GB1411418A - Signal generators - Google Patents

Abstract

1411418 Voice frequency signal generator; oscillators WESTERN ELECTRIC CO Inc 6 Sept 1972 [7 Sept 1971] 41248/72 Headings H4K and H3T A signal generator includes a frequency determining circuit which comprises a plurality of resistive elements and a plurality of frequency-selecting switches which are arranged in a ladder network such that when any one of said switches is actuated at least two of the resistive elements are operatively connected in series and when any two of said switches are actuated at least one of the resistive elements is operatively connected in series with two of the resistive elements connected in parallel. Such an arrangement is used in one of the T notch filter feedback networks of the signal generator disclosed in U.S. Specification 3,424,870 and two such signal generators are employed in a subscriber's sub-set of the pushbutton, two tone voice frequency signalling type. The arrangement allows 49 different two tone signals to be supplied at the output R, T for signalling to an exchange, 16 produced by the selection of single push-buttons and 33 produced by the selection of pairs of adjacent pushbuttons (horizontally, vertically or diagonally adjacent). As is well known each signal generator produces frequency tones in a frequency band which differs from that of the other and apart from component values the circuitry of the two signal generators is the same. Each signal generator comprises a pair of interconnected transistor oscillators each having a twin- T notch filter feedback network, and as shown the signal generator coupled to the row of buttons is diagrammatically represented as an amplifier 501 having two feedback T notch filters, one comprising resistor R3 and capacitors C1, C2 and the other comprising capacitor C3, resistor R2 and resistor network RN1. The arrangement of the resistors in the ladder network RN1 is such that if any one button is depressed, one of four principal frequencies is produced, and if two adjacent buttons are depressed which are not in the same row, a frequency is produced which is mid-way between two of the principal frequencies. Hence each signal generator can produce seven different frequencies, giving a total of 49 two tone frequency possibilities. For details of the circuitry of amplifiers 501 and 601 reference is directed to U.S. Specification 3,424,870 and the push-button dial DL is of the type disclosed in U.S. Specification 3,479,470. Integrated circuitry is used for the oscillator circuitry. The additional 33 tones may be used for data signalling.