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hello , what is the exact method to calculate or determine the impedance wof my board ??
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Ask PCB manufacturer or get the details of the materials and calculate yourself using some tool (Altium/Saturn PCB). Always prefer manufacturer data.
PS: most of these questions been already answered, so it's better if you could spend time and search here before posting which could save you time. -
I would also add direct link to one of my videos, maybe it can help: https://youtu.be/O4T9y_aIItAComment
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Width: track width is often defined by required impedance and PCB stackup (usually you may want to use 50OHMs for digital signals - so based on stackup you will know what width you should use to route by 50OHMs). For powers you would like to use wide tracks. Also, often I route with a width (for example 0.1mm) and by the end of the design I make all the 50OHM tracks thinner to meet the required 50OHMS (e.g. I route all digital tracks by 0.1mm and when the layout is finished I change all 0.1mm tracks to a width which is required for my stackup to achieve 50OHM impedance, for example I change all 0.1mm tracks to 0.08mm).
If you are using only 2 layer PCB ... then you can for example use a little bit wider track than what is the minimum track for a cheap PCB. In this case your tracks will not follow required impedance - but it may be just fine for some simple PCBs.
Length: depends on what you are routing, but generally I use the length required to connect the pins. However, for some interfaces you may need to do length matching.
I would recommend to have a look at some design guides, for example you can start with something like this - google for "com express design guide":
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