AD9857: Producing a sinc wave with DDS

Question I want to buy a DDS Model AD9857. In features description is written - SIN(x)/x correction (inverse SINC function) Does that mean that I can also produce a sinc signal with this DDS? Answer No - there is a sibc filter correction for the DAC build in - please see page 22 of the datasheet INVERSE SINC FILTER The sampled carrier data stream is the input to the digital-to-analog converter (DAC) integrated onto the AD9857. The DAC output spectrum is shaped by the characteristic sin(x)/x (or SINC) envelope, due to the intrinsic zero-order hold effect associated with DAC-generated signals. Because the shape of the SINC envelope is well known, it can be compensated for. This envelope restoration function is provided by the optional inverse SINC filter preceding the DAC. This function is implemented as an FIR filter, which has a transfer function that is the exact inverse of the SINC response. When the inverse SINC filter is selected, it modifies the incoming data stream so that the desired carrier envelope, which would otherwise be shaped by the SINC envelope, is restored. However, this correction is only complete for carrier frequencies up to approximately 45% of SYSCLK. Note also that the inverse SINC filter introduces about a 3.5 dB loss at low frequencies as compared to the gain with the inverse SINC filter turned off. This is done to flatten the overall gain from dc to 45% of SYSCLK. The inverse SINC filter can be bypassed if it is not needed. If the inverse SINC filter is bypassed, its clock is stopped, thus reducing the power dissipation of the part.

Question Subject: AD9857 Digital Up Converter and its evaluation board. 1) The rising edge of the FUD signal would to transfer the profile registers content in the DDS accumulator. Why Does the FUD signal in the board change its state continously during the data transfers ? 2) Is the reset signal anabling necessary any time the frequency is changed ? Answer 1) The rising edge of the FUD signal would to transfer the profile registers content in the DDS accumulator. Why Does the FUD signal in the board change its state continously during the data transfers ? Pin 79 on the AD9857 has two functions. In single tone more, pin 79 is an input and acts as a Frequency update pin (FUD) and operates just as you describe. When not in Single-Tone Mode, this pin is an output signal that should be used as a clock to synchronize the acceptance of the 14-bit parallel data words on Pins D13–D0. When in quadrature modulation mode, the part is expecting I and Q baseband data to be clocked into the 14bit parallel data bus. 2) Is the reset signal enabling necessary any time the frequency is changed ? No, when in single tone mode, you can change the frequency tuning word at any time via the serial port and output of the DDS will be phase continuous.