problem regarding decimation of samples

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shameem

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hi,
i am working on the dab receiver designing.i am having a signal containing 8.192 mega samples of 8 bit adc, but i want to use fft window size 2048. So i want to decimate the samples by a factor 4. i want to know that wheather a fir decimation filter is necessary to decimate or i can decimate the samples simply by taking 1 samples in group of 4 samples.
if anybody have an idea, please help me.
 

Hi

I can not understand why you should decimate by four. You can directly perfrom a 256-point fft on 8.192 MSPS signal.
 


It depends on tha ratio of the highest spectral component of the signal and the sampling frequency.
If this ratio is less than 1/8, then you can safely take one of four samples.
If this ratio is higher than 1/8, some form of filtering should be done before the decimation. The simplest filter would be to make an average of a group of four samples and teke this average as a new decimated sample.

Hope this helps.

Pop
 

It depends whether the sampling rate used is above 4 time the nyquist rate or below. If it is above, dropping three samples out of four, wont cause a problem.
 

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