Many different circuits have been devised to produce FM and PM signals. There are two different types of frequency modulator circuits, direct circuits and circuits that produce FM indirectly by phase modulation techniques. Direct FM circuits make use of techniques for varying the frequency of the carrier oscillator in accordance with the modulating signal. Indirect modulators produce FM via a phase shifter after the carrier oscillator stage. Frequency demodulator or detector circuits convert the FM signal back to the original modulating signal.