RE: ONLY one ref multiplier?: PMA clock reference
As a SerDes vendor, I can only say "Right On, Brother!"
At 08:57 AM 6/20/00 -0400, Stuart Brorson wrote:
>Please allow me to make a quick comment about 155 vs. 622 MHz clocks here.
>I was involved in OC-192 IO card design at my former employer, Nexabit
>Networks (now Lucent Technologies), and have had some experience in this
>It is not my desire to disparage the fine products of Giga here, so please
>accept my apologies in advance. However, the 10 Gig SERDES products from
>Giga (i.e. GD16555 and GD16554) had jitter gen problems, even on the
>company-supplied test board. Amongst other problems, Giga's test board
>incorporated a 155 MHz clock. Designing a low jitter PLL/SERDES chain is
>not very easy.
>It is noteworthy that the OIF has speced a 622 MHz reference clock freq for
>the 10 Gig framer/SERDES. That means that clueful PLL vendors have every
>reason to design low-jitter 622 MHz clock modules which can be used -- or
>modified for use -- with 10GigE also.
>In any event, most vendors with whom I am aware -- including Giga -- allow
>the user to select either a 155 or a 622 MHz reference clock. This allows
>the board designer freedom to choose the design problem he wants to tackle:
>either a lower speed 155 MHz PLL with stringent jitter specs (and a very low
>jitter SERDES), or a higher speed 622 MHz PLL with all the intricacies of RF
>design, but perhaps with an easier jitter (i.e. board noise) problem.
>Why not allow two clock frequencies and leave the board designer the freedom
>Marlborough, MA 01752
>From: Lysdal, Henning [mailto:firstname.lastname@example.org]
>Sent: Tuesday, June 20, 2000 5:17 AM
>To: 'Jscquake@aol.com'; email@example.com; firstname.lastname@example.org
>Subject: RE: ONLY one ref multiplier?: PMA clock reference
>In answer to your question:
>My company (formerly Giga) has been shipping OC-192 SerDes since 1997 and
>the majority of our customers use 155.52 MHz reference clock.
>From: Jscquake@aol.com [mailto:Jscquake@aol.com]
>Sent: 20. juni 2000 00:18
>To: email@example.com; firstname.lastname@example.org
>Subject: ONLY one ref multiplier?: PMA clock reference
>Your proposal sounds good,i.e. to have only a single clock multiple
>(1/4 division) for the reference clock, but I am not sure if this is wise.
>a lower rate frequency clock autmatically implies worse jitter performance
>the PLL's. This is not as much of an issue for the WDM case as it is for
>but every psec (or even sub-ps) counts for the serial versions. So I would
>to be NOT too restrictive in saying only 155-156Mhz xtal osc are allowed.
>Note that the present community of OC192 people use the higher clock rate
>for the reference. Are there any that uses the 155MHz as a reference for
>OC192? Having said all this ... are there readily available
>644.53125MHz xtal osc.?
>In a message dated 6/16/00 1:03:26 AM Pacific Daylight Time,
> > Henning,
> > Sorry about the confusion. I did mention in my note that there would have
> > be
> > two optional clock references specified in the XBI, one for the LAN PHY
> > the
> > other for the WAN PHY.
> > What I should have said is that only one clock MULTIPLE be specified. For
> > example, 161.1328125 MHz is 1/4 of 644.53125 MHz and 155.52 MHz is 1/4 of
> > 622.08
> > MHz. One fourth is a good multiple to use. This means that other
> > should not be required anywhere in the standard, even optionally (i.e.
> > 1/2,
> > 1/16, 1/1, etc.)
> > Best Regards,
> > Rich
> > --
> > "Lysdal, Henning" wrote:
> > >
> > > Rich,
> > >
> > > I don't see how you can avoid having separate reference clocks for LAN
> > > WAN (with realistic PLL design).
> > >
> > > In the LAN case there are several options
> > > 156.25 MHz (seems to be prefered among serial folks)
> > > 161.1328125 MHz
> > > 644.53125 MHz
> > >
> > > In the WAN case the OIF specifies 622.08 MHz. I know of a lot of people
> > who
> > > also like 155.52 MHz
> > >
> > > Now the problem is: how do you synthesize 9.95328 GHz and 10.3125 GHz
> > > the same reference. If you use a 10 kHz reference, it's easy, but you
> > > most likely have problems with transmit jitter.
> > >
> > > So I haven't been discussing the WAN case at all, since I was under the
> > > impression that WAN PHYs will use existing SONET SerDes using 622.08
> > > refck.
> > >
> > > Regards,
> > >
> > > Henning
> > >
Gigabit Product Marketing Manager