Art, No one would object to your analysis about T-line, after the popularity of the "black magic handbook". However, the issue is talking about the loading effect to the driver that the delay within the driver will be different when the T-line length is changing. In view of this theory you can say there is no added "C load" in short T-line, yet the EFFECT of cap loading is apparent to the driver, as analised both by you and Jon. Raymond Art Porter wrote: Well, that's the definitive behavior of a transmission line. A properly terminated transmission line "looks" resistive (i.e. voltage and current are in phase at all frequencies). If it isn't properly terminated, it still "looks" resistive at the time of the incident wave. The driver doesn't "see" the reflection of whatever is at the end of the transmission line until a time equal to twice the delay of the transmission line. A "short" transmission line doesn't have any inherent added "C load." With a short transmission line, there is less time between the incident edge and the reflection from the C (or whatever else) at the end of the transmission line. If the length of the transmission line is short compared to the transition time of the edge, then it's difficult to distinguish the incident edge from the reflection. That is the source of the "rule of thumb" that you can model transmission lines as lumped elements if their length is <1/5 (or 1/3, or 1/10, depending on which "a uthority" you prefer) of the transition time. Art -----Original Message----- From: Raymond.Leung@xxxxxxxxxxx [mailto:Raymond.Leung@xxxxxxxxxxx] Sent: Tuesday, June 10, 2003 6:20 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: delay vs. transmission line length I think it is more or less like a resistive load seen by the driver when the T-line is long enough. As what Jon has described below, the C load of a short line would cause longer Tpd. Raymond "Jon Powell" <jonpowell@xxxxxxxxxxxx> on 11/06/2003 01:07:31 Please respond to jonpowell@xxxxxxxxxxxx To: yonitz@xxxxxxxxxxxxx, "Si-List (E-mail)" <si-list@xxxxxxxxxxxxx> cc: (bcc: Raymond Leung/sdc) Subject: [SI-LIST] Re: delay vs. transmission line length In a lot of ways this relates to a previous question on how to measure time-of-flight and relate that properly to the CLK->Q data in a static timing program. Since the actual CLk->Q (Tpd or whatever) of the device is dependent on load, the datasheets will spec it into a specific load and then it becomes the job of the SI and Timing tools to figure out how to properly change that data for accurate total path timing. It used to be that most drivers were spec'd into 50pf loads because that happened to be the loads on the chip testers. Now days 50pf is so far away from the real load that that spec isn't good enough for many purposes. As other people have pointed out, if the load is closer to the driver than a round-trip, the C of the load is seen by the driver and can change the transition rate of the driver, which effectively changes Tpd. If the load is farther away than a round-trip it is effectively non-existent to the driver (at least starting from steady-state) and can have no effect on Tpd. -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Yoni Tzafrir Sent: Tuesday, June 10, 2003 5:45 AM To: Si-List (E-mail) Subject: [SI-LIST] delay vs. transmission line length hi, i run some simulations, for measuring Tpd from input of the of the = driver to the output of the driver.=20 |\ | \driver transmission line input------------| = \---------------------------------------------------LOAD | /out |/ I have noticed that the longer my transmission line, the Tpd becomes = shorter. does it make sense? as i understand it a longer transmission line means = more capacitance (and resistance) so the Tpd should be longer, doesn't = it? Yonatan (Yoni) Tzafrir *Tel: (972) - 3 - 7552300 (T/L: 351) *Fax: (972) - 3 - 6177130 *Mobile: (972) -54- 459469 *E-mail: <mailto:yonitz@xxxxxxxxxxxxx> ------------------------------------------------------------------ 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