The characteristic impedance (Zo) of a microstrip transmission line can be calculated using the active calculator or the formula at the bottom of the page.
The calculator gives very similar results to those in Table 1 of an article at http://www.polarinstruments.com/support/cits/IPC1999.pdf
The following formula is given in the above article by Polar Instruments.
Where εr is the Relative Dielectric Constant
W is the Width of track
t is the Thickness of track (1 oz/ft2 = 0.035 mm)
h is the Thickness of dielectric
Note that the "h", for dielectric thickness in this formula is omitted in some other Polar publications.
The original source of formula is IPC-2141: Controlled Impedance Circuit Boards and High-Speed Logic Design, April 1996.
IPC have their website at: http://www.ipc.org/ContentPage.aspx?pageid=PCB-Tools-and-Calculators
This calculator is provided free by Chemandy Electronics in order to promote the FLEXI-BOX
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