We have been using AD9910 DDS from Analog Devices in many projects. In one of
the applications, we needed to use the DDS with "Inverse-Sinc enabled" so that
the output is almost flat and doesn't have the Sinc pattern. To our surprise,
we found that the flatness was varying by almost by 2 dB over 20 to 315 MHz
band (that too not monotonic) whereas the data sheet shows less than 0.1 dB
variation with this feature enabled. Could you please explain this strange
phenomemnon and we have tried this on two custom designed DDS boards and even
on the AD9910 Evaluation Board bought from Analog Devices.
Kindly note that the output is as expected when Inverse-Sinc is not enabled.
when enabling the Inverse Sinc Filter, and output signal's frequency varies
from 20MHz to 315MHz, the output amplitude's flatness will be worse than what
we gives in the datasheet, 0.1dB. Attached is his test result.
Regardless the issue is probably the frequency response
of both/either the transformer or the DAC reconstruction filter, and not the
AD9910 w/ inverse sinc enabled.
If they need that much flatness, they’ll have to compensate for those devices.