Misunderstanding one
Think that the differential signal does not need a ground plane as a return path, or that the differential traces provide a return path for each other. The reason for this misunderstanding is that they are confused by superficial phenomena, or the mechanism of high-speed signal transmission is not deep enough. Similarly, for the differential circuit, and other projectile may be present in the power and ground planes on the noise signal is insensitive. The partial return cancellation of the ground plane does not mean that the differential circuit does not use the reference plane as the signal return path. In fact, in the signal return analysis, the mechanism of differential wiring and ordinary single-ended wiring is the same, that is, high-frequency signals are always Reflux along the loop with the smallest inductance. The biggest difference is that in addition to the coupling to the ground, the differential line also has mutual coupling. Which kind of coupling is strong, which becomes the main return path. In PCB circuit design , the coupling between differential traces is generally small, often accounting for only 10-20% of the coupling degree, and more is the coupling to the ground, so the main return path of the differential trace still exists on the ground plane. When there is a discontinuity in the ground plane , the coupling between the differential traces in the area without a reference plane will provide the main return path, although the discontinuity of the reference plane has no effect on the differential trace on the ordinary single-ended trace It is serious, but it will still reduce the quality of the differential signal and increase EMI , which should be avoided as much as possible. Some designers believe that the reference plane under the differential traces can be removed to suppress part of the common-mode signal in differential transmission . However, this approach is not desirable in theory. How to control the impedance? Not providing a ground impedance loop for the common mode signal will inevitably cause EMI radiation. This approach does more harm than good.
Misunderstanding 2
It is believed that keeping equal spacing is more important than matching the line length. In the actual PCB layout , the requirements of differential design are often not met 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 part of the differential pair cannot be parallel. The most important rule in the design of PCB differential traces is the matching line length. Other rules can be handled flexibly according to design requirements and actual applications.
Misunderstanding three
It is believed that the differential trace must be very close. Keeping the differential traces close is nothing more than to enhance their coupling, which can improve immunity to noise and 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 the purpose of suppressing EMI. How can we ensure good isolation and shielding of differential traces? Increasing the spacing with other signal traces is one of the most basic ways. The electromagnetic field energy decreases with the square of the distance . Generally, when the line spacing exceeds 4 times the line width, the interference between them is extremely weak. can be omitted. In addition, isolation by the ground plane can also play a good shielding role. 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).
Differential traces can also run in different signal layers, but this method is generally not recommended, because the differences in impedance and vias produced by different layers will destroy the effect of differential mode transmission and introduce common mode noise . In addition, if the adjacent two layers are not tightly coupled, it will reduce the ability of the differential trace to resist noise, but if you can maintain a proper distance from the surrounding traces, crosstalk is not a problem. In general frequency (GHz or less), EMI will not be a very serious problem, experiments show that, apart 500Mils differential traces of radiation energy in three meters outside the decay has reached 60dB, enough to meet the FCC 's standards for electromagnetic radiation , so The designer does not have to worry too much about the electromagnetic incompatibility caused by insufficient differential line coupling .
Misunderstanding four
Differential Manchester coding is not a kind of differential signal, it refers to the logic state "0" with the level transition at the beginning of each bit, and the logic state "1" without transition. But the transition in each bit is used to synchronize the clock, which has no logical meaning.
Misunderstanding 5
The twisted-pair cable is not necessarily a differential signal, and the electromagnetic radiation of the single-ended signal on the twisted-pair cable is smaller than that of the parallel cable.