[SI-LIST] Re: Question regarding current loop
- From: steve weir <weirsi@xxxxxxxxxx>
- To: doug@xxxxxxxxxx,si-list@xxxxxxxxxxxxx
- Date: Mon, 24 Apr 2006 14:09:31 -0700
Doug,
Pardon me, but I believe that your original statement was to the
effect that the field strength from line to plane versus line to line
changes significantly at different times during a one or two bit
cycle. If that were true, then by cutting the slot in the plane, we
should be able to observe those differences, should we not?
I am having a little semantic trouble here. You keep talking about a
point of interest having reached a location. If we are talking about
a properly terminated system, then there are no reflections. If we
have a mythical infinite impedance probe, we can observe anywhere
along the line and see the same result simply phase shifted. What
are you trying to get at with the statement: "I am assuming the
point of interest has reached the end of the trace at the receiver."
This sounds more and more like we are discussing the electron hose
analogy. I think this example shows just how easy it is to get
misled by that analogy.
Regards,
Steve.
At 01:47 PM 4/24/2006, Doug Brooks wrote:
>Gee,
>
>It seems to me you are off point.
>
>I asked about the specific current loop (current flows in a closed loop) at
>a specific point in time, not about radiation.
>You are correct that I needed to clarify where the wave form is if the
>trace is long (which I did on a follow-on post). I am assuming the point of
>interest has reached the end of the trace at the receiver.
>
>Doug
>
>
>
>
>
>At 01:30 PM 4/24/2006, steve weir wrote:
> >Doug:
> >
> >All of your assumptions are fine. I am also happy to band limit the wave
> >form rather than make it trapezoidal. Either arrangement is fine for me.
> >
> >I propose: 8mil lines 24mils center to center, 30" long, symmetric
> >stripline construction with plenty of stitch vias plane to plane. Assume
> >an eR of anything you like from 1.0 to 5.0 and we will set the dielectric
> >thickness as needed for 100 Ohms odd mode. We center the pair in a 10"
> >wide board.
> >
> >We consider X the 30" axis, Y the 10" board width, and the thickness of
> >the board Z.
> >
> >Now, lets also add a free standing antenna along the side of the board in
> >the X-Z plane and locate that 10" along the X axis from the transmitter.
> >
> >Finally, let's test with two plane arrangements.
> >
> >A. The image plane is solid
> >B. We cut a 0.100" wide slot in the plane end to end in the Y direction
> >10" along the X axis from the transmitter. ( Dead center of our antenna. )
> >
> >Let's drive with 50ps rise times, and 500ps bit intervals.
> >
> >I submit that under A, our antenna will be nice and quiet.
> >
> >I submit that under B, our antenna is going to light right up from the big
> >slot wave we excite crossing over the gap.
> >
> >Please comment:
> >
> >Do you agree that the slot wave will be there?
> >For the dimensions cited, do you think the slot wave be a little 50-100ps
> >pulse, or will it be a really messy thing with much lower frequency
> >content mixed in?
> >How does what you expect correlate with your original statements?
> >How does the fact that the line is much longer than the bit interval
> >relate to your statements concerning current loops?
> >
> >Regards,
> >
> >
> >Steve.
> >At 12:36 PM 4/24/2006, Doug Brooks wrote:
> >>Assume we have a differential pair, driven in odd mode with no timing
> >>offsets.
> >>Assume the pair is terminated with a single resistor between the
> two traces.
> >>Assume the signal is purely trapezoidal.
> >>Assume there is an underlying plane (but assume what you want about the
> >>spacing to the plane.)
> >>Assume what you want about the spacing between the traces.
> >>*If* current flows in a closed loop and *if* current is constant everywhere
> >>in the loop, and *if* current is a point (as opposed to an average) concept
> >>in time....
> >>
> >>What is the definition of the current loop at the instant when the current
> >>wave form is at the middle of the constant part of the wave form (between
> >>the rise and fall times)?
> >>
> >>Doug Brooks
> >>
> >>__________________________________________________________________
> __________-
> >>Check out UltraCAD's differential impedance and skin effect calculators at
> >>http://www.ultracad.com
> >>
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>____________________________________________________________________________-
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>http://www.ultracad.com
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