Hi Blala,
Ok - 800mm for a track? - in my books 800mm is about the width of the whole board! ;-)
So "The thickness of the track is determined by the passing current or are there more parameters that need to be considered?"
Yes - With low current circuits, its not primarily about current carrying capacity of the track.. its primarily about other factors.. And its real difficult for me to give you any numbers.. Lets look at factors you need to think about (these are not in order of importance):
1.) track has resistance, which causes voltage drop dependent on the current through it - Thicker the track, lower the resistance, lower the Vdrop, changing currents less likely to cause appearance of signals. The lower the current, the less Vdrop.
2.) Inductance (probably a lot more important than resistance for low power HF circuits) - narrower longer tracks have higher inductance than short fat ones - the same kind of effect as resistance, but frequency dependant - so high frequency signals pass through track less easily than low frequency ones.
3.) Capacitance. The fatter a track, and its proximity / overlap area to other tracks, will determine the capacitance between these tracks.
4.) Combined effects of all 3 above, which with HF can result in tuned circuits being created and cause active components like regulators or opamps to oscillate, or can cause HF ringing for fast transients.
In terms of ground and power supplies, these tracks can be laid close together because capacitive coupling is usually advantageous between them. You will be capacitively coupling these tracks anyway..
Because power supplies and ground are capacitively coupled, you regard them all as "ground" from a HF perspective, so you dont want any of these tracks to have capacitive coupling to antenna related circuits - or at least no unintentional coupling.
So, trying to think of numbers.. Just off the top of my head without any science I can quote to justify what I say, for the frequencies and currents in the EW, I would say..
Probably something like 5mm width track for a length of 50mm as a main ground "buss" - double this width for every doubling of length.
If you decouple every supply well (good 100n capacitor at supply points to IC's etc) then power tracks can be half this thickness.
But the trouble is that ive got lazy - I lay my board out instinctively, then if I have any doubts about inductance or whatever on a track, I port the design over to my PCB analysis software which tells me the inductances etc, and I can even insert signals and see radiation / coupling patterns etc.. But I dont do this often..
And in reality, unless one is going to manufacture boards, one can get away with a lot.. Hell, I have even had quick knock-up theremin circuits working on plug-in breadboard (NOT advised).
I tend to over-caution, in the belief that most people will try to get away with less than I suggest, and therebye get it "right" LOL ;-) .. In your case you seem to have done the opposite! I would say that for the EM design no track width above 10mm is probably "needed" and unless your layout is "unusual" 5mm is probably as wide as would ever be "required"... But theres nothing wrong with making power and ground tracks wider IF these dont cause coupling issues to other high impedence tracks carrying signals, particularly HF signals.
Fred.
Oh - for signal tracks, all the above apply (resistance, inductance, capacitance) but have different trade-offs... One wants low inductance for high frequency signals, and these are the most awkward, because you also want to avoid coupling these signals to other tracks - so best to keep these short and as narrow as possible (within reason) probably 2mm width max, but 1mm or less is about what I usually use - Audio signals are easy to handle, and short narrow (1mm or less) helps reduce capacitive coupling from HF signals.
But its an art form - all sorts of factors I dont even think of come into play - the angle of the bends on tracks, where one places vias and the relationship of vias to other vias or component pins (you can unintentionally create excessive inductive / capacitive coupling by wrongly placing vias and the tracks connecting these - effectively creating a "turn" or loop through the PCB) .. And I am by no means an artistic pro PCB designer - good analogue PCB design (as opposed to autorouter churned out digital board design) is an awesome feat IMO, as it combines good understanding of the circuit with art and incredible patience.
"You do not need a double sided pcb." - Betabox
Absolutely true! The main advantage of double sided is its easier to do a layout quickly, and it (often) enables smaller board size - And double sided is probably essential for all but the simplest SMD design... But for something like an EW board, no real need for DS.
If one is getting boards made by a Fab, there isnt any real price difference between SS and DS, so its worth doing a DS design - I often lay my DS board out as if its SS, with only straight tracks (links) on the component side - this allows me to etch my own SS prototype board and easily fit the links to replace the tracks on the component side, but then (after the board is proven) send the DS layout for manufacture so I then dont need to fit the (often extensive) links.
