I am using a DDS to transmit a broadband modulated RF output signal. For this application, the spectrum of the modulated waveform must be flat, but the DDS's sinc response distorts the spectrum. How can I get around this problem?
The cure for sinc response distortion is to use a filter with an inverse sinc response characteristic.
Sinc compensation can be done in the analog domain by modifying the response of the reconstruction filter so that it corrects for the sinc response. This technique can be a challenge, however, because not many filter design tools include sinc compensation.
Fortunately, a number of ADI's DAC/DDS products include digital sinc compensation. Usually, the user can enable or disable the sinc compensation feature via a programming register. To accomplish the sinc compensation a digital filter (an inverse sinc filter) predistorts the data before it arrives at the DAC input. The result is that the DAC output spectrum appears to be sinc-free.
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