The formula is saying that if you were to perform a Fourier transform on the
TDR step, then its 3-db down frequency
content is ~0.35/(10-90% Rise-time).
If the data rate is 28 Gb/s, then one Unit Interval is about 36 ps.
So, if you want to probe a channel (or a system) which is supposed to handle
36-ps pulses, then you'd want to hit it with a
TDR step that has a lot of energy at the 14 Ghz Nyquist. That would mean a DTR
rise-time of > "0.35/14 = 25 ps". But that
is barely enough, so personally, I would hit such a system with a TDR of at
least 10 ps, so I have a lot more energy in my
region of interest.
I reality, if you can de-convolve the incident signal out of the result
(output), then you can use a slower rise-time, but in order to
avoid having to know the exact shape and frequency content of your incident
wave, use a much faster rise than the formula gives
you at Nyquist, and make your life simple...if you can afford such a TDR :-)
To dig deeper, if you average the TDR signal a lot, and your Signal to noise
ratio is very high, then you can use a TDR rise/fall
slower than 25ps...But then again, why go into all that if you can afford a TDR
which is much faster than 25 ps!
Best Regards
Samie Samaan
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Mani
Sent: Thursday, March 03, 2016 3:44 PM
To: Bartlett, Josiah
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: TDR Rise time
Hi
Thank you all. I understand the theory behind TDR. But I am confused about
calculating the rise time to be used for simulation.
In the formula 0.35/BW. what is the bandwidth here? is it the data Nyquist
frequency?
Lets say the data rate is 28Gbps, how to calculate rise time based on 10%-90%
calculation and how does it change for 20%-80%?
Regards
Dan
On Thu, Mar 3, 2016 at 3:40 PM, Bartlett, Josiah <
josiah.bartlett@xxxxxxxxxxxxx> wrote:
Dan,
http://www.tek.com/document/fact-sheet/tdr-impedance-measurements-foun
dation-signal-integrity has some basics of TDR calculations and
applications that you may be interested in.
Best wishes,
Josiah Bartlett
Principal Engineer- High Performance Probes
Phone: 503.627.2946
www.tektronix.com
-----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
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