Dear Firdos, [quote userid="514977" url="~/cadence_technology_forums/f/custom-ic-design/51631/fft-of-a-signal/1383316#1383316"]Should the FFT response of output clock in both technique have some different? Ideally it should not. It should be same But in my plot frequency spurs are more spread out in 2nd case. Is it some FFT measurement issue?[/quote] I am afraid I must agree with Mr. Wiedmaan Firdos - the two spectra should not be the same as your two modulating signals differ in frequency by a factor 2.. You are describing a frequency-shift keying (FSK) modulator and comparing the DFT results for two different modulation rates. The separation of the two frequencies will be dependent on your modulation rate - which I believe is exactly one of the observations you when you stated: [quote userid="514977" url="~/cadence_technology_forums/f/custom-ic-design/51631/fft-of-a-signal/1383316#1383316"]But in my plot frequency spurs are more spread out in 2nd case. [/quote] FSK is a common modulation method used to encode digital information. You might study the spectrum of an FSK modulated signal to better understand the two spectrum you computed with the dft() function. I hope this helps Firdos, and thank you Frank!! Shawn
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