"Reference plane problem for PCB cabling"

Note: belong to reprint   Many people for the PCB traces of the reference plane is confused, often asked: for the inner layer traces, if the line side is VCC, the other side is GND, then which is the reference plane? To figure this out, you have to understand the concept of transmission lines. We know that the transmission line must be used to analyze the signal transmission on the PCB in order to explain the various phenomena occurring in the high-speed circuit. The simplest transmission line consists of two basic elements: a signal path, a reference path (also known as a return path). The signal is transmitted in the transmission line in the form of electromagnetic waves, and the two basic elements of the transmission line constitute the physical environment of the electromagnetic wave transmission. From the perspective of electromagnetic wave transmission, signal path and reference path together constitute a special physical structure, electromagnetic waves in the structure of transmission. From the current loop angle, the signal path carries the signal current, and the reference path carries the return current, so the reference path is also known as the return path. For the surface traces on the PCB, the traces and the plane layer below constitute the physical environment of the electromagnetic wave transmission. Here, the lines below the line is exactly what the network properties do not matter, VCC, GND, or even no network of isolated copper skin, can constitute such an electromagnetic wave transmission environment, the key lies in the following plane is a conductor, this is enough. The signal path is the surface line, so the plane below is the reference path. For this special structure on the 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, so it is called a return plane. The following figure shows the field distribution and current distribution of the surface contour. The function of the reference plane here should be clear: as part of the physical environment of the electromagnetic wave transmission (from the angle of the electromagnetic wave transmission), as the current return path (from the current loop angle). <ignore_js_op>  If you understand the above logic, then the inner layer of the reference plane of the line is very clear, the line, the upper plane, the lower plane 3 together constitute the electromagnetic wave transmission of the physical environment, so the upper and lower two planes are the signal reference path, That is, the reference plane, the relationship between the physical environment and the field distribution can be clearly seen from the field map below. From the point of view of the current loop, the current distribution diagram also clearly shows the distribution of the return current, if the spacing between the two plane and the line is approximately equal, then the return current on two planes is approximately equal, at this time, two planes are equally important. From this angle can also be very good understanding of two planes are reference plane. If still can not understand why two planes are reference plane, do not take a good look at the following figure, regardless of which aspect, two plane is completely symmetrical, why also tangled which is the reference plane, if one is, then another why not? <ignore_js_op>  the most straightforward way to understand the reference plane is to "form the physical environment in which electromagnetic waves are transmitted." Look at the confused knotTo meet this condition? Is it possible to have a wide network of isolated copper skins? Of course, the conductor must be "flat form" to be called the "reference plane", or how to say the plane!

"Reference plane problem for PCB cabling"