High-speed signal circuit pcb board design attention, PCB manufacturing

2019-08-28 14:53:03

We saw some board design principles for high-frequency, high-speed signals, including those at the edge of the PCB board. Do not take the high-speed signal line, and for the board of the onboard PCB antenna, it is recommended that the antenna be placed as close as possible to the edge of the board. What is the scientific reason?

We know in the junior high school stage that in the right-hand rule of Ampere, the wire current propagates in the direction of the thumb, and the 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 hand finger, and the charged charge in the conductor will The electric field, the electric field and the magnetic field are generated as a pair of good friends, collectively referred to as electromagnetic fields.

According to Maxwell's electromagnetic field theory, a changing electric field produces a changing magnetic field in its surrounding space, and a changing magnetic field produces a changing electric field. Thus, the varying electric field and the changing magnetic field are interdependent, mutually excited, alternately generated, and propagated in space from near and far at a certain speed. This is electromagnetic radiation. This has two diametrically opposite effects: on the positive side, all RF communications, wireless interconnects, and inductive applications benefit from the benefits of electromagnetic radiation; and the harmful side is that electromagnetic radiation causes crosstalk and electromagnetic compatibility. Aspects of the problem.

When the electromagnetic wave frequency is low, it can only be transmitted by the tangible electric conductor; when the frequency is gradually increased, the electromagnetic wave will overflow outside the conductor, and the medium can transmit energy without the medium. This is a type 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 energy radiation. However, in high-frequency electrical oscillations, the magnetoelectric interactions become very fast, and energy cannot be reversed back to the original oscillating circuit, so that the electrical energy and magnetic energy propagate into the space in the form of electromagnetic waves as the electric field and the magnetic field change periodically.

According to the above theory, each section of the wire flowing through the high-frequency current will have electromagnetic radiation, and the radiation intensity is proportional to the frequency. There are wires on the PCB for signal transmission, such as DDR clock signals, LVDS differential signal transmission lines, etc., it is not expected to have too much electromagnetic radiation loss energy and cause interference to other circuits in the system; and some wires are used as antennas. For example, PCB antennas, it is desirable to convert energy into electromagnetic waves as much as possible.

For high-speed signal transmission lines on PCBs (such as DDR clock signals, HDMI LVDS high-speed differential transmission lines), we always hope to reduce the radiation generated during signal transmission and reduce the electromagnetic radiation generated by signal transmission lines. Some design principles have been made. If the EMI of the signal transmission line is to be reduced, the distance between the signal transmission line and the reference plane constituting the signal return path should be 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 , can significantly reduce the external radiation intensity of the microstrip transmission line.

For the microstrip transmission line, the 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 should be at least three times the width of the transmission line, and the wider the reference plane, the better.

If the reference plane is not sufficiently wide relative to the micro-single transmission line, the coupling of the electric field to the reference plane is small, and the external radiation of the electric field is significantly increased.

Therefore, if the electromagnetic radiation of the high-signal transmission microstrip line is to be reduced, the reference plane corresponding to the microstrip transmission line needs to be as large as possible, and if the high-speed microstrip transmission line is parallel to the side of the PCB, it is relatively speaking. 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 ICs, crystal oscillators, etc. are also placed as far as possible from the edge of the board. High-speed ICs also require a complete and wide reference plane for electromagnetic coupling to reduce EMI.

For on-board antennas, we want to radiate electromagnetic waves as much as possible into the space. Therefore, the design of the on-board antenna is opposite to the principle of PCB design for high-speed transmission lines. The on-board antenna needs to be placed on the side of the board, and the antenna area is located. The position of the copper foil is prohibited from being placed, and all the layers need to be provided with a copper foil prohibited area. Moreover, the antenna should be spaced apart from the ground plane of the PCB.

The same theory has different PCB board design principles for different applications.

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