What exactly are you trying to do with it? Not knowing the specific problem, there are basically 2 options:
(1) create 75MHz as the master reference, and derive 15 MHz from it.
(2) create 15MHz as the master reference, and derive 75 MHz from it.
(1) is fairly easy - frequency division by an integer number is always easier than frequency multiplication, and can be done by purely digital circuits. It will also run from DC to some maximum frequency. Easiest if the ratio is a power of 2 (e.g. 2, 4, 8, etc.) but your divide-by-5 is still not too bad. You simply need a binary counter with a reset input, and add feedback that resets the counter whenever it reaches 4 or 5 (depends on the exact architecture, if the reset is synchronous or asynchronous). In most cases the reset circuit is a simple AND gate with 2 or more inputs. Fast CMOS gates (74HCxx series) may be up to the task at 75 MHz - need to check the specific data sheets. The oscillator does not need to be highly stable in this case.
(2) needs a phase locked loop (PLL) consisting of the same divider, a voltage controlled oscillator (VCO), a charge pump, and some minor components. Most of it is analog and thus much more complicated! Also any specific design only works over a limited frequency range.
I highly recommend Horowitz "The Art of Electronics" which is ECELLENT reading on all above subjects, and written in an easy-to-understand manner.
As for oscillators, dividers (only power-of-2) and charge pumps, a good source is www.minicircuits.com - they have all those components as integrated modules both for solder-on and with coaxial connectors; prices for the solder-on parts are low and they do small quantity orders. There are also quite a few ICs out that have some or all components of a PLL integrated.
Wolfgang
