RST Engineering (jw) wrote:
The top dielectric layer is 12 mils, so a 50 ohm trace is only 20 mils
wide.

My calculator says 22 mils at one Gig, and not knowing your frequency of
operation puts me at somewhat of a disadvantage. I wonder if the mismatch
between 20 and 22 is honking things up.

That's cheap to fab from a quick proto house, but not optimum
for losses. I want this thing to have a risetime below 150 ps, and an
inch of 50 ohm line on this cheap crud is about 70 all by itself.

Let me understand, please. I'm having a hard time relating line matching to
risetime. What does the loss tangent of the material or dielectric constant
have to do with risetime?



Take John's example. 150psec means that you'll have spectral components
in excess of 3GHz in there. If they are dissipated a lot more than the
lower ones your risetime goes to pots.


FR4 seems to have an Er around 4.5 at GHz speeds. Effective epsilon
for the microstrip is about 3.3.

I show 3.1, but that's picking at nits.


It's also losses that matter.

--
Regards, Joerg

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