Many people are confused about the reference plane of the PCB line, and it is often said that for the inner lining, if the line side is VCC, the other side is GND, which is the reference plane? To understand this problem, we must understand the concept of transmission line. We know that the transmission line must be used to analyze the signal transmission on the PCB, in order to explain the various phenomena in high-speed circuit. The simplest transmission line consists of two basic elements: the signal path, the reference path (also known as the return path). The signal is transmitted in the form of electromagnetic wave in the transmission line, and the two basic elements of the transmission line constitute the physical environment of electromagnetic wave transmission. From the point of view of electromagnetic wave transmission, the signal path and the reference path together constitute a special physical structure. In terms of the current loop, the signal path carries the signal current, and the reference path carries the return current.

On the surface of the PCB line, the line and the plane below the composition of the electromagnetic wave transmission of the physical environment. Here, the plane line below what the network property does not matter, VCC, GND, and even isolated copper skin without network, can constitute the electromagnetic wave propagation in such environment, the key lies in the plane is a conductor, it is enough. The signal path is the surface layer, so the following plane is the reference path. For the special structure of PCB, the reference path appears in the form of a plane, so it is also called the reference plane. From the point of view of the current loop, the reference plane carries the return current of the signal. The following figure shows the field distribution and current distribution. Here the role of the reference plane should be clear: as part of the electromagnetic wave transmission physical environment (from the electromagnetic wave transmission angle), as the current return path (from the current loop angle).

If you follow the above logic, then take the inner line of the reference plane in which it is clear, walk the line, above the plane below the plane, the 3 together constitute the physical environment of the electromagnetic wave transmission and so on two planes are the reference path signal, which is the reference plane, from the distribution chart below the can clearly see the physical environment and distribution relationship. From the perspective of a current loop, the current distribution is also clearly show that the distribution of the return current, if the distance between the two plane and line is approximately equal to two, then the plane of the return current is approximately the same, at this time, the two planes are equally important. From this point of view can also be a good understanding of the two planes are reference plane. If you still can not understand why the two planes are the reference plane, not to take a look at the proof below, no matter from which point of view, the two plane is completely symmetrical, which is why the back reference plane, if one is, the other one is why not?

The most direct way to understand the reference plane is the physical environment of electromagnetic wave transmission". Look at the structure of the puzzle, whether it meets this condition? Is there a wide network free copper skin? Of course, the conductor must be "flat form" can be referred to as the "reference plane", or how to say the plane! It is pointed out that if the distance between the two planes and the line is approximately equal (which is very common on the board in a dozen layers), the importance of the two planes is similar to that of the line. Another kind of situation we will encounter in the actual project, one of the two plane distance line close to another line far distance, such as the typical 6 layer configuration, a core plate in the middle (Core) in thickness is usually more than 1 mm. The following figure is an example of a 6 layer laminate, the inner layer of the two signal layers InnerSignal1 and InnerSignal2 are in this case, then the effect of the two plane on the inner line is certainly different. Which is the reference plane?

Look at the return current on the plane, and how much of the return current is observed for the VCC plane away from it by InnerSignal2. The following figure is a red signal current 10mA, blue is the return current on the VCC is about 1.2mA. Far from the InnerSignal2, the return current on the VCC is very small, and there is nearly 90% return current when it is returned from the GND plane next to InnerSignal2, and the effect of the GND plane on the InnerSignal2 is much larger than that of the VCC plane. The influence of VCC plane on the line impedance of InnerSignal2 layer. This will have the VCC plane and VCC plane no two case wire impedance made a comparison, even if the removed VCC plane did not have a fatal impact on line impedance, impedance variation of less than 1 ohms, the change rate is less than 2%. From the engineering point of view, it can be assumed that the InnerSignal2 layer is the closest GND plane.

Although this approximation can be, but the key is to be clear from the InnerSignal2 layer is far away from the VCC plane is not without influence, but little effect. Don't take any time of absolute, such is harmful for beginners but no good, if always pursue, either this or that black and white, is likely to be a dead end.

High speed circuit signal integrity design, a lot of problems are like this, you have to pay attention to the problem is not "yes" or "no", but "more" and "less" problem. When the circuit board in response to no unreasonable to have enough cards to prevent it Liaotiaozi temper, to maximize the control of it.