Leonard, Yes, I did mean path, not past. We won't get to that until we talk about the multi-universe theory of signal propagation ... kidding! 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 Teraspeed® is the registered service mark of Teraspeed Consulting Group LLC Leonard Dieguez wrote: > "The path that the wave takes is the "PAST" of lowest impedance at the > frequency of interest, which amounts to the path of least energy. " > > Scott, do you mean "Path" and not "past". > > Leonard. > > > > > > > -----Original Message----- > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] > On Behalf Of Scott McMorrow > Sent: Tuesday, April 25, 2006 6:03 PM > To: doug@xxxxxxxxxx > Cc: si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] Re: Question regarding current loop > > Doug > You're diverging into the charge hose analogy again, but I'll entertain > your thought experiment. > > * 1) driver switches from zero to one driving a trace over a > continuous plane (agreed) > o Lets attach that plane to ground. > * 2) current flows into the transmission line > o Well, some or most of the current does, depending upon > frequency. I know cases where all the power is transfered > down from the output pad of the die through to the substrate > ground, and never even passes out of the device. But in our > case, we'll agree that current travels through the output > transistors into the transmission line. > o There are actually several parts to this part of the > circuit. > + There is a power and ground feed to the output > transistors > # The power and ground feed are connected to a > power distribution system, which acts as an > imperfect AC short. > + The output transistor is "referenced" to a substrate > ground, or ground mesh. > + The output transistor is attached at the pad to a > transmission line > + The transmission line is referenced to a plane (lets > let it be ground for the sake of our discussion) > * 3) I prefer Maxwell's concept of electromagnetic field propagation > as the basis of AC current and charge flow. > o A field flows from the power/ground distribution system into > the output circuitry. > + This field has a direction (therefore a direction for > AC current and power flow) > + This field must be supported by some transmission > medium. > # In our case, it is the combination of the > power/ground distribution planes, grids and > capacitors, inside and outside the chip. > o The output circuitry redirects the EM field from the > power/ground system to the transmission line/ground system. > + In so doing, > # some of the power reflects back into the > transistors and back into the PDS > # most of the power is transferred forward > # because the size of the output is very small, it > appears to act as a lumped element switch, > moving the current train from the big PDS to the > small transmission line. > * 4) If the transmission line it is driving is infinitely long, > energy will continue to be transferred from the device to the > transmission line, and the electromagnetic wavefront will continue > to travel in the forward direction. > o Because the signal is traveling down a "well-defined" > transmission line, with power being transferred in the > forward direction, the EM field induces instantaneous signal > and return currents in the trace and the underlying ground > plane. > + All of the AC energy is contained in that wavefront > + To support the AC wavefront, the trace only needs to > know where the ground plane is. The wave has been > disconnected from the driver. > * 5) If the transmission line is not infinite, but is terminated to > ground by it's complex impedance, then the AC wavefront (which has > a positive and negative component on the trace and plane, > respectively), will cause the traveling wave to be fully absorbed. > o This matched termination does not need any knowledge of the > driver, and no current of any kind flows back to the driver. > * 6) If instead of a unit step excitation, a band-limited repeating > 1 0 pattern is driven, then this system can be decomposed using a > Fourier spectral analysis. > o The typical analysis shows a DC component, a fundamental AC > switching frequency, plus multiple harmonics above the > switching frequency. > + For all energy carried by the fundamental and it's > harmonics (The AC components), there is no need for a > current to return from the terminator back to the > driver. All energy is absorbed in a complex impedance > matched termination. > + This leaves us with DC. > # DC current does not induce a "return current" > field in the ground plane, since it is not time > varying. (It does, however, induce a magnetic > field) > # DC current does need to close the loop and find > it's way back to it's origin. > * In our case, the origin is the power > distribution system, and ultimately the > "battery" from which all current comes and > goes. > # So, when the DC current flowing through the > trace reaches the termination resistor, it does > not necessarily return back to the driver ground > to close the circuit, it takes the lowest > resistance path back to the battery (a.k.a. > power supply). > * 7) If we extend our thought experiment to two single-ended traces > well-separated on the board driven with a differential > excitation, then exactly the same steps can be followed until we > reach the terminator. > o When we reach the terminator, if both lines are terminated > to ground, the result is the same. Currents travel back to > the power supply through the termination resistors and the > ground plane. > o What if we detach the termination resistors from the ground > plane and tie them together, forming a differential > termination? > + The positive current enters one terminal of the > resistor and negative current enters the other > terminal of the resistor, and a DC loop is formed. > + NOTE: All AC current is still perfectly terminated by > a complex impedance matched terminator at the end of > the line, whether it is a terminator to ground or a > differential terminator. > + As far as the AC wave is concerned the middle of the > resistor forms a virtual ground. > + Implementation wise, this is a bit oversimplified. > For a differential terminator to work well at ultra > high frequencies, it has to be impedance matched to > what the wavefront sees. To do this the termination > must terminate all of the modes of propagation. > + Normally these are simplified into even and odd mode. > But, when a 3rd conductor exists (the ground plane > itself) there are actually 3 eigenmodes, not 2, and > all three must be correctly terminated to provide a > reflection free match. > + Practically, this is not an issue until we reach the > 25 to 50 GHz region. The region depends strongly upon > the distance from the trace to the plane. > > > In all cases that I can think of, single-ended or differential, any > signal driven on a transmission line from a device can be decomposed > into AC components and a DC component. The AC components can always be > treated with EM wave theory and can be terminated at the end of the > line, without any subsequent currents flowing back to the originating > device. (Energy is lost from the system.) For single ended > termination, the real inductance of attachment vias and plane spreading > inductance will cause significant disruptions when termination occurs. > This causes significant bounce. But for differential termination, with > > balanced currents, no such disruption occurs. > > As far as I am aware, only AC components of a signal radiate, and return > > to the universe. DC components do "flow" in the system too. But they > do not necessarily need to "flow" back to the driver. They flow back > through the lowest resistance path to the power supply. At DC the > driver is "just a switch" on a wire. > > In summary, once an AC signal is "released" to a transmission line, it > carries it's own instantaneous return reference, which I like to call > the AC, or RF return path, or image plane. The path that the wave takes > > is the past of lowest impedance at the frequency of interest, which > amounts to the path of least energy. This path can be terminated at the > > end of the transmission line without any additional currents flowing > back to the driver or power distribution system. This is energy that is > > "released" and then dissipated through termination and thermal > dissipation. In the limit, the universe forms a spherical transmission > line through which energy is radiated. Once it leaves the antenna, the > energy will not be returning to the driver, and travels off to infinity > and beyond. > > DC current, however, does flow in a loop back to the power supply, along > > the path of least resistance, which is the path of least energy. DC > current does not radiate, but does create a nice magnetic field. > > So, if we go back to the initial contention that somehow in the > non-switching portion of differential pair signaling, the radiation loop > > changes magically from between the trace and plane to between the traces > > and back to the driver, this is simply not true. DC components of the > positive and negative differential signal find their way back to the > power supply along the path of least resistance. The loop could be > huge, but who cares? DC don't radiate. All AC components still travel > down the transmission line and are either terminated correctly or are > reflected back towards the driver. > > Radiated energy is extremely low, because the traces are very close to > the image plane, which, as Lee Richey points out, pretty much fixes any > emissions problems anyway. (Formally, a trace over a plane can be > looked at like an antenna separated by it's mirror opposite 2X the plane > > spacing away. The mirrored antenna effectively cancels out any > appreciable radiation, as long as the plane is unbroken.) Whatever > residual radiation is left over will have additional cancellation from > the other signal in the differential pair. Differential pairs will have > > lower radiation than single-ended traces, but both will meet EMI > compliance if designed correctly. Because the image plane cancellation > of trace radiation is so good, there is negligible difference in > emissions between closely spaced and widely spaced differential traces. > > As a result, EMI is generally never a good reason for choosing close vs. > > wide spacing on differential traces. Density, routability, impedance > control, and manufacturability are good reasons. > > > Best regards, > > 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 > > Teraspeed(r) is the registered service mark of > Teraspeed Consulting Group LLC > > > > Doug Brooks wrote: > >> 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 >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> > ______________________________________________________________________ > >>>>> >>>>> >>> ______- >>> >>> >>>>> Check out UltraCAD's differential impedance and skin effect >>>>> >>>>> >>> calculators at >>> >>> >>>>> <http://www.ultracad.com>http://www.ultracad.com >>>>> >>>>> ------------------------------------------------------------------ >>>>> To unsubscribe from si-list: >>>>> <mailto:si-list-request@xxxxxxxxxxxxx>si-list-request@xxxxxxxxxxxxx >>>>> >>>>> >>> with >>> >>> >>>>> 'unsubscribe' in the Subject field >>>>> >>>>> or to administer your membership 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<http://www.qsl.net/wb6tpu>http://www.qsl.net/wb6tpu > >>>>> >>>>> >>>>> > _______________________________________________________________________ > >>>> >>>> >>> _____- >>> >>> >>>> Check out UltraCAD's differential impedance and skin effect >>>> > calculators > >>>> >>>> >>> at >>> >>> >>>> http://www.ultracad.com >>>> >>>> >>> ------------------------------------------------------------------ >>> To unsubscribe from si-list: >>> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >>> >>> or to administer your membership from a web page, go to: >>> //www.freelists.org/webpage/si-list >>> >>> For help: >>> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >>> >>> List FAQ wiki page is located at: >>> http://si-list.org/wiki/wiki.pl?Si-List_FAQ >>> >>> List technical documents are available at: >>> http://www.si-list.org >>> >>> List archives are viewable at: >>> //www.freelists.org/archives/si-list >>> or at our remote archives: >>> http://groups.yahoo.com/group/si-list/messages >>> Old (prior to June 6, 2001) list archives are viewable at: >>> http://www.qsl.net/wb6tpu >>> >>> >>> >> > ________________________________________________________________________ > ____- > >> Check out UltraCAD's differential impedance and skin effect >> > calculators at > >> http://www.ultracad.com >> >> ------------------------------------------------------------------ >> To unsubscribe from si-list: >> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field >> >> or to administer your membership from a web page, go to: >> //www.freelists.org/webpage/si-list >> >> For help: >> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >> >> List FAQ wiki page is located at: >> http://si-list.org/wiki/wiki.pl?Si-List_FAQ >> >> List technical documents are available at: >> http://www.si-list.org >> >> List archives are viewable at: >> //www.freelists.org/archives/si-list >> or at our remote archives: >> http://groups.yahoo.com/group/si-list/messages >> Old (prior to June 6, 2001) list archives are viewable at: >> http://www.qsl.net/wb6tpu >> >> >> >> > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > List FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > List technical documents are available at: > http://www.si-list.org > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > > > > ------------------------------------------------------------------ To unsubscribe from 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