I'm a real amateur on PLLs but some recollection of an
earlier post by someone who seemed knowledgable
suggests that 'locking' a PLL over a 15 to 1 freq. range
for weak signals, in the presense of strong inband interfering
signals, is not in the cards.  Digital techniques have ways
of 'cheating' some of the linear theories and methods, at the
expense of predictability and other good behaviors. But
not every project is rocket science, with the high penalties
on 'occasional' ooopses.

R.M.M.