An oversampling-based DTMF generator

A circuit is described which uses one oversampling frequency synthesizer in an interleaved mode to generate dual-tone multifrequency (DTMF) telephone dialing tones. The synthesizer is based on direct digital synthesis using phase accumulation. The digital-to-analog conversion is carried out using a first-order sigma-delta converter. This sigma-delta converter requires only a simple second-order RC filter, eliminating the need for a switched capacitor filter. Measurements on the circuit confirm that all specifications of the DTMF are fulfilled.<
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