[SI-LIST] Re: relation between signal rise time and scope rise time
- From: "Howard Johnson" <howie03@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 2 Feb 2011 20:21:19 -0800
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....
>
>
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