Hi, I am currently looking to develop my own GUI to calculate the required register values in order to output a certain frequency (as per my previous post). At the minute I'm using the ADF4350 GUI due to not having any specific 4351 software (from the datasheet it looks like the calcs are identical).

The datasheet specifies that the MOD register equals the reference input frequency / channel space resolution @ VCO...

so my current example would be...

ref_in = 24MHz

doubler bit and R div are disabled so pfd_freq=24MHz

channel spacing (output) = 1MHz

output divider = 4

therefore VCO spacing = 4MHz

24MHZ/4MHZ = MOD of 6...However, the software is giving a MOD of 3. In some instances the MOD calculated is correct so I presume there is a factor I'm missing here. The output frequency was 700MHz in this example.

By default, the software is designed to automatically use the greatest common denominator (GCD) for FRAC and MOD values. For example, if for a certain frequency the calculated FRAC/MOD = 500/1000, the software will change the values to FRAC/MOD = 1/2. Lower values of MOD result in less fractional spurs.

I'm not sure which version of software you are using, but in many of our PLL software applications, you can disable the GCD calculation by unchecking the option in the Tools menu (see image).