[ibis-macro] Re: clock_times .... yet again

  • From: Mike Steinberger <msteinb@xxxxxxxxxx>
  • To: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • Date: Wed, 14 Apr 2010 18:25:42 -0500

Scott-
Please consider that most CDR loops generate their output using a phase selector driven by a reference clock, usually through a PLL. The phase selector moves by at most one phase step in a clock cycle. Thus, the period of an individual clock cycle at the output of the CDR loop can vary by at most one phase step from the period of the reference clock. The period of the reference clock itself has a Guassian variation; however, other than that small effect, the period varies exactly between nominal reference clock period, reference clock period plus one phase step, and reference clock period minus one phase step. The circuit design allows no other possibilities.
So the bottom line is that the period of an individual clock cycle at the output of a CDR loop can only vary by one phase step plus a variation in the reference clock period. For most designs, that's a lot less than the variation you've described, much smaller than the sample interval used in most simulations, and too small to significantly affect the performance estimate. 

Mike S.

Scott McMorrow wrote:

Mike
We're speaking of the same thing. My numbers are in substantial agreement with Vladimir's. If you measure a CDR you will find that the jitter distribution is Gaussian. 

Scott



Mike Steinberger wrote:

Scott-
You're still addressing the jitter distribution rather than the period of an individual clock cycle. Vladimir's data was more to the point and is in general agreement with the information I have.
You should also know that the PDF of an individual clock cycle at the output of most CDR loops does not resemble a Gaussian at all, except in the sense that there is a very small amount of Gaussian phase that comes from the reference clock. Think about the actual circuit design of most CDR loops and you'll see what I mean. 

Mike S.

Scott McMorrow wrote:

Mike
For a well-designed CDR the jitter distribution is gaussian with a 0.007 to 0.01 UI sigma. This is +/- 0.05 UI over a 10-12 confidence interval. Over 10+12 bits you will see a minimum period of .95 UI and a maximum period of 1.05 UI. On top of this other deterministic jitter components will be convolved. 

Scott


Mike Steinberger wrote:

Scott-
You've addressed peak to peak jitter, but that's not the question at hand. Peak to peak jitter is the total deviation of a clock with respect to some reference clock. That deviation is typically built up over a number of clock cycles, however, and does not address the period of an individual clock cycle with respect to the reference clock period. What you've been focusing on is the time between a clock tick value and the time the data is actually sampled. That's affected by the period of the individual clock cycle and not some deviation built up over many clock cycles.
Can you please give us a probability density function for the period of an individual clock cycle with respect to the reference clock period? 

Thanks.
Mike S.

Scott McMorrow wrote:

Todd,
pk-pk jitter for good CDRs is designed to be around +/- 0.05 UI at a 10^-12 confidence interval. Deterministic jitter transfer and multiplication in the CDR loop will be added on top of this. CDR jitter can easily approach +/- 0.1 UI at a 10^-12 confidence interval under stress testing.
+/-10% is a reasonable number. Shortest period is 0.9 UI. Longest period is 1.1 UI. 

Scott




Todd Westerhoff wrote:

Vladimir,

What is the magnitude of the effect you're talking about?

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: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Dmitriev-Zdorov, 
Vladimir
Sent: Wednesday, April 14, 2010 4:06 PM
To: Mike Steinberger
Cc: fangyi_rao@xxxxxxxxxxx; scott@xxxxxxxxxxxxx; ibis-macro@xxxxxxxxxxxxx 
Subject: [ibis-macro] Re: clock_times .... yet again

Mike,


I question the practical relevance of your concern about missing or

duplicated waveform points at the edge of an eye diagram.

Perhaps the brief answer is that superimposing disconnected or
overlapped waveform pieces may cause distortions of the jitter PDF that accumulates at eye edges. I'm not ready to discuss the numbers but the 
approach does not seem technically clean. In particular, it cannot
correctly account for peak to peak jitter.

Vladimir

-----Original Message-----
From: Mike Steinberger [mailto:msteinb@xxxxxxxxxx] Sent: Wednesday, April 14, 2010 1:51 PM 
To: Dmitriev-Zdorov, Vladimir
Cc: fangyi_rao@xxxxxxxxxxx; scott@xxxxxxxxxxxxx;
ibis-macro@xxxxxxxxxxxxx
Subject: Re: [ibis-macro] Re: clock_times .... yet again

Vladimir-
The short answer is that your second statement is correct and always has 

been
"the standard way of building the eye histogram from Rx output is not 
defined in details and could be implementation specific"
I question the practical relevance of your concern about missing or duplicated waveform points at the edge of an eye diagram. It is certainly mathematically possible that there could be missing or duplicated waveform samples at the edge of an eye diagram. When this happens, how will it affect the results of the analysis? And if it doesn't affect the results of the analysis, is it worth discussing? 

Mike S.

Dmitriev-Zdorov, Vladimir wrote:

Mike,

We are talking about different things. My point was that the spec

should
be definite enough about what is a correct meaning and usage of clock 
times.
You are saying that since you do a specific type of simulation
successfully, there is no reason to worry about the accurate

definition.

Or did I take it wrong?
Any abstract EDA tool gets a long waveform and clock times output from Rx DLL. This is to be used to create eye "histograms". Do we need to 
formally define the standard way of doing this or remain silent on

this
point thus leaving the possibility that the result is implementation 
dependent?
The best I can figure out from the spec and our discussion so far is 
this:

"EDA tool will use the portion of the waveform from tick_time_N to
(tick_time_N+1UI) to fill the eye histogram. It is allowed that some portions of the Rx output waveform will be duplicated or missed in the 
histogram if the clock times are non equidistant"

or
"the standard way of building the eye histogram from Rx output is not 
defined in details and could be implementation specific"


Vladimir

-----Original Message-----
From: Mike Steinberger [mailto:msteinb@xxxxxxxxxx] Sent: Wednesday, April 14, 2010 11:35 AM 
To: Dmitriev-Zdorov, Vladimir
Cc: fangyi_rao@xxxxxxxxxxx; scott@xxxxxxxxxxxxx;
ibis-macro@xxxxxxxxxxxxx
Subject: Re: [ibis-macro] Re: clock_times .... yet again

Vladimir-
If you want to worry about missing or duplicated data, you might want 
to
consider the case of an odd/even receiver architecture. As you well know, the IBIS AMI API is based on the assumption that there is a 
single
data path, and yet we routinely and successfully use IBIS AMI to model 


receivers that have two or four data paths. In such cases, the

waveform
in each data interval is the waveform at the decision circuit whose decision is actually going to be used for that data bit. Thus, the receiver model output waveform and the resulting eye diagram is built 
up

from segments of waveform that come from different parts of the

circuit

altogether.
This all reproduces measured data quite well, so long as the switch in 


waveform segments always occurs at the edge of the eye.

Mike S.

Dmitriev-Zdorov, Vladimir wrote:
But here is a minor technical problem: if you do this way, and the clock times are not equidistant, then some portions of the waveform 
in
your histogram are counted twice and some could be missing. This is another Pandora box: what's a definition of an eye histogram built from waveform and non-equidistant clock times? 

Vladimir
--------------------------------------------------------------------- 
IBIS Macro website  :  http://www.eda.org/pub/ibis/macromodel_wip/
IBIS Macro reflector:  //www.freelists.org/list/ibis-macro
To unsubscribe send an email:
  To: ibis-macro-request@xxxxxxxxxxxxx
  Subject: unsubscribe




--
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC












---------------------------------------------------------------------
IBIS Macro website  :  http://www.eda.org/pub/ibis/macromodel_wip/
IBIS Macro reflector:  //www.freelists.org/list/ibis-macro
To unsubscribe send an email:
 To: ibis-macro-request@xxxxxxxxxxxxx
 Subject: unsubscribe

Other related posts:

  1. Home
  2. ibis-macro
  3. Archive
  4. 04-2010