In Reply to: Impedance Control posted by Sarkuna on 07/13/03 at 10:48 PM:
1) You are right, the main factors that influence the impedance of a board are the trace thickness, di-electric and trace width.
But when we design a impedance controlled board, we talk of the board as a whole and not just mere track . The track length does not play a significant role in controlling the board impedance.
Each and every layer has a certain impedance value based on its distance from the reference planes and the construction.
So when designing a multilayered controlled impedance board, the following things are to be done:-
1)Build the layer stack up :- Every signal layer will have a particular impedance associated with it based on its distance from the board reference power/ground plane. It also depends on the type of construction, whether a microstrip, stripline, embedded microstrip etc etc...
There are impedance calculators available at www.ultracad.com. These help you in determining your trace width based on the construction and impedance. You can also fix the impedance you want for different layers and the find out the other parameters. Just check this calculator out.
The stack up inturn will affect your board impedance. This will decide the trace widths.
2)Alternatively, build the stack up, fix the impedance you want to be maintained on the board, in different layers. Determine the trace widths.
The ultracad calculator will help you in this.
Always the length of a trace is much associated with the crosstalk, inductance effects etc etc rather than impedance. The length of a trace plays an important role with respect to the critical length on the board.
This does not mean that the length does not determine the impedance.Of course it does, but not very significantly.
Thus in this way we try to control the board impedance.
Finally, remember that when u talk of an impedance controlled board, it applies to the board as of a whole and not just the trace impedance.