Voltage controlled oscillators (VCOs) are widely used in all type of communication equipments. VCOs are one of the important building blocks of Phase locked loops which are used in frequency synthesizers for the generation of high performance signals. VCO performance matrices like frequency range, tuning sensitivity or tuning gain are very important parameters as compared to output power, Phase noise, supply pushing and load pulling for the performance a PLL based frequency synthesizer and are to be carefully considered while designing a frequency synthesizer. This article presents simulation exercises carried out to evaluate impact of VCO gain and frequency range on the performance of PLL based frequency synthesizer. The article also presents different performance parameters which are to be carefully considered for designing a VCO along with a practical design simulation of an 850 MHz VCO.