A digital rising or falling edge is a step function that can be modeled as a high-frequency wavefront. As the wavefront travels along the transmission line, it acts as a purely AC signal, encountering the characteristic impedance (Z0) of the transmission line. When the wavefront reaches the end of the path, if Z0 and the termination (Zt) do not match, portions of the wave are reflected. As the wave reflects back along the transmission line, it eventually reaches the original source impedance (Zs). If the transmission line characteristic impedance (Z0) and Zs do not match, then portions of the wave are re-reflected. The superposition of these reflected waves can cause significant signal degradation.

Reflection caused by an impedance mismatch at the end of a transmission line is quantified by the reflection coefficient. Reflection coefficient Γ is given by the following formula:

Γ = Vr/Vi = (Zt - Z0)/(Zt + Z0)

where:

Vr is the reflected voltage,

Vi is the incident voltage,

Zt is the terminating impedance,

Z0 is the characteristic impedance of the transmission line.

For example, by applying this formula, you can calculate that when a 3.3 V wave, traveling down a 50 Ω characteristic medium hits a 1 kΩ load impedance.

The reflection coefficient Γt, is equal to (1 kΩ - 50 Ω)/(1 kΩ + 50 Ω), or .90, and Vr equals 0.9 x 3.3 V = 2.97 V.

Thus, the reflected wave Vr is almost the same magnitude as the incident wave. At the load, this condition only has the effect of giving an erroneous voltage assuming that the circuit was originally calibrated with a 50 Ω load. While nearly the entire signal is reflected back, this reflection is eliminated at the source because the source and the transmission line are matched.

If the transmission line or cable be mismatched from the source and the load, the mismatch causes multiple reflections. These reflections cause aberrations at the load similar to what is shown in the improperly terminated signal in the Termination topic.
Note Ensure that the source impedance of the system matches the characteristic impedance of the transmission line as closely as possible. For generation operations, the source impedance is inside the device and is handled by the hardware architecture. For acquisition operations, however, you control the source impedance of the system. You must create a source impedance as close to the characteristic impedance of your device as possible.