[SI-LIST] Re: frquency limit of a channel
- From: "Zabinski, Patrick" <zabinski.patrick@xxxxxxxx>
- To: "Lakshmi N. Sundararajan - PTU" <lakshmi.s@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sat, 6 Mar 2010 15:20:54 -0600
> Suppose assume I have a high speed serial link at 6Gbps. The
> nominal rise time of the signals on this channel is 150ps.
> Given this rise time, the bandwidth required to transmit this
> signal is 0.35/tr = 2.33Ghz.
At 6 Gbps, the period of a single bit is 166 ps. Within that period,
the single must rise and fall. With a 150 ps edge rate, the rise and
fall is 300 ps, which exceeds that of the 166 ps bit period. The math
does not compute. As a general rule, 0.35/tr should always exceed (data
rate)/2.
The exception is if you're dealing with non-traditional signal protocols
such as PAM, QAM, QPSK, etc... where symbol rate must be considered.
> So, to study this channel behavior, is it correct to only look at
> s-param frequency output till say 3Ghz.
> Can any higher frequency data points on this s-param be
> ignored and still correctly model the channel behavior?
Ignoring the apparent discrepancy from above, it is useful to look
beyond 3 GHz. There is no general consensus in industry as to the max
frequency of consideration. 0.35/tr is a common, so is (data rate)/2.
My experience leads me to believe that either is inadequate in most
cases. Generally, it's best to consider at least 1/tr or even 1.5/tr
[that is, 3X or 5X of the edge rate].
If the passive channel is very clean, the most modeling will be
sufficient with 0.35/tr. However, "very clean" cannot be determined
unless you look beyond 0.35/tr. Resonances are prominent in high-speed
channels, and it is all to common to have a dramatic drop in S21 at
0.4/tr to 0.5/tr that will certainly affect the eye opening.
Pat Zabinski
Mayo Clinic
------------------------------------------------------------------
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
Other related posts:
