RF impedance This topic is actually discussed many times in the group, but many people are more blurred on this concept, explaining on the spot, and may have a design environment still do not know how to deal with it. The reason is that I don't know how it is. Everyone encounters a radio frequency signal, always conscious, is the compactment of all RF signals, the impedance control of all radio frequency signals Borrow the question of the study below, let's discuss this topic: Four-layer play-frequency signal compartment refers to 50 ohm impedance?
First let's take a look at the lamination of the regular four-layer board, as shown below.
The radio frequency signal generally takes the surface layer, and the separation reference is reference to the third layer Power03, then the thickness of the radio frequency signal distance from the radio frequency signal is 4 mil (PP thickness between 1, 2) when calculating the impedance. (The second layer of copper thickness) + XXX (the thickness of the middle Core). The thickness of the middle Core depends on the thickness. If we handle it according to the conventional plate thickness 1.6mm (63mil), the thickness of the intermediate Core = 63-1.6x2-4x2-1.2x2 = 49.4mil, the value of H1 is obviously H1 = 4 + 1.2 + 49.4 = 54.6mil. Then the 50 ohm radio frequency signal is made with the corresponding line width. We use the Si9000 to count, as shown below.
Obviously, this line is not a general thickness, and this line width cannot be achieved in the design process. Therefore, for the four-layer plates of conventional laminated, the radio frequency signal is not in the reference. Some students will ask, 1, 2 layers of PP thickness talents 4 mils are too thin to cause the middle Core too thick to make the value of H1 is large; if the 1, 2 layer PP thickness, the value of H1 is small, A reasonably radio frequency resistance should be obtained. It seems that it seems to be quite reasonable, then we will increase, add 10 mils should be similar, as shown below.
However, the thickness of the 1, 2 layer 10 mils is not used to make a partition reference, and it is not believed.
17.5 line width, 50 ohm impedance, RF signal refers to adjacent layers, which has been very compliant with the line width and impedance requirements of the RF signal, and no compartment reference is required. However, the data lines and address lines of our surface-layer DDR are also referring to the adjacent layers to do 50 ohm impedance, DDR is so densely routed 17.5 line width? It is definitely not suitable! If there is a large number of radio frequency signals on the board, there is no other important signals, the impedance requirements of the non-important signal signal can be ignored, and the impedance of the radio frequency signal can be given priority, which is suitable.
Therefore, in summary, the four-layer slave radio signal compartment refers to 50 ohm impedance. You can't do it, you can't refer to the adjacent layers, then? Please see the words of the figure with the ellipse.
Yes, do coplanar. For conventional laminated, refer to the second layer, 50 ohmic impedance lines are only 5mil, and the wire width requirements of the RF signal are not met. In order not to let it refer to the second floor, we have to dig the second layer or even the third floor, and the fourth layer retains a complete place, then do coplanar impedance, please see the picture below.
G1, G2 is the width of the co-impedance, the width is not a fixed value, a little bit, a little bit, the relationship is not large, don't be too fine. Do not believe that the value of G1, G2, the impedance change is not large, mainly the distance between the distribution of the radio frequency line D1 is relatively large, we are set to 5 mils. The final 50 ohm radio frequency line width is 22mil. Read the full article, original title: four-layer slave radio signal compartment reference does not do 50 ohm impedance?
Article Source: [Micro Signal: PCBTech, WeChat public number: EDA Design Smarthui] Welcome to add attention! Please indicate the source of the article.
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