Because the radio frequency (RF) circuit is a distributed parameter circuit, skin effect and coupling effect are prone to occur in the actual work of the circuit, so in the actual PCB design, it will be found that the interference radiation in the circuit is difficult to control, Such as: mutual interference between digital circuit and analog circuit, noise interference of power supply, interference caused by unreasonable ground wire, etc. Because of this, how to weigh the pros and cons in the PCB design process to find a suitable compromise, to reduce these interferences as much as possible, and even to avoid the interference of some circuits, is the key to the success or failure of the RF PCB design. From the perspective of PCB LAYOUT, this article provides some processing skills, which are of great use to improving the anti-interference ability of radio frequency circuits.
1.RF layout
What we discussed here is the layout of the components of the multilayer board. The key to the layout of components is to fix the components on the RF path. By adjusting its direction, the length of the RF path is minimized, and the input is far away from the output, and the high-power circuit and the low-power circuit are separated as far as possible. The signal is far away from high-speed digital signals and RF signals.
The following techniques are often used in the layout.
1.1 One-line layout
The components of the RF main signal are arranged in a straight line as much as possible, as shown in Figure 1. However, due to the limitation of PCB board and cavity space, it cannot be laid out in a straight shape in many cases. At this time, L-shaped layout can be used. It’d be better not to use U-shaped layout (as shown in Figure 2). When it is unavoidable, it’d be better to increase the distance between input and output to at least 1.5cm.
Figure 1 One-line layout
Figure 2 L-shaped and U-shaped layout
In addition, when using an L-shaped or U-shaped layout, the turning point should not turn as soon as it enters the interface, as shown on the left in Figure 3, but after a slight straight line, as shown on the right in Figure 3.
Figure 3 Two options
1.2 Same or symmetrical layout
The same modules should be made into the same layout or symmetrical layout as much as possible, as shown in Figure 4 and Figure 5.
Figure 4 The same layout
Figure 5 Symmetrical layout
1.3 Cross-shaped layout
The feed inductance of the bias circuit is placed perpendicular to the RF channel, as shown in Figure 6, mainly to avoid mutual inductance between inductive devices.
Figure 6 Cross-shaped layout
1.4 45° layout
In order to use the space reasonably, the devices can be arranged in a 45-degree direction to make the RF line as short as possible, as shown in Figure 7.
Figure 7 45° layout
2.RF Routing
The overall requirements for wiring are: RF signal traces are short and straight, reduce line abrupt changes, drill fewer holes, and do not intersect with other signal lines, and add as many ground vias as possible around the RF signal line.
The following are some commonly used optimization methods:
2.1 Gradient line processing
In the case that the RF line width is much larger than the width of the IC device pin, the line width of the contact chip adopts a gradual method, as shown in Figure 8.
Figure 8 Gradient line
2.2 Arc line processing
If the radio frequency line cannot be straight, treat it as an arc line, which can reduce the external radiation and mutual coupling of the RF signal. Experiments have shown that the corners of the transmission line are bent at right angles, which can minimize return loss. As shown in Figure 9.
Figure 9 Arc line
2.3 Ground wire and power supply
The ground wire should be as thick as possible. If it is possible, each layer of the PCB should be grounded as much as possible, and the ground should be connected to the main ground. More ground vias should be made to reduce the ground impedance as much as possible.
The power supply of the RF circuit should not be divided into planes as much as possible. The entire power plane not only increases the radiation of the power plane to the RF signal, but also is easily interfered by the RF signal. Therefore, the power cord or plane generally adopts a long strip shape and is processed according to the size of the current. It is as thick as possible under the premise of meeting the current capacity, but it cannot be widened without limit. When handling power lines, be sure to avoid loops.
The direction of the power line and the ground line must be parallel to the direction of the RF signal but not overlap. It is best to use a vertical cross where there is a cross.
2.4 Cross processing
The RF signal and the IF signal should be crossed with a ground if it is possible.
When RF signals cross other signal traces, try to arrange a layer of ground connected to the main ground along the RF traces between them. If it is not possible, make sure they are crossed. Other signal traces here also include power lines.
2.5 Coplanar impedance
Coplanar impedance of radio frequency signals, interference sources, sensitive signals and other important signals can not only improve the anti-interference ability of the signal, but also reduce the interference of the signal to other signals. As shown in Figure 10.
Figure 10 Coplanar impedance
2.6 Copper foil treatment
The copper foil processing requires smooth, no long lines or sharp corners are allowed. If it is unavoidable, fill a few ground vias at the edges of sharp corners, slender copper foil or copper foil.
2.7 Spacing
The RF line must be at least 3W wide from the edge of the adjacent ground plane, and there must be no non-ground vias within the 3W range.
Figure 11 Spacing
The radio frequency lines of the same layer should be have coplanar impedance, and ground vias should be added to the ground copper. The hole spacing should be less than 1/20 of the wavelength (λ) corresponding to the signal frequency, and they should be evenly arranged.
3.Cavity treatment
For the entire RF circuit, the radio frequency units of different modules should be isolated with a cavity, especially between sensitive circuits and strong radiation sources. In high-power multi-stage amplifiers, the isolation between stages should also be guaranteed.
After the branch of the entire circuit is placed, it is the treatment of the shielding cavity. The treatment of the shielding cavity has the following points:
The entire shielding cavity should be made into a regular shape as much as possible to facilitate casting. Try to make each shielding cavity rectangular, avoiding square shielding cavities.
The corners of the shielding cavity are arc-shaped, and the shielding metal cavity is generally formed by casting. The arc-shaped corners are convenient for drafting during casting. As shown in Figure 12.
Figure 12 Cavity
The periphery of the shielding cavity is sealed, and the interface line into the cavity is generally a strip line or a microstrip line
Place 3mm metalized holes on the corners of the cavity to fix the shielding shell, and evenly place the same metalized holes on each long cavity to strengthen the support.
The cavity is generally windowed to facilitate welding of the shielding shell. The cavity is generally thicker than 2 mm, and 2 rows of windowed via screens are added to the cavity. The vias are staggered. The distance between the vias in the same row is 150MIL.
4.Conclusion
In the design of RF circuit PCB, it is very important to reduce circuit radiation, thereby improving anti-interference ability. However, the handling of some problems in the actual layout and wiring is in conflict, so how to find a compromise point to optimize the overall performance of the entire radio frequency circuit is a problem that the designer must consider. This article summarizes some of the design experience in the work, which will help improve the anti-interference ability of the RF circuit PCB, and hope to help beginners in RF circuit design.
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