[SI-LIST] Re: PLL Loop filter
- From: Bill Wurst <billw@xxxxxxxxxxx>
- To: weirsp@xxxxxxxxxx
- Date: Tue, 07 Sep 2004 22:16:41 -0400
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
==============
steve weir wrote:
>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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>
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