Terminations in High speed design

Signal reflection occurs when an electrical signal encounters an impedance discontinuity along a transmission line, causing a portion of the signal to be reflected back towards the source. This can lead to interference issues like ringing and noise in high speed PCBs. Signal reflection can be avoided using Terminators

The system need the termination resistor at 2 cases

• Long reflection case
• short ringing case

Long reflection case:

When a line is long,meaning the cable length exceeds one-sixth of the electrical length of a rising edge,The cable needs terminators.

Short ringing case

When a line is short,it may still meed terminators is it drives a capacitive loads.The ringing phenomenon in short lines has the same practical effect as reflections in long lines.

Resistive terminators can cure problem with either reflection or ringing

Terminations Types:

• End Terminators
• Source Terminator

End Terminators:

When using the end terminations ,each driving gate connects directly directly to its transmission line,with the terminator located at the receiving end. End termination line have these properties .

The driving form propagates at full intensity, all the way down the cable.

All reflections are damped by the terminating resistor.

The received voltage is equal to the transmitted voltage.

Source terminators.

The source termination scheme connects each driving gate through a series resistor to each transmission line.the value of the series plus the output impedance the driving gate should be equal to Z,the characteristic impedance of the transmission line.reflection coefficient at the source will then be zero .

Source termination circuits have these properties.

• The driving wave form is cut in half by series termination resistor before it begins, propagating down the line.
• The driving signal propagates at half intensity to the end of the line.
• At the far end the signal reflection coefficient is +1. The reflected signal is half intensity. The half sized reflected plus the original incoming half sized signal together Bring the signal at the receiving end to a full level.
• The reflected signal propagates back along the line Toward the source where it damps out at the source termination.
• After the end reflection returns to its source ,the drive current drops to 0 where remains until the next transition.In fast systems, the next transition starts before the termination returns.