A modulator circuit comprises an input for receiving an audio frequency (AF) signal, an oscillator circuit for providing a carrier wave signal, a mixer circuit for mixing the AF input signal with the carrier wave signal to provide an AM modulated signal, a switch having a first position in which the...http://www.google.de/patents/US5155455?utm_source=gb-gplus-sharePatent US5155455 - AM/FM modulator in which AM can be converted to FM by vector addition
