Noisey (as in audio freq) PCB

[itdontgo]

Hi,

First post.  I have a PCB with a transistor amplifier which works in the audio to ultrasonic frequency range.  Quite often you can hear high pitch squeals coming from the board.  Unusually the input impedance is quite low ~1k and I think the noise is coming from a 100nF ceramic 0805 HPF capacitor.  Having said that touching almost any part of the board changes the sound so I can't be sure.

Any ideas how to get rid of it?

Michael

[alm]

A capacitor with a better dielectric (eg. X7R instead of Z5U) or film instead of ceramic might help. See this thread about a similar topic.

[amspire]

Michael,

You didn't say when this noise happened.

Does it happen when a particular signal is being amplified or with no signal?

[itdontgo]

It's the sound of the signal!  It's normally something like a 6-10kHz wave.  Sine-ish!

[GrumpyDave]

Are you saying that you are getting some kind of mechanical resonation from the PCB ??
I.E. Sound is comming from a component not from another source?

Or that the PCB is generating its own signal. I.E. Oscillating? If it is oscillation then pressing fingers across the track might change the frequency slighty..

[amspire]

It's the sound of the signal!  It's normally something like a 6-10kHz wave.  Sine-ish!

Then definitely look at the EEVBlog discussion that alm mentioned on the piezo effect in ceramic capacitors.

Replacing the capacitor with a different kind of capacitor will very likely cure the problem.  Even a different brand might solve it.

[itdontgo]

Yeah that's what it is.  It's hard to see which one is making the noise but it must be one the HPF/LPF caps as they must have reasonable current flowing through them (being low Z).

I think the procurement for 1000 boards has already been done but never mind I'll market it as a 'feature'!   

[ejeffrey]

Any high-k ceramic capacitor in an audio frequency signal path can cause this problem.  It is very likely that the problem is not one bad capacitor, but several.  One thing to keep in mind is that the reverse happens, too.  Vibrations will be picked up as signal by the capacitors.

[ivan747]

What is high K?
And, is low Z low impedance?

Thanks!

[slateraptor]

What is high K?

Solid-state process jargon. You might be more familiar with the symbol epsilon_r, or relative permittivity with respect to permittivity of free-space.

http://en.wikipedia.org/wiki/High-k_dielectric for more info.

And, is low Z low impedance?

Yup.