AnsweredAssumed Answered

ADF4350 phase noise vs output divider

Question asked by evgeny79 on Oct 12, 2012
Latest reply on Oct 12, 2012 by evgeny79


I am using evaluation board EVA-ADF4350EB1Z revision B and Agilent E5052B to measure phase noise. I have found that measured phase noise is very close to phase noise presented in  ADF4350 datasheet, but only  when output divider is set to 1.  For higher division coefficients phase noise doesn't match to reported results. According to figures 7-9 in datasheet phase noise decreases with increasing output divider coefficient. In practise I cant see this effect.  For example for PFD=25Mhz, VCO=4.4GHz, N=176 phase noise versus different output divider settings are in the table below:


Output divider
output frequency MHz
phase noise at 10kHz offset dBc/Hz
phase noise at 10MHz offset dBc/Hz
14400-89  (-90)-148 (-148)
22200-94 (-96)-148 (-150)

-95 (-101)

-147 (-152)
8550-93 (-108)-144 (-154)


Number in brackets are taken from figure 9 in datasheet. 

Unfortunately I don't have result for divider 16 because output frequency 275 MHz  significantly attenuated by output matching circuit in evaluation board and signal analyser uses 3th harmonic 825MHz as a crier.


Does anybody have any ideas why  difference so obvious, especially for output divider 4 and 8.


Thank you for any help.