Ring modulator

A ring modulator is an electronic device for ring modulation, used for amplitude modulation or frequency mixing.

The ring modulator includes an input stage, a ring of four diodes excited by a carrier signal, and an output stage. The input and output stages typically include transformers with centre-taps towards the diode ring. It is important to note that while the diode ring has some similarities to a bridge rectifier the diodes in a ring modulator all point in the same clockwise or counter-clockwise direction.

A particular elegance of the ring modulator is that it is bidirectional: the signal flow can be reversed allowing the same circuit with the same carrier to be used either as a modulator or demodulator, for example in low-cost radio transceivers.

The carrier, which alternates between positive and negative current, at any given time, makes one pair of diodes conduct, and reverse-biases the other pair. The conducting pair carry the signal from the left transformer secondary to the primary of the transformer at the right. If the left carrier terminal is positive, the top and bottom diodes conduct. If that terminal is negative, then the "side" diodes conduct, creating a polarity inversion between the transformers.

Ring modulators frequency mix or heterodyne two waveforms, and output the sum and difference of the frequencies present in each waveform. Multiplication in the time domain is the dual of convolution in the frequency domain, so the output waveform contains the sum and difference of the input spectral components. For the basic case where two sine waves of frequencies f_carrier and f_voice are multiplied, two new sine waves are created, one at f_carrier + f_voice and the other at f_carrier - f_voice.

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