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What's wrong in CIC filter spectrum?

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kuohsi

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I have designed sigma-delta followed by CIC decimator ,
and run FFT at CIC filter output.
The result is shown in attachment file.
what's wrong in the spectrum??
thank you!
-------------------------
spec:
fin=1.5625 Hz
fs=51200 Hz
decimator ration=256
 

No one knows the like "tone" around the notch frequency??
Thank you!
 

I don't understand, if this is a serious question. The output sampling rate is 200 Hz. How can you calculate spectral components above 200 Hz? The FFT is supposed to be cyclic with 200 Hz, this is simply the image of the low frequency output. Due to the logarithmic scale (not appropriate to FTT, actually), it looks somewhat distorted.
 

FvM said:
I don't understand, if this is a serious question. The output sampling rate is 200 Hz. How can you calculate spectral components above 200 Hz? The FFT is supposed to be cyclic with 200 Hz, this is simply the image of the low frequency output. Due to the logarithmic scale (not appropriate to FTT, actually), it looks somewhat distorted.
Click to expand...

Does the question be caused by "sampling theory"??
If it is caused by sampling theory,
how can I measure the SNR at digital output?
Thank you!
 

It doesn't like fft nor the image of the low frequency output~~~but i don't know the answer
 

I don't see a problem at all.

The CIC filter has an output sampling rate of 200 Hz. Thus the meaningful spectral range in SNR measurement is 0 to 100 Hz.
Above 100 Hz, you're simply displaying a non-existing signal in your above spectrum chart.
 

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