[SI-LIST] Re: PLL Loop filter

  • From: Geoff Stokes <gstokes@xxxxxxxxx>
  • To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 10 Sep 2004 13:52:53 +0100

You're welcome Girish

Actually I made a mistake - I said ESR when I meant ESL but that may have
been evident.

Geoff

> -----Original Message-----
> From: Girish Bangalore [mailto:bvg@xxxxxxxxxxx]
> Sent: 09 September 2004 18:57
> To: Geoff Stokes
> Subject: Re: [SI-LIST] Re: PLL Loop filter
> 
> 
> Geoff,
> 
> Thank you for the suggestions.
> 
> Girish
> ----- Original Message ----- 
> From: "Geoff Stokes" <gstokes@xxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Sent: Thursday, September 09, 2004 5:24 AM
> Subject: [SI-LIST] Re: PLL Loop filter
> 
> 
> > Girish
> >
> > If you can use NPO instead of X7R, that would also help, 
> for the lower
> > values up to 1 or 2 nF (typically 50 V rating 0603 or 
> 0805), which might 
> > be
> > needed for high frequency filtering.  NPO seem to be much less 
> > microphonic,
> > or at least if the ceramic forms a  much smaller proportion 
> of the overall
> > C, the effect will be proportionately less.  Then in 
> parallel connection, 
> > to
> > avoid piezo effect the larger values can be added in 
> plastic types, which
> > are not so good at high frequency.  For the ECHU, the ESL 
> is not given but 
> > a
> > graph here (page 3) shows a tendency to earlier series 
> resonance (which is
> > above 10 MHz) than NPO, suggesting higher ESR:
> >
> > 
> http://industrial.panasonic.com/ww/i_e/26550/smd_filmcap_e/smd
> _filmcap_e/app
> > li/elback-light_eng.pdf
> >
> > But my goodness, time flies - I can't help you any more 
> just now.  I need 
> > to
> > go and attend to my customers!
> >
> > Good luck
> > Geoff
> >
> >> -----Original Message-----
> >> From: Girish Bangalore [mailto:bvg@xxxxxxxxxxx]
> >> Sent: 08 September 2004 15:51
> >> To: reanderson@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> >> Cc: billw@xxxxxxxxxxx
> >> Subject: [SI-LIST] Re: PLL Loop filter
> >>
> >>
> >> Gentlemen & Ladies:
> >>
> >> Thank you very much for your time and insightful thoughts on
> >> this topic.
> >>
> >> Ray, sorry for the lack of details in my original post. Here
> >> is the exact
> >> scenario with as much detail as possible.. please let me know
> >> if you need
> >> more clarification with specifics,
> >>
> >> 1. The ADC & PLL in question was a ADI AD9884 used to
> >> digitize RGB VGA
> >> signal from a normal PC (application is LCD controller)
> >> 2. The PLL generates a clock and is synced to the Horizontal
> >> Sync input
> >> 2. The loop filter caps were SMD X7R/0603 of 39nF and 3.9nF values
> >> 3. Any kind of vibration or even tapping on the board with
> >> the back of a
> >> screw driver would make the digitized displayed (on an 
> LCD) image to
> >> momentarily shake, more like a ghosting effect with one image
> >> static and a
> >> ghost image moving away from the static image and then coming
> >> back to merge
> >> with it
> >>
> >> We have a similar product but with  a Philips ADC/PLL combo,
> >> TDA8752, which
> >> did not exhibit this problem.
> >>
> >> I hope I have given all the details as I know it.. I would be
> >> happy to
> >> provide some more info. if you could let me know.
> >>
> >> Also, we did some experiments as follows, we lifted both the
> >> SMD caps off
> >> the board and connected them through short wires. We found
> >> that the problem
> >> was more or less gone (no jumping).
> >>
> >> I am now looking to replace the X7R caps with Film capsand 
> found that
> >> Panasonic has an ECHU series which they specifically
> >> recommend for PLL
> >> filter applications and also talk about "no shock noise"/no
> >> piezo-electric
> >> effect. Are you aware of other similar caps from other 
> manufacturers?
> >> Panasonic ECHU 39nF comes in 1206 size but I would prefer
> >> 0603 as the board
> >> is already laid out for 0603.
> >>
> >> Again, all your inputs have been tremendous. I have learnt a lot.
> >>
> >> Girish
> >> ----- Original Message ----- 
> >> From: "Ray Anderson" <reanderson@xxxxxxxxxxxxx>
> >> To: <si-list@xxxxxxxxxxxxx>
> >> Cc: <billw@xxxxxxxxxxx>
> >> Sent: Wednesday, September 08, 2004 10:16 AM
> >> Subject: [SI-LIST] Re: PLL Loop filter
> >>
> >>
> >> > Bill Wurst wrote:
> >> >
> >> >>Steve, Ray,
> >> >>Certainly if the dielectric is piezo-electric, such as 
> ceramic, that
> >> >>could be the source of the problem.  A problem could also
> >> occur if the
> >> >>plate separation is changing with time, regardless of 
> the dielectric
> >> >>material.  One way to analyze such a time-varying capacitor
> >> is to start
> >> >>with the charge on the loop filter capacitor, i.e., q = CV.
> >>  To find the
> >> >>current in the circuit, one takes the time derivative of
> >> each side of
> >> >>the equation.  Normally, the capacitance is considered time
> >> invariant
> >> >>and we write i = C*dV/dt.  However, if C is not constant,
> >> in this case
> >> >>due to mechanical vibration, then the current must be
> >> expressed as the
> >> >>sum of the partial derivatives.  Makes for an interesting
> >> analysis.  Way
> >> >>back in the early days of electronic ignition, I designed a
> >> capacitive
> >> >>pickup to take advantage of this phenomena.  It worked to a
> >> degree but
> >> >>harmonic suppression proved difficult as I recall.  So
> >> proper material
> >> >>selection as well as mechanical damping may be required.
> >> >>    -Bill
> >> >>==============
> >> >>
> >> >>
> >> > Bill-
> >> >
> >> > Thanks for sharing your thoughts and experience on the topic. You
> >> > provide a good description of the mechanism that allows a
> >> time varying
> >> > capacitance to generate a changing voltage. (This effect is
> >> utilized in
> >> > capacitance microphones where the microphone's diaphram is
> >> one plate of
> >> > a capacitor. A DC bias is applied to the microphone's
> >> 'capacitor'. When
> >> > sound waves impinge on the diaphram and cause it to move an audio
> >> > voltage results as you've just described)
> >> >
> >> > The original poster described a problem where the timing 
> of a video
> >> > signal varied in response to mechanical vibration. _If_
> >> there were some
> >> > way for the plates in a ceramic capacitor to vary their
> >> physical spacing
> >> > in response to mechanical vibration I could see the time varying
> >> > capacitance causing a noise voltage. However, if you consider the
> >> > construction of a typical ceramic chip capacitor it is
> >> rather difficult
> >> > to visualize the plate to plate spacing changing due to
> >> vibration. The
> >> > whole part is rather monolithic in construction.  A stack
> >> of metallic
> >> > plates that are insulated from each other by thin layers 
> of ceramic
> >> > material and then encased in ceramic material doesn't seem
> >> to provide
> >> > much opportunity for mechanical movement.
> >> >
> >> > Stress and strain coupled from the PCB to the part can excite a
> >> > piezeoelectric effect, but  that doesn't cause the plate to plate
> >> > separation to vary. Perhaps if the part were stressed to
> >> the breaking
> >> > point there could be some internal movement in the part as it was
> >> > mechanically breaking, but that doesn't seem to be the case here.
> >> >
> >> > I guess the this whole discussion is predicated on several
> >> assumptions
> >> > of what was going on in the construction of the circuit
> >> that was alluded
> >> > to by the original posted as precious few details were given.
> >> > If anyone can relate further details on how a ceramic chip
> >> capacitor can
> >> > be induced to exhibit time varying capacitance due to mechanical
> >> > vibration coupled from a PCB I would be interested in
> >> learning. I agree
> >> > that if relative movement of the plates were to occur then
> >> a capacitance
> >> > change would occur which could induce a noise voltage, but
> >> it just seems
> >> > highly unlikely to me that the plate spacing would change due to
> >> > vibration.
> >> >
> >> > -Ray Anderson
> >> > 
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