I will answer to myself, as my previous email was unclear and buggy. Actually I was just doing some order of magnitude calculations which seem quite correct, but it is always better to dig bit more in the details. The whole idea is to find the waveform paths that will generate a zero crossing the most distant from the desired zero crossing. Spectral analysis is quite powerful for understanding the DDS general performance, but I believe that following the waveform path at every time instant gives an insight into the DDS behavior that is very hard to grasp with any other means.
Going to the matter. As the 1MHz is slowly drifting the waveform will be stable for some time and not cleaned by a low pass filter. Now take the instant when the DAC is about to ouput zero but it is still generating one LSB. The error here will be 0.5LSBs In the zero crossing the sinewave slope is: delta(V)=2pi/T*delta(t) (T=1microsecond) For a 10bit dac we will have that 0.5LSB represent an error of: 2^-10=2pi/T*delta(t) --> delta(t)=2^-11/pi*T=0.15ns The AD9850 uses 14 bits to calculate the cosine function. In any case the 14bits truncation can provoke an aditional 1 bit error on the DAC output. This would mean an error on the zero crossing of 0.45ns. Peak to peak would be 0.9ns, which is smaller than 2ns pk-pk actually observed by Ulrich. So the order of magnitude is somehow correct, but there is still something (most likely a lot) missing in this analysis. Any ideas? Best regards pablo On Fri, Jan 28, 2011 at 2:47 PM, pablo alvarez < [email protected]> wrote: > Hi, > > The AD9850 has a 10bit DAC. If the AD9850 does not dither the 10bit ADC > output the zero crossings for a 1MHz signal will have an aprox resolution of > 2^-10*1us~1ns on average. If the lookup table feeding values to the dac has > 10 address lines (just guessing, I do not see any anything on it on the > datasheet), you will have an additional uncertainty of 1ns to the phase > error. Adding these two we have the ~2ns Ulrich is measuring. > > What do you think? > > Pablo > > >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
