The "knee frequency" quoted in my book, High-Speed Digital Design: Handbook of Black Magic, was envisioned as a kind of upper limit for the maximum frequency that a digital engineer should normally have to worry about. It applies to digital waveforms. Technically, a Gaussian lowpass filter (having a gaussian step response) exhibits its -6dB attenuation point near the frequency 0.5/Tr, where Tr is the 10-90% rise/fall time of the step response. That's where I got F[6dB] <approx. equal to> F[Knee] <which is deifned as> 0.5/T (plus, as a practical matter, this seems to me a pretty good rule, and it's easy to remember). Scopes are normally specified in terms of a 3-dB attenuation point, not 6-dB. The relation between the -3dB point and Tr (10-90%), for a Gaussian filter, works like this: F[3db] = 0.338 / Tr That is the ordinary relation quoated by scope-manufacturers. In recent years, with the advent of digital-signal processing, some very advanced algorithms have been applied to the equalization of scope front-ends, with the result that the simple relations above may not longer always work. Still, given no other information, they make a reasonable starting place. I hope this information is helpful to you. Best regards, Dr. Howard Johnson, Signal Consulting Inc., tel +1 509-997-0505, howie03@xxxxxxxxxx www.sigcon.com -- High-Speed Digital Design seminars, publications and films -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Traa, Boris Sent: Tuesday, February 01, 2011 12:31 AM To: Tom Dagostino; 'Alfred Lee'; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: relation between signal rise time and scope rise time Dear Tom, I assume your calculation is only valid if the input of the oscilloscope is a first order circuit (i.e. the oscilloscope behaves as a first order system). I once determined the transfer of an oscilloscope and had to conclude that it was not a first order system. Kind regards Boris Traa System design engineer EMC It's the currents that make circuits work or fail. Philips Applied Technologies/EMC center Room 2.020 High Tech Campus 26 5656AE Eindhoven, The Netherlands Tel: ++ 31 40 27 43766 Fax: ++ 31 40 27 42224 E-mail: boris.traa@xxxxxxxxxxx -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Tom Dagostino Sent: 2011 Feb 01 6:43 AM To: 'Alfred Lee'; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: relation between signal rise time and scope rise time 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.pd >f > > >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. 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