Question

Difference between vco and ring oscillator

Answer 1

Voltage Controlled Oscillator (VCO) in concept is a voltage to frequency converter. It can be as simple as an Astable multivibrator, an LC oscillator or an odd numbered (min=3) chain of inverters (ring oscillators). 
Frequency can be changed by changing the delay in the circuit and delay can be changed by changing the current. So, all you need is a circuit which can do V2I (Voltage to Current) and a circuit which can change it's delay based on the input current. 
Below is an example of a 5 stage ring oscillator based VCO whose frequency (Fout) is controlled by the input voltage (Vctrl)