Precautions for laying high-frequency and high-speed signal lines on the side of the PCB board

We often see some design principles about high-frequency and high-speed signals in textbooks or the original PCB Design Guide. This includes avoiding high-speed signal lines at the edges of PCB circuit boards. For the design of on-board PCB antennas It is also recommended to place the antenna as close to the edge of the board as possible. What is the science behind this?

 

We already knew at the junior high school level that in the Ampere's right-hand rule, the current of a wire propagates along the direction of the thumb, and a corresponding magnetic field is generated on the wire. The direction of the magnetic field is the same as the direction of the fist of the right finger, and the charged charge in the conductor will generate Electric field, electric field and magnetic field are a good pair of base friends, collectively called electromagnetic field.

 

According to Maxwell's electromagnetic field theory, a changing electric field generates a changing magnetic field in its surrounding space, and a changing magnetic field generates a changing electric field. In this way, the changing electric field and the changing magnetic field are mutually dependent, mutually excited, and alternately generated, and propagate out of space at a certain speed from near and far, which is electromagnetic radiation. This has two diametrically opposite effects: On the plus side, all RF communications, wireless interconnects, and inductive applications benefit from the benefits of electromagnetic radiation. On the downside, electromagnetic radiation causes crosstalk and electromagnetic compatibility. Aspects.

 

When the frequency of electromagnetic waves is low, it is mainly transmitted by tangible electrical conductors; when the frequency is gradually increased, the electromagnetic waves will spill out of the conductor, and they can transfer energy without a medium, which is a kind of radiation. In low-frequency electrical oscillations, the mutual change between magnetoelectricity is relatively slow, and almost all of its energy is returned to the original circuit without radiating it out. However, in high-frequency electrical oscillations, magnetoelectricity changes very quickly, and energy cannot be returned to the original oscillating circuit, so electrical energy and magnetic energy are propagated to space in the form of electromagnetic waves with the periodic changes of the electric and magnetic fields.

 

According to the above theory, each section of high-frequency current flowing through the wire will have electromagnetic radiation, and the radiation intensity is proportional to the frequency. Some wires on the PCB are used for signal transmission, such as DDR clock signals, LVDS differential signal transmission lines, etc., do not want to have too much electromagnetic radiation to lose energy and cause interference to other circuits in the system; and some wires are used as antennas, Such as PCB antenna, it is hoped that it can convert energy into electromagnetic waves as much as possible and emit them.

 

For high-speed signal transmission lines on PCBs (such as: DDR clock signals, HDMI LVDS high-speed differential transmission lines), we always want to minimize the radiation generated during signal transmission, and the methods to reduce electromagnetic radiation generated by signal transmission lines have been summarized by bricklayers. There are some design principles. If you want to reduce the EMI of a signal transmission line, try to make the distance between the signal transmission line and the reference plane forming the signal return path as close as possible. If the ratio of the width W of the transmission line to the distance H of the reference plane is less than 1: 3 , It can significantly reduce the external radiation intensity of the microstrip transmission line.

 

For a microstrip transmission line, a wide and complete reference plane can also reduce the external radiation intensity of the electric field. The reference plane corresponding to the microstrip transmission line must be at least three times the width of the transmission line, and the wider the reference plane, the better.

 

If the width of the reference plane is not large enough compared to the micro-single transmission line, the coupling between the electric field and the reference plane is small, and the external radiation of the electric field increases significantly.

 

Therefore, if you want to reduce the electromagnetic radiation of the microstrip transmission line, you need to make the reference plane of the microstrip transmission line as large as possible, and if the high-speed microstrip transmission line is parallel to the PCB board side, relatively The coupling of the reference plane to the high-speed signal line is reduced, and naturally, the external radiation of the electric field is significantly increased.

 

Similarly, high-speed IC, crystal oscillators, etc. should be placed as far away from the board edge as possible. High-speed IC also need a complete and wide reference plane for electromagnetic coupling to reduce EMI.

 

For on-board antennas, we want to radiate as many electromagnetic waves into the space as possible, so the design of the on-board antenna is contrary to the design principle of high-speed transmission lines. The on-board antenna needs to be placed near the board, and the antenna area is located. To ban copper foil planes, yes, all layers need to have copper foil banned areas. And the antenna should be separated from the ground plane of the PCB.

 

The same theory, there are different design principles for different application designs.