A dual-tone multifrequency generator based on switched-capacitor filtering

An integrated dual-tone multifrequency (DTMF) generator is described. The circuit is based on the generation of sinusoidal signals by filtering square waves using low-pass switched-capacitor filters. The clock and input of the filter are derived from a programmable frequency divider. A change in the output frequency of the divider will simultaneously vary the frequency of both the input and clock of the filter thus providing a tuning mechanism as well as equal attenuation of undesired harmonics. The resulting sampled sinusoidal waves at the output of the filters are then combined to form the DTMF signal. This results in a simpler logic controller and approximately 25% reduction in chip area as compared to conventional DTMF dialers. Furthermore, a distortion performance of -34 dB was measured as compared to -23 dB of conventional counterparts. The circuit was fabricated in a double poly P-well 2 /spl mu/m CMOS process.