[SI-LIST] Re: frquency limit of a channel
- From: LD <lifeatthesharpend@xxxxxxxxx>
- To: "Lakshmi N. Sundararajan - PTU" <lakshmi.s@xxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Sat, 6 Mar 2010 19:35:06 -0800 (PST)
Patrick,
That is not true. If the period is 166ps the rise time needs to be less than
this figure and the fall time has to be less that that period. The signal will
generally need to complete a rise or fall time and stay high or low for a very
small fraction of that time. In fact most good receivers and recover the data
if the rise time where to be 166ps and the fall time was also 166ps or less and
there was zero high or low time.
If you are transmitting a signal of alternating ones and zeros then the signal
would look like a 3GHz clock. If the channel was limited to 3GHz then the
signal would start to look more like a sine wave. In general I recommend for a
6Gbps data rate of NRZ data the channel be designed to support 9GHz so to pass
the 3rd harmonic of the fundamental. Many channels do not do this and many
transmitters do not as well.
Leonard Dieguez
lifeatthesharpend productions / engineering
Spotted Love Savannahs :www.spottedlove.com
YouTube Videos: www.youtube.com/lifeatthesharpend
internet radio show: www.blogtalkradio.com/cattalkradio
“There are two kinds of engineers — those who have signal integrity problems,
and those who will.” – Eric Bogatin.
----- Original Message ----
From: "Zabinski, Patrick" <zabinski.patrick@xxxxxxxx>
To: Lakshmi N. Sundararajan - PTU <lakshmi.s@xxxxxxxxxxx>; si-list@xxxxxxxxxxxxx
Sent: Sat, March 6, 2010 1:20:54 PM
Subject: [SI-LIST] Re: 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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