Tone receiver comprising a switch capacitor zero crossing detect

Details Tone receiver comprising a switch capacitor zero crossing detect Tone receiver comprising a switch capacitor zero crossing detect

1993-03-01

1994-10-25

Dwyer, James L.

Telephonic communications

Supervisory or control line signaling

Signal receiver

307494, 307362, 307354, H03F 345, H03K 5153

Patent

active

053596528

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention relates to a tone receiver comprising a zero crossing detector which includes an amplifier circuit that has an inverting input and a non-inverting input. Such receivers are known, for example, from the article entitled "A monolithic dual tone multifrequency receiver" in IEEE Journal of Solid State Circuits, Vol. SC-14, No. 6, December 1979. These tone receivers may be used in telephone sets for detecting tone signals customary in telephony, such as DTMF signals (Dual Tone Multi Frequency).
Amplifier circuits having an inverting and a non-inverting input have the property of being capable of presenting an offset voltage i.e. it is possible that a voltage unequal to zero occurs at the output if there is a zero voltage difference between the two inputs. This offset voltage has a detrimental effect on the operation of the zero crossing detector, because the zero level of the amplifier/comparator input (that is, the input voltage which produces a zero voltage on the output) now no longer has a symmetrical position between the positive and negative input threshold voltages to be set. This may have the effect that the amplifier circuit acting as a comparator is energized by small interference signals which are smaller than the set threshold value.


SUMMARY OF THE INVENTION

It is an object of the invention to provide a tone receiver of the type mentioned in the opening paragraph, in which the offset voltage is compensated, so that the detrimental effect of the offset voltage is eliminated.
According to the invention, the tone receiver is characterized in that the zero crossing detector includes:
* an amplifier circuit used both as an amplifier and as a comparator, having an inverting input and a non-inverting input, the non-inverting input being coupled to a reference voltage source. The amplifier circuit has a feedback loop between the output and the inverting input, which includes a first on/off switch, inverting input of the amplifier, phase opposition.
In a first control phase of the two switches, the first switch is closed (i.e. conductive), and the second switch is open. Consequently, the second capacitor is not connected to the amplifier and the output of the amplifier is connected directly to the inverting input. This causes the first capacitor to be charged to the offset voltage. In the second control phase of the switches, the first switch is open and the second switch is closed. The original charge of the first capacitor is now divided between the first and the second capacitor. These phases are repeated, so that the second capacitor will accumulate charge from the first capacitor or capacitance and eventually be charged to the offset voltage. In this situation and in the event of a zero input signal on the zero crossing detector, the offset voltage will be present on both inputs i.e. the actual offset voltage on the non-inverting input and the voltage across the second capacitor having the same magnitude on the inverting input. The result of this is that with a zero voltage input signal the output voltage is also zero.
An additional advantage of said measures is that also any DC voltage component present in the input signal is blocked. Such a component may have the same disturbing effect as the offset voltage.
An embodiment of the tone receiver according to the invention is characterized in that the zero crossing detector comprises: non-inverting input of the amplifier circuit, source for producing a threshold voltage signal which consists of a fixed step of voltage level, the third capacitor and the third on/off switch and the other side of which is connected to the reference voltage source, and a fifth switch for disconnectably coupling the reference voltage source to the non-inverting input of the amplifier circuit, the fifth switch being opened and closed in phase with the fourth switch, second switch and the first switch respectively, by the control circuit.
These measures achieve that the zero crossing detector can produce a threshold voltage, so that

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P. E. Fleischer, "A Single Chip Dual-Tone and Dial-Pulse Signaling Receiver" IEEE International Solid State Circuits Conference, vol. 25, 11 Feb. 1982, pp. 212-213.
Jorg Walter and Dr. Ernst Habekotte, "Integrierbare Funktionblocke und Systeme", Elektronik, vol. 39, No. 5, 2 Mar. 1990, pp. 100-110.
"A Monolthic Dual Tone Multifrequency Receiver" In IEEE Journal Of Solid State Circuits, vol. SC-14, No. 6, Dec. 1979.

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