Geronimo said:
Ah, okay, what I am actually trying to find out is what makes FR4 act worse
than e.g. teflon at data rates beyond 2,5 Gbps. Is it the loss tangent or
the epsilon r?
It is the imaginary part of the dielectric constant. I believe that
can be described as a loss tangent, though I haven't done that.
How is the frequency-dependent attenuation physically
describable? Where does the energy go? Heat, ...?
Yes, heat. You can consider it as electrical friction.
It was my opinion that
higher frequencies can be transmitted over coax but not over FR4 because of
the geometry. Because in a coax there is (almost) no energy loss because the
TEM wave is "captured" by the outer shield and in a planar setup like
stripline or microstrip there are E-field and H-field lines vanish into the
air environment (or somewhere else...). Therefore I'm trying to design a
coax on a PCB. Am I right with my thoughts, anyway?
I don't believe that is true. It may be that coax tends to use better
dielectrics. Other than that, even if the wave isn't "captured" as you
say, as long as it doesn't find anything else to induce current into, it
doesn't contribute to loss. (Well, twisted pair cable is twisted to
minimize the radiation. Microstrip isn't twisted. Radiation will be
mostly in the plane of the board, and minimized by keeping the board
thin.) The very low loss coax cables use a spiral dielectric such that
most of the space is air and very little is plastic. There has to be
enough material to hold the center conductor in place.
The other loss increase with frequency is due to the skin effect, where
the current travels in a thin layer on the surface of the wire,
increasing the resistance as seen by the current. I believe FR4
is chosen for its thermal and strength properties, and ability to bind
to metal, all not normally required by a coax cable dielectric.
(And also cost.)
-- glen