ADF4350 PLL+VCO and AD9912 DDS power spectra

Update 2015-09-28: ADEV and Phase-noise measured with a 3120A:


Here's the 1 GHz output of an ADF4350 PLL+VCO evaluation board, when used with a 25 MHz reference.

The datasheet shows a phase noise of around -100 dBc/Hz @ 1-100 kHz, so this measurement may in fact be dominated by the Rigol DSA1030A phase noise which is quoted as -88 dBc/Hz @ 10 kHz.


The 1 GHz output from the ADF4350 is used as a SYCLK input for an AD9912 DDS. The spectrum below shows a 100 MHz output signal from the DDS with either a 660 MHz or 1 GHz SYSCLK. The 660 MHz SYSCLK is from a 10 MHz reference multiplied 66x by the AD9912 on-board PLL. The 1 GHz SYSCLK is from the ADF4350, with the AD9912 PLL disabled.

The AD9912 output is clearly improved when using an external 1 GHz SYSCLK. The noise-floor drops from -80 dBm to below -90 dBm @ 250 kHz from the carrier. The spurious peaks at +/- 50 kHz disappear. However this result is still far from the datasheet result where all noise is below -95 dBm just a few kHz from the carrier. It shouldn't matter much that the datasheet shows a 200MHz output while I measured a 100 MHz output.

Again I suspect the Rigol DSA1030A's phase-noise performance of -88dBc/Hz @ 10 kHz may in fact significantly determine the shape of the peak. Maybe the real DDS output is a clean delta-peak, we just see it like this with the spectrum analyzer?


Martein/PA3AKE has similar but much nicer results over here: 1 GHz refclock and 14 MHz output from AD9910 DDS. Amazingly both these spectra show a noise-floor below -90 dBm @ 50 Hz! Maybe it's because the spectrum analyzer used (Wandel & Goltermann SNA-62) is much better?