Thanks Stephen, This agrees with my simulation results (deck below) - I think grounding the conductors at both ends offers the best isolation (unless someone can find a fault in my sim). (Change the value of R_isolation_in/R_isolation_out to 50 to test the "terminate at Z0" case). _______________________________ Ribbon Cable isolation investigation w/ 0 ohm at isolation .param ribbon_L=3D2.5 .param vhi=3D2 .param tr =3D35p .param pw=3D5n .param per=3D1000n =20 .AC Lin 800 30K 20G .tran 1p 10n ******* Internal Field Solver Stuff ********** .MATERIAL fr4 DIELECTRIC ER=3D4.2 LOSSTANGENT=3D.018 .MATERIAL copper METAL CONDUCTIVITY=3D4.2E07 .FSOPTIONS myoptions ACCURACY=3DHIGH PRINTDATA=3DYES=20 + COMPUTEG0=3DYES COMPUTEGD=3DYES COMPUTER0=3DYES COMPUTERS=3DYES ** solve for G0, Gd, R0, Rs *** ribbon cable *** .LAYERSTACK ribbon_stack + LAYER=3D(fr4,'.0254*40e-3'), $ FR4 insulator, 1 mm (per mechanical dwg, rounded from 39.37 to 40 for simplicity) + LAYER=3D(air, '.0254*199e-3'), $ separate gnd and conductors by 200 mils + LAYER=3D(PEC,'.0254*1e-3'), $ GND plane, 1mils * NOTE: center of conductor should be at 220 .SHAPE s1_cond CIRCLE RADIUS=3D'.0254*10e-3' $ 10 mil radius .MODEL s1_Tline W MODELTYPE=3DFieldSolver LAYERSTACK=3Dribbon_stack + FSOPTIONS=3Dmyoptions RLGCFILE=3Ds1_Tline.rlgc + CONDUCTOR=3D(SHAPE=3Ds1_cond,=20 + ORIGIN=3D('.0254*0' ,'.0254*220e-3'), + MATERIAL=3Dcopper) + CONDUCTOR=3D(SHAPE=3Ds1_cond,=20 + ORIGIN=3D('.0254*40e-3' ,'.0254*220e-3'), + MATERIAL=3Dcopper) + CONDUCTOR=3D(SHAPE=3Ds1_cond,=20 + ORIGIN=3D('.0254*80e-3' ,'.0254*220e-3'), + MATERIAL=3Dcopper) **** ribbon stimulus at port 1 V_src N_src GND PULSE (0 vhi 0N tr tr pw per) AC =3D vhi R_src N_src N_p1 R=3D50 R_isolation_in N_ribbon_isolation_in GND R=3D0.0001 R_victim_in N_ribbon_victim_in GND R=3D50 T_src N_p1 GND N_ribbonag_in GND Z0=3D50 TD=3D180p W_ribbon N_ribbonag_in N_ribbon_isolation_in N_ribbon_victim_in GND N_ribbonag_out N_ribbon_isolation_out N_ribbon_victim_out GND + FSmodel=3Ds1_Tline N=3D3 L=3D'.0254*ribbon_L' T_out N_ribbonag_out GND N_load GND Z0=3D50 TD=3D180p R_isolation_out N_ribbon_isolation_out GND R=3D0.0001 R_victim_out N_ribbon_victim_out GND R=3D50 R_load N_load GND R=3D50 .END Jeff Loyer -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of stebla01 Sent: Tuesday, October 02, 2007 12:44 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: What to do with "isolation" conductors in ribbon cable? I've simulated this situation to look at the effect of inductively coupled noise into the ribbon cable. Each signal conductor was adjacent to a spare conductor. The result of the simulation was that the spare conductors should be tied to ground at both ends and that the inductively coupled noise was smallest on the signal conductors nearest the centre of the ribbon cable. However, in my case the inductive noise was at 20kHz and so the cable much less that a wavelength so I did not model the ribbon cable as a transmission line. However, it seems to me that the idea is to use the spare conductors to try to approximate a ground plane made out out a mesh of discrete wires; hence connecting the spare conductors to ground would still make sense at higher frequency. Stephen =20 ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net List archives are viewable at: =20 //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 =20 ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net 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