[SI-LIST] Re: frquency limit of a channel
- From: Todd Westerhoff <twesterh@xxxxxxxxxx>
- To: "Lakshmi N. Sundararajan - PTU" <lakshmi.s@xxxxxxxxxxx>
- Date: Mon, 8 Mar 2010 16:09:33 -0500 (EST)
Lakshmi,
Out of curiosity, where are you quoting 0.35/tr from? What source?
0.35/tr is one way of estimating the frequency content of an output signal, so
that you have some idea of which frequencies matter and which don't.
I've seen these figures most often when talking about high speed parallel
interfaces (e.g. DDR2, DDR3) with relatively short interconnect (i.e. 6 inches
or less). The loss in serial links is a lot higher: -15dB at the fundamental
frequency isn't uncommon, and the loss at the third harmonic will be much
higher than the loss at the fundamental. So ... in most cases, whatever energy
you had at the third harmonic at the driver has pretty much disappeared by the
time you get to the end of the link. As with everything in signal integrity,
the specifics will change based on your particular situation.
Bottom line, it's not just about the rise time of the driver ... it's about
what makes it to the receiver.
My apologies if I haven't stated this clearly enough; I'm having trouble
articulating it precisely.
Todd.
--
Todd Westerhoff
VP, Software Products
SiSoft
6 Clock Tower Place, Suite 250
Maynard, MA 01754
(978) 461-0449 x24
twesterh@xxxxxxxxxx
www.sisoft.com
----- Original Message -----
From: "Lakshmi N. Sundararajan - PTU" <lakshmi.s@xxxxxxxxxxx>
To: "Patrick Zabinski" <zabinski.patrick@xxxxxxxx>, si-list@xxxxxxxxxxxxx
Sent: Monday, March 8, 2010 2:01:40 PM
Subject: [SI-LIST] Re: frquency limit of a channel
Hi Patrick,
The tr/tf is indeed max 150ps. This is quoted from the datasheet.
Okay, then maybe I thought the datasheet is wrong, went back to my earlier
measurement results.
The tr/tf is anywhere between 45ps and 110ps.
So if we consider the fastest rise time tr=45ps as seen in the channel, then,
0.35/tr = 8Ghz.
The data rate of the channel is 6Gbps.
Now again I have seen different opinions on frq limit to be either 0.35/tr or
1/tr.
This indicates that the max frequency of interest is anywhere between 8Ghz to
22Ghz.
So, then is this the real frequency limit to keep tab for this channel.
22Ghz looks way too high.
Moreover, I have seen many emails pointing out 0.35/tr or even 1/tr.
But can somebody point me from which model do those numbers come from?
I am still unable to understand how come we use this limit for any channel.
Thanks,
-LN
-----Original Message-----
From: Zabinski, Patrick [mailto:zabinski.patrick@xxxxxxxx]
Sent: Saturday, March 06, 2010 1:21 PM
To: Lakshmi N. Sundararajan - PTU; si-list@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] frquency limit of a channel
> 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
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