If the AD9833 would be just a pure DAC (without internal clock or clock multiplication), then the output sine wave would look like a square wave for f=fclk/2, right? Or in other words the amplitude of the third harmonic would be about -10 dB. Similarly, for f=fclk/7 the amplitude of the third harmonic would be about -15 dB. But, this in in contradiction with figure 15 in the datasheet, where a 1.43 MHz sine wave with a 10 MHz clock (f=fclk/7) produces a second harmonic with an incredibly low amplitude of -60 dB. So, it seems that the AD9833 has an internal fast clock or clock multiplier in order to produce smooth sine waves, and the external clock is only for synchronization purposes? However, the datasheet doesn't say anything about an internal clock or clock multiplier... So, how is such an incredibly pure sine wave possible with f=fclk/7 or even f=fclk/2 ??