The frequency range of the RF circuit is approximately 10 kHz to 300 GHz. As the frequency increases, the RF circuit exhibits some characteristics different from the low frequency circuit and the DC circuit. Therefore, when designing the PCB of the RF circuit, special attention must be paid to the impact of the RF signal on the PCB. The RF switching circuit is controlled by the VXI bus. In order to reduce interference in the design, a cable connection is used between the bus interface circuit part and the RF switching function circuit. The following mainly introduces the design of the PCB board of the RF switching function circuit part.
1. The layout of components
Electromagnetic compatibility (EMC) refers to the ability of an electronic system to function properly in accordance with design requirements in a defined electromagnetic environment. For RF circuit PCB design, electromagnetic compatibility requires that each circuit module does not generate electromagnetic radiation as much as possible, and has certain anti-electromagnetic interference capability. The layout of the components directly affects the interference and anti-interference ability of the circuit itself. It also directly affects the performance of the designed circuit.
The general principle of layout: components should be arranged in the same direction as possible, by selecting the direction of the PCB into the molten tin system to reduce or even avoid poor soldering; at least 0.5mm spacing between components to meet the melting requirements of components If the space of the PCB is allowed, the spacing of components should be as wide as possible.
The rational layout of components is also a prerequisite for proper wiring, so it should be considered comprehensively. In this design, the relay is a channel for converting the RF signal, so the relay should be as close as possible to the signal input end and the output end, so as to minimize the length of the RF signal line, and make the next reasonable wiring. consider.
In addition, the RF switching circuit is controlled by the VXI bus, and the influence of the RF signal on the VXI bus control signal is also a problem that must be considered when laying out.
After the basic layout of the components is completed, the wiring must be started. The basic principle of the wiring is as follows: When the assembly density is allowed, the low-density wiring design should be selected as much as possible, and the signal traces should be as thick as possible to facilitate impedance matching.
For RF circuits, the unreasonable design of the direction, width and line spacing of the signal lines may cause cross-interference between the signal transmission lines. In addition, the system power supply itself also has noise interference, so it must be considered comprehensively when designing the RF circuit PCB. Reasonable wiring.
When wiring, all the traces should be away from the border of the PCB board (about 2mm), so as to avoid the hidden trouble of disconnection or disconnection when the PCB board is made. The power cable should be as wide as possible to reduce the loop resistance. At the same time, the direction of the power line and the ground line and the direction of data transmission are the same to improve the anti-interference ability. The signal line should be as short as possible, and the number of vias should be reduced as much as possible; the shorter the connection between the components, the better, to reduce the distribution parameters and mutual electromagnetic interference; for incompatible signal lines should try to stay away from each other And try to avoid parallel routing, and the signal lines on both sides should be perpendicular to each other: when wiring is required, it should be 135 degree angle, avoiding the right angle.
In the above design, the PCB board uses four layers. In order to reduce the influence of the RF signal on the VXI bus control signal, the two signal traces are placed in the middle two layers, and the RF signal lines are shielded by the ground via.
3. Power and ground
The wiring in the RF circuit PCB design requires special emphasis on proper wiring of the power and ground lines. Reasonable choice of power and ground wiring is an important guarantee for reliable operation of the instrument. A considerable number of sources of interference on the PCB board of the RF circuit are generated by the power supply and the ground line, and the ground line causes the largest noise interference. According to the current of the PCB board, the power line and ground line design should be as thick and short as possible to reduce the loop resistance. At the same time, the direction of the power line and the ground line are consistent with the direction of data transmission, which helps to enhance the anti-noise capability. When possible, multi-layer boards are used. The four-layer board is 20dB lower than the double-panel noise, and the six-layer board is 10dB lower than the four-layer board.
In the four-layer PCB board, both the top layer and the bottom layer are designed as ground layers. In this way, no matter which layer of the middle layer is the power layer, the physical relationship between the power layer and the ground layer are close to each other, forming a large decoupling capacitor, which reduces the interference caused by the ground line.
The ground layer is covered with copper in a large area. Large area copper plating has the following main functions:
(1) EMC. For a large area of ground or power supply copper, it will play a shielding role.
(2) PCB process requirements. Generally, in order to ensure the plating effect, or the laminate is not deformed, copper is laid for the PCB layer with less wiring.
(3) Signal integrity requirements, giving a high-frequency digital signal a complete return path and reducing the wiring of the DC network.
(4) Heat dissipation, special device installation requirements, copper plating, etc.
The VXI bus system is a modular instrument bus system that is completely open in the world and is suitable for multiple manufacturers. It is the latest instrument bus system in the world. The above mainly introduces the development of RF switch module based on VXI bus. The design of the bus interface and the design of the PCB board of the functional circuit part of the RF switch module are introduced. The RF switch is controlled by the VXI bus, which increases the flexibility of the switch operation and is easy to use.