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NCO simulation in MATLAB to high SFDR

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Mad I.D.

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nco sfdr

Hello.

I have a weird problem (or not?) with my NCO simulation in MATLAB (FixedPoint toolbox).
I have no need for freq. resolution and 2^9 (<1 deg angle resolution) accumulator suits my need.

So basically I don't need / have phase truncation. What bothers me is the SFDR, I don't understand why is it so good.

Look at the picture below. This is the (blackman windows) spectrum with only 10bit table precision (format 1.9).

I was expecting SFDR to be less than 1.76 + 10*6.02 cause of the periodic error problem when calling LUT table (discrete spectrum lines). Well it is >75dB ?

Is there an error in my simulation?
Thank you



BTW if you wanna see my MATLAB code no problem, just ask.
 

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