Q factor Of an inductor or capacitor the ratio of its reactance to its effective series resistance. An ideal inductor or capacitor has no series resistance and the Q factor is therefore infinite. Practical components inevitably have resistance and thus Q factors are finite: they are, in fact, a figure of merit indicating the extent to which the component approximates to a pure reactance. For inductors Q values of 100 to 300 are possible and for capacitors much higher values are common. For an inductor Q is given by Lω/R and for the capacitor 1/ωCR.

quadraphony In sound reproduction an extension of the stereo principle, in which two further channels are used to originate stereo sound behind the listener to give the illusion that he is surrounded by sound. Quadraphony is, in fact, sometimes called surround sound. The four channels are identified as front left, front right, back left and back right.

    A number of systems for broadcasting quadraphonic sound (compatible with existing stereo and monophonic FM broadcasting) and of recording it on disk (compatible with existing stereo and monophonic disks) have been proposed and used but so far there has been no international agreement. There is, of course, no difficulty in recording the four quadraphonic channels as separate tracks on magnetic tape and such tapes are available.

quadrature The relationship between two sinusoidal signals at the same frequency when there is a 90° (π/2 radians) phase difference between them. As an example the current in a loss-less inductor or a loss-less capacitor is in quadrature with the applied EMF, the current in the inductor lagging the EMF and the current in the capacitor leading the EMF.

quadrature detector A detector used to demodulate a quadrature modulated signal, i.e. to abstract from a quadrature-modulated carrier replicas of the two original modulating signals. Such a detector is an example of a synchronous detector and is used in an NTSC or PAL television receiver to derive the two colour-difference signals which together constitute the chrominance signal.

quadrature modulation Method of transmitting two modulating signals independently on one carrier by splitting the carrier into two components in quadrature and using each signal to modulate one of the components. This method is used in the NTSC and PAL colour TV systems to transmit the two colour-difference signals. They are transmitted by suppressed-carrier amplitude modulation of the two quadrature components of the colour subcarrier.

quadripole A network with two input terminals and two output terminals. It is sometimes termed a 4-terminal, 2-terminal-pair or 2-port network.

quality factor Same as Q factor.

quantising For a quantity which can have any value between certain limits the division of the range between the limits into a number of sub-ranges, any value within a sub-range being represented by an assigned value within the sub-range. The sub-ranges need not necessarily be equal. Thus when an analogue signal is quantised, the instantaneous amplitude of the resulting signals can have only a limited number of values—equal, in fact, to the number of sub-ranges originally decided. These values can be expressed, for example, in binary code and can thus be transmitted by a pulse modulation system.

quantising distortion (or noise) The distortion (or noise) introduced by quantising. An analogue signal after quantising jumps in amplitude from one quantising level to another and these jumps give rise to distortion the magnitude of which depends on the number of quantising levels employed. The greater the number of levels chosen the less is the quantising distortion but to reduce distortion to the level needed in high-quality sound reproduction requires at least 1000 levels and in practice 2¹² (4096) are employed.

quarter-wave line (or transformer) A length of transmission line with an electrical length of a quarter of a wavelength at the operating frequency, used for impedance matching and for isolating at radio frequencies. Such lines are used, for example, to match the characteristic impedance Z_o of a feeder to the driving-point impedance Z_a of an antenna, correct matching being achieved when the characteristic impedance of the quarter-wave section is the geometric mean of Z_o, and Z_a. If a quarter-wave line is short-circuited at one end, the other end presents an infinite impedance at the operating frequency. This isolating property enables short-circuited quarter-wave lines to be used for supporting transmission lines and antenna elements without effect on their performance.

quartz crystal Natural crystal of silicon dioxide with pronounced piezo-electric properties. Such crystals are used for controlling the frequency of oscillators where great stability is required.

quench frequency The frequency at which an oscillation is quenched. In a super-regenerative receiver, for example, the quench frequency must be ultrasonic to avoid audible interference with the received signal.

quenching The periodic suppression of an oscillation. The term was applied to the suppression of the primary wavetrain in the obsolete quenched-spark transmitting system and is now used to describe the process of terminating the discharge in a Geiger-Muller tube. It is also applied to the suppression of oscillation in a super-regenerative receiver.

quiescent-carrier modulation In telephony, modulation in which the carrier is suppressed when there is no modulating signal to be transmitted.

quiescent current The current taken by the anode (collector or drain) of an active device or by an equipment from the DC supply in the absence of an input signal. It is the current at the quiescent point.

quiescent point Of an active device, the position of the operating point when no input signal is applied. The quiescent point indicates the mean current and the mean voltage in the output circuit and so measures the dissipation within the active devices in the absence of an input signal.

quiescent push-pull operation (QPP) A push-pull circuit using two pentodes biased near anode-current cut off once popular in battery-operated equipment. The circuit is an example of a class B1 amplifier and its economy of HT consumption was its principal attraction.

quiet automatic gain control (QAGC) A combination of delayed AGC and signal suppression so designed that the audio output of the receiver is suppressed for input signals too weak to operate the AGC system. Quiet AGC has the advantage that only signals capable of operating the AGC system are received and these stand out from a quiet background. In particular the inter-station noise normally heard when the tuning control is operated is absent from a receiver with quiet AGC.