When you see the eye diagram above, you will know that it has an abnormality. What caused it? Experts will tell us that it may be caused by impedance mismatch. What is the impedance mismatch?
Usually the signal is transmitted on the wire. The transmission line itself has a resistor. A wire with parasitic inductance. The ground plane of the transmission line and the reference has parasitic capacitance.
Resistance, the uniform consumption of energy in the transmission medium. The signal is like a moving car. The transmission line is equivalent to a highway. As long as you drive, you have to burn oil.
Parasitic inductance and capacitance: hinder the AC signal. Reactance is a general term for capacitive reactance and electrical impedance, and reactance is related to frequency. The blocking effect of the capacitive reactance is like a car walking away from the pit, and the inductance resistance is like a car coming out of the pit, which is very laborious.
Electrical impedance, capacitive reactance, is related to the signal alternating frequency. The formula is as follows
If the inductance is small and the capacitance is large, the subsequent sub-item may be a negative value, called a capacitive load. Z = R - a value. Just like on a flat road, driving 10 kilometers requires 12 liters of oil. Capacitive resistance is like dropping into the pit when driving. On the way down the slope with a big pit, driving is bumpy, but it is more fuel efficient.
If the inductance is large, the capacitance is small, and the subsequent sub-item may be positive, called inductive load.
Z = R + value. The car was driving uphill, the road was pitted, the accelerator pedaled to the end, the car just came out of a pit, and saw a large step in front, only to continue to step on the gas pedal.
So what is the impedance mismatch?
There is nothing with impedance, as long as it is adapted, one signal followed by one signal, this is the case.
But, for example, if a car in front of you suddenly sees this road suddenly changing, the car behind does not know the road ahead, which is easy to get into trouble. Some cars don't want to continue exploring and want to go back (reflection). The signal is even worse before and after this.
Reflection is like this
The signal at one point increases, and the signal at the lower point decreases. From the time domain signal waveform, the original signal is added to the increase and decrease points caused by the reflection, that is, the signal seen when the impedance is not matched.
Or is like this
Standard signal wave + reflected wave = the signal that is seen
The eye diagram of the standard signal superposition will look good.
Superimposed with the reflected signal, the eye diagram looks irregular
Therefore, impedance matching is required.
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