Lets say we have a double sided PCB. On the top we have 2 chips and a signal track between the 2.
Both chips are connected to GND and decoupled. The ground plane is on the bottom side.
Ehm wrong…
Apparently it takes the route of less flux resistance (or something like that, please correct me on the terminology)
It follows the upper track EXACTLY but on the bottom GND plane like this:
What? So directly I got this question on why we were thought to make separate analog and digital planes and stuff like star grounds…
Maybe this might be neccesary for very high frequency signals, but for ‘normal’ signals it appears that the ground return can handle itself
-EDIT:
So why is this important? Remember the previous post about the twisted wires? The traces on a pcb are also wires.
In this case a pair of wires is the trace on top and the return is the path the return current flows through the GND plane.
In order to have a minimal effect of interferance, you want both ‘wires’ to be placed in the same magnetic field.
So if the return current follows the exact same path as the trace above it, this is (almost) the case. Just the distance to the GND plane difference (1.6mm or if the GND plane is in the middle of the PCB 0.8mm)
If this is not the case we create a loop and therefore a difference of the wires in the magnetic field and it becomes a receptor for interferance (read antenna).
In my next post there I explain how this can happen…
Now we put some more components on the PCB and of course we need to cross a track.
We go down to the ground plane make a track there and go up again:
Now we made an antenna! This is a drawing of an PCB antenna which can send a signal to a sattelite:
It just consists of a track and a cut out of the ground plane. With the size of the cut you can set the frequency…
The signal gets picked up in the track crossing the open ground plane. (in the previous picture, the track from top left to bottom right)
So this is very bad and also very very common!!
Now how to fix this?
Lets make a 4 layer PCB… We were thought we can use the inner layers for GND and V+ right?
Now lets put a component on the top and one on the bottom and connect both with a signal track:
The first part right bottom part of the track ‘sees’ the GND plane and the return signal follows the exact path of this track.
But the other part of the track is blocked by the V+ plane !! So it takes the other shortest route.
And now we are back to the theory of the wires and picking up magnetism. As you see the signal and return form a loop, the space between is big and therefore both paths will lay in a different magnetic field, different sizes of current will be picked up and not cancelled out. So noise will appear on this signal path…
This eliminates the need for removing copper from the GND plane and therefore no antenna is formed, signals can just follow the tracks back on the GND plane.
So far for EMI now, I hope to find time for ESD later on
Hi to everyone, I have a question related to what yoiu Jan actually prepared here. The return signal can not go back in the V+ plane?, only in the ground plane? Thank you for you reply :"D
