[SI-LIST] Re: Do you really ship products at BER 10e-xx ?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Chris.Cheng@xxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 19 Apr 2005 00:24:46 -0700
Chris, the jitter transfer specs in SONET come from a need to keep from
underflowing / overflowing the rather shallow buffers traditionally used in
TDM equipment.
For loops that close unity Fo <= Fpd/3, I agree that the loop response can
be observed at the error amplifier output to the VCO following an impulse
or step, such as start-up. Start-up will render the large signal response
as well as the small signal response.
Best Regards,
Steve.
At 07:36 PM 4/18/2005 -0700, Chris Cheng wrote:
>I thought that's the reason why some specs like Sonet spec out jitter
>transfer Fc to roll off way before the loop bandwidth. How do you get there
>is another problem.
>I will be intrested to know what parasitics in the PLL can approach the
>unity gain frequency to impact the stablity. Long long time ago there were
>concerns about the PD output rc time constant to the loop filter or the
>discrete Z sampling pole but I don't know how they look like with internal
>filters and faster sampling rate. Still, I claim you can always observe the
>problem by the cold start step response and thereby going back to the
>drawing board. No suprise there.
>
>Here's what really get me started on this thread. Customer service always
>want us to give an MTBF estimate for everything so as to generate a support
>cost structure. I think I understand the power supply, fan fail, bad chips,
>bad discrets and bad disks problem. But I never quite "get" the 10e-12 link
>error rates. Like Paul says, I've seen peta bytes of disk humming for weeks
>(minus the disk or real components failure not related to the link itself).
>I don't want to scare myself silly for a statistical number that no one
>really care but everyone put it in the book. But if you tell me that's real,
>I will get phone calls from CS engineers. How does one budget CS service
>calls for the BER down the road at 8G FCAL for a peta byte installation ?
>Even if the BER is a million times better than 10e-12 ? My money is on
>device (disk,fan,power supply etc) failures will dwarf the link failure
>rate. But I can't proof that on a satistical basis.
>
>Al, I will appreciate the papers off line also.
>TIA,
>Chris
>
>-----Original Message-----
>From: Alfred P. Neves [mailto:al.neves@xxxxxxxxxxx]
>Sent: Monday, April 18, 2005 4:20 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Do you really ship products at BER 10e-xx ?
>
>
>When we had to optimize the loop characteristics for PLL's an overriding
>consideration has been the conflicting requirements of low jitter
>generation (GR-253, Sonet) and jitter transfer. If you make the loop BW
>large, and the loop gain high you often have poor loop stability of
>phase/frequency, such that you reject intrinsic VCO noise, but have poor
>jitter transfer performance. Remember that there are parasitics in the
>loop and they cause peaking in the loop response, along with higher
>order poles in VCO, so when the loop BW increases loop stability may
>suffer. Verify that the equations are correct (they are usually fairly
>lousy linearized approximations of a charge-pump sampling system)and the
>loop is set correctly, we can check the loop dynamics with Spectrum
>Analyzer and autocorrelation analysis often using Wavecrest instruments.
>
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>On Behalf Of Chris Cheng
>Sent: Monday, April 18, 2005 2:46 PM
>Cc: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Do you really ship products at BER 10e-xx ?
>
>
>My PLL design is getting rusty so if I am mumbling nonsense please
>correct me. If you are dealing with classical PLL design, the loop
>filter is always a trade-off between the high pass VCO phase noise and
>the low pass input phase noise. My money is always on the VCO phase
>noise and I optimize it for such. Afterall, if I read Geogre's response
>in the same thread, "But if you solve all these problems to the extent
>that there is sufficient eye-opening inside the receiver, then you are
>dealing with errors caused by the second-order effects, mainly the RJ
>from TX PLL, RX PLL / CDR circuits." That's sounds like a controlled
>input phase noise (good eye opening) vs. a out of control VCO phase
>noise problem (you have non-zero BER). And that's why our employers pay
>us peanuts to design a power distribution that ensure no "large supply
>glitches" or at least some good PLLVDD filters to avoid that to happen,
>right ?
>As to making sure the loop filter damping factor, one can either observe
>directly the VCO control voltage or monitor the VCO output frequency in
>the modulation domain during a cold start to deduce the stability
>factor, there is no magic about it. You either get it right or you are
>back to the drawing board. There is no 10e-xx probably you are either
>right or wrong. And if you are talking about these bang bang PLL's.
>AFAIK, if you are operating under the slew rate limit, your hunting
>jitter is bounded and is related to the metastability limit of the FF
>you use. And they have the added bonus of even if the input phase noise
>is large, they are limited by Jwalk which is sqrt of input jitter. I
>can't argue that a 1ps rms xtalk MAY have a 100ps jitter within the life
>time of the universe but it is hard to extend that to say a properly
>design system is one big 10-exx distribution.
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
>List technical documents are available at:
> http://www.si-list.org
>
>List archives are viewable at:
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
