FreeLists / si-list / [SI-LIST] Re: PLL Loop filter

[Ray Anderson <reanderson@xxxxxxxxxxxxx>]

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
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu