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Posted by GG on April 16, 2008, 9:40 am
If you were Registered and logged in, you could reply and use other advanced thread options > Robert Redelmeier wrote:
> >>GG wrote:
> >>>How long can the pcb trace length for the TX and RX differential
> >>>pairs of a 10/100 PHY interface to the Magnetics be? Using
> >>>Intel's LXT971A with Pulse transformer and need to go from one
> >>>side of the pcb to the other with a maximum length of 8-9 inches.
> >>How close can you maintain impedance and balance?
> >>That would seem to depend on the tolerance of the PCB processing.
> > I would also be very concerned about the signals on the
> > nearby traces. Crudely seen, it is like having 8-9 inches of
> > untwisted parallel conductors. 100 only really tolerates 0.5
> > inch, and those 8-9 inches might pick up excessive crosstalk.
>
> To maintain 100 ohms the traces will be pretty close together,
> so nearby has to be pretty close to bother it. Probably best
> to put the transformer as close as possible to the connector,
> I am not sure if that is what the OP meant.
>
> > I believe the usual layout solution is to use lots of vias to
> > essentially twist the traces. Have to be careful about the
> > impedence discontinuities.
>
> On PCB scale I don't believe that twist is so important,
> and the extra vias will create extra impedance discontinuities.
> Keeping balance means equal distance to anything else nearby,
> so it is best that there not be anything sufficiently nearby.
>
> -- glen
The transformers are next to connector, however the PHYs are on the
opposite side
of the board about 8" away. The pairs will be internally routed to the
transformers, how far
away do these lines have to be to avoid crosstalk? The board has a
cpu, memory and fpga all
running >100Mhz, theses devices sit in the middle of the board above
the pair lines.
Thanks for the help.
GG
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