Signal Integrity: How is reflection generated?
Some friends have read the previous article and said that it is not enough to explain in depth, think about it, and then write an article, trying to explain how white reflection is formed.
To illustrate the white reflection, it is necessary to refer to the concept of impedance and matching mentioned above. In terms of image, as shown in the following figure, like a jigsaw puzzle, the red square is too big, or too small to fit into the space, which will cause signal integrity problems. Only when it matches, can it be put in, no reflection.
Specifically, the foregoing describes the characteristic impedance. We know that the maximum power transfer theorem in the actual circuit is that the load can obtain the maximum power when the load matches the power supply. Migrating into high-speed circuits is characterized by excitation circuit characteristics and transmission line characteristics that greatly affect the integrity of the signal transmitted from one device to another.
Specifically, in the high-speed circuit, in order to effectively transmit the signal energy from the source end to the load end, the characteristic impedance of the transmission line must be matched with the source impedance and the load impedance of the signal, otherwise the signal will be reflected, resulting in a signal waveform. A series of problems such as distortion.
Before, I also read articles written by other big cows on the Internet. The relationship between impedance and reflection is very easy to understand. It is probably that the current is analogized to the water flow, and the height of the water level is regarded as the voltage. This is with us. The physical knowledge of junior high school contact is consistent. The speed of the water flow is regarded as the frequency of the signal. If the width of the water in the channel is the impedance, then the wide impedance of the river must be smaller. This should be well understood. The same is true for our traces. The wider the trace, the smaller the impedance.
Therefore, for a river, if the water suddenly flows into a narrow road and the water channel narrows, the impedance will increase. At this time, if the water flow speed is very fast, that is, the signal frequency is very high, then Imagine if it will splash, as shown below, this is the reflection. And if the river is wide to narrow, then the water level in the small ditch will definitely rise. (It should be clear to the canal that has been to Suzhou and Hangzhou.) This shows that the positive reflection and the voltage become high. It is.
On the other hand, if the river enters the river and the river is obviously wider, then the impedance will become smaller and the water level will become lower. At this time, a negative impedance will be generated, and the voltage will be low after superposition.
Through the above analogy, we can see the following is very easy.
Reflection is the echo on the transmission line. A portion of the signal power (voltage and current) is transmitted to the line and reaches the load, but a portion is reflected, as shown in the following figure. A mismatch between the source and load impedances causes on-line reflections, and the load reflects a portion of the voltage back to the source. If the load impedance is less than the source impedance, the reflected voltage is negative. Conversely, if the load impedance is greater than the source impedance, the reflected voltage is positive. Such reflections can be caused by variations in wiring geometry, incorrect wire termination, transmission through the connector, and discontinuity in the power plane.
Well, the reason for the formation is probably here. The partners who are interested in the formula can push to a 3-5 reflection, and the understanding is deeper.
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