[time-nuts] need help with LPF

[Joseph M Gwinn]

Said,


time-nuts-bounces at febo.com wrote on 04/12/2010 07:42:39 PM:

> From:
> 
> SAIDJACK at aol.com
> 
> To:
> 
> time-nuts at febo.com
> 
> Date:
> 
> 04/12/2010 07:52 PM
> 
> Subject:
> 
> [time-nuts] need help with LPF
> 
> Sent by:
> 
> time-nuts-bounces at febo.com
> 
> 
> Hello guys,
> 
> need some help please:
> 
> I am looking for suggestions on a problem I have with a 20MHz LPF.  This 

> part is used to reduce harmonics when generating an nice 10MHz 
> sine-wave. Any help is appreciated.
> 
> 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