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:
Now how would the return signal go?
Following the tracks exactly?
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..
So how to make a PCB which follows the rules:
1. Make a GND plane in one of the inner layers.
2. Do not interrupt this plane (near tracks)
3. Trace all V+ as traces
To take the previous example and add a GND layer:
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