Dec. 05, 2019
Because the radio frequency (RF) circuit is a distributed parameter circuit, it is easy to produce skin effects and coupling effects in the actual work of the circuit. Therefore, in actual PCB design, it will be difficult to control the interference radiation in the circuit, such as digital circuits and analog Interference between circuits, noise interference from power supply, interference caused by irrational ground, etc. Because of this, in the PCB design process, how to weigh the advantages and disadvantages to find a suitable compromise point, minimize these interferences, and even avoid the interference of some circuits, is the key to the success of RF circuit PCB design. From the perspective of PCB layout, this article provides some processing techniques, which is of great use to improve the anti-interference ability of RF circuits.
What is discussed here is the component location layout of the multilayer board. The key to the component location layout is to fix the components located on the RF path. By adjusting its direction to minimize the length of the RF path and keep the input away from the output, separate the high-power circuits and low-power circuits as far as possible. Keep the signal away from high-speed digital and RF signals.
The following techniques are often used in layouts.
1.1 Glyph layout
The components of the RF main signal are arranged in a zigzag pattern as much as possible. However, due to the space constraints of the PCB board and the cavity, many times cannot be arranged in a zigzag shape. At this time, an L-shape can be used. It is better not to use a U-shape layout. Sometimes it can not be avoided. The distance between them should be at least 1.5cm. In addition, when using an L-shaped or U-shaped layout, the turning point is best not to turn just after entering the interface, but to turn after a short straight line.
1.2 Identical or symmetrical layout
The same modules are made as much as possible or symmetrical.
1.3 Cross layout
The feeding inductance of the bias circuit is placed vertically with the RF channel, mainly to avoid mutual inductance between inductive devices.
1.4 45 degree layout
In order to use space reasonably, the device can be laid out in a 45-degree direction to keep the RF line as short as possible.
2. Cavity treatment
For the entire RF circuit, the RF units of different modules should be separated by a cavity, especially between sensitive circuits and strong radiation sources. In high-power multi-stage amplifiers, isolation between stages should also be guaranteed. After the tributary of the entire multilayer PCB circuit is placed, it is the treatment of the shielding cavity. The handling of the shielding cavity has the following precautions:
The whole shielding cavity is made into a regular shape as far as possible to facilitate the casting. For each shield cavity, try to make it rectangular, avoid square shield cavity.
The corner of the shield cavity is arc-shaped, and the shield metal cavity is generally cast-molded. The arc-shaped corner is convenient for drafting during casting.
The periphery of the shielding cavity is sealed. The lead-in of the interface is generally a strip line or a microstrip line. The different modules inside the cavity are microstrip lines. For 3mm, the microstrip line goes in the middle.
3mm metallization holes are placed at the corners of the cavity to fix the shielding shell. The same metallization holes should be evenly placed on each long cavity to strengthen the supporting effect.
The cavity is generally opened to facilitate the welding of the shielding shell. The cavity is generally thicker than 2 mm. Two rows of window openings are added to the cavity.