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.