[time-nuts] phase noise in digital divider, 2nd harmonic

life speed life_speed at yahoo.com
Thu Jul 28 19:39:24 EDT 2016


Been a while since I visited, I recall there are many, well, time nuts here.  I am trying to track down a source of phase noise in a frequency synthesizer design.  One part of the frequency reference upconverts a DDS and then divides it down again using a digital divider - standard technique for DDS angle modulation spurious reduction.

The DDS tunes over more than an octave, so obviously the single low pass filter isn't going to cut it.  I am noticing up to 3 dB phase noise degradation at the output of the divider as the DDS frequency decreases and the 2nd harmonic is in-band to the LPF.  I suspect this is disturbing the threshold crossing in the high speed digital logic divider, as described in "The Effect of Harmonic Distortion on Phase errors in Frequency Distribution and Synthesis" by F.L. Walls et al at NIST.

What do you think?  I should probably put in a switched filter to get my 3 dB back  ;)

- Lifespeed

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