I'm trying to multiply 21.4MHz to 1005.8MHz using an ADF4108 and a low-noise VCO, using R=1 and N=47 (P=8, B=5, A=7). It works properly if I set N=48 (B=6, A=0) and reduce my input frequency to 20.95MHz. But when I set MUXOUT to the N divider output and go back to B=5, I can see that the divide ratio doesn't change when I increase A from 6 to 7. So N remains at 46, even though 8*5+7=47.

The only oblique reference to holes in the divide ratio range is where the datasheet notes (page 9) that "A minimum divide ratio is possible for contiguous output frequencies. This minimum is determined by P, the prescaler value, and is given by (P^2-P)." But I'm not trying to generate a range of contiguous output frequencies. Or am I misunderstanding what this is trying to say?

Ah, I forgot that with conventional dual modulus prescaler, the value loaded to the B counter must always be greater than that loaded to the A counter. Thanks to MT-086.