[time-nuts] need help with LPF

[Bob Camp]

Hi

NPO dielectric caps are not inherently microphonic. They only pick up audio / vibration if they are cracked or are placed under a lot of strain. 

At the roughly a micro henry range, a powdered iron core coil is the  thing to use I would try for a single layer coil.  Powdered iron is not microphonic. Windings can be if they are loose. Coils have the same issue as caps if they are cracked. Since they have a field, stuff moving close to them can be a very real issue. 

If you have a question about a part the normal thing to do is to first swap it out and next to burry it in potting compound. Hot wax is the old school thing to use. It doesn't take much to stop a part from wobbling around. If it's a field issue, copper foil and wax is the traditional debug approach. 

You mention three caps and one coil. Is one cap across the coil? 

How wide is the resonance? 

Lots of fun.

Bob






On Apr 12, 2010, at 9:10 PM, SAIDJACK at aol.com wrote:

> Hi Joe/Bob,
> 
> the board is mounted extremely rigidly into the enclosure, so I don't think 
> it's flexing. But even if there were flexing I wonder what components 
> would give  the best results. I am also sure the caps are COG as specified, this 
> is from a  small proto-run. Low values anyway (47pF etc).
> 
> The vibration amplitude is less than 3g rms. The noise increases from less  
> than -145 at 1KHz under vibration to more than -120dBc due to the  
> resonance.
> 
> If I bypass the passive filter the problem goes away. The passive filter is 
> very simple, three COG caps, one inductor. This is why I suspect the  
> inductor.
> 
> Testing this at much less than -120dBc at 1KHz is not easy, since I need a  
> phase noise analyzer that takes more than 30 seconds to give results for a  
> measurement. So tapping with a hammer etc won't work.
> 
> But I will try using my 8561E spectrum analyzer, maybe that will pick up  
> the tapping noise?! I think we are close to the noise floor of that SA.
> 
> Maybe I could use a mixer and feed the audio output into an audio amp to  
> see if I can hear the tapping noise. But at -120dBc I need massive 
> amplification  to hear anything...
> 
> I like the idea of using leaded parts, since the leads will act as dampers. 
> Now if those parts will stay on the PCB during shock testing..
> 
> bye,
> Said
> 
> 
> 
> 
> In a message dated 4/12/2010 17:14:22 Pacific Daylight Time,  
> gwinn at raytheon.com writes:
> 
>> We  are using a ceramic 560nH 0603 inductor, and believe that 
>> this part  is picking up lots of noise around 500Hz to 1KHz when on the 
>> vibration table.  Makes the Phase Noise of the Oscillator more 
>> than 20dB higher than without it!
> 
> What is the vibration  amplitude?
> 
> I would guess that the inductor has a ferrite core, and that  the ferrite 
> is magnetostrictive.
> 
> High-K ceramic bypass capacitors  are usually piezoelectric.
> 
> SMD components pick up more strain from  board flexing than other kinds of 
> component.  Something with flexible  leads may be less sensitive to 
> vibration.
> 
> If you unsolder and  remove the inductor, what happens?
> 
> More generally, why are you  suspicious of this inductor, versus 50 other 
> components?
> 
> 
>> The caps used in the filter are small COG types, so I don't 
>> think  they are the ones causing the microphonic sensitivity.
> 
> I would try  tapping the components with a plastic stick hit by a small 
> hammer while  watching.
> 
> 
>> I am thinking that a wire-wound inductor fully  encased in epoxy
>> would work better.
> 
> Only if well shielded, to  prevent EMI problems.
> 
> It may work better simply because the leads are  more flexible.
> 
> 
> Joe
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