What Leads to Signal Integrity Issues in a PCB?
Perhaps the most important cause of signal integrity issues in a PCB is faster signal rise times. When circuits and devices are operating at low-to-moderate frequencies with moderate rise and fall times, signal integrity problems due to PCB design are rarely an issue. However, when we are operating at high (RF & higher) frequencies, with much shorter signal rise times, signal integrity due to PCB design becomes a very big issue.
Generally speaking, fast signal rise times and high-signal frequencies increase signal integrity issues.
For analytical purposes, we can divide various signal integrity issues into the following categories:
Signal degradation due to uncontrolled line impedance discipline
Signal degradation due to line impedance discontinuities
Signal degradation due to propagation delay
Signal degradation due to signal attenuation
Crosstalk on one conductor due to other conductors
Issues caused by power and ground distribution network
EMI and radiation from the system
Signal degradation due to uncontrolled line impedances:
Signal quality on a net depends on the characteristics of the signal trace and its return path. During travel on the line, if the signal encounters changes or nonuniformity in the impedance of the line, it will suffer reflections that cause ringing and signal distortion. Moreover, the faster the signal rise time, the greater will be the signal distortion caused by changes in uncontrolled line impedances.
We can minimize signal distortion due to reflections by reducing or eliminating line impedances changes by:
Ensuring that signal lines and their return paths act as uniform transmission lines having uniform controlled impedances.
Having signal return paths as uniform planes placed close to signal layers.
Ensuring that controlled impedance signal lines see matched source impedances and receiver impedances – same as the characteristic impedance of the signal line. This may call for addition of proper terminating resistors at the source and receiver ends.
Signal degradation due to other impedance discontinuities:
As we mentioned earlier, if the signal encounters a discontinuity in impedance during its travel, it will suffer reflections which cause ringing and signal distortion. Discontinuities in the line’s impedance will occur at the point of encountering one of the following situations:
When a signal encounters a via in its path.
When a signal branches out into two or more lines.
When a signal return path plane encounters a discontinuity, like a split.
When line stubs are connected to signal lines.
When a signal line starts at the source end.
When a signal line terminates at the receiver end.
When signal and return paths are connected to connector pins.
And, faster the signal rise time, the...
