How fast the ad9858 can respond the order ?


  AD9858 chip will take how long  to respond the order it receive and begin to generate the wave?  If I already use FPGA chip Cyclone_EP1C6T144C6 and AD9858   generate a single_tone mode wave , in there I use 80MHZ clock to deliver control word 、frequency word and phase word.when I want a 0.3us width wave ,my   solution is counter  0.3us time ,then set the control register CFR[20] and CFR[19] to logic 1,and updata this control word to DDS core .But there don't generate  the wave I expect. When I cut the deliver clock  down to 20MHZ ,the same set will be generate the expect wave .But observe the wave on oscilloscope  its quality is not good ! why cut down clock can generate? How to do this type wave?And how to improve the wave's quality?

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