How do minute deviations in the characteristic impedance of a CAN bus cable for communication affect signal integrity in specific ways?
Release Time : 2026-02-11
In automotive electronics, industrial automation, and intelligent equipment, CAN bus cables for communication serve as a reliable real-time communication backbone, widely used for data exchange between nodes. Their physical layer relies on a twisted pair to transmit differential signals, which can subtly yet destructively affect signal integrity, leading to communication delays, bit errors, and even bus failure. This impact is not simply amplitude attenuation, but rather a gradual erosion of system reliability through reflection, ringing, and timing jitter.
1. Impedance Discontinuities Causing Signal Reflection: Invisible "Echo Interference"
An ideal CAN bus requires uniform impedance throughout the cable, matched with 120Ω terminating resistors at both ends. If a section of the cable experiences a local impedance deviation due to uneven twist pitch, fluctuations in the dielectric constant of the insulation material, or poor crimping, this point becomes an impedance discontinuity. When a high-speed CAN signal passes through this point, some energy is reflected back to the source, forming an "echo."
2. Ringing Phenomenon: High-Frequency Oscillations Induce EMI and False Triggers
Reflections caused by impedance deviations, if they repeatedly travel back and forth between the two ends of the bus, will form standing waves with the original signal, manifesting as high-frequency ringing on the received waveform. This ringing not only prolongs the time required for signal stabilization but may also cause the voltage to cross the threshold multiple times between logic "0" and "1," resulting in a single bit being misread as multiple transitions—the "false edge" phenomenon. Although the CAN controller has bit stuffing and CRC check mechanisms, high-frequency ringing also radiates electromagnetic interference, which may couple to nearby sensors or control lines, indirectly causing system-level failures.
3. Timing Jitter Accumulation: Threatening the Synchronization Accuracy of High-Speed Communication
High-speed CAN relies on precise bit timing for sampling. Signal edge distortion caused by impedance deviations introduces deterministic jitter, causing the actual sampling point of each bit to shift. In long frame transmission or multi-node response scenarios, jitter accumulation may cause the sampling window to deviate from the optimal position, reducing noise margin.
4. Decreased Common-Mode Noise Suppression: Weakened Shielding Effectiveness
High-quality CAN cables require not only precise differential impedance but also a good common-mode rejection ratio (CMRR). Impedance deviations often disrupt the symmetry of the twisted pair, introducing common-mode components into the differential signal. This not only reduces the CAN transceiver's immunity to common-mode noise but can also cause an imbalance in the shielding current distribution, creating a noise coupling path and further deteriorating signal quality.
5. System-Level Latent Faults: The Root Cause of Intermittent Communication Anomalies
The most challenging issue is that problems caused by minute impedance deviations are often intermittent—manifesting only under specific temperature, humidity, or bus load conditions. For example, low temperatures cause PVC insulation to shrink, slightly altering conductor spacing and increasing impedance; high loads increase bus current, exacerbating reflection effects. These "intermittent frame drops" are extremely difficult to locate through conventional diagnostics and are often misdiagnosed as software or node faults, delaying maintenance.
In summary, while the characteristic impedance of a CAN bus cable for communication appears to be a static parameter, even minute deviations can disrupt signal integrity in complex and subtle ways through various physical mechanisms such as reflection, ringing, jitter, and common-mode rejection degradation. Therefore, during the selection, installation, and acceptance phases, it is essential to rigorously verify the impedance consistency of the entire link using tools such as a time-domain reflectometer (TDR) to ensure the long-term stable operation of the high-speed CAN system in harsh environments. Only in this way can the core advantages of the CAN bus—high reliability and strong real-time performance—be truly realized.
1. Impedance Discontinuities Causing Signal Reflection: Invisible "Echo Interference"
An ideal CAN bus requires uniform impedance throughout the cable, matched with 120Ω terminating resistors at both ends. If a section of the cable experiences a local impedance deviation due to uneven twist pitch, fluctuations in the dielectric constant of the insulation material, or poor crimping, this point becomes an impedance discontinuity. When a high-speed CAN signal passes through this point, some energy is reflected back to the source, forming an "echo."
2. Ringing Phenomenon: High-Frequency Oscillations Induce EMI and False Triggers
Reflections caused by impedance deviations, if they repeatedly travel back and forth between the two ends of the bus, will form standing waves with the original signal, manifesting as high-frequency ringing on the received waveform. This ringing not only prolongs the time required for signal stabilization but may also cause the voltage to cross the threshold multiple times between logic "0" and "1," resulting in a single bit being misread as multiple transitions—the "false edge" phenomenon. Although the CAN controller has bit stuffing and CRC check mechanisms, high-frequency ringing also radiates electromagnetic interference, which may couple to nearby sensors or control lines, indirectly causing system-level failures.
3. Timing Jitter Accumulation: Threatening the Synchronization Accuracy of High-Speed Communication
High-speed CAN relies on precise bit timing for sampling. Signal edge distortion caused by impedance deviations introduces deterministic jitter, causing the actual sampling point of each bit to shift. In long frame transmission or multi-node response scenarios, jitter accumulation may cause the sampling window to deviate from the optimal position, reducing noise margin.
4. Decreased Common-Mode Noise Suppression: Weakened Shielding Effectiveness
High-quality CAN cables require not only precise differential impedance but also a good common-mode rejection ratio (CMRR). Impedance deviations often disrupt the symmetry of the twisted pair, introducing common-mode components into the differential signal. This not only reduces the CAN transceiver's immunity to common-mode noise but can also cause an imbalance in the shielding current distribution, creating a noise coupling path and further deteriorating signal quality.
5. System-Level Latent Faults: The Root Cause of Intermittent Communication Anomalies
The most challenging issue is that problems caused by minute impedance deviations are often intermittent—manifesting only under specific temperature, humidity, or bus load conditions. For example, low temperatures cause PVC insulation to shrink, slightly altering conductor spacing and increasing impedance; high loads increase bus current, exacerbating reflection effects. These "intermittent frame drops" are extremely difficult to locate through conventional diagnostics and are often misdiagnosed as software or node faults, delaying maintenance.
In summary, while the characteristic impedance of a CAN bus cable for communication appears to be a static parameter, even minute deviations can disrupt signal integrity in complex and subtle ways through various physical mechanisms such as reflection, ringing, jitter, and common-mode rejection degradation. Therefore, during the selection, installation, and acceptance phases, it is essential to rigorously verify the impedance consistency of the entire link using tools such as a time-domain reflectometer (TDR) to ensure the long-term stable operation of the high-speed CAN system in harsh environments. Only in this way can the core advantages of the CAN bus—high reliability and strong real-time performance—be truly realized.