Jitter in DLLs vs PLLs

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
Just wondering if anyone can point me to some articals or papers that analyse
the characteristics of xilinx DLLs vs altera PLLs.


Re: Jitter in DLLs vs PLLs

Is a side by side bench test of the DLL vs the PLL in a "real" situation
where IOs and logic are switching, not just a quiet side by side
back-off of a do-nothing design.

As frequencies increase, delays decrease, jitter fast becomes the number
one problem in a design.

Realizing this, we have many tools, design notes, suggestions, and
techniques to minimize jitter, and maximize performance.



is an excellent newly written description of the DCM (includes DLL) for
Spartan 3 which also makes for good reading.


Michael Chan wrote:
Quoted text here. Click to load it

Re: Jitter in DLLs vs PLLs

Thanks, that's a decent and concise treatment of jitter.  Not particularly
easy to do.

Quick question that perhaps you (or anyone who's had experience w/ the
RocketIO receivers) could answer...

Is the Peak-Peak Jitter tolerance spec'd in the rocketIO user's guide (.65
UI) spec'd in the 14-sigma fashion described in the xapp462 or is some
measurement interval (such as loop lock time) assumed?

As it's given in the RocketIO User's guide, there's no BER, no sigma factor,
and no specific measurement interval.  Table 3-3, p.103 of the guide only
says "peak-to-peak"

Any help/ experience would be greatly appreciated

--Josh Model
MIT Lincoln Laboratory

Quoted text here. Click to load it

Re: Jitter in DLLs vs PLLs

The p-p jitter for the V2P Rocekt IO is specified by using the 14 sigma
case of broadband (DC to daylight, no filters) RMS jitter as measured by
the Agilent DCA.

This method defines the "eye" as that which has 1E-12 BER or less
extrapolated from the 14 sigma points (no actual BER measurements are
done with this method).

As most folks know, if you filter out the low frequency phase noise
(which is tracked out by the receiver) then the RMS jitter is even less.

Hence the claim that we are error free when used correctly is valid (as
the 1E-12 contour was grossly pessimistic).


Josh Model wrote:
Quoted text here. Click to load it

Re: Jitter in DLLs vs PLLs
Quoted text here. Click to load it

Altera has published a document comparing PLLs to DLLs, specifically
looking at jitter.

We also publish specs for PLL jitter in the appropriate device
datasheet, so you can take a look at those as well.

Greg Steinke
Altera Corporation

Re: Jitter in DLLs vs PLLs

Two things:  first, the spec is not peak to peak, but cycle to cycle, so
we do not "exceed" our spec as you erroneously state, and second, the
jitter from the DLL is decidely non-gaussian so any statistical
estimates are wrong.  Can't "multiply by 6" or any other shortcut.

The peak to peak jitter is entirely random, but not a normal
distribution as the taps are discrete, and the selection of the taps is
from a digital control system.

The intrinsic jitter is not very interesting, as the "when busy" jitter
of our part is 1/3 of yours under the same conditions.

Choosing the 2X output is also not entirely honest, as the 1X outputs
have half the jitter (something you fail to mention, but we freely
advise our customers of).

Other than that, we also have LeCroy equipment, and use it often.  The
+/- 6 ps intrinsic measurement capability for jitter is selling you
short, however, as the Wavecrest with its +/- 3 ps peak to peak worst
case intrinsic jitter would make the PLL look better.


Greg Steinke wrote:
Quoted text here. Click to load it

Site Timeline