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Impedance control for high speed circuit design
Posted:10:29 AM July 24, 2018 writer: G

Impedance control purposes:

In order to minimize the negative effects of reflection, there must be a solution to control them. In essence, there are three ways to mitigate the negative effects of reflection.

1. The first method is to lower the system frequency so that the reflection of the transmission line reaches a steady state before another signal is applied to the transmission line, which is generally not possible for high speed systems because it requires lowering the operating frequency to become a low speed system.

2. The second method shortens the PC B trace so that the reflection reaches a steady state in a short time, which is also impractical because it usually increases the PC B layer and increases the cost. In addition, shortening the trace is physically impossible under certain circumstances.

3. The third method is to terminate the transmission line at both ends of the transmission line with an impedance equal to the characteristic impedance of the line to eliminate reflections.

PCB trace impedance control:

The component itself can display the characteristic impedance, so the PC B trace impedance must be chosen to match the characteristic impedance of all logic families in use (the range of characteristic impedances is 50 to 110 ohms for CMOS and TTL). In order to best transfer the signal from the source to the load, the trace impedance must match the output impedance of the transmitting device and the input impedance of the receiving device.

If the impedance of the PCB trace connecting the two devices does not match the characteristic impedance of the device, multiple reflections will occur before the load device can enter the new logic state. The result will likely result in increased switching times or random errors in high speed digital systems. To this end, line design engineers and PCB design vendors must carefully specify trace impedance values and their errors.

Therefore, impedance control technology is especially important in high-speed PCB design. Impedance control technology includes two meanings: 1 Impedance-controlled PCB signal line refers to continuous impedance along the high-speed PCB signal line, that is, the impedance on the same network is a constant. 2 Impedance-controlled PCB board means that the impedance of all networks on the PCB is controlled within a certain range, such as 20~75Ω. The key to making a board a "controllable impedance board" is to make the characteristic impedance of all lines meet a specified value, typically between 25 ohms and 70 ohms.

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