Solution:

1) voltage controlled oscillates are an integral part of PLL.

2) $\mathrm{VCO}$ generates an o/p frequency that is directly proportional to its IP voltage.

3) Block diagram of VCO is shown in fig.

4) Typical IC SE(NE 566 VCO is shown in a fight) It provides simultaneous square and triangular wave op as a function of IP controlling voltage.

5) Frequency of oscillation is determined by external resistor $R_1$, capacitor $C_1$, and voltage $V_C$ applied at the control terminal (5), The freq n of Op waveform is approximated by,

$ f_0 \cong \frac{2\left[+V-V_c\right]}{R_1(,(+V)}\\ $