[SI-LIST] Re: Question regarding current loop

Apparently this is a pretty tricky concept.
Let's break it down in very small increments and see what sort of agreement 
we can get in stages.
The first thing is to see if there is agreement at every step along the way.

To start:
1. Assume a driver switches from zero to one, driving a single ended 
transmission line (trace over a continuous plane).
2. At the first increment of time there will be a current into the line 
from the driver that equals Vout/Zo
3. Since current is the flow of charge, there will be charge flowing into 
the line.
4. If the driver doesn't change state, charge will continue to flow into 
the line.

Here's a punch line

If charge leaves the driver (at any instant of time), the same amount of 
charge must instantaneously be returning to the driver. (current flows in a 
loop)

Do we all agree with this statement? If not, why?

Charge is something physical (it has mass). Do we all agree with that?

If so, then as the signal travels down the line (forget about the end of 
the line) how and where is this physical charge flowing?

If we can agree on that, then the next question will be, when the signal 
has propagated to the far end of the line, where (we assume) there is a 
proper terminating resistor connected between the trace and the 
plane,  charge is still flowing onto the line from the driver (which is 
still driving a resistive load, Zo). The same amount of charge must 
instantaneously be returning to the driver. The same amount of charge must 
be flowing through the terminating resistor (R = Zo). In the first 
increment of time after the signal reaches the terminating resistor, where 
and how is this physical charge flowing?

It's not clear to me that everyone sees this the same way.

Doug











At 10:05 AM 4/25/2006, Rick Brooks \(ricbrook\) wrote:
>Only a current loop in a lumped (sized) circuit is the same at every
>point (sums to zero).
>For distributed circuits this is not true.
>But a cutset at a single physical point in any circuit would have a sum
>of currents equal to zero.
>EG, for a long Tline, the current at the load is not the same as at the
>driver.
>
>I don't know if this also misses the point.
>Also, DC is an ideal concept just as infinite frequency is.
>
>just my 2 cents.
>
>
>
>
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>On Behalf Of Doug Brooks
>Sent: Tuesday, April 25, 2006 9:22 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Question regarding current loop
>
>Ahhhhhh...............
>Poor choice of words.
>What I meant is that current is the *same* at every point in the loop at
>any point in time.
>Doug
>
>
>
>At 09:09 AM 4/25/2006, Scott McMorrow wrote:
> >Doug
> >
> >Constant current flowing in a closed loop would be a DC circuit, by
> >definition.
> >
> >Scott
> >
> >
> >
> >
> >Scott McMorrow
> >Teraspeed Consulting Group LLC
> >121 North River Drive
> >Narragansett, RI 02882
> >(401) 284-1827 Business
> >(401) 284-1840 Fax
> >
> ><http://www.teraspeed.com>http://www.teraspeed.com
> >
> >Teraspeed(r) is the registered service mark of
> >Teraspeed Consulting Group LLC
> >
> >
> >Doug Brooks wrote:
> >>
> >>Well,
> >>I happen to believe these assumptions,  but I'm not sure everyone
>does.
> >>Do you?
> >>Doug
> >>
> >>
> >>
> >>At 07:05 AM 4/25/2006, Andrew Ingraham wrote:
> >>
> >>>
> >>>Doug,
> >>>
> >>>I'm having trouble with your assumption:
> >>>
> >>>
> >>>>
> >>>>*If* current flows in a closed loop and *if* current is constant
> >>>>
> >>>
> >>>everywhere
> >>>
> >>>>
> >>>>in the loop
> >>>>
> >>>
> >>>Regards,
> >>>Andy
> >>>
> >>>
> >>>
> >>
> >>
> >>______________________________________________________________________
>______-
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