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ADF4350 Frequency error due to finite resolution

Question asked by mukuru on Sep 26, 2013
Latest reply on Oct 4, 2013 by rbrennan

Hi all,

I have an ADF4350 setup to generate 3177.216MHz and 1588.608MHz derived from a 26MHz reference clock and would like to work out the frequency error down to a fraction of a Hz. Assume we have a perfect 26MHz reference clock.


Using the following settings:

input ref clock - 26MHz

pfd freq = 2.6MHz


Prescaler: 8/9

N counter: 1222

frac: 4

Modulus: 650

R counter: 10

doubler : 0

BS counter: 21

RF divider: 2


Given these settings how can I calculate back the actual frequency of the VCO output? The frac-n synth has only a finite resolution so the VCO will not be exactly 3177.216MHz. 12bits for the fractional and 12bit modulus. If I plug this in the formula from the datasheet I will get back exactly the frequency I want. The formula though uses floating point numbers though so you won't see the rounding errors.

target RF output freq: 1588608000.000 Hz

Aux output freq: 3177216000.000 Hz

calculated Rf freq: 1588608000.000 Hz


Many thanks