For example, crystal-controlled transmission systems can be expected to attain frequency offsets as low as a few hundred parts per million. This sort of specification is measured with a frequency counter, averaging all clock pulses over a period of perhaps many seconds. Clock wander refers to the tendency of a clock reference to exhibit short-term frequency variations. A PLL is designed to track the short-term wander, provided that it does not slew too fast or wander too far afield. The permitted amount of wander, the rate of which a signal may wander up and down across the permitted frequency rate, and the slew rate of the wander are often key components of a good wander specification.
Jitter refers to the fastest variations in clock frequency.
It comprises those variations too fast to expect the PLL to track, and therefore jitter always directly affects the accuracy of the timing relation between the reference clock and the local oscillator. In a data communications application, excessive jitter causes bit errors. These new clock generators are flexible, fast, and packed with features. In a typical clock multiplier application, the reference clock is often sourced at about 10 MHz from a traditional crystal oscillator. 10 MHz is a very comfortable range for crystals, and it's a good bet you already have one in your system.