[SI-LIST] Re: Dead Band in PLL
- From: Chris Cheng <chris.cheng@xxxxxxxxxxxx>
- To: "'Guo Yawei'" <ywguo527@xxxxxxxxxxx>,Chris Cheng <chris.cheng@xxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Thu, 20 Nov 2003 12:47:20 -0800
I really don't know what to say, the PLL I worked on even in early 90's have
pulses less than 400ps. And while your claim of xtals from several to dozen
MHz may describe the very first stage of a clock distribution tree in a
system, there are many more PLLs down stream locking to a much much higher
speed reference clock. Just ask any designer working on DDR II, GDDR3 or
zero delay fan out buffers for clock distribution.
I also claim it is easy to predict where the zero charge phase point is for
a charge pump given a known VCO operating point. We can simulate and measure
it all the time.
May be techniques to analyze PLL performance and designing phase detector
pulses smaller than 400ps are core competencies of some of the companies I
work for/with but rest assure they are here and well understood.
Unfortunately, you will probably have to work for those companies to learn
it. Good luck in finding one in your country. It is indeed fun to work on
PLLs.
-----Original Message-----
From: Guo Yawei [mailto:ywguo527@xxxxxxxxxxx]
Sent: Wednesday, November 19, 2003 9:00 PM
To: chris.cheng@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Dead Band in PLL
Chris,
I don't agree with you on the phase offset. Theoretially it is right that
the phase offset is due to the difference between the VCO control voltage
at the desire frequency and the net voltage where the charge pump will
settle at when both up and down pulse are turn on. But please note that we
are talking about charge PLL when we say up and down current in charge
pump. Idealy, charge pump PLL has zero phase offset. So one of its
important applicaton is deskew on PC board or on chip. However, due to
mismatch current in charge pump, there must be very small phase error in
practice.
If you ask why charge pump PLL has zero phase error idealy, just because
PFD+charge pump acts as an integrator, which means infinite DC gain. Any
difference between the VCO control volatge at the desired frequency and the
net voltage where the charge pump settles is divided by infinite DC gain.
Thus the phase error is zero idealy.
Furthermore, we cannot determine where the charge pump settles because the
up and current are sourced from and sinked in current source, which has
very high impedance.
For may PLLs used as frequency synthesizer, 400ps pulse is not very high
because the refernece frequency is usually several to dozens MHz. Crystals
with resonant frequency higher than 100MHz is more expensive.
400ps is not uncommon because it is safe to avoid deadband in charge pump
PLL.
Very glad to discuss PLL with you.
Best regards,
Yawei Guo
>From: Chris Cheng <chris.cheng@xxxxxxxxxxxx>
>Reply-To: chris.cheng@xxxxxxxxxxxx
>To: "'Guo Yawei'" <ywguo527@xxxxxxxxxxx>,Chris Cheng
<chris.cheng@xxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Dead Band in PLL
>Date: Wed, 19 Nov 2003 18:27:52 -0800
>
>Not exactly. You can have current mis-match in the up and down charge
pump.
>But the phase offset is due to the difference between the VCO control
>voltage at the desire frequency and the net voltage the charge pump will
>settle at when both up and down pulse are turn on. That delta will result
in
>excess charge transfer in or out of the charge pump. However, if you
>consider the short duration of the pulse compare with the cycle time, it
is
>only a fraction of the peak delta current (averaged over one cycle).
>400ps sounds very high. Many PLL's have cycle time less than that
(although
>they may not compare phase every cycle).
>
>-----Original Message-----
>From: Guo Yawei [mailto:ywguo527@xxxxxxxxxxx]
>Sent: Wednesday, November 19, 2003 6:07 PM
>To: chris.cheng@xxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
>Subject: Re: [SI-LIST] Re: Dead Band in PLL
>
>
>Hello, Chris,
>
>About the said ~10ps static offset.
>
>I think it is due to the unbalance up and down current in the charge pump.
>Is my understanding right? If so, the static phase error depends on the
>mismatch of the up and down current in the charge pump. Of course it is
>very difficult to make the up and down current equal in practice. 10
>percent current mismatch is not uncommon. So the up and down pulse
>generated by PFD must have about 10 percent mismatch to compensate the
>mismatch current in the charge pump. If the up and down pulse generated by
>PFD have about 400ps (That is already very small. Do you think so), The
>static phase error will be about 40ps.
>
>Of course, ~40ps is still better that what clock tree can get in most
large
>ASIC.
>
>
>Best Regards,
>Yawei Guo
>
>
>
> >From: Chris Cheng <chris.cheng@xxxxxxxxxxxx>
> >Reply-To: chris.cheng@xxxxxxxxxxxx
> >To: si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: Dead Band in PLL
> >Date: Wed, 19 Nov 2003 11:48:36 -0800
> >
> >If you can tolerate a slight ~10ps static offset, the classic non-dead
>zone
> >design extend both the up and down pulse to a non-zero pulse width at
zero
> >phase error by slightly delaying the reset circuits of the PFD. A
>carefully
> >design PFD and VCO combo can optimize the natural operating point of VCO
>at
> >the desire frequency to be close to what the charge pump wants to stay
>when
> >both up and down pulse are on during zero phase error. Phase error
around
> >10ps is not uncommon. This is usually acceptable to most large ASIC
>designs
> >since the clock tree balance tolerance is worst than that anyways.
> >
> >-----Original Message-----
> >From: Parthasarathy Sampath [mailto:parthsv@xxxxxxxxx]
> >Sent: Wednesday, November 19, 2003 3:46 AM
> >To: si-list@xxxxxxxxxxxxx
> >Cc: analogcmos_vlsi@xxxxxxxxxxxxxxx
> >Subject: [SI-LIST] Dead Band in PLL
> >
> >
> >Hi All,
> > How does one reduce/avoid Dead Band in PFD. The
> >usual one is to rely on small glitch generated by Two
> >Flip-Flops (in case PFD is based on FF design). Is it
> >the standard in the industry
> > Also, if the PFD is different from FF's,say based on
> >NOR gates, how does one reduce the dead band?
> >
> >Thanks in Advance,
> >
> >Regards,
> >Partha!
> >
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