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Fdac/8 modulation in AD9142A

Question asked by sebastiencoudreau on Jul 16, 2018


I am trying to use the NCO in the AD9142A as a Fdac/8 modulator. I use the following settings :

- Fdac = 800MHz

- Fdci = 100MHz (obtained form Fdac by a division by 8)

- Interpolation = 8

- DAC uses external clock directly (internal PLL clock multiplier disabled)

- NCO is enabled and FTW = 0x20000000


For testing purposes, DAC input is set to 0xFFFF for both I and Q data (constant). I am expecting to obtain at the output the pure modulation signal generated by the NCO which should be at Fdac/8 = 100MHz given my settings (in my understanding, FTW=0x20000000 is a pure divide-by-8 factor). Thus I am expecting a sinusoidal signal that is the exact same frequency as my DCI signal.

But what I observe is that both signals (DCI and DAC output) drift in phase : DAC output frequency is 100MHz, but its frequency seem not to be exactly the same as DCI as both signals beat.

What could I be missing, what can I check ? Is the NCO not supposed to allow perfect integer division of Fdac to obtain its modulation frequency ?


Thanks in advance for any help/suggestion.