Dual tone and modem frequency generator with on-chip filters and voltage reference

DTMF Generator based on Interleaved Oversampling

“…Designing a DTMF generator in 1989 may seem like reinventing the wheel, but the market for telephone dialler chips is still sufficiently large that, finding new solutions to this problem with smaller silicon area and better performance is still of great interest Optimized frequency synthesizers are also of interest because of their wide area of application in modems, telephone ringing , clock generators etc Previous solutions for the DTMF generator use either switched capacitor [2] or resistor string synthesizers [3] with one synthesizer per frequency group These synthesizers use oversampling ratios in the range from 8 to 16, resulting in mirror frequency components relatively near the base band This requires the use of precision smoothing filters to remove the unwanted frequencies Conventionally these filters are constructed using a switched capacitor filter, followed by an RC smoothing filter The DTMF generator proposed here uses onl> one synthesizer in a interleaved mode to generate both frequencies The synthesizer is based on direct digital synthesis [5,6] An oversampling rate of approximately 1000 is used This prevents the problem of mirror frequency components near the base band and permits the use of a simple first order sigma-delta digital to analogue converter 2. Principle of Operation.…”