It is well known that cyclotrons are very good mass separators, specially when the number of turns in the machine is large. This property is particularly interesting if the cyclotron unavoidably accelerates multiple species of radioactive beams simultaneously, which is the case for the cyclotron CIME at GANIL. We propose to improve the natural mass separation power by using a vertical resonance effect: it consists of putting two small electrodes between the poles, which provide a vertical electric field operating at two frequencies close to twice the RF frequency and which are tuned with respect to the vertical betatron oscillation. A prototype has been designed and built at GANIL, and tested successfully in the cyclotron CIME this September.