Misunderstandings in PCB Fifferential Signal Design (1)

The current popular LVD (ow voltage differential signaling) refers to this small amplitude differential signaling technology. For PCB engineers, the most concern is how to ensure that these advantages of differential wiring can be fully utilized in actual wiring. Perhaps anyone who has been in touch with layout will understand the general requirements of differential wiring, that is, “equal length and equal distance”. The equal length is to ensure that the two differential signals keep the opposite polarity at all times and reduce the common mode component. The equal distance is mainly to ensure that the differential impedance of the two is consistent and reduce the reflection. “As close as possible” is sometimes one of the requirements of differential wiring. But all these rules are not used to mechanically apply, and many engineers seem to still not understand the essence of high-speed differential signal transmission. The following focuses on several common misunderstandings in PCB differential signal design.

Misunderstanding 1: The differential wiring must be very close.
Keeping the differential traces close is nothing more than to enhance their coupling, which can not only improve immunity to noise, but also make full use of the opposite polarity of the magnetic field to offset electromagnetic interference to the outside world. Although this approach is very beneficial in most cases, it is not absolute. If we can ensure that they are fully shielded from external interference, then we do not need to use strong coupling to achieve anti-interference and suppressing EMI. How can we ensure that the differential trace has good isolation and shielding? Increasing the distance between other signal traces is one of the most basic ways. The electromagnetic field energy decreases with the square of the distance, and the general line spacing exceeds 4< times. When the line is wide, the interference between them is extremely weak and can basically be ignored. In addition, isolation through the ground plane can also play a good shielding effect. This structure is often used in the design of high-frequency (above 10G) IC package PCBs. It is called a CPW structure, which can ensure strict differential impedance control (2Z0).

Misunderstanding 2: Keeping equal spacing is more important than matching line length.
In the actual PCB layout, it is often unable to meet the requirements of differential design at the same time. Due to the existence of factors such as pin distribution, vias, and wiring space, the purpose of line length matching must be achieved through proper winding, but the result must be that some areas of the differential pair cannot be parallel. What should we do at this time? Before we draw conclusions, let’s take a look at the following simulation results. From the above simulation results, it can be seen that the waveforms of Scheme 1 and Scheme 2 are almost coincident, that is to say, the impact caused by the unequal spacing is minimal. In comparison, the line length mismatch has a much greater impact on the timing(Scheme 3). From the theoretical analysis, although the inconsistent spacing will cause the differential impedance to change, because the coupling between the differential pair is not significant, the impedance change range is also very small, usually within 10%, which is only equivalent to one pass. The reflection caused by the hole will not have a significant impact on the signal transmission. Once the line length does not match, in addition to the timing offset, common mode components are introduced into the differential signal, which reduces the quality of the signal and increases EMI. It can be said that the most important rule in the design of PCB differential wiring is the matching line length, and other rules can be flexibly processed according to design requirements and practical applications.