It is said that the cyclotron frequency must be equal to the voltage oscillator frequency for resonance condition. But i feel that the voltage oscillator frequency must be half the cyclotron frequency because the voltage needs to change every time the ions complete one half rotation. Please explain where am I going wrong.

The frequency of cyclotron and frequency of oscillator should same because in half the time period the charge particle complete half revolution and in this time the oscillator reverse its polarity( it wil take half the time period for a sin function to change from + to - polarity). Thats why

for the complete revolution the time taken should be equal to the time period for the complete cycle of oscillating voltage.