Importance of Impedance Matching

Okay two separate things.

1 - Reflections - When a signal propagates along a transmission line of a certain impedance (this includes board traces) and encounters a change in impedance (this could be due to a change in trace width, a "T" in the line, a change in ground referencing, a pin on an IC, a pin on a discrete component, etc.), a portion of the energy in the signal is reflected back down the transmission line in the opposite direction. What doesn't get reflected, continues to propogate down the transmission line or lines. When that reflected portion of the signal reaches it's source or another impedance mismatch, the same thing will occur (part of the signal will reflect back in the original direction of the signal).

At the signal receiver, all of these reflections in combination with the original signal will add together either constructively or destructively depending on the point in time. This adding together of waves can be a cause of ringing.

2 - Ringing due to resonance.-Capacitive and Inductive sources within a circuit have a characteristic frequency. One way to think of this is like a pendulum or a swing. If you continue to push on the swing at the proper instance, the energy of your push will add to the energy of the swing and the swing will continue to gain energy and will swing higher and higher. The timing with witch you push on the swing is the swings characteristic frequency. This isn't a perfect analogy but it's the general idea. If you add energy to those sources of capacitance and inductance at the right frequency, they will resonate.

Typically, if ringing is due to a resonant circuit, it can be compensated for (this is what I meant by uncompensated capacitance and inductance). By compensating for the resonant circuit you will more than likely will make a tradeoff in the rise/fall times of the signal. You are essentially trying to filter out of the signal those frequency components that cause the resonance (which are at a higher frequency than your signal rate).

Another way to think of this would be to look at the circuit as a filter (that's what it is). Those capacitances and inductances in the circuit are poles and/or zeros in the transfer function of the filter. If the circuit is resonant, that means the filter has a low dampening effect at a specific frequency (It may even act as an amplifier at certain frequencies). You can change the filter's behavior by adding more poles/zeros to the transfer function (compensation).

Typically you compensate by adding more capacitive, resistive and/or inductive sources to the circuit.

Hope this helps. Maybe somebody else has something to add.

Jake