With diode D2 reversed , O is positive with respect to b, so that Vab is a sum voltage rather than the difference it was in the discriminator it is now possible to connect a large capacitor between a an b to keep this voltage constant . Once C5 is connected Vab is longer the output voltage thus the output voltage is now taken between O and O’
The above equation shows that the ratio dector output voltage is equal to half the difference between the output voltage from the individual diode. It behaves identically to the discriminator for input frequency changes. The ratio detector itself consists of limiter stage due to large capacitor C5 .Hence, limiter stage is not used in ratio detector.