I'm getting a noisy FFT peak when I put a stable sine wave in. Where is this noise coming from ?

[nickev]

I'm using the Labwindows FFT function with a 14 bit 400KHz
Powerdaq card. I'm putting a ~ 70mV pk-pk 4kHz sine wave in from a very good quality signal generator - I get about
1 part in a 1000 noise on the resulting peak in the FFT. I've checked everything and can't find any noise sources getting into the DAQ card. Are FFTs inevitably this noisy or is there something else I can do to get a stable amplitude. I'm sampling at 20kHz with a 2048 point sample.

[Chris Matthews]

1 part in a 1000 is pretty low noise for an FFT with a 70mV amplitude signal. You should probably consider increasing the sampling rate. Usually 10x oversampling gives you an adequate representation of the wave. Try increasing to 40kHz. Also, what kind of shielding are you doing on cabling and connectors? It could be getting some low noise there.

Best Regards,

Chris Matthews
National Instruments