Page 120 - Radar Technology Encyclopedia

110 delay line, quartz delay line, ultrasonic

The maximum delay time is usually restricted by the sub-
strate length and typically is about 50 ms, and the maximum
frequency about 1.5 GHz. Surface-acoustic-wave delay lines
are widely used in dispersive filters because of the simplicity
in getting the required frequency responses. IAM
Ref.: Skolnik (1970), p. 20.6; Morgan (1985), pp. 16, 175.
A tapped delay line is one in which required signal-process-
ing function is achieved through proper choice of the loca-
tion, frequency response, and coupling of the taps. In surface-
acoustic-wave delay lines, interdigital transducers are
employed as taps. Modern analog tapped delay lines also can
be built using charge-coupled devices and fiber-optics. (See
Figure D3 A programmable delay line (from Wiegand, 1991,
charge-coupled delay line and fiber-optic delay line.)
Fig. 2.20, p. 84).
Tapped delay lines feature a large number of taps offer-
ing a choice of delays and are used to process complex sig-
makes it possible to obtain long delays (1 to 3 ms) at frequen-
nals. One type of tapped delay line is based on the
cies of 10 to 80 MHz, with a bandwidth on the order of 30%.
propagation of longitudinal and twisting waves in a wire
To reduce the absorption of acoustical energy significantly,
waveguide, usually with magnetostriction transducers. A
monocrystal quartz delay lines may be cooled to the tempera-
tapped delay line may also be based on the propagation of
ture of liquid helium, which also results in highly stable
surface waves along a piezoelectric crystal. (See waveguide
parameters. The total loss in a quartz delay line is 40 to 60
delay line.)
dB. IAM
A tapped magnetostriction delay line can provide delays
Ref.: Skolnik (1962), p. 120; Lukoshkin (1983), pp. 247, 254.
up to 100 ms with relatively small dimensions. An advantage
A strip delay line is an ultrasonic delay line made from a to this type of delay line is the ability to tune the phase and
long, thin strip of metallic material (typically, aluminum or gain of each tap continuously and independently. They are
steel) with transducers at opposite ends (Fig. D4). Aluminum used when filtering narrowband signals, with bandwidth less
lines operate at lower frequencies than steel, with lower than 1 MHz and delay up to 1 ms.
losses. Strip delay lines have linear delay versus frequency Due to the simplicity of the electrodes, a piezoelectric
characteristics and are used in generation and processing of tapped delay line may have up to 10,000 taps. An advantage
linear frequency-modulated waveforms. SAL to this type of delay line is the ability to construct them in an
Ref.: Skolnik (1970), p. 20.6; Brookner (1977), p. 138. integrated fashion. They are used to process complex signals
with bandwidths on the order of several hundred megahertz
and delays up to 100 ms.
Tapped electrical delay lines, usually of coaxial cable, are
used to process wideband signals of short duration. It is nec-
essary, however, to consider the large size and weight of such
devices. SAL, IAM
Ref.: Lukoshkin (1983), p. 217; Fink (1977), p. 9.73; Brookner (1977),
pp. 163, 172, 182, 186–194, 384.
An ultrasonic delay line exploits the low propagation speed
of ultrasonic waves along an acoustic waveguide. Such a
delay line consists of an acoustic waveguide made from a
hard elastic material and two or more transducers that convert
radio signals to acoustic waves and vice-versa. The acoustic
Figure D4 Metallic strip dispersive delay line (from Brookner, waveguide may be a piezoelectric crystal, a metal with low
1970, Fig. 29, p. 138). ultrasonic propagation speed, or a ferromagnetic sample. The
transducers are placed on the surface of the delay line, at a
A surface-acoustic-wave (SAW) delay line is an ultrasonic separation corresponding to the desired delay. Piezoelectric
delay line based on the effect of the propagation of acoustic and magnetostriction delay lines differ in the way in which
wave in the surface layer of the solid body. Typically, the the waves are excited, relying on the coupling between
acoustic-wave guide is made from a piezoelectric mono- mechanical deformation and electric and magnetic fields,
crystal with electrodes deposited as metal components respectively.
through photocoating technology. When ac voltage is applied The most common ultrasonic delay lines are strip delay
to these electrodes, the acoustic wave arises as the result of lines, diffraction-grating delay lines, and yttrium-iron-garnet
piezoelectric effect. When the quantity of electrodes is large, (YIG) delay lines. The frequency and bandwidth performance
the tapped delay lines can be formed.