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We are using a current output DAC to perform waveform generation in the band of $20\textrm{ MHz}$ to $500\textrm{ MHz}$ and sampling at $1.2\textrm{ GHz}$. SFDR and harmonics suppression requirement for our application is $60\textrm{ dBc}$. During initial testing for the above mentioned band we are able to achieve the specifications of SFDR and harmonics suppression, until outputs were generated in following manner,

  • For $200.3456\textrm{ MHz}$ - SFDR and harmonic suppression is $>60\textrm{ dBc}$.

  • For $200.001\textrm{ MHz}$ - suppression is below $50\textrm{ dB}$.

Could anyone differentiate between the above the outputs generated?

Also these spurious levels are high at only at specific points such as: $40.001, \ 80.001, 120.001,\ 160.001,\ 200.001,\ 240.001,\ 280.000,\ldots, 440.001,\text{and}\ 480.001 \textrm{ MHz}$.

  • Spurious at $200.001\textrm{ MHz}$

    Spurious at 200.001 MHz.

  • Spurious at $240.001\textrm{ MHz}$

    Spurious at 240.001 MHz

So anyone could suggest a requirements in DDS for clean spectrum at DAC output?

DDS in implemented in a FPGA (Kintex 7).My sampling frequency is 1.2GHz and it is anything to do with decimal frequency outputs such as 200.001?

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  • $\begingroup$ This question may be impossible to answer (except by the ADC's manufacturer), but in any case you should provide more details about your device and how you're generating the digital samples. $\endgroup$ – MBaz Nov 9 '16 at 4:23
  • $\begingroup$ Part used is DAC38J82IAAV from TI is 16 bit DAC and maximum sampling rate of 2.5 GHz and four parallel DDS logic is implemented at 300 MHz sampling rate to meet sampling frequency of 1200 MHz. $\endgroup$ – BALAJI YOKI Nov 9 '16 at 9:46

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