how to overcome microphonic disturbances in electronics..?

[bulerias]

does someone know,how to overcome microphonic disturbances in electronics..?for example:
an elektronic unit mounted in a closed metal box.when knoking on the box,it causes ber...,
what electronic component can be sensitive to this phenomenon..?how one can overcome this behaviour..?
pls help
bull

 

[alledauser]

Seems to me that it is related to touching the metal box rather than knocking. Does the same thing happen when you knock the box with a dielectric material?
regards

 

[flatulent]

connector problem

This looks like a connector problem. The electrical contacts are becoming open and then reconnecting with the vibration. Are there any connectors that have the signals going through them?

 

[Beetroots]

Problems can occur with some quartz crystal oscillator designs subjected to vibration (even very low levels of vibration) - especially in equipment using temperature compensated oscillators. It can also occur do to wire movement resulting in displacement current.

Although probably not a cause in your application cable runs (such as coax) subject to vibration can generate charge due to movent of the dielectric if unsuitable cables are used. Valves are also microphonic, although once again they are probably not used in your device.

 

[bulerias]

thanks all for your help..but..:

to be more specific,the box is located on the roof.any kind of nature "knocking" ,like rain,wind etc,causes the problem.
its not a mechnical problem (connector etc...)...
i also suspected on the quartz & osciltors. is there any way to protect them or the circuits from these knocking vibration...?
thanks,
bull

 

[bulerias]

also,..yes i use coax inside the box...

i use mcx coax cables (very short)..they carry if 7 rf signals...
what makes the coax cables suspecious...?
how can i protect them?
what kind of coax should i look for...?
thanks,guys..for your help..
bull

 

[flatulent]

other connectors

If it is a connector problem, it may be inside the box with the connectors going to PC cards. RF cables and RF connectors are very rigid and have large multiple contact points on the connectors. Trouble shoot by taking the lid off the box and gently hitting each internal connector. Also look for cables which are supported only at their ends by the connectors. Try moving them about.

One of my trouble shooting methods is to assume nothing. Measure the signal along the path including every exposed metal part along the path and you will blunder into the real problem.

 

[FANT]

vibrations

I survived to some similar problems.
It depends on the speed of the correction loop for the PLL; if it is fast ( mean more than 2 KHz ) the vibration is caming from the quartz oscillator ( especially low price oscillators with poor mechanical suspension system, usually you can here the crystal touching the side of the case shacking it kindly ) due to the fact the vibrations propagate through the PCB until a frequency about 800-900 Hz ( of course depends on the size of the PCB and the quantity of the components installed over )
If the PLL loop frequency is lower and especially if the coils are printed to PCB ( overall the one of the oscillator ) you will have problems with vibrations due to the fact the PCB is vibrating around the spacers.
Putting a screen over the oscillator coil makes the situation worse, due the relative moving ( different resonating frequency ) between the coil and the screen.
Better results are obtained fullfilling the cavity with a hard glue ( but no more adjustment available ).
Better is to avoid any spacer for the PCB, installing it directly on the base of the box and using a center screw on the PCB.
Try to leave some cm ( at least 2 ) between the top of the PCB and the cover of the box, to reduce the stry capacitance between coil and cover.
If the cables inside the box are not secured firmly, you will have problems due the resonance; foresee a good locking method.

Hope to be helpful

Mandi

 

[TomAss]

Me too had similar problems on a PLL synthesizer board,
and in my case were the piezolelectric behaviour of
some SMD capacitors. I really proved this phenomenon
desoldering the capacitors and fixing them on a small
piece of PCB then excitating it with a vibrating tip.
An oscilloscope connected to the capacitors under test
showed the waveform of the vibration as a few millivolts
peak to peak signal.
Since these caps was located in the loop amplifier/filter, the
disturbing signal was amplified and FM modulated the VCO
(which was suspended by means of rubber strips)
I reduced the microphonic effects of at least 20dB mounting
in place of the SMD caps some traditional caps of the same value.
Note that, in my case, the phenomenon was present only in
high valued (47-100nF) caps. Other caps ranging from 4.7 pF to
4.7nF seemed to be immune.
Lastly, remeber that also coil inductors are microphonic and, when
possible, it is convenient to block their windings with some glue or
use a piece of short circuited semirigid cable in place of them.

Hoping this could help.

 

[bulerias]

thanks all for your help.

i see i have a long way infront of me...
few plls,few quartz,few spacer & screws,few coax...
not to talk about the amount of the caps & inducators...
thanks for giving me a these clues ,ill check them...
again thanks for your help...
bull