Said,
[email protected] wrote on 04/12/2010 09:10:36 PM: > From: > > [email protected] > > To: > > [email protected] > > Date: > > 04/12/2010 09:13 PM > > Subject: > > Re: [time-nuts] need help with LPF > > Sent by: > > [email protected] > > 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, > [email protected] 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 > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/ > listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
