[Repost] Brief analysis of the points to pay attention to in RF circuit design

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[Repost] Brief analysis of the points to pay attention to in RF circuit design [Copy link]

I have been engaged in RF R&D for many years. I would like to share my experience in RF circuit design with you. I hope that the following brief discussion will be helpful to engineers who do RF design work. Let's get straight to the point. Let's learn about the relevant content together.

The reference design of the EVB board makes us get twice the result with half the effort

When we design a new RF chip, we are required to quickly understand the performance indicators of the RF circuit of this chip and the requirements for peripheral devices, and quickly complete the design work of this part. The first thing we need to do is to carefully read and understand the chip specification and the design and precautions of the reference board. This plays a key role in the success or failure of our first version of the design, especially some RF chips and some specific RF devices (such as external PA LNA BPF, etc.) of the RF peripherals. This is particularly important.

Let's look at an example of RF circuit in the figure below:

[font= [Microsoft YaHei]This circuit is a circuit that combines a high-performance RF chip and an integrated BALUN (the IC in the yellow part of the figure above). Since there is no matching circuit between the main chip and the BALUN, this part is completely matched by the line width and line length of the RF routing and the impedance control of the differential signal. It cannot be debugged later through peripheral devices. Therefore, there are strict requirements for the routing of the differential part (as shown in the red mark in the figure above). The quality of the routing and impedance control will directly affect the quality of the later debugging indicators of the entire RF part. Therefore, the design requirements of the EVB board of the RF chip must be referred to in the early design stage, so that you can get twice the result with half the effort.

RF circuit layout should be done in mind

In the design process of many of our projects, the design cost will be an important part of our design process that we pay more attention to. The cost of our RF circuit part is mainly reflected in whether the peripheral circuit is built with discrete devices or integrated ICs. The cost of using integrated circuits for the peripheral RF part is higher than that of using discrete devices. So here we mainly talk about what matters need to be paid attention to in the peripheral circuit of discrete devices.

The following figure is a partial circuit of a path in a design:

Since the peripheral circuits are built with capacitors and inductors, the layout of this part of the circuit becomes more important under the premise that the size of our board is getting smaller and smaller.

• RF components should be placed on the same layer as much as possible, and RF routing should be avoided from drilling to the other side. This layout is mainly to reduce the risk of RF index problems caused by drilling on the PCB board.
• Considering the integrity of RF routing and grounding, if large-package devices cannot be guaranteed, try to use small-package devices to save more space for complete grounding.
• If the designed circuit has multiple WLAN, BLE, and Zigbee circuits, the mutual interference and coexistence of multiple signals require us to consider the routing layout in the early stage of design, and try to do a good job of board routing and device placement to leave a safe distance, so as to reduce our requirements for uncontrollable performance of the product in the later stage.

Control the process to make the product performance in one step

All the efforts in the early stage of circuit design are to make the product a qualified product. For RF products, process control is particularly important. The specific points are as follows:

• PCB process, RF board has strict requirements on PCB board material and RF part trace impedance control. The requirements of RF trace impedance of 50 ohms and differential impedance of 100 ohms need to be confirmed with the PCB board factory whether the board factory can do it. Try to make boards in a factory that has done RF PCB.
• Preliminary debugging has requirements for the brand of materials used. The brand of debugging materials and mass production materials must be consistent.
• For PCB suppliers, the early R&D proofing and mass production must ensure the consistency of PCB manufacturers. If you want to import multiple PCB suppliers, you need to verify the RF indicators of PCB boards provided by different manufacturers.

RF circuit design requires R&D engineers to have a long-term work experience. There is no shortcut. An excellent RF engineer must have rich experience in RF product development and debugging. This article is only for some work experience, welcome to communicate with everyone.

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懒猫爱飞

The impedance of the antenna is mainly the impedance to the ground. The wiring around the antenna has high requirements for the ground.