[time-nuts] need help with LPF

SAIDJACK at aol.com SAIDJACK at aol.com
Mon Apr 12 21:10:36 EDT 2010


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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