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Characteristic impedance of the transmission line
Posted:03:28 PM July 16, 2018 writer: G

For the battery, when the signal propagates forward along the transmission line, a new 0.06-inch transmission line segment is charged to 1V every 10 ps interval, and the newly added charge from the battery ensures a stable current is maintained from the battery. To absorb a constant current from the battery, the transmission line is equivalent to a resistor and the resistance is constant. We call this the surge impedance of the transmission line.

Similarly, as the signal propagates forward along the transmission line, each time a certain distance is propagated, the signal continuously probes the electrical environment of the signal line and attempts to determine the impedance of the signal as it travels further forward. Once the signal has been added to the transmission line and propagates forward along the transmission line, the signal itself is constantly examining how much current is needed to charge the length of the transmission line that propagates during the 10 s time interval and keep charging this portion of the transmission line segment to 1V. This is the instantaneous impedance value we want to analyze.

From the perspective of the battery itself, if the signal propagates in the direction of the transmission line at a constant speed, and assuming that the transmission line has a uniform cross-section, then each time the signal propagates a fixed length (such as the distance traveled by the signal during a 10 ps time interval), then Obtain the same amount of charge from the battery to ensure that this segment of the transmission line is charged to the same signal voltage. Each time the signal propagates a fixed distance, it will get the same current from the battery and keep the signal voltage consistent. During the signal propagation, the instantaneous impedance is consistent across the transmission line.

During the propagation of the signal along the transmission line, if there is a consistent signal propagation speed throughout the transmission line and the capacitance per unit length is the same, then the signal always sees a completely uniform instantaneous impedance during propagation. Since the impedance remains constant throughout the transmission line, we give a specific name to indicate this characteristic or characteristic of a particular transmission line, which is called the characteristic impedance of the transmission line. Characteristic impedance is the value of the instantaneous impedance seen by the signal as it propagates along the transmission line. If the signal is consistent with the characteristic impedance seen by the signal during the propagation of the signal along the transmission line, then such a transmission line is called a transmission line with controlled impedance.

Transmission line characteristic impedance is the most important factor in design

The instantaneous impedance of the transmission line or the characteristic impedance is the most important factor affecting the signal quality. If the impedance between adjacent signal propagation intervals remains the same during signal propagation, the signal can travel very smoothly forward, making the situation very simple. If there is a difference between adjacent signal propagation intervals, or if the impedance changes, a portion of the energy in the signal will be reflected back and the continuity of the signal transmission will be destroyed.

To ensure optimal signal quality, the goal of the signal interconnect design is to ensure that the impedance seen by the signal during transmission remains as constant as possible. Here mainly refers to keeping the characteristic impedance of the transmission line constant. Therefore, it is becoming more and more important to design and manufacture PCBs with controlled impedance. As for any other design, such as minimizing the length of the gold finger, terminal matching, daisy chain connection or branch connection, etc. are to ensure that the signal can see a consistent instantaneous impedance.

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