confused by PLL figures

[seanwu]

I have just read a PLL specification. There are two figures: open loop gain and open loop phase.
I am confused by the X-coordinate. Its range is from 1e+5 to 1e+9 with unit of (rad/sec), but it runs at 400MHz~600MHz VCO frequency.
Anybody who can give me a detailed explain? Thanks in advance.

 

[flatulent]

These graphs are at a point in the loop that is low frequency. Think of the loop being broken just at the VCO frequency control input. This is the graph of the gain and phase from the control input to the dangling wire you just cut in you imagination.

 

[seanwu]

The closed loop is from Ref_Clk ---> PFD ----> LPF ---> VCO ----> DIV -----> Fbk_Clk. You mean the loop is opened befor VCO. Ref_Clk and Fbk_Clk is 75MHz~400MHz. What is the low frequency refered to ?

 

[flatulent]

A PLL is a classical feedback control system. It is best to break the loop at an analog point for visualization purposes. The frequencies at the point of the VCO frequency control pin are very low compared to the VCO frequency and extend to DC.