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delay line[di′lā ‚līn]
a device used to delay signals by some predetermined time. The delay time τ is determined by the path length of an electromagnetic or acoustic wave within the delay line divided by the propagation velocity of the wave (however, this rule does not apply to artificial delay lines with lumped parameters).
Delay lines are used in color television equipment, in oscillograph equipment with a driven sweep, in radar stations with moving target selection, in devices providing optimum filtering of complicated radar signals, in coding, decoding, and selector devices, in memories, and in control devices of electronic computers. Delay lines are manufactured with values of τ ranging from fractions of a microsecond μsec) to tens of thousands of microseconds. They may have one or more outputs with different τ (multioutput delay lines); and τ may be constant or may depend on the frequency of the signal (dispersion delay lines). Delay lines have also been developed with adjustable τ (variable delay lines), with trimming of τ (magnetoelastic delay lines), with a low temperature coefficient of τ (thermally stable delay lines), and with internal signal amplification (active delay lines with phonon-photon or phonon-magnon interaction).
To produce small τ, electric lines with distributed parameters are used, such as conductive lines, strip lines, coaxial cables (particularly with an external helical conductor), and radio wave guides. In very long delay lines (several dozen meters), attenuation and wave dispersion (associated with electrical losses) distort the shape of the transmitted signal. The pass band of such delay lines does not exceed 10 megahertz (MHz). A greater τ (of the order of 01.-20.0 μsec) can be obtained in an electric artificial line with lumped parameters. The circuit of such a line is a series of networks, each of which consists of induction coils and capacitors. In such a line the value of τ depends on the number of networks, on the connection scheme of induction coils and capacitors in each network, and on the values of inductance and capacitance.
To produce τ of the order of 10 μsec to 10 msec, ultrasonic delay lines are used. In such lines electric input signals are first converted into ultrasonic signals by means of piezoelectric or magnetostrictive conversion, after which they pass through special matching layers (indium, epoxy resins, or glues) into an acoustic line. Acoustic lines may be of the cavity type (in the shape of a polyhedron), of the wave guide type (made of a ribbon or wire, usually wound as a helix), or of the multioutput type (bars made of piezoelectric materials with electrodes deposited on the bar). The signals propagate in the acoustic line with a velocity lower than the propagation velocity of electric signals by a factor of 105; they are converted back into electric signals by an output converter, which is analogous to the input converter. Materials used for acoustic lines include special steels; magnesium-aluminum alloys; single crystals of sodium chloride, potassium chloride, and potassium bromide; and fused quartz. To produce large τ in a small volume, an acoustic line is frequently shaped as a polyhedron (cavity acoustic line). In such a polyhedron the path length of ultrasonic waves is increased significantly because of multiple internal reflections of waves from the walls of the cavity.
The most commonly used delay lines are various kinds of electric delay lines with lumped parameters and some individual types of wave guide ultrasonic delay lines and ultrasonic delay lines with cavity acoustic lines, especially those with τ = 64 μsec, which are used for color television receivers.
The best parameters are exhibited by ultrasonic delay lines with cavity acoustic lines made of single crystals or fused quartz (τ ~ 1–5 msec; operating frequency, 20–60 MHz; pass band, 5–15 MHz; signal attenuation, of the order of 40–70 decibels [dB]; spurious signal level, 35–40 dB).
REFERENCESEveleth, J. “Obzor ul’trazvukovykh linii zaderzhki, rabotaiushchikh na chastotakh nizhe 100 Mgts.” Trudy Instituta inzhenerov po elek-trotekhnike i radioelektronike, 1965, vol. 53, no. 10.
Mason, W. “Ul’trazvukovye linii zaderzhki s mnogokratnymi otra-zheniiami.” In Fizicheskaia akustika, vol. 1, part A. Moscow, 1966.
May, J. “Volnovodnye ul’trazvukovye linii zaderzhki.” Fizicheskaia akustika, vol. 1, part A. Moscow, 1966.
E. I. KAMENSKII and V. M. RODIONOV