Noise at zero frequency

[imperial-aero]

Hi,

I have a VI that reads in readings from 8 microphones. Even with no input signal, my FFT waveform shows a peak at zero frequency (with an amplitude of 0.008) which I'm assuming is the DC component. On my waveform, without any connections made to my BNC-2090 terminal, I can still see this component.

The problem is when I connect my microphones, the magnitude of this zero frequency signal is sometimes higher than my microphone readings and I get incorrect results in my calculations

Does anyone know why I'm getting this signal at zero frequency, is it the DC component? And also how I could get rid of it.

Thanks.

[Ian Colman]

Hi,

Yes this is the DC component of your signal being inputted. This is probably caused by the biasing of your microhone, what type of microphone is it (eg. electret) and does it need an excitation voltage? Also what card do you have conencted to your BNC-2090 terminal block, are you using differential analogue input?

Best Regards,

Ian C
Applications Engineer
National Instruments UK & Ireland

[imperial-aero]

Hi,

Thank you for the reply. My microphones are just simple tie-clip microphones that uses button-type batteries (I think it's 1.5V). I'm using a PCI-6229 card and I have to use single-ended input as I need to use all 16 channels. I guess I won't be able to get rid of this 0 Hz signal, so I'm just neglecting the first few frequency bins when I do my FFT calculations and it seems to work.

I've also had some other problems with noise from my BNC-2090A terminal block. Here's a link to it. I'd be very grateful if you could have a look at it and give me a suggetion on how to remove the noise.

http://forums.ni.com/ni/board/message?board.id=250&message.id=38253#M38253

[Ian Colman]

Hi,

Noise experienced when using BNC is not overly common but is something that we have noticed. There is a Knowledge Base artical on ni.com which has been written to help solve this issue so if you could have a look through that, try the suggestions, and let me know how you get on with it that would be great.

http://digital.ni.com/public.nsf/allkb/CE719E5A9409ADCC8625714100033433?OpenDocument

Best Regards,

Ian