I am designing a digital temperature sensor using a ring VCO. The supply voltage is 1.2V and should be constant without any changes due to noise or other parameters. How can I have this constant supply voltage?

You need a regulator with zero ohm output impedance, and/or an infinite capacitance at the supply. Or at least try to achieve a performance which mimics these ideal conditions.

The current fluctuations in your circuit -- if it's a digital ring VCO, there will be current spikes for every transition -- will drop a voltage on the output impedance of your regulator. A regulator typically has a feedback loop and the regulator is effective only within the bandwidth of this loop. At frequencies above that you can combat voltage droops with a low ESR capacitor on your supply. If the cap is small, then charges/current taken from it will drop its voltage. The current from it will drop a voltage through its ESR.