In high-speed systems, why must the extremely thin coaxial cables be strictly the same length?

In high-speed signal transmission systems, extremely thin coaxial cables (Micro Coaxial Cable) are widely used in AI devices, camera modules, AR/VR terminals, and various high-speed data links due to their excellent shielding performance, stable impedance control, and low crosstalk characteristics. However, in actual projects, many high-speed link problems do not come from the chips or protocols themselves, but from the details of cable assembly control. Among them, the consistency of cable length is the most easily overlooked but highly influential key factor. In high-speed, multi-channel parallel transmission scenarios, even the smallest cable length differences may be amplified into system-level signal hazards.

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One, timing skew problem caused by inconsistent line length

The propagation speed of high-speed signals in extremely thin coaxial cables is usually about 70% to 80% of the speed of light, which means that the signal is extremely sensitive to physical length changes. In multi-channel parallel interfaces, if there are differences in the length of the lines between different channels, it will introduce inconsistent propagation delays (Skew). When the speed reaches several Gbps or even higher, even a few millimeters of length difference may result in picosecond-level delay deviations, directly destroying the time alignment of the data at the receiving end. This problem is particularly common in camera module and AI data collection systems, manifested as frame misalignment, data jitter, or intermittent packet loss, and is often difficult to detect in the early stages.

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Differential signal imbalance and decreased anti-interference capability

In high-speed differential systems such as MIPI, LVDS, and SerDes, ultra-thin coaxial cables are usually used in pairs. Their original design intention is to enhance anti-interference capability through strict electrical symmetry. However, when the lengths of the two lines are not consistent, the differential signal cannot maintain ideal phase and amplitude matching, and some differential energy will be converted into common-mode noise. This not only weakens the system's ability to suppress external interference but also increases its electromagnetic radiation level, leading to deterioration of the EMI index. At the whole machine level, this problem is often directly reflected in the increased difficulty of EMC testing, even requiring the modification of harness or structural design.

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Impedance discontinuity and signal reflection superposition

The extremely thin coaxial cables usually take 50Ω or 45Ω as the target characteristic impedance to ensure the smooth transmission of high-speed signals. However, in systems with multi-segment cables or multiple interface connections, if the cable lengths are inconsistent, there are often hidden impedance discontinuities at connectors, transition areas, or bending points. When high-speed signals pass through these locations, the reflected signals will superpose with the main signal, causing echo noise and cumulative jitter, which ultimately manifests as eye diagram contraction, increased jitter, and rising bit error rate. Compared with single link issues, this kind of reflection often has randomness, making system debugging more difficult.

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The consistency of the length of extremely fine coaxial cables is not just an optimization item for embellishment, but rather one of the fundamental conditions for the stable operation of high-speed signal systems. From the routing planning in the early design stage, to the length tolerance control in the cable assembly process, and to the consistency management of the wiring in the assembly stage, matching and consistency should be taken as the core principle. Only by controlling delay, impedance, and symmetry at the physical level can the performance advantages of extremely fine coaxial cables in high-speed and low crosstalk applications truly be brought into play.

I amSuzhou Huichengyuan Electronic Technology Co., Ltd.,Long-term focus on the design and customization of high-speed signal cable harnesses and ultra-fine coaxial cable harnesses, committed to providing customers with stable and reliable high-speed interconnection solutions. If you have any related needs or want to learn more, please feel free to contactManager Zhang: 18913228573 (WeChat same number)。