# Phase lock loop frequency of reference signal

I am writing a Matlab code for phase locked loop and up to now I assumed that frequency of input signal is given (known) and I choose frequency of my VCO close or exactly the same as reference signal. However I am wondering what if I do not know the reference signal frequency, how can I approach the problem in that case? How can I choose the frequency of VCO?

• The whole point of the reference signal is that it is one known frequency from which to stabilize the frequency of the VCO at the desired value using the PLL. You have to assume that the reference is one value and compute VCO and PLL parameters from there. – user2943160 Jul 12 '16 at 2:28

## 1 Answer

However I am wondering what if I do not know the reference signal frequency, how can I approach the problem in that case ? How can I choose the frequency of VCO?

The VCO frequency should be equal to the desired output frequency, or cover the range of desired output frequencies. It isn't related to the reference frequency.

But to go any further, you do have to know your range of accepted reference frequencies. Because you'll need to choose a divide ratio to divide down the output frequency, and possibly the reference frequency, to obtain two equal frequencies for comparing to each other.