It all depends on how accurate you want your measurements to be. You can do the math yourself by remembering that the measured risetime = SQRT[(Scope system risetime)^2 + (DUT risetime)^2]. The scope system includes the scope plus probe plus whatever is between the scope and the DUT. If DUT and scope have the same risetime the error will be 41.4% If the scope is half the DUT the error is 11.8%. At one forth the risetime you are down to 3% and at 5 times faster you have 2% error when measuring from the scope screen. Tom Dagostino Teraspeed Labs 13610 SW Harness Lane Beaverton, OR 97008 503-430-1065 tom@xxxxxxxxxxxxx www.teraspeed.com Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 401-284-1827 www.teraspeed.com -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Alfred Lee Sent: Monday, January 31, 2011 9:20 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: relation between signal rise time and scope rise time Here's my take. With HJ's rule, at near the limit, what you see is not what you get, but you could figured out what it was. With Tek's rule, who is in the interest of selling scopes, at the limit, what you see is for all practical purposes what you get. - Alfred "prasad" <hariprasad.palli@xxxxxxxxx> wrote: >hi All, >i was going through one of the note from an oscilloscope vendor saying > >*oscilloscope rise time < signal rise time/5.* its available at link >below >http://www.tek.com/Measurement/scopes/selection/pdf/55W_13768_2_rise.pdf > > >but when i looked at the howard Johnson's book for High speed Digital >Design, where the BW of a signal is calculated as FKnee= .5/Tr, and >then >BW of scope is chosen as around 1.4 to 2 times of BW. > >am i missing anything while trying to interpret what the above >highlighted >statment? request your comments. > > >Thanks for taking time to read this.... > > > >Best Regards, >Hari.... > > >------------------------------------------------------------------ >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 > >Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > -- Sent from my Android phone with K-9 Mail. Please excuse my brevity. ------------------------------------------------------------------ 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 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 technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu