# What's wrong in CIC filter spectrum?

1. ## What's wrong in CIC filter spectrum?

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 •

2. ## Re: What's wrong in CIC filter spectrum?

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

3. ## Re: What's wrong in CIC filter spectrum?

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. •

4. ## Re: What's wrong in CIC filter spectrum? Originally Posted by FvM
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.
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! 5. ## What's wrong in CIC filter spectrum?

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

6. ## What's wrong in CIC filter spectrum?

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. --[[ ]]--