[SI-LIST] Re: single end and bused trace question
- From: ma mu <sitaskina@xxxxxxxxx>
- To: weirsi@xxxxxxxxxx, a.ingraham@xxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 17 Feb 2005 15:52:50 -0800 (PST)
Hi steve,
My board is slow, 32 mhz and it is a 5 slot board with
connector only (hm hard metric connector). it is kind
of CPCI board. my question was when I measured the
bused impedance , it( the software that i used) didn't
ask me how long the length of my trace and where will
be the device / stub. so in what basic /equation it
calculated the impedance, which is low compare to
single end impedance measurement. could anyone explain
me the equation behind it!! Thanks.
taskina
--- Steve Weir <weirsi@xxxxxxxxxx> wrote:
> Andy, that is certainly possible. Perhaps Taskina
> can clarify his situation.
>
> Regards,
>
>
> Steve
> At 11:31 AM 2/17/2005 -0500, Andrew Ingraham wrote:
> > > Taskina, what you are witnessing is the energy
> that couples between your
> > > lines as crosstalk. If you move the lines
> further apart, the impedance
> > > will rise back towards the impedance of a single
> line.
> >
> >Depends on what he means by "bussed."
> >
> >The above answer is correct if he means the trace
> is just one trace in the
> >middle of a wider bus (several traces in parallel),
> and the traces are close
> >together.
> >
> >But "bussed" could also mean that he has a single
> trace that gets bussed
> >from one device to another to another. That's what
> I thought he meant, and
> >might make more sense considering how much smaller
> his impedance got.
> >
> >In that case, you have some capacitive loading at
> each device along the way,
> >and this is what reduces the effective (also called
> "loaded") characteristic
> >impedance. This effect should be described in any
> good textbook description
> >about characteristic impedances of traces with
> devices attached.
> >
> >A simple way to think about it is this: The raw
> trace (without devices
> >attached to it) has some capacitance per unit
> length and some inductance per
> >unit length. The characteristic impedance equals
> the square root of the
> >inductance (per length) divided by the capacitance
> (per length). By adding
> >devices (or stubs) to the trace, you've increased
> the average (effective)
> >capacitance per unit length by some amount, and
> from that you can
> >re-calculate the effective characteristic
> impedance.
> >
> >It makes sense to think of it this way when the
> distance between adjacent
> >devices on the "bus" is much less than a wavelength
> at your frequencies of
> >interest, so that the effect of adjacent loads
> blends together. At higher
> >frequencies or bigger distances between loads where
> that is no longer true,
> >it is better to treat the trace as having the
> original (unloaded)
> >characteristic impedance with a discontinuity at
> every device.
> >
> >If you are going to simulate the trace, use the
> larger unloaded
> >characteristic impedance and make sure to include a
> model for each attached
> >device, with its capacitance.
> >
> >The time to use the loaded bus characteristic
> impedance, is when doing
> >back-of-the-envelope calculations about the
> effective load seen by the
> >driver. You could use it in simulations too, but
> only if your simulation
> >doesn't include the capacitance of each attached
> device, and only if you are
> >sure the frequencies are low enough so that the
> loaded trace "looks"
> >continuous.
> >
> >Regards,
> >Andy
> >
> >
>
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