[SI-LIST] Re: PLL Loop filter
- From: steve weir <weirsp@xxxxxxxxxx>
- To: reanderson@xxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Tue, 07 Sep 2004 10:32:19 -0700
Ray, either way it is effectively microphonic. Under vibration, and
depending on the board location and mounting scheme the device will
generate a piezo-voltage. Stress relief can largely prevent this and make
a ceramic useful under all but very demanding conditions of combined high
vibration and very high spectral purity.
Steve.
At 10:21 AM 9/7/2004 -0700, Ray Anderson wrote:
>David Greig wrote:
>
> >Hi Girish
> >
> >I would hazard a guess that the filter cap is ceramic, and another guess
> >that it is surface mounted in a mechanically unsupported area of the board.
> >
> >Most ceramics exhibit a piezoelectric effect. The value will also alter on
> >board deflection, in the order of 0.1% to 0.5% of value in the extreme
> >(before popping off!).
> >
> >Film caps do not have a piezoelectric effect.
> >
> >
> >Best Regards
> >
> >David Greig
> >
> >
>.I concur with David on this. In previous work on satcom synthesizers,
>the inclusion of ceramic capacitors in the loop filter was the kiss of
>death with regards to vibration.
>
>The original poster theorized that the capacitance might be changing
>with vibration. I believe that the piezoelectric voltage generated by
>the ceramic dielectric material is injecting an undesired noise signal
>into the loop which is causing the difficulties.
>
>Note that most ceramic capacitors use (or at least used to) some varient
>of barium titanate as the dielectric material. This is the same material
>that is used to fab sonar transducers because it exhibits piezoelectric
>properties.
>
>-Ray
>
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