Signal Integrity

Signals in PCB traces and connectors or wires in cables can be coupled when routed in parallel because of proximity effects. The problem is known as cross-talk

How To deal with the inductive cross talk the ground plane or the return plane should be placed close to the conducting trace as much as possible. This is because the magnetic flux generated by the track tends to cancel with the magnetic flux in the return path

Give preference to stripline over microstrip when routing critical signals sensitive to cross talk.
Reduce the separation between the signal and ground layers as far as possible while still achieving the required impedance.
Space out the critical high-speed traces as much as possible. A minimum spacing of twice the signal width is a good number.
Pay special attention to high-speed clock signals. Keep other traces away from clock signals.
Use guard traces if a very high immunity to noise is required.
If a guard trace is used, use vias to connect the guard trace to the ground.
For traces running in parallel, the cross talk level increases with the increase in length of parallelism. The near-end crosstalk or NEXT saturates after a certain length (depending upon the rise time of the signal) and the cross talk does not increase further. There is no gain in trying to minimize the length of parallelism beyond the critical length.
Make sure traces in adjacent layers run perpendicular to each other.
Add thicker solder masks on the top and bottom layers.
Use lower dielectric constant material. The lower dielectric constant reduces the separation between the signal and the return plane, thereby reducing the crosstalk.
Use a design with lower characteristic impedance.

ringing is a voltage or current output that oscillates like a ripple on a pond when it’s seen on an oscilloscope. The oscillation is a response to a sudden change in the input signal, like turning it on or switching.
Because of the characteristic shape of the output signal, ringing is sometimes called “ripple.”
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If you have a short trace, ringing is caused by parasitic inductance and capacitance. A pulse or sudden change in the input causes the parasitic components to resonate at their characteristic frequency domain, creating the ringing effect in your output. On long traces, the cause of ringing is more likely to be signal frequency reflection from an impedance mismatch.
For long traces, it can be caused by impedance mismatch/reflection or parasitic inductance/capacitance effects, or both. “long” if the round trip propagation time (to the load and back) is comparable to the signal rise time in printed circuits. If you happen to be working with strip lines or microstrips,

Increased EMI: Ringing can, and often does, produce noise and interference. This can radiate or conduct across your ground plane, with all the associated performance problems.
Increased current flow: Ringing causes increased current to flow through your circuit. Not only does that cause a corresponding increase in power consumed by your product
Decreased performance: Along with the accumulated performance drops from the increased current and heating, ringing decreases performance across a range of metrics. Because you have a lag in output due to the settling time, the vias and responsivity of your circuit board will drop. The resolution of your outputs will also be much poorer.
digital circuits, ringing is especially damaging. You still have all the problems we’ve covered, and the threshold is much lower. Combine this with any supply rail noise, and you’re likely to have errors and corrupted data.

should minimize node lengths, especially around power stage components on your ground plane.
use impedance matching to minimize any signal reflection.

Maximum frequency in a signal is a property of rising time and fall times
f(max GHz) = 0.5/T(rise time, ns)
When PCB trace length exceeds 1/12th in the wavelength the dielectric
Always have a ground (or relevant power) plane adjacent to the signal plane
Important for controlled impedance and return path
The return path should be directly under the signal trace (fs > 1kHz). there should not be any splits in the ground plane
Always have the GND plane adjust to the signal plane or power plane.

PCB trance + reference plane directly underneath (return path) = transmission line

Keep high-speed traces as short as possible - to reduce EMI
Keep differential high-speed traces as far away from each other to minimize cross talks - at least 3xH (H = height of dielectric)