I want to generate a 4.25 GHz using a PLL and 50 MHz crystal/oscillator.
I am confused as to what I need to look for in a PLL. In a integer-N PLL, the prescalers are 16/17, 32/33, 64/65, etc. So, my understanding is that these PLLs take in the reference 50 MHz signal and then multiply it by the prescaler to generate 800/850 MHz, 1600/1650 MHz, 3200/3250 MHz, etc respectively. Most integer-N PLLs have a prescaler upto 64/65 only and hence I am limited to 3200/3250 MHz. So, I wont be able to generate a 4.25 GHz signal using 50 MHz crystal.
So, is my understanding correct? If not, can someone guide me as to what the mistake is.
If I am correct, do I need to use a higher frequency crystal or are there other PLLs available which can be used to generate the desired frequency using 50 MHz crystal?
Note: I have taken the data of prescalers from the Analog Devices ICs
Best Answer
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.