RF circuit layout

PCB Stack Up

 

Before wiring, we have to make sure the circuit board is stacked, just like building a house with walls. The determination of circuit board stack structure is related to the complexity of circuit design and the consideration of electromagnetic compatibility. The following diagram shows the common stacking methods of four-layer, six-and eight-layer .

 

stack up

 

Problems that should be paid attention to in the Design of double side Board

 

However, in the case of two layers, it is not the same. In two layers, in order to ensure the strength of the circuit board, we can't do it with a very thin circuit board, and the distance between the top floor and the bottom layer (the reference plane) is going to be very large, if the characteristic impedance of 50 ohms is still controlled in the same way. Then the top line must be wide. For example, let's assume that the thickness of the board is 39.6mil (1mm), which is normally designed in Polar, as shown in the figure below

   

pcb stack up

Impedance control

 

After our principle design and simulation, one of the important things in Layout is impedance control. As we all know, we should try our best to ensure that the characteristic of the line is 50 ohms, which is mainly related to the linewidth. In this case, it is two and a half layers. In Polar, the Surface Coplanar Line model is used to calculate the impedance. A group of ideal values: Height (H) = 39.6 milt, Track (W) = 30milr, Track (W1) = 30milr Thicknessessi 1OZZN 1.4mill, Separation (S) = 7milr, Dielectric (Er) = 4.2. The corresponding characteristic impedance is 52.14 ohms, which meets the requirements. The highlighted line below is such a radio-frequency line.

 

circuit line

 

Placement of RF components

 

I believe that people who have done RF design should know that we should make the wiring as short as possible, the more compact the components are arranged (except for special requirements), and, at the same time, As much as possible to ensure that the placement of components is very conducive to wiring (do not make running around). As shown below, the devices around the RF Power Amplifier (PA,Power Amplifier) are arranged, and we can see that the distance between the components is very small.

PAD

 

The problems that should be paid attention to in Radio Frequency Line

 

Radio-frequency line length should be as short as possible, line width is strictly in accordance with the calculated value to set. It is particularly important to note that there is no cusp in the radio frequency line. At the turning point of the line, it is best to use an arc, as shown in the figure below

PAD

 

Placement of VIAs

The ground wire next to the radio frequency line should preferably be perforated through a hole and be connected to the ground plane of the bottom or middle layer. This can be the shortest way to the ground of any interference signal or radiation, but, The distance between the crossing hole and the radio frequency signal line can not be too close, otherwise, it will seriously affect the quality of the radio frequency signal, and can be grasped flexibly in the actual design process. As shown in the following figure, we can see that there are many holes in the two layers of the highlight signal line.