# [SI-LIST] Re: Circle bus topology; Circular Firing Squad?

• From: steve weir <weirsi@xxxxxxxxxx>
• To: Vinu Arumugham <vinu@xxxxxxxxx>
• Date: Mon, 30 Jul 2007 16:53:36 -0700

```Vinu but for the discussion at hand:

First:  The driver is back terminated in the example so both wavefronts
are completely absorbed and the characteristic impedance is the
effective impedance of the line everywhere.  Energy propagating forward
or backwards in the line does not change the impedance.

Second:  At the point in time where the apparent reflection occurs, no
wavefront has reached an impedance discontinuity.  And in fact as stated
above, if the source matches perfectly, never will.  There are no
reflections in this system at all.  Each wavefront launches, goes its
merry way around the path and gets identically absorbed back at the
driver.

To an observer monitoring the line two equal and opposite wave fronts
will indeed appear to bounce like a perfectly elastic mechanical
collision.  So let's ask ourselves which is the illusion:  the apparent
100% reflection, or the continuous propagation of each front.  Several
useful experiments have been offered to resolve the issue.  In each we
send two wavefronts which are not identical and monitor the behavior.
What do we find?  We find that rather than each waveform reflecting
identically as predicted by the reflection model, the difference
continues to propagate forward.  IE, the observation EXACTLY matches the
wave propagation model, while it does not match an unmodified reflection
model.  In order to fix the reflection model we have to artificially
create a short to the odd mode at the same point where we have an open
to the even mode INCIDENT waveforms.

Best Regards,

Steve.
Vinu Arumugham wrote:
> "There is only one impedance at any given point on the line, and for
> constant line parameters, the impedance is constant throughout."
> Yes, that's the characteristic impedance of the line.
>
> The input impedance of an unterminated line can vary from zero to
> infinity depending on the frequency of the driving signal. In other
> words, the line driver "sees" a high or low impedance that is a
> function of the magnitude and phase of the reflected wavefront. The
> same thing happens when wavefronts meet in a loop. The effective
> impedance seen by each wavefront is a function of the magnitude and
> phase of the other wavefront. So, why is this interpretation
> "nonsensical"?
>
> Thanks,
> Vinu
>
> olaney@xxxxxxxx wrote:
>> If you suppose that the waves meet and rebound like billiard balls,
>> that would be incorrect.  Each passes through the other as if it was
>> the only wave on the transmission line.  Only a real open circuit (or
>> other impedance discontinuity) can cause reflection.  Though
>> identical wavefronts might create the illusion of a "virtual open
>> circuit" to the viewer, that is not the physical reality.  The
>> simultaneous "high impedance / low impedance" interpretation is
>> nonsensical.  There is only one impedance at any given point on the
>> line, and for constant line parameters, the impedance is constant
>> throughout.  Especially note that the impedance of a linear xmsn line
>> has nothing to do with the shape or direction of the waves that
>> happen to be traveling on it.  To suppose otherwise wrenches the laws
>> of physics.  Sorry if I have to be blunt.  Wavefronts passing through
>> each other is the bedrock reality, all else is armwaving.
>>
>> Orin Laney, PE, NCE
>>
>> On Mon, 30 Jul 2007 10:47:33 -0700 Vinu Arumugham <vinu@xxxxxxxxx
>> <mailto:vinu@xxxxxxxxx>> writes:
>>
>>     When identical wavefronts are sent through the two branches of
>>     the loop and meet at the far end, each wavefront can be described
>>     as being reflected by the virtual open circuit.
>>     When one wavefront is "marked", the wavefronts do not encounter a
>>     virtual open circuit. One wavefront encounters a high impedance
>>     and the other a low impedance compared to the line impedance. The
>>     subsequent reflections of opposite polarity can be described as
>>     producing the illusion of the wavefronts flowing through rather
>>     than being reflected at that point.
>>
>>     In other words, it seems to me that both the reflection and
>>     reinforcement descriptions are perfectly valid and each is as
>>     real or illusory as the other.
>>
>>     Thanks,
>>     Vinu
>>
>>     olaney@xxxxxxxx wrote:
>>>     There is a difference, Ron, and my experiment illustrates it.  It is
>>> that
>>>     rather than bouncing back as a relection on the same trace, the loop
>>>     return signals are the result of a round trip without reflection.  Two
>>>     open ended lines in parallel will show an impedance profile similar to
>>>     that of the loop *only* if the trace lengths are matched.  The fact that
>>>     this special case is indistinguishable from a loop at the driving point
>>>     is interesting, but does not make it equivalent in terms of the origin
>>> of
>>>     each return signal.  If you have a means to mark the driving signals so
>>>     that they can be distinguished from each other, the difference between
>>>     double open ended traces and with the ends shorted together can be
>>>     observed.  As you say, try it with a couple of pieces of coax and a TDR
>>>     if you disagree.  It'll work best if you use a separate series
>>>     termination for each trace rather than a single backmatch resistor for
>>>     both so that you can see the return signals separately.  I mentioned
>>>     ferrite but a high frequency LC trap on one leg to notch out a specific
>>>     frequency might be more convincing.  With two traces, the marked signal
>>>     returns on the same trace.  Create a loop by shorting the ends (making
>>>     sure that the short maintains the correct path impedance), and the
>>> marked
>>>     signal returns on the other trace.  With identical traces (or coax) and
>>>     identical driving signals, as you propose, the difference is there but
>>>     you can't see it.  That does not mean that the cases are equivalent,
>>> just
>>>     that your experimental setup cannot distinguish between them.  Hence,
>>> the
>>>     need to mark the signals.  Steve explained it well.  This would make a
>>>     good question for the electrical engineering professional licensing
>>> exam.
>>>
>>>     Orin
>>>
>>>     On Sat, 28 Jul 2007 23:29:35 -0700 steve weir <weirsi@xxxxxxxxxx>
>>> writes:
>>>
>>>>     Ron, yes if the signals exactly match then Ron's description of the
>>>>     apparent open end matches the illusion.  It is an illusion just the
>>>>
>>>>     same.  This is where Orin's proposed experiment can provide insight.
>>>>
>>>>     Any difference between the two wavefronts is not accounted for by
>>>>     the
>>>>     open end model.  That odd mode if you will encounters the illusion
>>>>     of a
>>>>     dead short at the same juncture where the even mode Ron and you
>>>>     describe
>>>>     encounters the illusion of an open.  Account for both the even and
>>>>     odd
>>>>     signal modes and you will get the right answer from the illusion
>>>>     just as
>>>>     you will if you follow the formal, exact, and I think simpler view:
>>>>     that
>>>>     the two wavefronts continue to propagate until they are absorbed.
>>>>
>>>>     Steve.
>>>>     ron@xxxxxxxxxxx wrote:
>>>>
>>>>>     Consider for a moment a 50 ohm source driving two equal length 100
>>>>>
>>>>     ohm
>>>>
>>>>>     lines unterminated(open circuit)
>>>>>     TDR will show the open circuit at the end of the lines just as if
>>>>>
>>>>>     there were one 50 ohm open ended line.
>>>>>
>>>>>     Next consider what will happen if you connect the open ended lines
>>>>>
>>>>>     together.  No change.  It will still reflect back as an open.
>>>>>
>>>>>     Ponder that for a little and try it with a couple pieces of coax
>>>>>
>>>>     and a
>>>>
>>>>>     TDR if you disagree.
>>>>>
>>>>>
>>>>>
>>>>     --
>>>>     Steve Weir
>>>>     Teraspeed Consulting Group LLC
>>>>     121 North River Drive
>>>>     Narragansett, RI 02882
>>>>
>>>>     California office
>>>>     (707) 780-1951 Fax
>>>>
>>>>     Main office
>>>>     (401) 284-1840 Fax
>>>>
>>>>     Oregon office
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>>>>
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>>>>     This e-mail contains proprietary and confidential intellectual
>>>>     property of Teraspeed Consulting Group LLC
>>>>
>>>>
>>>
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>>
>>
>>
>

--
Steve Weir
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882

California office
(707) 780-1951 Fax

Main office
(401) 284-1840 Fax

Oregon office
(503) 430-1285 Fax

http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of
Teraspeed Consulting Group LLC
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