I'm evaluating the ADF4350 and comparing it to the datasheet figures. I'm seeing some strange results that don't seem to match for Figure 11 and 12. I am using the ADF4350 Evaluation Board.

For Figure 11 conditions, I'm seeing many spurious tones compared to the datasheet, and for Figure 12 conditions, I see spurs well above the phase noise level. I should also say that for Figure 11, I see intermittent spurs, but I assume those are environmental. Can you review my settings and let me know what I could be doing wrong?

I have the E5052A in Trace->Spurious->dBc mode, so the spurs are in dBc, not dBc/Hz. fREF=50MHz, fPFD=25MHz, fVCO=4227MHz, fRFOUTA=2113.5MHz, loop band width = ~40kHz.

Figure 11 register settings:

00580005

009C82FC

0000000B

00009E42

000083E9

00548050

Figure 12 register settings:

00580005

009C82FC

0000000B

60009E42

000083E9

00548050

Thank you !

It looks like you are not using the optimum FRAC and MOD values.

For the frequency mentioned above, you have FRAC = 10 and MOD = 125 (10/125). The larger the MOD number, the more spurs you will see in a phase noise plot.

Instead of using 10/125, you should use FRAC = 2 and MOD = 25. This will give you less spurs.

You can find the optimum FRAC/MOD number by doing a greatest-common-denominator calculation on your FRAC/MOD numbers.

Here is the code used in our software:

private uint GCD(uint a, uint b)

{

if (a==0)

return b;

if(b==0)

return a;

if(a>b)

return GCD(a % b, b);

else

return GCD(a, b % a);

}