Precautions for running high-frequency and high-speed signal lines near PCB board - basic principles for high-frequency and high-speed signal design

We often read in textbooks or in the original factory PCB Design Guide See some design principles for high-frequency and high-speed signals , It includes PCB Do not use high-speed signal lines at the edge of the circuit board , For on-board PCB Antenna design , It is also recommended that the antenna should be placed as close to the board edge as possible . fuck , What is the scientific truth ？ Old here wu Combined with some of my shallow knowledge , Let's talk about the high-frequency and high-speed signal lines in PCB What happens to the edge of the board , The following represents the old wu My personal point of view , There is no guarantee of correctness , Please identify by yourself ,O(∩_∩)O~

We already know in junior high school , The right hand rule tells us that , The current in the wire travels in the direction of the thumb , Then a corresponding magnetic field will be generated on the conductor , The direction of the magnetic field is the same as that of the fist clenched by the fingers of the right hand , The electric charge in the conductor will generate an electric field , Electric and magnetic fields are good friends , Collectively referred to as electromagnetic fields .

Ampere right hand screw rule

According to Maxwell's electromagnetic field theory , A changing electric field produces a changing magnetic field in its surrounding space , And the changing magnetic field will produce the changing electric field . such , There is an interdependence between the changing electric field and the changing magnetic field , Mutual excitation , Alternately produce , And spread out in space from near to far at a certain speed , This is electromagnetic radiation . This has two diametrically opposite effects ： The good , be-all RF signal communication 、 Wireless Internet 、 Inductive applications benefit from the benefits of electromagnetic radiation ; The harmful aspect is , Electromagnetic radiation leads to crosstalk and electromagnetic compatibility problems .

When the frequency of electromagnetic wave is low , It is mainly through a tangible conductor that it can be transmitted ; As the frequency increases , Electromagnetic waves will spill out of the conductor , It can transfer energy without medium , This is a kind of radiation . In low-frequency electrical oscillations , The mutual change between magnetoelectricity is relatively slow , Almost all of its energy is returned to the original circuit without energy radiation . However , In high frequency electrical oscillations , Magnetoelectric interconversion is very fast , It is impossible for the energy to return to the original oscillation circuit , So electricity 、 With the periodic change of electric field and magnetic field, magnetic energy propagates to space in the form of electromagnetic wave .

According to the above theory , Each section of wire flowing through high-frequency current will have electromagnetic radiation , The radiation intensity is proportional to the frequency .PCB Some wires on the are used for signal transmission , Such as DDR Clock signal ,LVDS Differential signal transmission line, etc , You don't want too much electromagnetic radiation to lose energy and cause interference to other circuits in the system ; Some wires are used as antennas , Such as PCB The antenna , I hope to convert energy into electromagnetic waves and send them out as much as possible .

about PCB For high-speed signal transmission lines on （ Such as ：DDR Clock signal ,HDMI LVDS High speed differential transmission line ）, We always want to minimize the radiation generated during signal transmission , Some experts have summarized some design principles to reduce the electromagnetic radiation generated by signal transmission lines , To reduce the of the signal transmission line EMI, Make the distance between the signal transmission line and the reference plane constituting the signal return path as close as possible , If the width of the transmission line W Distance from reference plane H The ratio of is less than 1：3, The external radiation intensity of the microstrip transmission line can be significantly reduced .

For microstrip transmission lines , Using 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 the 3 Times the width , The wider the reference plane, the better .

If the reference plane is not wide enough relative to the micro single transmission line , The coupling between the electric field and the reference plane is small , The radiation from the electric field increases significantly .

So , If we want to reduce the electromagnetic radiation of high signal transmission microstrip line , The reference plane corresponding to the microstrip transmission line should be as large as possible , If the high-speed microstrip transmission line is close to PCB If the board edge is parallel to the line , Relatively speaking , The reference plane is less coupled to the high-speed signal line , Naturally, it will cause a significant increase in the external radiation of the electric field .

Empathy , High speed IC, The crystal oscillator shall also be placed away from the board edge as far as possible , High speed IC It also needs a complete and wide reference plane for electromagnetic coupling , To reduce EMI.

For onboard antennas , We hope to radiate electromagnetic waves into space as much as possible , Therefore, the design principle of onboard antenna is opposite to that of high-speed transmission line , The onboard antenna needs to be placed at the edge of the board , Moreover, the position of the antenna area shall be free of copper foil plane , Yes , All layers need to be set with a copper foil prohibition zone . And the antenna should be connected with PCB The ground level of .

summary ：

The same theory , There are different design principles for different applications , The knowledge of electronic design is really broad and profound , It's not something that happens overnight . It's just like that old saying , Three generations of poor farmers , Electrons destroy life … Be careful, be careful ！ The old wu I advise you to be cautious when entering the industry O(∩_∩)O~

Reprinted from Wu Chuanbin's blog http://www.mr-wu.cn/