[SI-LIST] Re: Clock Jitter
- From: Eddy <eddyvk@xxxxxxxxx>
- To: si list freelist <si-list@xxxxxxxxxxxxx>
- Date: Sun, 5 Jul 2009 09:47:57 -0700 (PDT)
The clock generator IC is most likely a synthesizer that multiplies the
frequency up from a crystal reference. These synthesizers can be very sensitive
to noise frequencies near their loop bandwidth, usually in the region
500KHz~5MHz. Most RF chokes have trouble stopping these relatively low
frequencies and I find an ordinary resistor of a few ohms to work better
sometimes.
The synthesizer can also be very sensitive to the noise it creates itself on
its power lines. Therefor power supply decoupling is VERY important. Move the
decoupling capacitor 1mm further away from the clock chip and the jitter may go
up with more than 20ps. Because of the sensitivity to decoupling 'quality' I
agree with Jose below that a simple change of capacitor manufacturer and/or
capacitor dielectric can cause the jitter change that you see. CM's are always
seeking to lower cost....
The jitter numbers you are speaking of are most likely PkPk unit of the Period
Jitter kind. PkPk numbers are not very exact. You can do one measurement and
get for example 80ps PkPk. Then you do another measurement with the same setup
and you get 90ps. If 90~100ps is already causing real trouble then a nominal
78ps is too close for comfort. You may be able to clean up the environment
around the current chip such that it improves or else chose another clock chip
generator.
You mentioned you were able to fix some PCB's. What did you do to fix them?
Possibly the root cause of what caused the jitter to go up is in the area of
what you did to bring it back down.
Good luck fixing your jitter problem!
Eddy van Keulen
Fremont CA, USA
--- On Tue, 6/30/09, Moreira, Jose <jose.moreira@xxxxxxxxxx> wrote:
>
> From: Moreira, Jose <jose.moreira@xxxxxxxxxx>
> Subject: [SI-LIST] Re: Clock Jitter
> To: "si list freelist" <si-list@xxxxxxxxxxxxx>
> Date: Tuesday, June 30, 2009, 8:16 AM
>
> Why are you assuming that the problem is with the bare
> PCB?
>
> Is the Bill of Material exactly the same as your reference
> board. i.e. the values part numbers and vendors of all used
> parts (IC's, Cap's, etc.) are the same from the US and Far
> East manufacturers? You would not be the first to have a
> part changed to a different vendor (usually much cheaper)
> that impacts your design.
>
>
>
>
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Richard Jungert
> Sent: Tuesday, June 30, 2009 5:01 PM
> To: hermann.ruckerbauer@xxxxxxxxxxxxx;
> greg pietz; si list freelist; steve weir
> Subject: [SI-LIST] Re: Clock Jitter
>
> I don't quite get how small changes in board construction
> can increase jitter.
>
> If one isolates the oscillator with good power filters and
> good ground plane splits then how can the noise get in to
> modulate the oscillator?
>
>
>
> Jitter is all about randomness right?
>
>
>
> Richard Jungert
>
>
>
> > Date: Tue, 30 Jun 2009 16:48:00 +0200
> > From: hermann.ruckerbauer@xxxxxxxxxxxxx
> > To: r_jungert@xxxxxxxxxxx
> > CC: greg.pietz@xxxxxx;
> si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: Clock Jitter
> >
> > Content-Type: text/plain; charset=ISO-8859-1
> > Content-Transfer-Encoding: quoted-printable Hi,
> >
> > although some explantions came back I guess the
> orignal question was
> > not really answered. What can be the difference in PCB
> manufacuring to
> > increase jitter ?
> >
> > I think the hint to power/ground noise is quite good.
> We have seen,
> > that the Copper thickness is varying quite a bit with
> different
> > vendors, what might cause a difference in power noise
> between the different boards.
> > But what would worry me is, that such a small change
> should change
> > your design from pass to fail. If your design is that
> critical it is
> > not stable enough for HVM (high volumen
> manufacturing). In a volume
> > production you will see a certain distribution of the
> Power plane
> > thickness what could be a big problem for sutch a
> design.
> >
> > As you measured your impdance and confirmed the same
> value I would not
> > really expect the issue to come from the dielectric
> properties. If
> > they would change too much this would influence er and
> therefore the
> > impedance. Overall I would not really see this to
> cause a bigger
> > change for the Jitter (but is 20ps really a bigger
> change ??)
> >
> > What I would see as another possiblilty is e.g. the
> size of Anti-pads.
> > If there are some Anti-pads along the current return
> they might even
> > create a cut in the Ground plane when the PCB
> manufacterer does some
> > overetch. Basically it might be possible to see such
> things in the
> > TDR, but maybe the effect is big enough to shift a
> reflection a bit,
> > but not easily see it as difference in the TDR.
> >
> > regards
> >
> > Hermann
> >
> >
> >
> >
> > EKH - EyeKnowHow
> > Hermann Ruckerbauer
> > www.eyeknowhow.de
> > hermann.ruckerbauer@xxxxxxxxxxxxx
> > Veilchenstrasse 1
> > 94554 Moos
> > Tel.: +49 (0)9938 / 902083
> > Mobile: +49 (0)176 / 787 787 77
> > Fax: +49 (0)721 / 151 258 230
> >
> >
> >
> > Richard Jungert schrieb:
> > > Greg.
> > >
> > >
> > >
> > > Its more than likely power noise or noise from
> the ground plane getting> into the oscillator.
> > >
> > > You might need to put a moat in the ground plane
> and power plane on the> oscillator to cut the jitter cuz
> noise can sneak in any connection thru > the ground or
> the power.
> > >
> > >
> > >
> > > I was suprized how much the noise on the ground
> can sneak into an oscil> lator when we had to fix this
> probklem type years ago on a video graphics> board.
> Originally I thought it was power noise but turned out to be
> grou> nd noise causing the problem.
> > >
> > >
> > >
> > > Are the any splits in your ground plane around
> the oscillator? Is the o> scillator in the middle of the
> board is another issue that can make it wo> rse.
> > >
> > >
> > >
> > > If you free run the oscillator with a battery how
> much jitter is presen> t? Getting the jitter within spec
> is hard work! Try free running it by i> tself totally
> away from the circuit board to see what stand alone best
> ca> se jitter can be.
> > >
> > >
> > >
> > > Richard Jungert
> > >
> > >
> > >
> > >
> > >
> > >
> > >> Subject: [SI-LIST] Re: Clock Jitter
> > >> Date: Tue, 30 Jun 2009 10:06:28 -0400
> > >> From: mrose@xxxxxxxxxxxx
> > >> To: greg.pietz@xxxxxx;
> si-list@xxxxxxxxxxxxx
> > >>
> > >> Greg,
> > >>
> > >> Is the additional jitter DJ or RJ? If it's
> uncorrelated, you could
> > >> loo> k at power noise. If it's DJ, you may
> want to look at the
> > >> relative magnitude of the impedance
> discontinuities (traces, vias,
> > >> connectors).> Did you check trace coupons?
> Also, you might check if
> > >> there is any additional duty cycle distortion
> from the oscillator.
> > >>
> > >> Good luck
> > >>
> > >> -----Original Message-----
> > >> From: si-list-bounce@xxxxxxxxxxxxx
>
> > >> [mailto:si-list-bounce@xxxxxxxxxxxx>
> g] On Behalf Of Pietz, Greg P
> > >> Sent: Tuesday, June 30, 2009 9:55 AM
> > >> To: si-list@xxxxxxxxxxxxx
> > >> Subject: [SI-LIST] Clock Jitter
> > >>
> > >> We have a board design using PCIe. The PCIe
> reference clock for
> > >> each ASIC comes from a clock generator IC. On
> initial proto boards,
> > >> build i> n the USA, the clock jitter was
> about 78ps or less on all
> > >> boards tested.> We then had boards built
> overseas and are seeing jitter of 90 to 100ps> .
> > >> This is greater then the spec allows and is
> causing problems with
> > >> the link. I did a TDR of the clock traces and
> they are 100 ohms
> > >> differential.
> > >>
> > >> For two of the board vendors I was able to
> make some changes to the
> > >> design to get their jitter in spec. For the
> third vendor I have
> > >> been unable to fix the jitter problem.
> > >>
> > >> The board stackup is the standard 6 layer
> design. The clocks all
> > >> reference the ground plan.
> > >> My question is what can a board vendor do to
> increase clock jitter.
> > >>
> > >> Thanks,
> > >> Greg
> > >>
> > >>
> > >>
> > >>
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