LTC2000 output spurious


I see a spuriouse on the RF output of LTC2000Y-16 that I can't seem to get rid of.

The spuriouse that I'm having trouble with is related to the folded back 2nd harmonic. For example:

I'm generating a 933.888 MHz sine wave, using a sampling frequency of 2490.368 MHz. The 2nd order product will then be located at 1867.776 MHz and this is then folded back to 622.592 MHz. The folded 2nd overtone is attenuated by the transformers so it is lower than 70 dB relative to the sine wave. So far everything is perfectly fine. However, at exactly half the folded 2nd harmonic (622.592 / 2 = 311.296 MHz), I have a spurious that is only 63 dB lower than the sine wave.

63 dB is within my specification (> 60 dB) but I would like to have a little more margin. In the "Single-Tone Spectrum at fOUT = 990MHz, fDAC = 2.5Gsps" figure in the datasheet, this spurious is invisible.

The spurious (half the folded back 2nd harmonic) is always there. I used 933.888 MHz just as an example.

The spurioses exists (and have about the same level) both on my own board and when using Demo Circuit 2303A. I'm using the DAC's internal pattern generator to generate the sine wave. On  my own board I use both the DAC's pattern generator and our own FPGA's sine wave generator.

So my questions are:

  • Do you also see this spurious when you use your evaluation board?
    • If not, can you please send me the content of the entire register map so I can be sure that we are using the same settings. I'm currently not using your UI to control my 2303A Demo Circuit.
  • What causes this spurious and is there any way to reduce it?

Regards, Jonas