[SI-LIST] Re: PLL Loop filter
- From: Ray Anderson <reanderson@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 07 Sep 2004 10:21:13 -0700
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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