ratio of bandwidth and reference frequency

ram77,

This device is fairly old so let me get back to you on the origins of the specific ratios.   However, violating the ratios such that the carrier is closer to the loop bandwidth (smaller ratio) will result in additional velocity ripple which could introduce errors in the position output.  This additional ripple comes from any residual carrier energy at the output of the PSD into the velocity integrator.

Hope this helps

Sean

I wonder that carrier ripple will matter because at lower angles, the magnitude of ripple will be quite lower

Ram,

In general if it is bounded the ripple may not adversely affect your system.  However, in the context of providing a guarantee that the system will perform as specified the user must adhere to the recommendations in the datasheet as that is what ADI will support based on our characterization and validation efforts. 

Remember, the loop is analog in nature and therefore there could be all sorts of issues with saturation of specific elements (integrator) or other dynamic or other non-linear effects (insufficient phase margin, etc.) that we may not be considering based on our simple discussion that could occur outside those bounds and lead to loss of accuracy or function.  For this reason I would highly recommend adhering to the document bounds.

Sean