On Sat, 17 May 2008 16:31:32 -0700, "RST Engineering \(jw\)"
wrote:

Good lord, the effective epsilon of FR-4 ought to be something on the order
of 2 or so (real er should be around 4.5 or so) and the propagation delay
ought to be femtoseconds or so. What is slowing down the risetime? 50 ohms
on FR4 ought to be a conductor about as wide as the thickness of the board
material as I vaguely remember.

Jim



The top dielectric layer is 12 mils, so a 50 ohm trace is only 20 mils
wide. 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. A
real microwave laminate would help, and make the traces wider, but
that's expensive... this is really just a prototype. As Joerg says,
you can do fast stuff on junk boards, but you have to keep it short.

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


John