Electronic – How to generate a high-frequency clock

So a PLL uses 2 oscillators. One, the reference, is generally lower frequency, and the other needs to be able to operate at the output frequency. In this case, you'll need a 4.25 GHz VCO as well. Using a PLL IC, the 50 MHz Crystal, and the 4.25 GHz VCO, you'll be able to do what you've described you need. You should download Analog Devices SimPLL application, that will walk you through the calculations. There's no frequncy multiplication taking place in a PLL, only frequency division

National Semiconductor has an Excellent application Note http://www.national.com/en/clock_timing/pll_designbook.html that covers the details of PLLs.

The Prescaler details (64/65) for example are called "Dual Modulus" prescalers, and they work in conjunction with another divider to provide the correct divide ratio.

What gave you the idea that Si530 is a VCXO? It says in the datasheet:

The Si530/531 IC based XO is factory configurable for a wide variety of user specifications including frequency, supply voltage, output format, and temperature stability. Specific configurations are factory programmed at time of shipment, thereby eliminating long lead times associated with custom oscillators.

Maybe you should be looking at this page: http://www.silabs.com/VCXO