Jeremy, I think if you follow the thread Ravinder was responding to Lee's comment about flattened rise times. Respectfully, this evening when you brought it up is the first that I have seen on this particular thread about propagation time. The assumption of an unadulterated wave shape relies on perfect impedance matching at the load, which is a bit rare. Regards, Steve. At 08:14 PM 10/6/2003 -0700, Jeremy Plunkett wrote: >Hi Steve, >That was my point, that in these discussions we should be careful to >distinguish between amplitude effects (things that affect measured prop >delay by changing rise time or waveform shape) and delay effects (which are >only based on the effective dielectric constant). > >In Ravinder's simulation, he got the result that the tightly coupled traces >have a faster propagation delay than the loosely coupled, which is what we >would expect based on the different effective dielectric constants for the >two cases. That's the thing he was missing. > >Your response that tight coupling should slow down the transitions is true, >if you don't keep the total differential impedance constant while you vary >the coupling. But it was a non-sequitur, since Ravinder was measuring >shorter propagation delay, not transition time. > >And actually, the rise time will not change due to changes in impedance, >only the slew rate. That's another situation where keeping the amplitude >and timing effects separate in your head can be very important. The only >things that will change rise time are frequency-dependent effects that >create phase shifts, amplitude differences, or both between different >frequency components in your signal (examples include reactive loads, >dielectric constant that varies with frequency, skin effect, and dielectric >loss). Impedance discontinuities by themselves do not have any frequency >dependence and will not change the rise time, only the slew rate (by >changing the amplitude of the entire signal). > >Best regards, >Jeremy > >-----Original Message----- >From: steve weir [mailto:weirsp@xxxxxxxxxx] >Sent: Monday, October 06, 2003 7:05 PM >To: Jeremy Plunkett; Ravinder.Ajmani@xxxxxxxx; leeritchey@xxxxxxxxxxxxx >Cc: si-list@xxxxxxxxxxxxx >Subject: RE: [SI-LIST] Re: Diff. Pairs > > >Jeremy, I was referring to percentage rise-time, not propagation >velocity. To the extent that each line in a pair couples to the other, the >opposing transition on each requires more energy for any given excursion >than without coupling, and that flattens the transitions at any real load. > >Regards, > > >Steve. > >At 06:40 PM 10/6/2003 -0700, Jeremy Plunkett wrote: > >Steve, > >Ravinder's edges propagate faster in the 1st case because tighter spacing > >between microstrip lines results in a greater portion of the field > >propagating in air above/between the traces vs in the fiberglass under the > >traces. The exact amount of speedup depends on the details of his trace > >geometry, soldermask geometry, and dielectric constants of the prepreg and > >soldermask. > > > >Outside of the special case of a mixed dielectric (as above), changes in > >coupling will not affect velocity one way or another (I mean "pure" > >propagation velocity here, not measured delay--see below). However, if >you > >do not hold impedance constant, increasing coupling with make the impedance > >will go down, which will result in "slower" edges throughout the system (in > >V/ns) due to reduced signal amplitude (and vice versa, less coupling -> > >higher Z -> "faster" edges due to larger signal swing). > > > >Maybe not everyone agrees that there should be quotes around slower and > >faster in the sentence above. When I think about signals propagating on a > >transmission line, I prefer to keep the "delay" effects separate from the > >"amplitude" affects, even though they may both affect the measured delay. > >For example there is one effect of changing coupling even in a uniform > >dielectric while keeping Z constant; skin effect losses change due to the > >proximity effect. This will slightly affect the measured delay (if we > >measure at 50% of the transition) because it changes the waveshape at the > >receiving end of the line, but I don't consider it as changing the velocity > >because if we measure at the earliest recognizable point on the transition, > >it does not create any extra delay. > > > >There are 2nd order effects (variation of Er with frequency) that can > >complicate this mental separation of delay and amplitude, but I find it >does > >a nice job of clarifying the 1st order effects (impedance changes and > >attenuation). I'm interested to hear any comments people have on it's > >usefulness or things I should watch out for. > > > >best regards, > >Jeremy > > > > > > > >|>--/\/\/--((((((((()--|> > > > >Jeremy Plunkett > >Signal Integrity Engineer > >Broadcom Corp > >www.serverworks.com > > > >|>--/\/\/--((((((((()--|> > > > > > >-----Original Message----- > >From: si-list-bounce@xxxxxxxxxxxxx > >[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir > >Sent: Friday, October 03, 2003 4:40 PM > >To: Ravinder.Ajmani@xxxxxxxx; leeritchey@xxxxxxxxxxxxx > >Cc: si-list@xxxxxxxxxxxxx > >Subject: [SI-LIST] Re: Diff. Pairs > > > > > >Ravinder, something seems very wrong with the physics here. I don't know > >if it is round-off error in your simulation, but Lenz' law agrees with > >Lee. Differential coupling resists any change. The more tightly you > >couple the two nets in a diff pair, the more it slows down the > >transitions. To cause a speed-up, they would have to switch in the same > >direction. If we could manufacture a machine that accelerated transitions > >in opposite directions we could solve the world's energy needs. > > > >Regards, > > > > > >Steve. > > > >At 01:12 PM 10/3/2003 -0700, Ravinder.Ajmani@xxxxxxxx wrote: > > >One obvious benefit is the reduction in EMI because of reduced loop area. > > >I have verified this through simulation. However, I am not able to > > >understand the phenomenon of edge degradation due to close coupling. I > > >ran some quick simulations on six inch long differential Microstrip nets > > >under the following conditions: > > >Case 1: Trace width 9 mils, separation 4 mils, differential impedance >99.3 > > >ohms > > >Case 2: Trace width 12.5 mils, separation 40 mils, differential impedance > > >98.8 ohms > > > > > >The driver had a rise time of 250 ps. The only difference I observed > > >between the two waveforms was that in the first case the propagation time > > >was 63 ps less than the second case. This is understandable since the > > >signals in the two branches of differential net have opposite polarity, > > >the coupling effect will speed them up. > > > > > >Next I tried the same simulation for Stripline nets. In this case, there > > >was practically no difference between two waveforms (less than 5 ps > > >difference in propagation delay). > > > > > >Am I missing something here. > > > > > >Regards, Ravinder > > >Server PCB and Flex Development > > >Hitachi Global Storage Technologies > > > > > >Email: Ravinder.Ajmani@xxxxxxxx > > > > > > > > > > > > > > > > > > > > >"Lee Ritchey" <leeritchey@xxxxxxxxxxxxx> > > >Sent by: si-list-bounce@xxxxxxxxxxxxx > > >10/03/2003 11:41 AM > > >Please respond to leeritchey > > > > > > > > > To: "Duane Takahashi" <duanet@xxxxxxxxxxxxxxxxxxxxxx>, > > > si-list@xxxxxxxxxxxxx > > > cc: > > > From: si-list-bounce@xxxxxxxxxxxxx > > > Subject: [SI-LIST] Re: Diff.Pairs > > > > > > > > > > > > > > >More than that, it does not have any benefit. Tight coupling of > > >differential pairs forces the traces to be narrower increasing the skin > > >effect losses. Also, this tight coupling is going to result in good old > > >cross talk that actually degrades the edges. > > > > > >How the notion of tight coupling of differential pairs as beneficial got > > >started is a mystery to me. There are several references that show that > > >tight coupling is not beneficial, one of them is Howard Johnson's latest > > >book, at least one column he has written and my recently released book. > > > > > >Lee Ritchey > > > > > > > > > > [Original Message] > > > > From: Duane Takahashi <duanet@xxxxxxxxxxxxxxxxxxxxxx> > > > > To: <si-list@xxxxxxxxxxxxx> > > > > Date: 10/2/2003 3:58:59 PM > > > > Subject: [SI-LIST] Re: Diff.Pairs > > > > > > > > Hi Juergen: > > > > > > > > Aligning the stack up for the broadside coupled diff lines is >expensive. > > > > > > > You can do this, but it drives up the cost of the board. > > > > > > > > Duane > > > > > > > > > Hi Juergen, > > > > > You can find lots of application notes > > > > > especially with respect to process variation > > > > > on differential pairs here: > > > > > > > > > > > > > > > www.polarinstruments.com/support/cits/cits_index.html > > > > > > > > > > In particular this one may be of interest: > > > > > > > > > > > > > > > How measured impedance may vary from field solver calculations when > > > > > using woven glass reinforced > > > > > <http://www.polarinstruments.com/support/cits/AP139.html>laminates > > > > > > > > > > www.polarinstruments.com/support/cits/AP139.html > > > > > > > > > > > > > > > And this note: > > > > > > > > > > Copper thickness, edge coupled lines and > > > > > characteristic > > > > > <http://www.polarinstruments.com/support/cits/AP151.html>impedance > > > > > > > > > > > > > > > www.polarinstruments.com/support/cits/AP151.html > > > > > > > > > > > > > > > > > > > > Hope this helps.... > > > > > > > > > > > > > > > Kind regards > > > > > Martyn Gaudion > > > > > www.polarinstruments.com > > > > > T: +44 1481 253081 > > > > > F: +44 1481 252476 > > > > > M: +44 7710 522748 > > > > > E: martyn@xxxxxxxxxxxxxxxxxxxx > > > > > > > > > > ============================================ > > > > > Controlled Impedance & Signal integrity tools > > > > > for the Printed circuit fabrication industry > > > > > ============================================ > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > At 19:00 02/10/2003, you wrote: > > > > > > > > > >>I am seeking help in finding enlightenment regarding electrical > > > > >>performance pros and cons and how manufacturing tolerances play a >role > > > > > > > >>when comparing side by side and tandem differential pairs. I'd > > >appreciate > > > > >>your opinion, experience, analysis, pointers to papers and articels, > > >etc. > > > > >> > > > > >>In return, I would offer to share a summary of the >finding/discoveries > > > > > > > >>with interested parties. > > > > >> > > > > >>Thanks > > > > >> > > > > >>Juergen > > > > >> > > > > >> > > > > >> > > > > >>------------------------------------------------------------------ > > > > >>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 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 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 > > > > > > > > > > > > > -- > > > > Duane Takahashi phone: 408-720-4200 > > > > Greenfield Networks fax: 408-720-4210 > > > > 255 Santa Ana Court email: duanet@xxxxxxxxxxxxxxxxxxxxxx > > > > Sunnyvale, CA 94085 > > > > > > > > * MOVING! 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