Why does USB4 use coaxial cable instead of stranded cable?

Nov 11, 2020

   With the development of USB 3.2, USB4, Thunderbolt 3, Thunderbolt 4, HDMI 2.1 cables, the data transmission rate is getting faster and faster, but the diameter of the cable is getting smaller and smaller. When the cable diameter becomes smaller, the differential impedance of the conductor becomes larger, and high-frequency signal attenuation is serious (the differential impedance of the conductor is inversely proportional to the conductor area and proportional to the length of the cable). The traditional stranded conductor structure cable can no longer meet the data transmission requirements of the new standard,so the previously unpopular coaxial cable return to people's field of vision 

   But, Why does USB4 use coaxial cable instead of stranded cable?

   The coaxial cable has self-shielding characteristics. Its obvious feature is that there is a metal braided shielding layer outside the conductor, the metal net can shield the electromagnetic interference. Part of the electromagnetic interference of the cable comes from the outside, and the other part comes from the magnetic field generated by itself when transmitting changing signals, When the coaxial cable has a metal shielding net, external electromagnetic interference cannot pass through the shielding layer, and the internal magnetic field cannot pass through the shielding layer too. Therefore, the attenuation of the signal when it is transmitted in the coaxial cable is only related to the transmission distance and the frequency of the signal itself
   Another obvious difference between the coaxial cable and the traditional stranded cable is that the loop of the coaxial cable is not twisted. the stranded cable will have a twist rate parameter, that is, if the twist rate is 1.1, then we see when the length of the cable is 1.0 meters, the actual core wire length is 1.1 meters, and when we see that the length of the coaxial cable is 1.0 meters, the actual core wire length is still 1.0 meters. The length is a very important factor affecting high-frequency impedance. The shorter the cable, the better the performance. Therefore, the length of the USB 3.1 GEN2 we see is usually less than 1.0M, the Thunderbolt 3 is generally 0.7 meters, and the Apple Thunderbolt 3 is 0.8 meters

   So, what are the advantages of coaxial cable compared to stranded cable?                                                                                                                                                                                                 
tranded cable is a traditional signal transmission mode. The high-frequency impedance of the stranded cable has a large difference in the range of plus or minus 10%, and the coaxial wire can be plus or minus 3%. Theoretically, USB4 can use stranded cables, but in the actual production process, it will be very difficult for the stranded cables to meet the requirements
    1. The coaxial cable is super soft;

    2. Bending resistance: the swing life can be more than 5000 times, and the stranded wire is about 3000 times;

    3. Small diameter; the traditional stranded wire, a loop is twisted together by two wires, which is almost equivalent to two wires side by side, but the coaxial wire uses the metal braided shielding layer as its loop, and the diameter is only slightly larger than a single wire. the diameter is much smaller than the stranded wire with two wires side by side;

    Disadvantages of coaxial cables:

    1. Expensive: Because each high-frequency signal wire needs a separate braided shielding layer, the production efficiency is low, and the more expensive Teflon is used to replace the original PE insulation material;

    2. The processing technology is complicated: because the conductor is very thin, when stripping the inner and outer insulation layer and the metal braid of the core wires , cannot use traditional machines, but use laser cutting;

    With the advent of 5G even 6G big data era, high-speed data transmission requires high bandwidth and high speed. Only coaxial cable can meet this requirement. Coaxial cable will be the trend of the next generation of high-speed cables.