Page 119 - Radar Technology Encyclopedia

delay line, digital delay line, quartz 109

A digital delay line is implemented in digital circuity and is The folded-tape meander delay line consists of a thin con-
used in the processing of digital waveforms. Arbitrarily long ducting tape extending back and forth between two ground
delays may be achieved with digital delay lines, subject only planes. The space between tape and ground planes is filled
to the limitations imposed by the cost of the associated cir- with dielectric material (Fig. D2). The time delay per mean-
cuitry. The minimum delay is determined by the speed of the der is a function of the dimensions of the loop and the dis-
digital components. tance from the ground plane. This configuration is the
The digital delay line is the fundamental element of digi- microwave analog of the all-pass time-delay network and can
tal filters, which are widely used in digital signal processing. be used for generation of frequency-modulated waveforms.
Digital delay lines have considerable advantage over analog SAL
lines, principally in flexibility, stability, repeatability, and reli- Ref.: Skolnik (1970), p. 20.12.
ability of performance. IAM
Ref.: Kuz’min (1986), p. 44.
A dispersive delay line has a frequency-dependent delay.
The dispersion can be natural or artificial. Natural dispersion
exists in ultrasonic delay lines and yttrium-iron-garnet (YIG)
delay lines. Delay lines with artificial dispersion are surface-
wave delay lines and charge-coupled delay lines. IAM
Ref.: Skolnik (1970), p. 20.6; Sloka (1970), p. 190; Shirman (1974), p. 92.
An electrical [electromagnetic] delay line is implemented
with electromagnetic wave transmission lines. They are clas-
Figure D2 Tapered folded-tape meander line (from Skolnik,
sified on the basis of the type of transmission line used. The
1970, Fig. 10, p. 20.12, reprinted by permission of McGraw-
most common types of electrical delay lines are the all-pass
Hill).
time-delay network, the folded-type meander delay line, the
coaxial cable delay line, the waveguide delay line, and the
A magnetostriction delay line is an acoustic delay line that
charge-coupled delay line. Electrical delay lines have less
transforms the electrical signal into an acoustic (ultrasonic)
insertion loss than acoustic (-wave) delay lines, but the delay
wave using the phenomenon of magnetostriction, which is a
is less for a given size. IAM, SAL
change in the geometric dimensions of a sample of ferromag-
Ref.: Skolnik (1970), p. 20.6; Sloka (1970), p. 180.
netic material placed in an alternating magnetic field. The
electromechanical delay line (See ultrasonic delay line). acoustic waveguide is usually constructed from a thin iron-
nickel alloy.
A fiber-optic delay line is a delay line based on a fiber-optic
The magnetostriction delay line is distinguished by loose
transmission line. The simplest fiber-optic delay line includes
coupling between the transducer and the acoustic waveguide,
a laser with a modulator to apply the electrical signal, a fiber
which makes them suitable for tapped delay lines. IAM
circuit of given length, determining the amount of delay of
the signal, and a photodetector to recover the signal. More Ref.: Sloka (1970), p. 187.
complex fiber-optic delay lines with variable delay use inte- A piezoelectric delay line is based on the propagation of sur-
grated optical switches for discrete changing of the fiber- face acoustic waves along a piezoelectric crystal. They are
optic length. For weighted processing of radar signals, tapped distinguished by the simplicity of their transducers, which are
delay lines are widely used. These consist of an optical fiber small metallic electrodes (with dimensions on the order of
with taps along its length, or a group of fibers with progres- microns) placed against the surface of the crystal. Due to this
sively increasing lengths. Weighting of the optical signals is feature they are used as tapped delay lines. IAM
done either by a frequency shift of the signal in each of the Ref.: Lukoshkin (1983), p. 218.
taps, or by the introduction of losses in each of the fibers. The
A programmable delay line is a delay line with electroni-
introduction of sections with variable delay into fiber-optic
cally controlled coherent delay. The typical approach to build
delay lines makes it possible to control the point of connec-
such a device is the tapped delay line and switch-matrix com-
tion of the taps in accordance with a set program. Such delay
bination (Fig. D3). The main range of usage of programmable
lines are called transversal filters and allow rapid switching
delay lines is in ECM techniques to delay in a coherent man-
both of weight functions and tap positions. ner radar pulses for the repeater jamming mode. SAL
The basic application of fiber-optic delay lines is for
Ref.: Wiegand (1991), p. 83.
highly stable precision delay of wideband signals. In compar-
ison with delay lines of other types, fiber-optic delay lines A quartz delay line is an ultrasonic delay line, using a quartz
ensure lower transducer losses, lower losses per unit of delay, (SiO ) acoustic waveguide. Despite the fact that quartz is not
2
wider bandwidth, higher signal-to-noise ratio, higher temper- distinguished by low acoustic absorption, large crystals of
ature stability, and greater flexibility. IAM synthetic quartz may be produced industrially, and are easily
processed. Quartz is usually used in volume-wave delay lines,
Ref.: Montgomery, J. D., and Nixon, F. W., Microwave J., 1985, vol. 28,
no. 4; Zmuda (1994), p. 481. which exploit multiple reflections from the crystal walls. This