Chassis gnd plane on PCB to make comms circuitry more noise immune?

[treez]

Hi,
We all know the chassis can't float otherwise electrical noise abounds......so we bond chassis to circuit ground.
But why would we then need a chassis ground plane on a PCB that contains sensitive comms and control circuitry?

2nd page of this says a chassis gnd plane is needed..

https://www.newark.com/pdfs/techarticles/natSemi/national_AN-1881.pdf

..why is this?

 

[barry]

It says nothing of the sort. It explicitly says that your chassis ground plane is decoupled from your pcb.