Re: Jitter work for clause 52

[Geoffrey Garner <gmgarner@lucent.com>]

Jonathan,

The purpose of setting the golden PLL BW equal to the sinusoidal tolerance
breakpoint is to make the transmitter and receiver specs consistent.  It does not
mean that the actual BWs of the transmitter and receiver in equipment would be
equal.  One would be free to use a wider bandwidth receiver if one wanted; it
would then be able to tolerate more jitter at lower frequencies and would exceed the requirement.  Similarly,
use of a wider BW golden PLL would allow the transmitter to generate more jitter (more precisely, would all
more jtter at the measurement point); use of a narrower BW golden PLL would allow less jitter at the measurement point.
If we made the golden PLL BW larger (and thus allowed there to be more jitter at the measurement point coming
from the transmitter side), it would be correct to also make the tolerance mask breakpoint higher (keeping
the flat level of the tolerance mask the same) and require the receiver
to tolerate this.  Alternatively, if we made the tolerance breakpoint smaller and required the receiver to tolerate less jitter
(keeping the flat level the same), we should also decrease the golden PLL BW to require the transmitter side to produce less
jitter.

The relation between the breakpoint of a sinusoidal jitter tolerance mask and
the bandwidth of the jitter measurement filter (the golden PLL BW) actually is now
documented in the 2000 version of ITU Recommendation G.825 (Appendix I).
I attempted to summarize this material in the presentation I gave at the October, 2000 ad hoc jitter meeting
(in Austin).  The presentation is available at:

 http://www.ieee802.org/3/ae/public/email_attach/sonet-sdh-jitter.pdf

(see VGs 11-15).

In addition, the material in Appendix I of the new (2000) G.825 essentially came from work
done by Dan Wolaver of Tektronix; he had a contribution to committee T1X1.3 in 1996 which had
a more detailed presentation of this.  This contribution is available via FTP from the committee T1 web site:

 ftp://ftp.t1.org/pub/t1x1/x13old/6x130640.ww6

The relation between output jitter and sinusoidal jitter tolerance is in roughly the first 5 pages.
(Note:  The document is a word document, so it is probably best to download it with an ftp tool (make sure you're
in binary mode), change the
suffix to .doc, and then open it; I found if I used my web browser it didn't open properly.  In any case, I also have a pdf
version, which I can send to this list if desired; I did not do this automatically because the pdf file is just over 400K and
I wasn't certain about file size limitations).

Regards,

Geoff

Jonathan Thatcher wrote:

 Geoff,As you say, there is no change in this document for the Rx. We discussed this on the conference call. Sorry you weren't available. I need to write it up for any discussion to make sense.We are planning to specify the Golden PLL, etc. But, not quite for the same reasons you list.Prior to getting this done, I would like to pursue a point you make below for general discussion. You mention that you think the Golden PLL B/W "should be equal to the breakpoint on the sinusoidal jitter tolerance mask..."  and that "...this ensures compatibility of the transmitter and receiver."This is an interesting statement to me for a couple of reasons:1. I have been taught that for a system (link) to work well, the B/W of the PLL in the Tx and the PLL in the Rx must be different. The Rx must be able to follow variations caused by the Tx PLL. Given that this should be true across all chip and system conditions and variations, the delta could be significant. The result is that there is an inherent jitter caused by the difference in the breakpoints. To my knowledge, while this has been discussed in a variety of forums in the past (e.g. the 10b chip specification), I know of no case where this is documented or specified.2. The Golden PLL should be made to closely approximate the performance of a well designed Rx PLL and also provide a definitive standard for measurement. These two requirements conflict to some degree. Historically, we have shown preference to the later. Probably the worst mistake we could make would be to create a situation where the optimization of the Tx ends up in conflict with the optimization of the Rx design.3. I don't remember ever seeing an analysis aimed at defining the optimal specifications for the Golden PLL wrt other specifications. This, therefore, reduces to engineering judgment. Unfortunately, it is rare that we have any PLL designers amongst attending the PMD (optical) meetings.I assume that the SONET mask will be used to test the Rx. I would similarly assume that the Golden PLL will be used to test the Tx. If what I surmise above is correct, we would not want these to have the same break point (close perhaps, but not the same).Curiously, my understanding is that most SONET PLLs are designed not against the mask, but against a corner frequency that has a certain probability of occurrence (maximum run length). Since this becomes a design trade-off consideration, not a standards requirement, we see a wide variety of specifications for the corner frequencies. If we design the specifications and conformance testing to continue to allow this kind of "personal initiative" and "market differentiation" it has the potential to further complicate an already complicated scenario.jonathan-----Original Message-----
From: Geoffrey Garner [mailto:gmgarner@lucent.com]
Sent: Thursday, March 08, 2001 12:15 PM
To: Jonathan Thatcher
Cc: Serial PMD reflector (E-mail)
Subject: Re: Jitter work for clause 52
 

Jonathan,

I have a few comments on the jitter draft you had attached.  While I did not attend the conference call this week, I did discuss this with my colleague, Juergen Rahn.  My comments concern the entire draft, though I can see that the receiver portion was
already there (you indicated you had written up the transmitter part).  I do plan to attend the meeting next week.

1) Golden PLL, Golden receiver, and receive eqiupment in transmitter test,  referred to in Figure 52-3 and lines 4-15 (approximately) on p. 357 (just above section 52.8.2).  I believe it is appropriate to specify this equipment, as this defines how
the transmitter measurement is to be made.  If, as indicated in the note beginning on line 9 of p. 357, these are outside the scope of the document, then it will be difficult to achieve interoperability.  This could happen if 2 different vendors used
different test equipment for transmitter and reciever, respectively, one of which forced a greater burden on the system under
test than the other.  In addition, it would be difficult to verify compliance if the developer/vendor of equipment used one set of
test equipment and the customer used another.

Regarding the golden PLL, this is in essence equivalent to the high-pass jitter measurement filter.  Its bandwidth should should be equal to the breakpoint on the sinusoidal jitter tolerance mask.  I don't know if the sinusoidal tolerance mask has been decided on yet; however, using as examples the ones that have been discussed in the past, the breakpoint would be 4 MHz for the ITU (SONET/SDH) mask (which has flat level of 0.15 UIpp sinusoidal jitter that must be tolerated) and 6 MHz for the Fibre Channel mask (which has flat level of 0.1 UIpp sinusoidal jitter that must be tolerated.   The sinusoidal jitter tolerance mask is effectively a requirement on the minimum bandwidth of the receiver (For example, if the Fibre Channel mask is used,
it says the reciever must have a minimum BW of 6 MHz if it just tolerates 0.1 UIpp sinusoidal jitter for higher frequencies; if one wants to use a 4 MHz BW, the tolerance must be increased to 0.15 UIpp for frequencies above this).  By choosing the golden
PLL BW to correspond to this jitter tolerance mask breakpoint, this ensures compatibility of the transmitter and receiver.

Also in the paragraph on the golden PLL (lines 4 - 7 on p. 357), it is mentioned that there will be sufficient wander or drift
in the system under test to result in eye closure.  If the intent is to really talk about wander here, then this effect is unimportant.
Wander is low-frequency variation in the bit timing (zero crossings of the bit slopes).  This is well below the bandwidth of the golden PLL (or of a receiver used in practice) and is easily tracked.  However, it does say that the frequency draft will occur over the time it takes for the optical signal to traverse the fiber.  If this time is sufficiently short (I don't know what this time is),
then this frequency change would actually be jitter.  Since it is occurring in the transmitter clock of the system under test, I would classify it as part of jitter generation.  What is most important in determining whether this is jitter or wander is the time scale over which this occurs compared to the golden PLL time constant (1/BW).

2) For the receiver conformance test (Section 52.8.2), I see that this is the same as what is in the D2.1 and D2.2 drafts.  My main comment here is that there needs to be consistency between this, the transmitter test, and the receive sensitivity measurements given in Section 52.8.8 and the tables referenced there (Tables 52-6,7, 10,11, 14,15).  Related to this, it would be useful if a power penalty were specified for the sinusoidal jitter component in item 8) at the bottom of page 357 (Section 52.8.2); the usual penalty for this is 1 dB optical.  On the consistency issue, Tables 52-7, 11, and 15 indicate that stressed receive senstivity is measured with 1e-12 BER at the eye center.  They do refer, however, to Section 52.8.11 (in D2.1, which
is 52.8.2 in your document), which has the DDJ, DCD, and sinusoidal jitter added.  In any case, my main point is that
if we define stressed receive sensitivity to include a set of impairments with a certain power budget at 1e-12 BER with sampling
at the eye center, then if we add the effect of jitter, the BER will go up.  Or, to put it differently, if the BER is 1e-12 with all the
jitter sources present, then it would be much lower if we remove these sources but keep all the other impairments and the same
power level.  If the sinusoidal jitter has a specified power penalty, then this can be included in the budget.  In addition, it means that sinusoidal jitter tolerance can be measured by measuring the jitter associated with this power penalty.

Thanks.

Regards,

Geoff Garner

Jonathan Thatcher wrote:

Here is my humble attempt at writing up the transmit side of the jitter specs for clause 52.\As you can see, there are a number of places where help is needed.Will join the PMD serial teleconference at about 15 to 20 minutes after the start.I wrote this based on a version of 52 that was somewhere between 2.0 and 2.1. Sorry if there is any confusion because of this.jonathanJonathan Thatcher
Principal Engineer, World Wide Packets
Chair, IEEE P802.3ae Task Force
Office: 509.242.9228  Fax: 509.242.9001
jonathan@wwp.com

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