Ali1990
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Thanks for your reply Sheldon.
In the spectrums, I have used just Hanning windowing. Still I didn’t find why we have higher noise floor for signals with higher frequencies which is generated by ideal sinusoidal source and how we can avoid this issue. To clarify my question, I have attached another photo. The two spectrums in the attached photo is for 2 different frequencies generated by ideal sinusoidal source.
Sampling frequency is 9.7090370370370MHz (this is the sampling frequency of my CT delta sigma modulator) and the strobe period for transient simulation is equal to sampling time (I activated the conservative mode for simulation as well). Nfft is 32768 and start and stop times in spectrum measurement is defined based on frequency resolution of fft (Tstop-Tstart=Nfft/Fs=3.375ms).
In the attached photo, the red spectrum is for the case that the input frequency is at (13/32768)*Fs and the yellow one is for fin=(3375/32768)*Fs. As you can see, the noise floor is higher for higher input frequency. So, I wanted to know the reason for higher noise floor at fin=(3375/32768)*Fs, and possible solution to reduce noise floor. Still the noise floor is very low enough and can be considered ideal, but when I apply this source to an amplifier, implemented by transistors, the noise floor becomes around -160dB. Since I have to down convert the amplified signal and then apply it to a sigma delta modulator, I have a concern that I will not be sure that I’m calculating SNR in a correct way or not….
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