[SI-LIST] Re: Questions on PLL response
- From: steve weir <weirsp@xxxxxxxxxx>
- To: Michael_Poimboeuf@xxxxxxxxxxxxxx, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 05 Feb 2004 18:25:01 -0800
Michael, just one other item to toss in is that most people look at the
frequency response, but for jitter sensitive applications, the phase
response matters quite a bit. The effective time constant of the phase
response varies closely as 1/D^2.
Regards,
Steve.
At 06:11 PM 2/5/2004 -0800, Michael Poimboeuf wrote:
>Geoff references F.M. Gardner "Phaselock techniques," which does express
>PLLs using classic control theory as do most references including Best
>"Phase Locked Loops" and others. While it's important to understand the
>classic second order treatment given in Gardner's and Best's works, I've
>found the approach used by Wolaver "Phase-Locked Loop Circuit Design" to
>be more practical when designing PLLs such as the type Chris describes,
>i.e. ones that have good jitter attenuation characteristics. I think
>this gets to the root of what you are wondering about Partha, i.e. why
>not make the PLL overdamped?
>
>Wolaver points out that for heavily overdamped systems where the 2nd
>pole frequency is much less than the loop bandwith (or zeta is much less
>than 0.5) the response of the PLL (peaking frequency and peaking
>magnitude) "hardly depends" on the classic second order natural
>frequency, but is the geometric mean of that 2nd pole and the loop
>bandwidth.
>
>See the text for more details and background, but in a nutshell, the
>numbers I find effective in almost all cases are those given in
>Wolaver's table 3-1 "Peaking Parameter Approximations" and specifically
>the equation for peaking frequency in the overdamped case which is
>equivalent to the total PLL bandwidth wp=3D1.2((w2)2^3/4)((K)^1/4).
>
>Also, don't confuse "overdamped" with "low bandwidth." One can have an
>underdamped PLL with low bandwidth or an overdamped PLL with wide
>bandwidth, although generally I like to keep my damping (defined in
>Wolaver as w2/K to less than 0.25).
>
>See Wolaver if you have followup questions.
>
>--
>mkp@xxxxxxxxxxxxxx
>
>-----Original Message-----
>From: Chris Cheng [mailto:Chris.Cheng@xxxxxxxxxxxx]=20
>Sent: Thursday, February 05, 2004 1:27 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Questions on PLL response
>
>
>There are a few more considerations :
>a) If you believe you have a highly jittered input source, I would agree
>that you are better off with a critically damped loop filter to give the
>PLL the fastest response time to reject the jitter. I still won't go
>underdamp since it usually results in larger jitter at the output.
>b) If you believe you have a noisy power source (especially with low
>frequency components), it is advantageous to overdamp the PLL so that
>the jitter induced by the power noise can be rejected at a lower
>frequency. You will need to have external power filters below that
>frequency. That's usually consists of those funny ferrites and caps
>combo in these PLL app notes you see in oh so many highspeed VDDA pins.
>
>In all of my PLL designs, I always have more problem in b) so I always
>overdamp the loop filter.
>
>Classic PLL loop dynamics depends on the PFD/VCO gain and the 2nd order
>filter. But I would caution many new designs does not falls into this
>simple analysis.
>
>-----Original Message-----
>From: Geoff Stokes [mailto:gstokes@xxxxxxxxx]
>Sent: Thursday, February 05, 2004 3:41 AM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Questions on PLL response
>
>
>Dear Partha
>
>Answering question 2 first, the PLL bandwidth depends on the loop gain
>parameters and their base band frequency responses. It's all described
>in F.M. Gardner "Phaselock techniques".
>
>The optimal practicable damping factor will give the fastest response
>time with minimal overshoot and ideally no ringing, so that the settling
>time window is minimised for which you can expect phase disturbance and
>any associated timing uncertainty. For a second order loop, a damping
>factor zeta=3D1 gives the fastest settling time with no overshoot.
>Zeta=3D0.707 gives the fastest settling time with a single overshoot, =
>and
>slightly faster than for zeta=3D1. In addition, the 0.707 value gives a
>second order Butterworth type filter response to any input phase
>disturbances. If the response is underdamped, that settling time window
>is lengthened. If the response is overdamped, the settling time is also
>lengthened, but also the stop-band phase-filter rejection is reduced.
>In practice, because of VCO characteristic non-linearity and other
>uncertainties, the damping factor cannot be controlled precisely. If
>you can get the value between about 0.5 and 2 you're doing quite well.
>I would aim for a value of around 1 because you have less margin for
>error on the low side.
>
>Cheers
>Geoff
> > -----Original Message-----
> > From: Parthasarathy Sampath [mailto:parthsv@xxxxxxxxx]
> > Sent: 05 February 2004 10:17
> > To: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Questions on PLL response
> >=20
> >=20
> > Hi All,
> > 1. The PLL response time depends on 'damping
> > factor'. The common value of it used is '0.707', which
> > makes the system underdamped since for damping factor
> > < 1, the system is underdamped.=20
> > My question is why not make the system overdamped
> > since the response will be smooth and straight?
> >=20
> > 2. What are the parameters on which PLL bandwidth
> > depend?
> >=20
> > Thanks in Advance,
> >=20
> > Regards,
> > Partha!
> >=20
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