[SI-LIST] Re: PLL Loop filter
- From: Ray Anderson <reanderson@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 08 Sep 2004 07:16:46 -0700
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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