On Aug 14, 3:33=A0pm, Joerg <inva...@invalid.invalid> wrote:> Folks, > > This is close to voodoo but repeatable. Unfortunately I can't disclose > the schematic since it is for a client. Just wondering , anyone had this > before? > > At the far end of a TX line I used to have a diode connected across > because a previous version of a chip would have such a substrat path. > Nice waveforms, fast sims. Everything as expected and peachy. Now the > new iteration of the chip design won't have this diode path so I chopped > off its cathode connection. Sims fine. So I deleted the diode -> > ka-crunch ... sim slows down and the ouput is junk. > > If I put the diode back in and connect only its anode -> fine. If I > leave the anode open and only connect the cathode it still sims but the > results are different. > > <scratching head>I really hate to ask this, but have you "simulated" your LTSpice results in hardware yet? Also, what's the RF voltage level on the line? Mark L. Fergerson
Unconnected part LTSpice seems to need (weird ...)
Started by ●August 14, 2011
Reply by ●August 14, 20112011-08-14
Reply by ●August 15, 20112011-08-15
On Sun, 14 Aug 2011 15:33:20 -0700, Joerg <invalid@invalid.invalid> wrote:>Folks, > >This is close to voodoo but repeatable. Unfortunately I can't disclose >the schematic since it is for a client. Just wondering , anyone had this >before? > >At the far end of a TX line I used to have a diode connected across >because a previous version of a chip would have such a substrat path. >Nice waveforms, fast sims. Everything as expected and peachy. Now the >new iteration of the chip design won't have this diode path so I chopped >off its cathode connection. Sims fine. So I deleted the diode -> >ka-crunch ... sim slows down and the ouput is junk. > >If I put the diode back in and connect only its anode -> fine. If I >leave the anode open and only connect the cathode it still sims but the >results are different. > ><scratching head>The Spice transmission line is weird in that it has, essentially, an ideal 1:1 transformer built in. There's no continuity in the "shield" from end to end. That can have weird effects, including "singular matrix" types of errors that you wouldn't expect from a real hunk of coax. John
Reply by ●August 15, 20112011-08-15
petrus bitbyter wrote:> "Joerg" <invalid@invalid.invalid> schreef in bericht > news:9ar2nqF3ocU1@mid.individual.net... >> Jim Thompson wrote: >>> On Sun, 14 Aug 2011 15:33:20 -0700, Joerg <invalid@invalid.invalid> >>> wrote: >>> >>>> Folks, >>>> >>>> This is close to voodoo but repeatable. Unfortunately I can't disclose >>>> the schematic since it is for a client. Just wondering , anyone had this >>>> before? >>>> >>>> At the far end of a TX line I used to have a diode connected across >>>> because a previous version of a chip would have such a substrat path. >>>> Nice waveforms, fast sims. Everything as expected and peachy. Now the >>>> new iteration of the chip design won't have this diode path so I chopped >>>> off its cathode connection. Sims fine. So I deleted the diode -> >>>> ka-crunch ... sim slows down and the ouput is junk. >>>> >>>> If I put the diode back in and connect only its anode -> fine. If I >>>> leave the anode open and only connect the cathode it still sims but the >>>> results are different. >>>> >>>> <scratching head> >>> Show just the netlist of that portion. Diode? In CMOS? The "diode" has >>> other parts ;-) >>> >> Ok, it's attached, I cut our the proprietary swaths. The results are >> very different but the basic effect is still there: >> >> Note that D2 is connected at the anode only. Run the sim, look at >> V(n001), three nice clean pulses. Now delete the anode connection and >> the diode D2 -> first pulse is hosed. >> >> -- >> Regards, Joerg >> >> http://www.analogconsultants.com/ > > <snip> > > I got an extra 120V peak on the second pulse and a sharp dip in the third > though not after removing the diode but before removing it. (See abse) > Removing the diode makes the extra's disappear. Connecting the diode gives > the weird pulses back. It make's a difference what type diode is chosen. A > 1N914 and a 1N4148 give the same results. Several Schottkys and other > silicons however seem to do no harm. >I have no access to abse, but that kind behavior is exactly what I meant. Different sim results while the netlist should not have caused a difference. Maybe I stumbled upon a bug here and if this is corroborated by others I should send it in for Mike's crew. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●August 15, 20112011-08-15
Martin Riddle wrote:> "Joerg" <invalid@invalid.invalid> wrote in message > news:9ar2qaF3a2U1@mid.individual.net... >> Martin Riddle wrote: >>> "Joerg" <invalid@invalid.invalid> wrote in message >>> news:9ar0piFma8U1@mid.individual.net... >>>> Folks, >>>> >>>> This is close to voodoo but repeatable. Unfortunately I can't >>>> disclose >>>> the schematic since it is for a client. Just wondering , anyone had >>>> this >>>> before? >>>> >>>> At the far end of a TX line I used to have a diode connected across >>>> because a previous version of a chip would have such a substrat >>>> path. >>>> Nice waveforms, fast sims. Everything as expected and peachy. Now >>>> the >>>> new iteration of the chip design won't have this diode path so I >>>> chopped >>>> off its cathode connection. Sims fine. So I deleted the diode -> >>>> ka-crunch ... sim slows down and the ouput is junk. >>>> >>>> If I put the diode back in and connect only its anode -> fine. If I >>>> leave the anode open and only connect the cathode it still sims but >>>> the >>>> results are different. >>>> >>>> <scratching head> >>>> >>>> -- >>> Convergence problems? >>> >>> What if you terminate it with a 1000G ohm resistor? ( or a very high >>> resistance), instead of the diode. >>> >> I have a 1MEG in there. It's not really a convergence issue (I think >> ...). Looks more like a etlist thing. On the schematic there clearly >> is >> no connection in either case but the results are vastly different. >> >> -- >> Regards, Joerg >> >> http://www.analogconsultants.com/ > > Its an unterminated tx line. You need some R for spice to behave. > See this..... Cheers >Thanks ... but ... if you add a diode in your version and disconnect the cathode you get a change again: A Mohawk hair-style on the third pulse. That's not supposed to be there. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●August 15, 20112011-08-15
John Larkin wrote:> On Sun, 14 Aug 2011 15:33:20 -0700, Joerg <invalid@invalid.invalid> > wrote: > >> Folks, >> >> This is close to voodoo but repeatable. Unfortunately I can't disclose >> the schematic since it is for a client. Just wondering , anyone had this >> before? >> >> At the far end of a TX line I used to have a diode connected across >> because a previous version of a chip would have such a substrat path. >> Nice waveforms, fast sims. Everything as expected and peachy. Now the >> new iteration of the chip design won't have this diode path so I chopped >> off its cathode connection. Sims fine. So I deleted the diode -> >> ka-crunch ... sim slows down and the ouput is junk. >> >> If I put the diode back in and connect only its anode -> fine. If I >> leave the anode open and only connect the cathode it still sims but the >> results are different. >> >> <scratching head> > > The Spice transmission line is weird in that it has, essentially, an > ideal 1:1 transformer built in. There's no continuity in the "shield" > from end to end. That can have weird effects, including "singular > matrix" types of errors that you wouldn't expect from a real hunk of > coax. >You do need an R to GND on TX lines, else they cause SPICE to abort. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●August 15, 20112011-08-15
nuny@bid.nes wrote:> On Aug 14, 3:33 pm, Joerg <inva...@invalid.invalid> wrote: >> Folks, >> >> This is close to voodoo but repeatable. Unfortunately I can't disclose >> the schematic since it is for a client. Just wondering , anyone had this >> before? >> >> At the far end of a TX line I used to have a diode connected across >> because a previous version of a chip would have such a substrat path. >> Nice waveforms, fast sims. Everything as expected and peachy. Now the >> new iteration of the chip design won't have this diode path so I chopped >> off its cathode connection. Sims fine. So I deleted the diode -> >> ka-crunch ... sim slows down and the ouput is junk. >> >> If I put the diode back in and connect only its anode -> fine. If I >> leave the anode open and only connect the cathode it still sims but the >> results are different. >> >> <scratching head> > > I really hate to ask this, but have you "simulated" your LTSpice > results in hardware yet? >No, can't do that yet. It's an IC and that is not taped out yet.> Also, what's the RF voltage level on the line? >About 100V, and then from microvolts up to a volt during receive depending on signal strength coming in. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●August 15, 20112011-08-15
"Joerg" <invalid@invalid.invalid> wrote in message news:9asojgFipoU2@mid.individual.net...> Martin Riddle wrote: >> "Joerg" <invalid@invalid.invalid> wrote in message >> news:9ar2qaF3a2U1@mid.individual.net... >>> Martin Riddle wrote: >>>> "Joerg" <invalid@invalid.invalid> wrote in message >>>> news:9ar0piFma8U1@mid.individual.net... >>>>> Folks, >>>>> >>>>> This is close to voodoo but repeatable. Unfortunately I can't >>>>> disclose >>>>> the schematic since it is for a client. Just wondering , anyone >>>>> had >>>>> this >>>>> before? >>>>> >>>>> At the far end of a TX line I used to have a diode connected >>>>> across >>>>> because a previous version of a chip would have such a substrat >>>>> path. >>>>> Nice waveforms, fast sims. Everything as expected and peachy. Now >>>>> the >>>>> new iteration of the chip design won't have this diode path so I >>>>> chopped >>>>> off its cathode connection. Sims fine. So I deleted the diode -> >>>>> ka-crunch ... sim slows down and the ouput is junk. >>>>> >>>>> If I put the diode back in and connect only its anode -> fine. If >>>>> I >>>>> leave the anode open and only connect the cathode it still sims >>>>> but >>>>> the >>>>> results are different. >>>>> >>>>> <scratching head> >>>>> >>>>> -- >>>> Convergence problems? >>>> >>>> What if you terminate it with a 1000G ohm resistor? ( or a very >>>> high >>>> resistance), instead of the diode. >>>> >>> I have a 1MEG in there. It's not really a convergence issue (I think >>> ...). Looks more like a etlist thing. On the schematic there clearly >>> is >>> no connection in either case but the results are vastly different. >>> >>> -- >>> Regards, Joerg >>> >>> http://www.analogconsultants.com/ >> >> Its an unterminated tx line. You need some R for spice to behave. >> See this..... Cheers >> > > Thanks ... but ... if you add a diode in your version and disconnect > the > cathode you get a change again: A Mohawk hair-style on the third > pulse.Well, you normally will not have the diode in the circuit like that anyway. Same goes for the resistor. This is just a SPICE thing. I would not worry too much about it. Cheers
Reply by ●August 15, 20112011-08-15
On 08/15/2011 11:57 AM, Martin Riddle wrote:> "Joerg"<invalid@invalid.invalid> wrote in message > news:9asojgFipoU2@mid.individual.net... >> Martin Riddle wrote: >>> "Joerg"<invalid@invalid.invalid> wrote in message >>> news:9ar2qaF3a2U1@mid.individual.net... >>>> Martin Riddle wrote: >>>>> "Joerg"<invalid@invalid.invalid> wrote in message >>>>> news:9ar0piFma8U1@mid.individual.net... >>>>>> Folks, >>>>>> >>>>>> This is close to voodoo but repeatable. Unfortunately I can't >>>>>> disclose >>>>>> the schematic since it is for a client. Just wondering , anyone >>>>>> had >>>>>> this >>>>>> before? >>>>>> >>>>>> At the far end of a TX line I used to have a diode connected >>>>>> across >>>>>> because a previous version of a chip would have such a substrat >>>>>> path. >>>>>> Nice waveforms, fast sims. Everything as expected and peachy. Now >>>>>> the >>>>>> new iteration of the chip design won't have this diode path so I >>>>>> chopped >>>>>> off its cathode connection. Sims fine. So I deleted the diode -> >>>>>> ka-crunch ... sim slows down and the ouput is junk. >>>>>> >>>>>> If I put the diode back in and connect only its anode -> fine. If >>>>>> I >>>>>> leave the anode open and only connect the cathode it still sims >>>>>> but >>>>>> the >>>>>> results are different. >>>>>> >>>>>> <scratching head> >>>>>> >>>>>> -- >>>>> Convergence problems? >>>>> >>>>> What if you terminate it with a 1000G ohm resistor? ( or a very >>>>> high >>>>> resistance), instead of the diode. >>>>> >>>> I have a 1MEG in there. It's not really a convergence issue (I think >>>> ...). Looks more like a etlist thing. On the schematic there clearly >>>> is >>>> no connection in either case but the results are vastly different. >>>> >>>> -- >>>> Regards, Joerg >>>> >>>> http://www.analogconsultants.com/ >>> >>> Its an unterminated tx line. You need some R for spice to behave. >>> See this..... Cheers >>> >> >> Thanks ... but ... if you add a diode in your version and disconnect >> the >> cathode you get a change again: A Mohawk hair-style on the third >> pulse. > > Well, you normally will not have the diode in the circuit like that > anyway. Same goes for the resistor. > This is just a SPICE thing. I would not worry too much about it. > > Cheers >SPICE uses iterative techniques to solve large, sparse, nonlinear systems. That's a hard problem in general, and disconnected nodes make even the linearized matrices singular. LTSPice is generally very good at finding those sorts of things and ignoring them, but like other solver packages, it isn't perfect. A nearly-singular system of equations is a very powerful noise amplifier, so having a bit of fur show up now and again may simply be amplified roundoff, or it may be actual numerical instability. What integrator are you using? Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●August 15, 20112011-08-15
petrus bitbyter wrote:> "Joerg" <invalid@invalid.invalid> schreef in bericht > news:9ar2nqF3ocU1@mid.individual.net... > >>Jim Thompson wrote: >> >>>On Sun, 14 Aug 2011 15:33:20 -0700, Joerg <invalid@invalid.invalid> >>>wrote: >>> >>> >>>>Folks, >>>> >>>>This is close to voodoo but repeatable. Unfortunately I can't disclose >>>>the schematic since it is for a client. Just wondering , anyone had this >>>>before? >>>> >>>>At the far end of a TX line I used to have a diode connected across >>>>because a previous version of a chip would have such a substrat path. >>>>Nice waveforms, fast sims. Everything as expected and peachy. Now the >>>>new iteration of the chip design won't have this diode path so I chopped >>>>off its cathode connection. Sims fine. So I deleted the diode -> >>>>ka-crunch ... sim slows down and the ouput is junk. >>>> >>>>If I put the diode back in and connect only its anode -> fine. If I >>>>leave the anode open and only connect the cathode it still sims but the >>>>results are different. >>>> >>>><scratching head> >>> >>>Show just the netlist of that portion. Diode? In CMOS? The "diode" has >>>other parts ;-) >>> >> >>Ok, it's attached, I cut our the proprietary swaths. The results are >>very different but the basic effect is still there: >> >>Note that D2 is connected at the anode only. Run the sim, look at >>V(n001), three nice clean pulses. Now delete the anode connection and >>the diode D2 -> first pulse is hosed. >> >>-- >>Regards, Joerg >> >>http://www.analogconsultants.com/ > > > <snip> > > I got an extra 120V peak on the second pulse and a sharp dip in the third > though not after removing the diode but before removing it. (See abse) > Removing the diode makes the extra's disappear. Connecting the diode gives > the weird pulses back.I got the same results as you. I didn't try different diode types. Put a .001 R in series in the cathode path of that "floating diode" (for lack of a better term) and you get a ~ 3.6 KV spike a bit after .3 uS. I tried a number of different values for that added R and the only value that affected the thing was .001 ohms. .000999 did not, nor did .0011. Weird. Ed> It make's a difference what type diode is chosen. A > 1N914 and a 1N4148 give the same results. Several Schottkys and other > silicons however seem to do no harm. > > petrus bitbyter > >
Reply by ●August 15, 20112011-08-15
On 8/15/2011 7:29 AM, Joerg wrote:> nuny@bid.nes wrote: >> On Aug 14, 3:33 pm, Joerg<inva...@invalid.invalid> wrote: >>> Folks, >>> >>> This is close to voodoo but repeatable. Unfortunately I can't disclose >>> the schematic since it is for a client. Just wondering , anyone had this >>> before? >>> >>> At the far end of a TX line I used to have a diode connected across >>> because a previous version of a chip would have such a substrat path. >>> Nice waveforms, fast sims. Everything as expected and peachy. Now the >>> new iteration of the chip design won't have this diode path so I chopped >>> off its cathode connection. Sims fine. So I deleted the diode -> >>> ka-crunch ... sim slows down and the ouput is junk. >>> >>> If I put the diode back in and connect only its anode -> fine. If I >>> leave the anode open and only connect the cathode it still sims but the >>> results are different. >>> >>> <scratching head> >> >> I really hate to ask this, but have you "simulated" your LTSpice >> results in hardware yet? > > No, can't do that yet. It's an IC and that is not taped out yet.Dang. Do you have an example of the old setup handy on which you can try your diode changes as above?>> Also, what's the RF voltage level on the line? > > About 100V, and then from microvolts up to a volt during receive > depending on signal strength coming in.I thought it might be high. Think; what happens with a real diode connected as described above in such a field? Anyway, if it were me I'd stop fiddling with the diode and just use a terminating resistor. Of course, once the chip was ready I'd try it with the diode, just to see. ;>) Mark L. Fergerson
