[SI-LIST] Re: PLL Loop filter -Feedback
- From: "Girish Bangalore" <bvg@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Thu, 9 Sep 2004 14:13:53 -0400
I just wanted to let you all know that I got rid of the problem completely
by mounting a thru' hole cap (I choose a Film cap to be safe) on the SMD
pads. I still haven't got hold of SMD Film caps to test. My customer is
very happy and so am I. Thank you all for your generous comments &
feedbacks.
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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>
>
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