped layout can be adopted. It’s advisable to avoid U-shaped layouts (see Figure 2) whenever possible, as they may inadvertently increase the distance between the input and output, preferably maintaining a separation of at least 1.5cm or more.
1. Figure 1: One-line Layout
2. To ensure optimal performance and functionality of the printed circuit board (PCB), meticulous attention must be paid to its layout design.
3. The one-line layout, depicted in Figure 1, illustrates a systematic arrangement of components and traces on the PCB.
4. At the core of this layout is the strategic placement of components, considering factors such as signal integrity, thermal management, and ease of assembly.
5. By organizing components in a logical manner, signal paths are minimized, reducing interference and enhancing the overall efficiency of the PCB.
6. Furthermore, the one-line layout facilitates streamlined assembly processes, minimizing the risk of errors and optimizing production timelines.
7. Thermal management is also a critical aspect addressed in this layout. Components generating significant heat are strategically positioned to promote effective dissipation and prevent overheating.
8. In conclusion, the one-line layout exemplifies the importance of thoughtful design in achieving PCBs that meet stringent performance requirements while ensuring reliability and manufacturability.Figure 2: L-shaped and U-shaped Layout
Furthermore, when employing an L-shaped or U-shaped layout, it is advisable not to turn immediately upon entering the interface, as depicted on the left in Figure 3. Instead, it is preferable to proceed with a slight straight path before turning, as illustrated in the right image of Figure
3.Figure 3: Two Options
The same modules are arranged in either identical or symmetrical layouts wherever possible, as depicted in Figures 4 and Figure 5: Symmetrical Layout
The feed inductance of the bias circuit is positioned perpendicular to the RF channel, as depicted in Figure 6. This orientation is primarily intended to mitigate mutual inductance between inductive devices.
Figure 6: Cross-shaped Layout
1.4: 45-Degree Layout
To optimize space utilization, consider arranging the devices at a 45-degree angle, minimizing the length of RF lines, as illustrated in Figure 7.
Figure 7: 45-degree Layout
The overall requirements for wiring are as follows: RF signal traces should be short and straight, minimize abrupt changes in line direction, minimize the number of vias, avoid intersections with other signal lines, and add ground vias around RF signal lines wherever possible. Below are some commonly used optimization methods:
When the width of the RF line is significantly larger than the width of the IC device pin, a gradual approach is adopted for the line width at the chip contact, as illustrated in Figure 8.
Figure 8: Gradient Line
If the radio frequency line cannot be straight, consider treating it as an arc line. This approach can help in reducing external radiation and mutual coupling of the RF signal. Experiments indicate that bending the corners of the transmission line at right angles can minimize return loss, as illustrated in Figure 9.
Figure 9 Arc Line
1. The ground wire should be made as thick as possible. Ideally, each layer of the PCB should be grounded extensively, with connections made to the main ground. Additional ground vias should be incorporated to minimize ground impedance.
2. It is advisable to avoid dividing the power supply of the RF circuit into planes whenever possible. Dividing the power plane not only increases radiation towards the RF signal but also renders it susceptible to RF interference. Hence, the power line or plane should preferably adopt a elongated strip shape, tailored according to current requirements. While striving for maximum thickness to accommodate current capacity, it’s crucial to avoid excessive widening without constraint. When routing power lines, ensure to circumvent creating loops.
3. The orientation of power lines and ground lines should run parallel to the direction of the RF signal without overlapping. Where intersections occur, employing a vertical cross configuration is optimal.
4. For effective isolation, RF signals and IF signals should intersect with a ground whenever feasible.
5. When RF signals intersect with other signal traces, efforts should be made to sandwich a layer of ground connected to the main ground between them along the RF trace. If this is impractical, ensure that they cross paths. Here, “other signal traces” encompass power lines as well.
Packetizing the processing of radio frequency signals, interference sources, sensitive signals, and other critical signals not only enhances signal immunity to interference but also mitigates signal interference with other signals, as depicted in Figure 10.
Figure 10: Package Land Processing
1. The processing of copper foil necessitates a smooth and uniform surface, devoid of lengthy traces or abrupt corners. In cases where such occurrences are unavoidable, it is advisable to fill a few ground vias at sharp corners, slender copper foil sections, or along the edges of the copper foil.
2. The RF line should have a minimum width of 3W measured from the edge of the adjacent ground plane. Furthermore, there should be no non-ground vias present within this 3W boundary.
Figure 11: Spacing
1. Radio frequency (RF) lines within the same layer should be grounded, with ground vias added to the ground copper. The spacing between holes should be less than 1/20 of the wavelength (λ) corresponding to the signal frequency, evenly distributed. The width of the ground-clad copper edge should be either 2W or 3H in height from the RF line, where H represents the total thickness of adjacent dielectric layers.
2. For the entire RF circuit, isolation between RF units of different modules should be ensured with cavities, particularly between sensitive circuits and strong radiation sources. In high-power multi-stage amplifiers, isolation between stages must also be maintained. Once the entire circuit branch is positioned, attention shifts to processing the shielding cavities, with the following precautions:
3. Aim to fashion the entire shielding cavity into a regular shape to ease casting, favoring rectangular over square configurations.
4. The corners of the shielding cavity should be arc-shaped, with the shielding metal cavity typically cast. Arc-shaped corners facilitate drafting during casting, as depicted in Figure 12.
Figure 12 12: Cavity
The periphery of the shielding cavity is sealed. The interface line is typically introduced into the cavity using either a strip line or a microstrip line, while different modules inside the cavity utilize a microstrip line. The joints of different cavities are processed with grooves, each with a width of 3mm, and the microstrip line runs through the middle.
To secure the shielding shell, a 3mm metalized hole is positioned at the corner of the cavity. Equally spaced metalized holes should be placed along each long cavity to reinforce support.
The cavity is generally windowed to facilitate welding of the shielding shell. Typically, the cavity is thicker than 2mm, and two rows of windowed via screens are added. These vias are staggered, with a distance of 150MIL between vias in the same row.
2. This section discusses the arrangement of components on multilayer boards. An essential aspect of component layout is securing them along the RF path. By optimizing their orientation, the length of the RF path is minimized. Moreover, positioning the input far from the output and maintaining a considerable separation between high-power and low-power circuits are important. Additionally, sensitive analog signals should be isolated from high-speed digital signals and RF signals.
3. Various techniques are commonly employed in component layout:
4. **Linear Layout for RF Main Signal Components:** Whenever feasible, RF main signal components should be arranged linearly, as depicted in Figure 1. However, due to constraints posed by the PCB board or cavity space, achieving a straight layout may not always be possible. In such cases, an L-shaped layout can be adopted. It’s advisable to avoid U-shaped layouts (see Figure 2) whenever possible, as they may inadvertently increase the distance between the input and output, preferably maintaining a separation of at least 1.5cm or more.
1. Figure 1: One-line Layout
2. To ensure optimal performance and functionality of the printed circuit board (PCB), meticulous attention must be paid to its layout design.
3. The one-line layout, depicted in Figure 1, illustrates a systematic arrangement of components and traces on the PCB.
4. At the core of this layout is the strategic placement of components, considering factors such as signal integrity, thermal management, and ease of assembly.
5. By organizing components in a logical manner, signal paths are minimized, reducing interference and enhancing the overall efficiency of the PCB.
6. Furthermore, the one-line layout facilitates streamlined assembly processes, minimizing the risk of errors and optimizing production timelines.
7. Thermal management is also a critical aspect addressed in this layout. Components generating significant heat are strategically positioned to promote effective dissipation and prevent overheating.
8. In conclusion, the one-line layout exemplifies the importance of thoughtful design in achieving PCBs that meet stringent performance requirements while ensuring reliability and manufacturability.Figure 2: L-shaped and U-shaped Layout
Furthermore, when employing an L-shaped or U-shaped layout, it is advisable not to turn immediately upon entering the interface, as depicted on the left in Figure 3. Instead, it is preferable to proceed with a slight straight path before turning, as illustrated in the right image of Figure
3.Figure 3: Two Options
The same modules are arranged in either identical or symmetrical layouts wherever possible, as depicted in Figures 4 and Figure 5: Symmetrical Layout
The feed inductance of the bias circuit is positioned perpendicular to the RF channel, as depicted in Figure 6. This orientation is primarily intended to mitigate mutual inductance between inductive devices.
Figure 6: Cross-shaped Layout
1.4: 45-Degree Layout
To optimize space utilization, consider arranging the devices at a 45-degree angle, minimizing the length of RF lines, as illustrated in Figure 7.
Figure 7: 45-degree Layout
The overall requirements for wiring are as follows: RF signal traces should be short and straight, minimize abrupt changes in line direction, minimize the number of vias, avoid intersections with other signal lines, and add ground vias around RF signal lines wherever possible. Below are some commonly used optimization methods:
When the width of the RF line is significantly larger than the width of the IC device pin, a gradual approach is adopted for the line width at the chip contact, as illustrated in Figure 8.
Figure 8: Gradient Line
If the radio frequency line cannot be straight, consider treating it as an arc line. This approach can help in reducing external radiation and mutual coupling of the RF signal. Experiments indicate that bending the corners of the transmission line at right angles can minimize return loss, as illustrated in Figure 9.
Figure 9 Arc Line
1. The ground wire should be made as thick as possible. Ideally, each layer of the PCB should be grounded extensively, with connections made to the main ground. Additional ground vias should be incorporated to minimize ground impedance.
2. It is advisable to avoid dividing the power supply of the RF circuit into planes whenever possible. Dividing the power plane not only increases radiation towards the RF signal but also renders it susceptible to RF interference. Hence, the power line or plane should preferably adopt a elongated strip shape, tailored according to current requirements. While striving for maximum thickness to accommodate current capacity, it’s crucial to avoid excessive widening without constraint. When routing power lines, ensure to circumvent creating loops.
3. The orientation of power lines and ground lines should run parallel to the direction of the RF signal without overlapping. Where intersections occur, employing a vertical cross configuration is optimal.
4. For effective isolation, RF signals and IF signals should intersect with a ground whenever feasible.
5. When RF signals intersect with other signal traces, efforts should be made to sandwich a layer of ground connected to the main ground between them along the RF trace. If this is impractical, ensure that they cross paths. Here, “other signal traces” encompass power lines as well.
Packetizing the processing of radio frequency signals, interference sources, sensitive signals, and other critical signals not only enhances signal immunity to interference but also mitigates signal interference with other signals, as depicted in Figure 10.
Figure 10: Package Land Processing
1. The processing of copper foil necessitates a smooth and uniform surface, devoid of lengthy traces or abrupt corners. In cases where such occurrences are unavoidable, it is advisable to fill a few ground vias at sharp corners, slender copper foil sections, or along the edges of the copper foil.
2. The RF line should have a minimum width of 3W measured from the edge of the adjacent ground plane. Furthermore, there should be no non-ground vias present within this 3W boundary.
Figure 11: Spacing
1. Radio frequency (RF) lines within the same layer should be grounded, with ground vias added to the ground copper. The spacing between holes should be less than 1/20 of the wavelength (λ) corresponding to the signal frequency, evenly distributed. The width of the ground-clad copper edge should be either 2W or 3H in height from the RF line, where H represents the total thickness of adjacent dielectric layers.
2. For the entire RF circuit, isolation between RF units of different modules should be ensured with cavities, particularly between sensitive circuits and strong radiation sources. In high-power multi-stage amplifiers, isolation between stages must also be maintained. Once the entire circuit branch is positioned, attention shifts to processing the shielding cavities, with the following precautions:
3. Aim to fashion the entire shielding cavity into a regular shape to ease casting, favoring rectangular over square configurations.
4. The corners of the shielding cavity should be arc-shaped, with the shielding metal cavity typically cast. Arc-shaped corners facilitate drafting during casting, as depicted in Figure 12.
Figure 12 12: Cavity
The periphery of the shielding cavity is sealed. The interface line is typically introduced into the cavity using either a strip line or a microstrip line, while different modules inside the cavity utilize a microstrip line. The joints of different cavities are processed with grooves, each with a width of 3mm, and the microstrip line runs through the middle.
To secure the shielding shell, a 3mm metalized hole is positioned at the corner of the cavity. Equally spaced metalized holes should be placed along each long cavity to reinforce support.
The cavity is generally windowed to facilitate welding of the shielding shell. Typically, the cavity is thicker than 2mm, and two rows of windowed via screens are added. These vias are staggered, with a distance of 150MIL between vias in the same row.
