Abstract: The invention relates to a circuit for testing digital logic circuitry to determine the rise and fall time associated with a particular circuit. The circuit is tested by sending a test signal to the input of the circuit and measuring the area under a waveform taken at the output of the circuit.

Abstract: A unity gain or buffer amplifier having a low offset voltage. The amplifier uses two emitter followers of different conductivity types (PNP and NPN) in an up-down emitter or voltage follower configuration to provide high input impedance and low output impedance. By using both PNP and NPN transistors in a current mirror, the base-emitter voltages of the input and output transistors are forced to be substantially the same, reducing the offset voltage. N- and P-channel MOSFETs can be substituted for the NPN and PNP transistors. Single ended and push-pull arrangements are shown.

Abstract: An electronic tone ringer provides a distinctive ringing capability through the use of two oscillators which allow selection of four tones and four modulation rates. In determining the type of tone generated, formattable inputs select between frequency shift or amplitude modulation modes or disabling the modulation altogether to generate a single tone. In the frequency shift mode, two fundamental frequencies are generated which are related by a predetermined ratio, the output tone alternating between these two frequencies at one of four selectable modulation rates. In the amplitude modulation mode, a single frequency is switched on and off at the selected modulation rate. All of the tones are produced with a duty cycle appropriate for driving an electromagnetic acoustic transducer.

Abstract: An electronic tone ringer responds to ringing signals on a telephone line while being unresponsive to dialing pulses or other signals on the line. Powered directly from the telephone line or alternatively from an external DC supply, the ringer uses a digital integration and detection arrangement for distinguishing between valid and invalid ringing signals. A distinctive ringing capability for the tone ringer is also available through the use of two oscillators which allow selection of four frequencies and four modulation rates.

Abstract: In an oscillatory signal generator circuit exceptional frequency stability is insured by the use of interrelated constant current sources (520, 521, 522) that provide an insensitivity to supply voltage variations over a wide range. Two of the current sources determine the duty cycle by controlling the charge and discharge current of a capacitor (C) in the circuit. A third current source determines the voltage swing on the capacitor. By varying the current output from the third current source, the generator circuit provides a variable frequency signal.

Abstract: An electronic tone ringer responds to ringing signals on a telephone line while being unresponsive to dialing pulses or other signals on the line. Powered directly from the telephone line the ringer uses current sources (223-226) to provide a high impedance to the ringer circuitry and an insensitivity to line voltage variations so that reliable operation of the ringer is maintained on long loops and with multiple ringers terminated on the line. A multitone signal for activating an electroacoustic transducer (300) is provided by frequency modulating a single oscillator (500) within the tone ringer.

Abstract: A relay driver circuit is disclosed in which a relay coil is operated by enabling the collector-emitter path of a switching transistor. In order to avoid damage to the transistor from voltages induced in the relay coil when the relay is turned off, an isolating semiconductor device is inserted between the transistor and the relay coil. This semiconductor device is operated in synchronism with the driving transistor and thus isolates the driving transistor from transients during the de-energization of the relay.