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I'm using a Xilinx FPGA (Virtex) with 4 DDS cores (each supplied a 250MHz clock) used in parallel to provide a samples to a DAC38J82IAAV from TI, 16 bit DAC running at 1 Gsps. The four cores super sample (interleave their samples) to provide samples at 1 Gsps (250MHz clk * 4 cores = 1 Gsps) for the DAC. I'm trying to output frequencies between 20MHz and 500MHz with the DAC.

The DDS cores are configured in rasterized mode with modulus of 10000. I set the Phase Increment (PINC) to be (desired_freq * 10000)/250 and the Phase offset for each core to be 0, PINC/4, PINC/2, 3*PINC/4.

This works great for generating frequencies above 250MHz, but for frequencies below 250MHz, I see harmonics at Fout, 2*Fout, 3*Fout and so on. I would expect to see harmonics at Fout, Fsample-Fout, Fsample+Fout and so on, which I do see for frequencies above 250MHz.

Why am I seeing multiple harmonics at Fout interval for frequencies below 250MHz?

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2 Answers 2

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I think that with a 250 MHz sampling clock, you should be able to generate a signal with a 125 MHz max frequency.

Also, what type of DDS do you use? Is it based on a table of sine/cosine or is it a CORDIC algorithm? It has an impact on precision.

Is the signal path from DDS to DAC ok? Do they work at the same sampling frequency? If not, a resampling module should be used or correctly configured.

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  • $\begingroup$ I'm super sampling across 4 cores running at 250Mhz to get 1GHz with a max frequency of 500MHz. $\endgroup$
    – Mark73
    Jul 13, 2020 at 13:38
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How high are your harmonics?

They are probably frequency spurs caused by the limited precision of your DDS.

https://www.analog.com/media/en/training-seminars/tutorials/MT-085.pdf

They could also be caused by the analog electronics, i.e. the DAC itself, the power supplies, etc.

Try generating a frequency that has a period not equal to a whole number. If the spurs dissapear, it's probably your DDS. IIRC, there are probably some options in the Xilinx and Altera DDS cores to attenuate the spurs.

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  • $\begingroup$ Intended Fout is 250MHz and I'll see harmonics at 0MHz, 500MHz, 750MHz. Similarly if I choose Fout of 200MHz, I see harmonics at 0MHz, 200MHz, 400MHz. Bascially just integer multiples of my intended frequency, but my power level at Fout is still the strongest. If I change Fout in 1 Hz steps I will see the power level of the harmonics decrease as I approach 250MHz (the clock rate of my individual DDS cores and the clock rate of my DAC). $\endgroup$
    – Mark73
    Oct 14, 2019 at 23:30
  • $\begingroup$ 1 - Try changing the rounding mode if possible. 2 - Try to add phase dithering (should be a core option) 3 - Change the frequency to something like 249.8 MHz if you can... $\endgroup$
    – Ben
    Oct 15, 2019 at 3:19

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