Hi Karan, Ray: I think comments below referenced to Geoff were from my posting of 7/18. As Ray points out velocity in microstrip is only dependent on effective dielectric constant and given you scale line for same impedance when changing the dielectric thickness, velocity shouldn't change. So this effect is moot if you are maintaining line impedance. Epsilon effective will be slightly larger (& velocity slightly slower) for low impedance lines, than for high imedance lines. According to equation previously referenced, Epsilon effective is slightly dependent on W/h and even less on t/h-in many stack-ups, we can often ignore thickness. Although this is equation-based and probably limited in accuracy to reasonable values of W/h & t/h intuitively, one should be able to imagine the difference in field distribution between a narrow, high-Z line and that of a wide low-Z line. Ray also correctly points out that stripline's velocity is solely dependent on material dielectric constant and W/h, t/h doesn't matter because all fields are contained in the dielectric. OK-so how bad is this effect for microstrip?? I dug and found a very old design guideline, plotting Epsilon effective vs W/h. For Alumina (thin-film), it varies from 5.9 (W/h=0.1, Z=130 ohms) to 6.5 (W/h=1.0, Z=50 ohms) to 7.4 (W/h=3.0, Z=26 ohms) The only reference on the page is "Report No. 03-68-85", probably funded by (D)ARPA. Hope this can help, Jeff LaT. -----Original Message----- From: Ray Anderson [mailto:Raymond.Anderson@xxxxxxx] Sent: Thursday, July 24, 2003 9:28 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Power planes > >This is quite a general question. > >--------------------------------------------- > >If I place the GND plane too close to the track wouldn't it affect the >velocity of signal on track ? > >Possible reason is the capacitance b/w track and GND will increase which will cause the delay in changing signal(rising edges) as it will need more time to charge the increased capacitor value.Geoff just told us that placement of GND affects the velocity of Microstrip and Not Strip Line. How far is it true ? > > The distance between a signal trace and a reference plane in a stack-up does not effect the velocity of the signal propagating on the signal trace. It can effect the impedance of the signal trace. The velocity of propagation is dependent only on the effective dielectric constant (Er) or electric permittivity of the substrate material. Yes, there will be more distributed capacitance between the signal trace and the plane for small spacings, but this does not effect the velocity of propagation as the PUL inductance of the signal trace will decrease as well. some relevant equations: ------------------------ Velocity = C/sqrt(Er) <= velocity is a function of Er (where C is speed of light in this equation) Velocity = 11.8/sqrt(Er) inches/nSec <= numeric version of above Velocity = 1/sqrt(L*C) hence: L = (delay^2)/C and C = (delay^2)/L -Ray Anderson Sun Microsystems ------------------------------------------------------------------ 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