Download Help (Windows Only) |

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 (Z_{0}) of the
transmission line. When
the wavefront reaches the end of the path, if Z_{0}
and the termination (Z_{t}) 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
(Z_{s}). If the transmission line characteristic
impedance (Z_{0}) and Z_{s} 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 = *V _{r}/V_{i} = (Z_{t} - Z_{0})/(Z_{t} + Z_{0})*

where

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, Γ_{t}, is equal to (1 kΩ - 50 Ω)/(1 kΩ + 50 Ω), or .90,
and V_{r} equals 0.9 x 3.3 V = 2.97 V.

Thus, the reflected wave V_{r}
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.

However, should the transmission line/cable be mismatched from the source and the load, the mismatch causes a scenario of multiple reflections resulting in aberrations at the load similar to what is shown in the improperly terminated signal in the Termination topic.

Note NI strongly recommends that you take great care to ensure that the source impedance of the
system is matched as closely as possible to the characteristic impedance of the transmission
line. For generation operations, the source impedance is inside the NI device and is handled
by the hardware architecture. For acquisition operations, however, you control the source
impedance of the system. You should create a source impedance as close to
the characteristic impedance of your device as possible. |

Helpful

Not Helpful