[time-nuts] need help with LPF

Joseph M Gwinn gwinn at raytheon.com
Tue Apr 13 12:27:24 EDT 2010


Said,


time-nuts-bounces at febo.com wrote on 04/12/2010 09:10:36 PM:

> From:
> 
> SAIDJACK at aol.com
> 
> To:
> 
> time-nuts at febo.com
> 
> Date:
> 
> 04/12/2010 09:13 PM
> 
> Subject:
> 
> Re: [time-nuts] need help with LPF
> 
> Sent by:
> 
> time-nuts-bounces at febo.com
> 
> 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.

At 500 to 1000 Hz, 3 g amplitude, even a stiff board will flex a lot.  The 
flexing need not be visible to be an issue.


> 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.

Ahh.

 
> 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.

Use a doorbell buzzer touching the rod?  (Be sure to shield the buzzer 
well - build it inside a closed metal box with feedthrough capacitors on 
the power leads.)

What may also work is to turn the problem inside out:  With the board on 
the shake table being vibrated, touch the suspected component with a 
handheld soft plastic rod while observing the effect on phase noise.

 
> 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...

Someone else suggested using a SSB receiver, which sounds workable.

 
> 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.

The leads can be fairly stout, so long as they can bend and relieve the 
stress when the PCB flexes.  The component can also be glued to the board 
with soft conformal coat material or a dab of silicon rubber adhesive.


Joe

 

> 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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