Basic question

[Stefan Nielsen]

> > Ok, time for a surprisingly rookie question.
> > I've seen this term before.  What EXACTLY does it mean:
> > 
> > "Ground Plane"
> 
> Well, instead of just pulling small tiny wires of "ground" signal every here
> and there you can make one side of the PCB to be the ground plane. Basically
> you avoid etching of that copper which would otherwise just be around filling
> up space. You actually let all (well almost all) groundings be done to this
> plane of copper. When you want to draw a line you actually remove some copper
> around the line. Same thing goes for holes etc.
> 
> > How do I make one in a circuit?  Is it the thick border of copper you
> > are supposed to leave around a pcb which you hook to ground and make
> > all of your "internal" grounds go to?
> > (Are you supposed to completely surround the board, or leave a gap?
> > I've seen both.)
> 
> I would be very carefull in letting the ground of a board be directly connected
> to the chassi of the box. Also, haveing ground interconnections every here and
> there migth not always be good. I personally think that all mechanical support
> (screws, supporters etc) should NOT be connected to any cursuit (good physical
> marginals.. I have seen screws eating throuth the laquier of a PCB and "rewire"
> the box) without lot's of thougth have went trougth. So to answer your question
> I would not let the groundplane near the edge... and not near the screw holes
> either... you never know what they will connect for you...
> 
> Magnus

Well, I have an entirely different approach to ground planes:

I take a two-layer PCB card and let the copper on the COMPONENT side 
stay on. This drastically reduces track lengths, because every time a 
component goes to ground, i simply solder it on the component side. 
Down side is, that you have to hand-drill the copper away from the 
holes, where you don't want a connection. I usually keep track of this 
whith a different pad form on the track side.

Furthermore, this reduces noise drastically, because all current 
loops are much smaller. The current doesnot follow the shortest 
return-path through the copper, but the one that takes the least 
*energy*. The result is, that lesser magnetic flux passes through the 
current loop (the area of the loop is smaller), thus inducing much 
less noise. And, not to forget, it has a much smaller impedance than 
a standard PCB track, which reduces crosstalk.

But be warned: Hand-drilling is a bitch, and it is difficult to 
connect ICs to ground. Plus you have to have a high-temperature 
solder iron, 'cause copper is a *very* good heat conductor.

Stefan
-------- URL: http://www.iae.dtu.dk/u/932674/home.htm --------
------------- E-mail: 932674 at student.iae.dtu.dk --------------