Abstract: A method and apparatus for generating dual tone multiple frequency signals employing a low frequency quartz oscillator, wherein a phase accumulation dual tone multiple frequency generator is comprised of a low frequency quartz oscillator. Two phase selectors are connected to produce digital phase signals in response to a signal input. Two phase accumulators, each composed of an adder and a register, are connected to accumulate the phase signals. Two ROM based sine wave tables are addressed by the outputs of the accumulators. An adder adds the outputs of the ROMs, and a D/A converter produces an analog output from the output of the adder.

Abstract: A Dual-Tone Multi-Frequency (DTMF) Generator produces a row and column tone frequency for application to the input of two source follower buffer stages. The outputs of the buffers stages are combined in summation operational amplifier to generate the DTMF Signal. There is a voltage level shift between the input and output terminal of the source followers which can be compensated with a self-compensated circuit. With a perfect match between the compensated signal and the source follower, the operational range of working voltage can be about 1.8V to 5V, and the generator can tolerate threshold voltage variation of the transistors.