You got to put some noise in your circuit, e.g. vcc noise, or the jitter number you got from your eye diagram is only the intrinsic jitter (that is, from your contorl voltage ripple).
About the jitter frequency, you can choose, say,
1. digital system clock frequncy, e.g. 120MHz in USB2 or 125MHz in Gigabit Ethernet PHY
2. PLL peaking frequency, this jitter will be amplified.