On Wed, 22 Aug 2001 16:24:37 -0500, Tim Hutcheson wrote:


>Since my source resistance is 53 ohms, I have less than 2% mismatch.

Just to inject a little reality here. Board houses can not control layer
thickness to anything like 2%. The can mic up the cores and maybe get close
to what you want but prepreg layers it depends how long, how hot, and how
hard they squeeze them in the press. 

Add to that the dielectric constant of FR4 is variable by about 10% and
varies over commercial temperature range by about another 10%. 

Then I doubt they can give you 5 mil tracks within 10% and what is the
actual effective track width when it is under etched at an angle anyway? 

So yes it is wise to aim for what you worked out but expect your mismatch
to be more like +/-20%

You could put some test traces outside the board and find a board house
with (an expensive) impedance analyser - expect that to be expensive
because if the impedance isn't right all they can do is scrap and start
again. 


Cheers, Terry.

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