I have designed a simple PLL with current-starved ring VCO. The minimum frequency of oscillation for this VCO is zero since there is no current fed to ring of inverters when control voltage is zero.
As described in Chapter on Digital Phase Locked Loops in Jacob Baker's CMOS Circuit Design, Layout, and Simulation (Second Edition, Page 554)
"The frequency of the square wave output of the VCO is Fcenter when Vin(= Vcenter) is VDD/2 (typically). It is important that the VCO continues to oscillate with no input data."
Why is it essential for VCO to continue oscillating when input is zero? It would not be so if a current-starved VCO is used.
Best Answer
Why is it essential for VCO to continue oscillating when input is zero?
I disagree with the statement that it is essential to keep the VCO oscillating for any input signal.
You could look at that statement in a broader sense: Is it needed to have the VCO oscillating for all possible input signals ?
A PLL is a feedback loop in which part of the signal is a frequency. What happens if that frequency becomes zero, i.e. DC ?
In my opinion this depends on the implementation of the frequency detector in the PLL. As long as that frequency detector considers DC to be a lower frequency than the reference frequency and therefore tries to set the chargepump such that the VCO will be steered to a higher frequency. Then the loop will eventually lock.
More dangerous is if the VCO stops because of a too high input signal (tuning voltage) because then the sign of the loop suddenly reverses (a slightly higher tuning voltage makes the VCO frequency go to DC). This will lock-up the loop as negative feedback (good) has changed to positive feedback (unwanted).
Note that this also might happen if the VCO produces such a high frequency that the frequency divider stops working !
To make the PLL lock quickly it is indeed more desirable to have the VCO oscillating for the full input signal range as it takes more time to start up a VCO than it takes to change its frequency. So desirable but not essential ! And depending on the phase-frequency detector of course.