With the improvement of the working speed of digital circuits, the transmission rate of signals on PCB boards is also increasing. For example, the signal rate of PCI Express has reached 2.5Gb/s, the signal rate of SATA has reached 3Gb/s, and the working rate of new standards such as PCI Express II, XAUI, and 10G Ethernet is even higher. With the increase of data rate, the rise time of signals will become faster. When signals with fast rising edges encounter impedance discontinuities on the circuit board, they will produce significant reflections, which will change the waveform of the signal. Therefore, line impedance is a key factor affecting signal integrity.
For high-speed circuit boards, ensuring the continuity of impedance on the signal transmission path is crucial to avoid significant signal reflections. Correspondingly, for testing, it is also necessary to test the impedance changes on the signal transmission path of the high-speed circuit board, analyze the cause of the problem, in order to better locate the problem. For example, both PCI Express and SATA standards require measuring the impedance of transmission lines.
A fast and effective method for impedance testing is the Time Domain Reflectometer (TDR) method. Its working principle is based on transmission line theory, and its working mode is somewhat similar to radar. As shown in the figure below, when a step pulse is added to the tested line, reflection will occur at the impedance discontinuity point. If the source impedance (Z0) is known, the impedance (ZL) of the measured point can be calculated based on the reflection coefficient (ρ).
Through TDR impedance testing, PCB manufacturers can ensure that their products meet all impedance requirements without the need for lengthy and repetitive testing processes. It provides fast and accurate impedance measurement, and helps ensure reliable PCB quality and signal transmission. By using TDR impedance testing, PCB manufacturers can save time, money, and resources while ensuring high-quality PCBs for customers.
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