[SI-LIST] Re: frquency limit of a channel
- From: steve weir <weirsi@xxxxxxxxxx>
- To: "Lakshmi N. Sundararajan - PTU" <lakshmi.s@xxxxxxxxxxx>
- Date: Mon, 08 Mar 2010 12:16:41 -0800
Lakshmi, the amount of energy that you have above 8Ghz will be limited
but not zero. As I believe Istvan and perhaps others have pointed out,
there is not a hard band-stop based on rise-time. Evaluating your
channel substantially beyond the rise time knee insures that you capture
all significant effects.
Steve.
Lakshmi N. Sundararajan - PTU wrote:
> 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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Steve Weir
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