Hello!

Im trying to learn/understand how this spurs get generated. So maybe someone here can try to explain.

I have ADF4351 with PDF=32MHz (max for frac-n mode). For example at 550.006MHz i get spurs every 4kHz.

In this example i have PFD=32MHz, INT=68, FRAC=3003, MOD=4000, OutDIV=4, channel spacing is 0,002MHz.

At same frequency 550,006, if i set R=2, so PFD=16MHz, INT=137, FRAC=1003, MOD=2000, they dissapear.

Is there any formula, i can calculate how they are generated or predict where i can expect them in whole frequency range with PFD=32MHz or any info on what kind of spurs are there and how are they generated inside of the chip?

Look at attached images. Last image is example at 550.002MHz with PDF=32MHz.

Best regards,

Zoran

Hello!

I'm using my own custom made board with both diferential outputs, pulled up with 50Ω to Vcc and then connected to custom made balun from ferrite pearls and then connected to wideband amplifier. It's for learning purposes.

Yes, i've been revieving the spur mechanism on page 23, but im not sure if i understand the formula in table.

It seems this spurs are not integer boundary spurs, since with PDF=32MHz i'm far away from 32MHzx17=544MHz and 32x18=576MHz. Loop filter is at 12.5kHz.

So they must be fractional spurs that are caused by sigma-delta modulator, right? Now in my case at 550,006 MOD=4000, so looking at Table 7 in datasheet, it is divisible by 2 and not by 3. So

"repeat lenght"is 2xMOD=8000 and"spur interval"is 2kHz/2=1kHz?I'm not sure what is ment as spur interval in Table 7.

Equation in text above the table says "interval between spurs" shoud be 32MHz/8000=0.004MHz? And get them every 4kHz from carrier at 550,006(first picture).

In next example at 550,020MHz i have INT=68, FRAC=301, MOD=400, PDF=32MHz. Spurs appear every 40kHz.

Accodring to table: MOD=400 so divisible by 2, spur interval 2kHz/2.

So if i try to calculate that: 32MHz/2x400=0.040MHz. Is that correct?

Now, i'm not sure about case at 550,004MHz:

INT=68, FRAC=1501, MOD=2000 .

Here i get first spur at 16kHz and next repeat repeat every 8kHz, so next one is at 24kHz.

32MHz/2x2000=0.008MHz. Shouldn't here we see first spur at 8kHz?

Best regards

Zoran