High speed digital design fundamentals - Signal Intergrity Issues-Overview

Signal Integrity

Signal Integrity Overview

Circuit designers face Signal Integrity problems when the signal loses it integrity. losing integrity means both change in shape than desired and having degraded Signal to Noise ratio.

Distortion of the signal usually means that the shape of the waveform begins to change. Typically digital signals can withstand a fair amount of distortion without obscuring  the bit of information present every clock cycle. Decrease in rise time is the main source of all issues related to signal integrity. 

The bad effects of inductance of the trace had it affect from the time the PCB begun but its ugly head is being shown up only in the recent past. Increased rise time and speed had made the integrity problem worse as it follows the equation:

V = L x di/dt OR V = L x di/Tr where Tr is the rise time of the signal. Hence it is obvious that as the rise time rises voltage droop across trace due to its inductance too rises and starts to become a problem.

The four main areas where signal integrity problems falls into are 

1. EMI
2. Reflections on single net
3. Crosstalk between two or more nets
4. Un-stability of power supply during component switching 

The solutions are more simpler than expected yet difficult to achieve because of the general practice by circuit designers to visualize the net/trace only as one wire rather than two, Signal path and return path.

Collectively, solutions below will help a designer to mitigate the signal integrity issues at the source level. They are

• Common sense of appreciating the truth that Current always flows in the closed loop and every signal has a return path.
• Have good control over the planes through which impedance can be controlled over the board thus by bringing down EMI
• Minimize the distance between trace and the reference plane and also the length of the trace
• Minimize inductance