The principle of high-speed signal lines on the edge of the print circuit board

We often see some design principles about high-frequency and high-speed signals in textbooks or original PCB Design Guide, which includes not to take high-speed signal lines on the edge of the PCB circuit board. But for the design of on board antenna, then recommended should be placed as close as possible to the edge of the board. 

What is the scientific truth?

We already knew in the middle school that in the Ampere's Right Hand Grip Rule, the wire current propagates along the direction of the thumb, and the corresponding magnetic field will be generated on the wire. The direction of the magnetic field is consistent with the direction of the fist of the right hand, and the electric charge in the conductor will generate an electric field, the electric field and magnetic field are a pair of good friends, collectively called electromagnetic field.

According to Maxwell's electromagnetic field theory, a changing electric field produce a changing magnetic field in its surrounding space, and a changing magnetic field produce a changing electric field. In this way, the changing electric field and the changing magnetic field depend on each other, excite each other, generate alternately, and propagate out in space from near to far at a certain speed, which is called electromagnetic radiation. Here are two diametrically opposed effects: on the good side, all RF communications, wireless interconnection, and inductive applications all benefit from the electromagnetic radiation; on the harmful side, electromagnetic radiation causes problems such as cross-talk and electromagnetic compatibility.

When the frequency of the electromagnetic wave is low, it can mainly be transmitted by the tangible electrical conductor; when the frequency is gradually increased, the electromagnetic wave will overflow out of the conductor, and the energy can be transmitted outward without the 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 returns to the original circuit without energy radiation. However, in high-frequency electrical oscillations, magnetoelectricity changes very quickly, and energy cannot return to the original oscillatory circuit, so electrical energy and magnetic energy propagate out into space in the form of electromagnetic waves as the electric field and magnetic field change periodically.

According to the above theory, each section of wire passing high-frequency current 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; while some wires are used as antennas, Such as PCB antenna, it is hoped that the energy can be converted 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 want to minimize the radiation generated during signal transmission. Some experts have summarized some design principles. If you want to reduce the EMI of the signal transmission line, try to make the distance between the signal transmission line and the reference ground plane forming the signal return path as close as possible. If the ratio of the width of the transmission line to the distance of the reference ground plane is less than 1:3, the external radiation intensity of the micro strip transmission line can be significantly reduced.

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

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

Therefore, if you want to reduce the electromagnetic radiation of the height signal transmission microstrip line, the reference ground plane corresponding to the microstrip transmission line needs to be as large as possible. The coupling of the reference ground plane to the high-speed signal line becomes less, which naturally causes a significant increase in the external radiation of the electric field.

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

For onboard antennas, we want to radiate electromagnetic waves into space as much as possible, so the design of onboard antennas is contrary to the design principle of high-speed transmission lines. Onboard antennas need to be placed on the edge of the board and the position of the antenna area need prohibit the copper foil ground plane, yes, all layers need to set copper foil prohibited area. And the antenna should be separated from the ground plane of the PCB.

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