# [SI-LIST] Re: Concept of Voltage

*From*: Vinu Arumugham <vinu@xxxxxxxxx>*To*: Yuriy Shlepnev <shlepnev@xxxxxxxxxxxxx>*Date*: Tue, 19 Jul 2011 10:43:34 -0700

Yuriy, "Considering use of multiport models in a SPICE circuit simulator, the only possible and legitimate use of the local reference terminal is to connect it to a common reference node (global ground)." Connecting both local references to global ground seems like a non-physical connection. We know that in the real world, the voltage difference between the local references is defined at DC and non-zero. But in the circuit you are forcing it to zero for DC and all frequencies (gA=gB=0). It seems Shawn's circuit with only gA tied to global ground is better. I of course agree that voltage on gB measured w.r.t to global ground is undefined at high frequencies. Could you please explain why gA=gB=0 the only legitimate use of the model? Thanks, Vinu Yuriy Shlepnev wrote: > Shawn, > > You have described a perfect example of what should never be done with > multiports described with S-parameters (or equivalent t-line models). It is > typical model misuse (garbage in - garbage out). > Formally, each single port in a multiport has 2 terminals (or nodes) - > signal and local reference. There is no information on the potentials of > each terminal - only information on the difference of potentials or voltage. > Moreover, as I pointed earlier, the voltage may be defined not as the > difference of potentials (or line integral of electric field), but as a > projection of the electric field on the electric field of a wave-guiding > eigen-mode. Current flowing out of the reference terminal must be exactly > opposite to the current flowing into signal terminal. Again, current may be > defined either locally or as a projection of magnetic field on the > eigen-mode field. The projection definition must be used even for MTL with > quasi-TEM modes at high frequencies. > > The only possible way to use a port is to connect it to another port that is > defined identically. Lumped components can be connected to local or lumped > ports defined in electromagnetic analysis. Wave-ports can be connected only > with the identically defined wave ports. An exception is quasi-TEM > transmission lines at lower frequencies. Such ports can be connected with > the lumped or local ports as long as the cross section stays much smaller > than the wavelength (due to similarity of the voltage definition for MTL and > lumped element). In cases when distance between strips becomes comparable > with the wave-length, connection of a quasi-TEM t-line port with a lumped > port becomes ambiguous and erroneous. > > Considering use of multiport models in a SPICE circuit simulator, the only > possible and legitimate use of the local reference terminal is to connect it > to a common reference node (global ground). The signal terminal must be > connected to a signal terminal of the identically defined port. In case of > lumped port, the lumped element must be connected between the signal > terminal and the global reference node. If a lumped element contains more > than two nodes, there must be a separate lumped port (possibly with common > reference) constructed to connect each signal node of the lumped element. > There are no restrictions in the circuit analysis as long as one follows the > port construction and connection rules and build or collect a sufficient > number of multiport models for each interconnect component. Not only > quasi-TEM, but optical or substrate integrated waveguides for instance can > be analyzed with a SPICE solver without any problems. > > Best regards, > Yuriy > > Yuriy Shlepnev > www.simberian.com > > -----Original Message----- > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On > Behalf Of Shawn Hermite > Sent: Monday, July 18, 2011 5:32 PM > To: si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] Re: Concept of Voltage > > Hi, > Thank you all for adding your explanations. > > For those of you with full faith in the traditional definition of voltage, > I'd invite you to conduct the following simulation-based experiment: > - Collect a 2-port S-parameter from either measurement or simulation of a > physical transmission-line structure. It can be PCB trace or co-ax cable; > - Let's label the following nodes: (1) sA: the signal node on the left side, > (2) gA: the reference node on the left side, (3) sB: the signal node on the > right side, and (4) gB: the reference node on the right side. > - By definition, the 2-port S-parameters use two voltage variables: > vA=phi(sA)-phi(gA) and vB=phi(sB)-phi(gB). Here phi(.) is the potential > function w.r.t. infinite, i.e., phi(infinity)=0. > - With a simple mathematical transformation, we can convert the 2-port > S-parameter block into a 4-port block, making use of the potential variables > at the nodes. > - In your favorite circuit simulator, HSPICE or ADS ..., you apply an AC > voltage source between nodes sA and gA (and possibly a source termination), > and gA is tied to GND. You need put a termination resistor between sB and > gB. But, gB is not tied with GND. The rationale is: the circuit has only one > true GND. > - Run you AC simulation, with any reasonable frequency band. Plot voltage at > gB, which is phi(gB). It is likely NOT ZERO. Wah lah! you have just > extracted the "ground bounce" effect at the receiver. > > Again, thanks for bearing with my hung-ups on the trivial concept. > > Regards, > > Shawn > > On Mon, Jul 18, 2011 at 12:26 PM, Luciano Boglione > <l.boglione@xxxxxxxx>wrote: > > >> Yuri, Shawn, >> >> Basic questions are always interesting. May I bring up a couple of >> additional points to mull over: >> >> - the definition of voltage should be unique, the integral of E over a >> path l. Whether the field is conservative or not, depends on the >> field, not on the definition. >> - the fact that the field is always conservative locally seems a >> logical assumption to make; however, it may butt heads with the >> necessary condition for a field to be conservative (dEz/dy=dEy/dz; >> dEx/dz=dEz/dx; dEy/dx=dEx/dy where the derivative d/dx, etc. are >> partial) >> - the modes in a guided propagation structure are determined by the >> transversal (x,y) section of the structure (since its cross section is >> considered "constant" in order to handle it analytically). The >> propagation that occurs in the orthogonal z direction is uniquely >> described by a wave equation (e.g. V1*exp(k*z)+V2*exp(-k*z) with k >> complex for a sinusoidal field over time, exp(jwt)) independently of >> the cross section of the waveguide (e.g. coax cable, waveguide, etc.). >> If one can calculate/determine the cross section integral at z=0 and >> z=L (say start and end of the structure), the constants V1 and V2 >> should also be determinable uniquely for each mode. >> - when the structure supports DC (lowest cutoff frequency is 0Hz), TEM >> modes (strictly speaking) are not part of the mode solution, although >> the above procedure (field is split into transverse and longitudinal >> components) still applies. In particular, the E field should not be >> conservative - although, some approximations can be made to tweak the >> solution and use a quasi-TEM approximation... memory may not help me >> here, but I believe a lot of more precise information can be found in >> Collins, Foundation for Microwave Engineering, McGraw Hill. >> >> Regards, >> Luciano >> >> >> ----- Original Message ----- >> From: "Yuriy Shlepnev" [shlepnev@xxxxxxxxxxxxx] >> Sent: 07/18/2011 08:44 AM MST >> To: "'Shawn Hermite'" <shawn.hermite@xxxxxxxxx>; >> <si-list@xxxxxxxxxxxxx> >> Subject: [SI-LIST] Re: Concept of Voltage >> >> >> >> Shawn, >> There are two possible definitions of voltage. >> >> The first one is classical with the integral of electric field between >> two points - that definition assumes that the field is conservative >> (no difference in voltage value if the integration path is changes). >> This definition can be applied to lumped elements or TEM transmission >> lines only in case if distance between the integration (measurement) >> points is much smaller than the wavelength (locally the field is >> always conservative). It pairs with the current definition through a >> surface integral over a conductor cross-section (or port cross-section). >> >> The second definition is specific to waveguiding structures (or >> transmission >> lines) in microwave theory. Electric field in a cross-section of a >> waveguide (or MTL) can be expressed as a sum of eigen-waves with some >> coefficients. >> Those coefficients are treated as the voltages in the theory of >> > multiports. > >> Technically voltages is this case are Fourier coefficients with the >> base functions defined by a set of eigen-modes (each wave has its own >> > voltage). > >> Currents in the multiport theory are introduced in a similar way >> through the projections of magnetic field on the magnetic fields of >> the eigen-waves. >> There are no restrictions on the size of cross-section with such >> definition of voltage and current. Both wave-guide ports and local or >> lumped ports can be used to define a multiport. This allows to build >> multi-port models to combine distributed and lumped structures. >> Circuit theory can be used to analyze connections of such multiports >> without any restrictions as soon as the voltages and currents are defined >> > identically for the connected ports. > >> Considering multi-conductor transmission lines, if it is analyzed with >> a static field solver, the electric field is conservative by >> definition and voltage can be uniquely defined in the model. Though, >> the model breaks if the distance between strips or reference >> conductors becomes comparable with the wavelength. In case of >> electromagnetic analysis of multi-conductor line, the voltage can be >> defined following the first definition only in case if cross-section >> size is much smaller than the wavelength. Such solution will be >> identical to the obtained with the static field solver. See more on >> estimations of frequency boundaries in I.V. Lindell: On the quasi-TEM >> modes in inhomogeneous multiconductor transmission lines, IEEE >> Transactions on MTT, vol.29, no.8, pp.812-817, 1981 or Electromagnetic >> waveguides and transmission lines By Frank Olyslager. In case if >> cross-section becomes comparable with the wave-length, the second or >> projection definition of voltage can be used to turn MTL into a >> multiport. Note that the current definition through the a conductor >> cross-section integral stays valid up to higher frequencies than the >> conservative voltage definition. This fact can be used to define >> voltage through the current and power transmitted by wave. >> >> Finally, at high frequencies only power of propagating waves is >> > measurable. > >> That is why waves and scattering parameters are used for interconnect >> analysis at microwave frequencies. See more on definitions of >> multiports and S-parameters in presentation #2010_01 at >> http://www.simberian.com/TechnicalPresentations.php (tutorial >> materials from DesignCon2010). >> >> Best regards, >> Yuriy >> >> Yuriy Shlepnev >> www.simberian.com >> >> >> >> -----Original Message----- >> From: si-list-bounce@xxxxxxxxxxxxx >> [mailto:si-list-bounce@xxxxxxxxxxxxx] >> On >> Behalf Of Shawn Hermite >> Sent: Sunday, July 17, 2011 11:06 PM >> To: si-list@xxxxxxxxxxxxx >> Subject: [SI-LIST] Concept of Voltage >> >> To SI experts, especially those used Maxwell's equations in their books: >> The concept of voltage has been deeply engrained in our minds. It's >> the foundation of the circuit theory and electrical engineering at large. >> Recently, when collecting materials for a presentation advocating the >> importance of signal integrity, I realized that the concept of voltage >> is based on the assumption of the electric field being conservative. >> The general differential form of Maxwell's equations (the Faraday's >> Law part in >> particular) contains the dB/dt term that ruins the validity of curl >> {E} = 0. >> >> There are two categories of arguments: >> (1) With the integral form of the Faraday's Law, we can treat the >> time-varying magnetic flux term as an electromotive-force (like >> battery), hence fix the KVL, the general idea of voltage is still >> being the line integral of the electric field between two points. >> (2) Switching to the frequency-domain, the surface integral of the >> B-field is related to the characteristic dimension of the system (D) >> and the operating frequency. It goes with some hand-waving arguments, >> and the claim is that the unfriendly term is nearly zero when D << >> wavelength. This argument also goes hand-in-hand with validity of >> lumped vs. distributed element modeling. >> >> I have also observed that in a multi-conductor transmission-line (MTL) >> system where the TEM mode is propagating, the E-filed is conservative >> on each cross-section, thus voltage is well defined between a signal >> conductor and the common-reference conductor. Here, the general >> electrodynamic Maxwell's equations are all satisfied. The MTL theory >> explains why RF/microwave testing has to be done using co-ax cables. >> It also makes sense why 'port' (instead of circuit node) voltage is >> used in constructing S-parameters. >> >> If the second explanation holds true, I am really worried about what >> the frequency limit is before we get into trouble with freely applying >> knowledge developed with DC or low-frequency circuits. >> >> Any way, looking for a better answer or proof on the validity of the >> concept of voltage. >> >> Thanks >> >> >> ------------------------------------------------------------------ >> 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: >> http://www.freelists.org/webpage/si-list >> >> For help: >> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >> >> >> List technical documents are available at: >> http://www.si-list.net >> >> List archives are viewable at: >> http://www.freelists.org/archives/si-list >> >> 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: >> http://www.freelists.org/webpage/si-list >> >> For help: >> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >> >> >> List technical documents are available at: >> http://www.si-list.net >> >> List archives are viewable at: >> http://www.freelists.org/archives/si-list >> >> 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: >> http://www.freelists.org/webpage/si-list >> >> For help: >> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field >> >> >> List technical documents are available at: >> http://www.si-list.net >> >> List archives are viewable at: >> http://www.freelists.org/archives/si-list >> >> 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: > http://www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > List technical documents are available at: > http://www.si-list.net > > List archives are viewable at: > http://www.freelists.org/archives/si-list > > 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: > http://www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field > > > List technical documents are available at: > http://www.si-list.net > > List archives are viewable at: > http://www.freelists.org/archives/si-list > > 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: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: http://www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu

**Follow-Ups**:**[SI-LIST] Re: Concept of Voltage***From:*Albert Ruehli

**[SI-LIST] Re: Concept of Voltage***From:*Albert Ruehli

**[SI-LIST] Re: Concept of Voltage***From:*Yuriy Shlepnev

**References**:**[SI-LIST] Re: Concept of Voltage***From:*Peter . Pupalaikis

**[SI-LIST] Re: Concept of Voltage***From:*Luciano Boglione

**[SI-LIST] Re: Concept of Voltage***From:*Shawn Hermite

**[SI-LIST] Re: Concept of Voltage***From:*Yuriy Shlepnev

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