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G geosynchronous orbit: similar to a geostationary orbit, but the satellite traces a figure-of-eight orbit thus appearing to move up and down in one-day cycles to an observer on earth. germanium: semiconductor element used in the majority of early transistors and diodes. giga- : unit prefix which means a multiplication factor of 10^12. Abbreviation: G. graphics: display of graphical symbols and scenes, generated by a computer. ground: synonym for earth. Abbreviation: Gnd. guard band: range of frequencies between two ranges of transmission frequencies, left un occupied to minimize interference. H half-adder: elementary digital circuit composed of logic gates. See adder. half-duplex: a pair of transmission channels over which two-way communications may take place, although only one channel is operational at any one time, is said to allow half-duplex communications. half wave rectifier: a circuit which rectifies only one half of each cycle of an applied AC wave. Hall effect: an electromagnetic phenomenon which occurs when a current carrying conductor is placed in a magnetic field, the direction of which is perpendicular to the directions of both the current and its own magnetic field. hardware: physical parts of a computer system eg, printer, keyboard, VDU etc. harmonic: a signal present in a complex periodic waveform, which is a multiple of the fundamental frequency. The second harmonic is times three, etc. head: transducer of a magnetic recording system which allows electrical signals to be changed into a magnetic field to write data onto the medium, or converts magnetic data into electrical signals. heatsink: metal attachment mechanically connected to a heat producing element in a circuit (eg, a power transistor) to ensure heat is dissipated away from the element, preventing damage by excessive heat. henry: unit of magnetic inductance. Symbol: H. hertz: unit of frequency. Equivalent to one cycle of a periodic wave which occurs in one second. Symbol: Hz. heterodyne: production of beats by combination of two signals which interfere. Used in a super heterodyne radio receiver to produce an inter mediate frequency. HF: abbreviation for high frequency. hifi: acronym for high fidelity. high fidelity: commonly used term denoting audio reproduction equipment of good quality. high frequency: bands of radio transmissions around 10 MHz. Abbreviation: HF. high level programming language: a computer programming language which is more like human language or mathematical notation than the machine code used by the central processing unit of the computer. high logic level: term denoting a logic 1 level (in positive logic). high pass filter: a filter which allows signal frequencies above a specific corner frequency to pass without attenuation. Signal frequencies below the corner frequencies are attenuated. high tension: voltages in the range between about 50V to 250V. Abbreviation: HT. holding current: the value of current which must be maintained to hold a thyristor in its on state. If the current through the thyristor falls below the holding current, the thyristor turns off and ceases conduction. hole: an empty space in a semiconductor material due to a 'missing' electron. As electrons are negatively charged, holes are positive. Holes, like electrons, may be thought of as charge carriers, moving through the semiconductor material thus forming a current. hole current: the current through a semiconductor due to the movement of holes under an applied voltage. howl: colloquial term for the sound caused by acoustic feedback. HT: abbreviation for high tension. hum: capacitive or magnetic interference between a mains powered device such as a power supply, and local equipment such as an amplifier. Often heard in audio frequency systems as a low drone of mains supply frequency, or a harmonic of that frequency. hunting: a system's oscillation about its desired point, caused by over-correction. hybrid integrated circuit: an integrated circuit comprising a number of discrete components attached to a substrate and interconnected to form a circuit. See integrated circuit. hybrid-it : a type of equivalent circuit used to show transistor operation. hysteresis: phenomenon occurring in some circuits or systems, in which the output lags behind a changing input. A hysteresis loop is formed-a graph of output against input--which shows that the value of output depends on whether the input is increasing or decreasing in value. Hz: abbreviation for hertz. Continued next month Also see: Letters |
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