Hi
The 10-90% rise time comes from the oscilloscope specifications that have
been around since the 1940s? The oldest 20-80% specifications I'm aware of
came from the edges on the old ECL logic families from the 1960s for the
reason already stated. The fastest slew rate area did not encompass the
full 10-90% area but the fast edge typically in the 20-80% areas had the
most impact on getting the signal from point A to point B.
There are several kinds of things that people do with TDR. Finding the
location of a fault and defining the characteristics of a feature are two of
them. To fully characterize two features near each other the TDR risetime
at the location of the feature has to be smaller than the feature size
and/or the distance between them. To localize the location of an open or
short then the risetime and timebase come into play. It is easy to locate
the position of a short or open with a fast edge but you do not need a 1
psec edge to see if the location of an open is 523 psec into a traces vs.
520. Any moderate speed TDR will give you that location if the timebase can
support a psec/div or better. Here you are measuring phase not bandwidth.
Your spacial resolution is dependent on the risetime but also the dielectric
constant of the medium. 1 nsec in air is about 300 mm. In FR4 with Er of
4, 1 nsec is about 150mm. In a ceramic with Er of 9, 1 nsec is about 100
mm. So you can resolve smaller features in higher dielectric constant
transmission line environment.
Regards,
Tom Dagostino
971-279-5325
tom@xxxxxxxxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Samaan, Samie
Sent: Thursday, March 03, 2016 3:33 PM
To: gmanikandan20@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: TDR Rise time
Dan,
The faster the rise-time, the more special resolution a TDR has. For
instance, a TDR which has a 10-90% rise-time of 100 ps, might be able to
resolve features that are only about 0.5-1" long on a PCB. Roughly speaking.
You want fast rise-time.
But, if the medium being probed is highly attenuating, and electrically
long, then even if you start with a fast rise-time, the edge will be blunted
pretty quickly, and the resolution will drop, no matter how fast of an edge
you start with. This would be very true of PCBs. So, it should all be
looked at as a whole system.
10-90% rise time, versus 20-80% is a matter of convenience and convention.
Some folks prefer 20-80% because they want to capture the "linear part of
the rising edge". Generally speaking, the Frequency content of the TDR
signal can be approximated by using
Signal Bandwidth in GHz = 0.35/(10-90% Rise time in
nanoseconds).
So, if you want to probe a medium whose band-width is say 3 GHz, and you
want spatial resolution, then you'd want to use a TDR whose rise-time is
better than 0.35/3 Ghz ~= 0.1 ns, so I would go with something like 30 ps
rise-time. etc.
Regards
Samie Samaan
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Mani
Sent: Thursday, March 03, 2016 3:20 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] TDR Rise time
Hi,
Can I get help in understanding TDR rise time calculation? I came across 10%
to 90% calculation and 20% to 80% as well. Just wanted to know what is the
difference between them and how to calculate the TDR rise time.
Thanks for the help.
Regards
Dan
------------------------------------------------------------------
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 forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
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 forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
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 forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
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
