The objective is to avoid adding any noise to the clock between the oscillator and PLL input pin. Below are some guidelines when co-locating the oscillator with the PLL.
1. Connect the oscillator directly to the PLL and avoid using a buffer; if a buffer must be used, select a technology that will add as little noise as possible.
2. Minimize the oscillator's output clock signal trace length, as the oscillator output is single ended and thereby vulnerable to crosstalk.
3. Provide an option for a series termination resistor at the oscillator output, in case impedance matching is required to eliminate signal over/undershoot.