Electrical transmission or interconnection systems – Nonlinear reactor systems – Parametrons
Patent
1981-05-18
1984-05-01
Heyman, John S.
Electrical transmission or interconnection systems
Nonlinear reactor systems
Parametrons
328143, 328186, 307529, 307260, 377 76, 340347DA, H03K 402, H03K 600, H03K 1302
Patent
active
044464363
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention pertains to digital-to-analog conversion circuitry and more particularly to the synthesis cf sinusoidal signals.
BACKGROUND OF THE INVENTION
In DTMF (dual tone multifrequency) telephone signaling systems specified frequency sinusoidal signals must be generated when selected touch keys of the telephone are activated. The original and still widely used approach for generating these tones has been the use of conventional LC, discrete component, oscillators. However, oscillators of this type are bulky and expensive.
Integrated circuit tone dialing equipment has been developed in response to the demand for economical and low power devices. However, integrated circuit tone dialers have suffered from a number of serious drawbacks. These include the difficulty of working at the extremely low voltages which can be supplied to telephone receivers and the relative complexity and resulting high cost of tone synthesizing circuits. Conventional tone generation circuits utilize a large number of components which increases the size of the integrated circuit. This in turn increases the cost of the circuit. Therefore there exists a need for a tone synthesizing circuit which has a minimum number of circuit elements in order to reduce the area required for the integrated circuit.
SUMMARY OF THE INVENTION
A selected embodiment of the present invention comprises a circuit for generating an analog signal. This circuit receives power from first and second power terminals. The circuit includes a multi-tap resistor which is connected between the first and second power terminals to produce a plurality of discrete voltages. A plurality of first switches are connected respectively to the taps of the resistor. Circuitry is provided which is responsive to a digital input signal for generating a plurality of first control signals each controlling a separate group of the first switches. A plurality of second switches are provided with each connected to a plurality of the first switches wherein each of the second switches is connected to no more than one of the first switches within each of the groups of first switches. Further, each first switch is connected to no more than one of the second switches. Circuitry is provided which is responsive to the digital input signal for generating a plurality of second control signals each controlling a separate group of the second switches. A plurality of third switches are provided with each connected to a plurality of the second switches and to an output terminal wherein each of the third switches is connected to no more than one of the second switches within each of the groups of second switches. Each second switch is connected to no more than one of the third switches. Circuitry is provided which is responsive to the digital input signal for generating a plurality of third control signals for controlling the third switches wherein the operation of the third switches connects the taps one at a time to the output terminal to produce an analog signal at the output terminal. The analog signal can have any desired waveform depending upon the weighting and connection of the taps to the resistor. In a telephone application for producing DTMF signaling the taps are weighted and connected such that a sinusoidal type of analog output signal is produced.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a schematic logic circuit illustrating the tone generating circuit of the present invention; and
FIG. 2 is an illustration of selected waveforms which occur in the circuit illustrated in FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
There is illustrated in FIG. 1 a tone synthesizing circuit which is designated generally by the reference numeral 10.
Circuit 10 includes input terminals 12 and 14 which receive respectively a digital clock signal and its complement. This
REFERENCES:
patent: 3626076 (1971-12-01), Uchiyama
patent: 3641442 (1972-02-01), Boucher
patent: 3657657 (1972-04-01), Jefferson
patent: 3992680 (1976-11-01), Herzel
patent: 4366470 (1982-12-01), Takanashi et al.
Heyman John S.
Mostek Corporation
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