Techniques for RF PCB Design & Layout

Because the radio frequency (RF) PCB is a distributed parameter circuit, it is easy to produce skin effect and coupling effect 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: digital circuit and analog Problems such as mutual interference between circuits, noise interference of power supply, and interference caused by unreasonable grounding. Because of this, how to weigh the pros and cons to find a suitable compromise in the PCB design process, reduce these interferences as much as possible, and even avoid the interference of some circuits, is the key to the success or failure of the RF circuit PCB design. From the perspective of PCB LAYOUT, the article provides some processing skills, which are of great use to improving the anti-interference ability of radio frequency circuits.

1. RF Layout

Discussed here is the layout of the components of the multilayer board. The key to the layout of the 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, sensitive analog The signal is far away from high-speed digital signals and RF signals.

The following techniques are often used in the layout.

• 1.1 In-line layout

The components of the RF main signal adopt a linear layout 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 is best not to use U-shaped layout (as shown in Figure 2). Sometimes it is unavoidable. May increase the distance between input and output, at least 1.5cm or more.

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 is best not to turn as soon as it enters the interface, as shown on the left in Figure 3, but after a slight straight line, as shown in the right image of Figure 3.

Figure 3 Two options

• 1.2 Same or symmetrical layout

The same modules are made into the same layout or symmetrical layout as much as possible, as shown in Figure 4 and Figure 5.

Figure 4 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 degree 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-degree layout

2. RF Wiring

The overall requirements for wiring are: RF signal traces are short and straight, reduce line abrupt changes, drill fewer vias, and do not intersect with other signal lines, and add ground vias as much as possible around RF signal lines. The following are some commonly used optimization methods:

• 2.1 Gradient line processing

In the case where 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 the return loss. As shown in Figure 9.

Figure 9 Arc line

• 2.3 Ground wire and power supply

The ground wire is as thick as possible. Under conditions, 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 minimize the ground impedance.

The power supply of the RF circuit should not be divided into planes as far as possible. The whole 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 line 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 should 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 as far as possible.

When the RF signal crosses other signal traces, try to arrange a layer of ground connected to the main ground between them along the RF trace. If it is not possible, make sure that they are crossed. Other signal traces here also include power lines.

• 2.5 land parcel processing

Packet processing 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 Package land processing

• 2.6 Copper foil treatment

The copper foil processing requires smooth and smooth, no long lines or sharp corners are allowed. If it cannot be avoided, fill a few ground vias at the sharp corners, slender copper foil or the edge of the 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 grounded, 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. The width of the edge of the ground-clad copper is 2W or the height of 3H from the RF line, and H represents the total thickness of adjacent dielectric layers.

3. Cavity Handling

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 entire circuit branch is placed, it is the processing of the shielding cavity. The processing of the shielding cavity has the following precautions:

The whole shielding cavity should be made into a regular shape as far as possible to facilitate casting. Try to make each shielding cavity rectangular, avoiding square shielding cavity.

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. The interface line is usually introduced into the cavity using a strip line or a microstrip line, while the different modules inside the cavity use a microstrip line, and the joints of different cavities are processed with grooves. The width of the grooves It is 3mm, and the microstrip line runs in the middle.

A 3mm metalized hole is placed at the corner of the cavity to fix the shielding shell, and the same metalized hole should be evenly placed 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 same row of vias is 150MIL.