EMC Concept (3)

EMC Concept (3)

Let us think about the difference between the real circuit and the circuit in the book. The following two figures, the first one is we see in the book and the other is the real circuit on the PCBA. Apparently the real circuit involves the transmission line conditions.

Consider the parameters: 

When the reflective coefficients are the contrary sign, it causes ring. If the overshoot is too large, will it damage the system?
When the  reflective coefficients are the same sign, the curve will achieve to V_L  slowly.  

For eliminating the ring effect, we do some experiments.
1. Here is the initial photo:

     

If we add a serie resistor R and let Rs + R = Zc, we see the ring effect is disappeared.

This is why we could find the 22 or 33 ohm resistors on the reference circuit shown below.

2. Slow down the rising time of Vs:

 

We find out it would eliminate the ring effect.

Conculsions:
In this chapter, it is not easy to understand. So we could just remember the results that adding series resistor or modifing the rising time. Both two ways could remove the ring effect.

The theory is totally a big different than realistic application. As I remember being a junior hardware engineer, I saw a lot of predecessor's circuits, they put the series resistors on the DDR signals. It is also named damping resistors. But we know the layout job is after designing a circuit.  Imagine that the trace maybe on the inner layer → via → outer layer → damping resistor →.... What a terrible impedance matching.