The ratio detector

A discriminator with built in limiting effect is called as a ratio detector. This type of FM detector was developed by RCA and is used in high receivers and in the audio portions of TV receivers.A simple ratio detector circuit is shown in the figure given below. A transformer splits signal into 2 components.

                                                                               Figure--A ratio detector for demodulating FM.


The change in signal amplitude causes equal level changes in both the halves of circuit. These effects cancel as they are always 180 °out of phase. This nullifies amplitude variations on signal.The change in signal frequency creates a phase shift in circuit. This unbalances it, such that the outputs in the 2 halves of circuit become different, which generates an output in direct proportion to instantaneous phase shift. The output signal is a duplication of modulating waveform on FM signal.