http://hackaday.com/2016/03/23/michael-ossmann-makes-you-an-rf-design-hero/#comments

Prodigity says:	
March 24, 2016 at 5:47 am 

Perhaps the most important and overlooked topic is the current return path; every trace carrying a current has an associated path that returns the current to it’s source.
This path is always the path of least impedance, which for signals faster then ~1MHz translates to the path of least inductance.
The path of least inductance for a trace is parallel directly beneath it on the adjacent power/gnd plane which is the main reason why split planes are the bane of RF; a split plane underneath a trace greatly increases the inductance as the current return path has to move away from under the trace to ‘find’ an alternate route.
This can cause all sorts of fun such as ground bounce, signal ringing/overshoot/undershoot, sometimes the alternate route happens to be another signal trace..

Ground bounce is an alternating voltage potential across your ground plane which causes voltages on signal traces to appear lower or higher depending on which part of the ground you are referencing to and at what time.
It results from the change of magnetic flux and the inductance of your trace; the faster the rise and fall time of your signal and the higher the inductance the more ground bounce you’ll encounter.
Digital circuits generally don’t mind ground bounce because a small deviation on the signal trace still translates to either a ‘0’ or a ‘1’ but analog circuits on the other hand are heavily reliant on the voltage of a trace at any given time and cannot tolerate such deviations.

I firmly believe that splitting your ground plane to divide digital and analog circuitry is a bad strategy to stop ‘high frequency noise leaking into your analog circuitry’ because it does not take into account WHY it’s happening and could potentially increase the impedance of your traces because your current return path is being neglected thus making things WORSE.
Instead I suggest that you focus on keeping the current return paths of > ~1 MHz signals out of the way of analog circuitry to ensure that the ensuing ground bounce does not create a voltage potential across the analog section of the ground plane and to keep your ground plane whole to ensure that your trace inductance stays at a minimum which reduces ground bounce.

Signal ringing / overshoot / undershoot are deformities that appear on your square wave if your signal encounters different impedances, takes up a significant portion of the wave length of the frequency of your signal (though typically for square waves we tend to refer to rise and fall times to determine the threshold) and does not have a terminating resistor.
They are the result of not treating your trace as a transmission line and can be detrimental to your signals which can cause some of the following behaviour: false clock/data, missing clock/data and in the worst case destroying components
(e.g. ICs, FETs, etc.) because the voltages are too high / low for the specs.

Analog devices has a white paper(MT-097) which has a nice rule of thumb to determine whether you should treat a trace as a transmission line and use termination:
Multiply the rise/fall in nanoseconds by 2, this is your max trace length in inches before you need to treat it as a transmission line.

Which brings me to the next relevant topic; decoupling caps.
As we have seen the current return path is important in ensuring the quality of your signals and keeping your ground plane quiet and decoupling caps play a significant role in this; decoupling caps decrease the size of your current return loops which decreases inductance and can improve ground bounce and signal degradation problems.
Every current wants to return to it’s source and without decoupling caps this would normally be a regulator on the PCB or an external power supply which increases trace length and thus inductance quite a lot.
So to combat this problem you use decoupling caps as close as possible to your ICs… no closer…. even closer…. that’s it ;)
As to which cap you should choose the general rule is; place the smallest cap with the highest value with preferably double the voltage rating of what you need.


Dodo says:	

March 25, 2016 at 9:05 am
If you have to use 2 layers (cost reasons for example) it is often better to use a coplanar waveguide with ground compared to a microstrip.