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THERE
HAS BEEN a tremendous growth in the computer, television and radar
fields. As a result, the demand for components that give time control over
pulse information has led to the development of a great variety of delay
lines; delay lines that find use in systems that relate electrical
information to time. Computers, television studios, telemetering systems,
guided missiles, navigation systems, identification coders and decoders,
radar systems and video tape recorders are typical systems that use delay
lines.
The selection of the proper
delay device for a given application is not only important from an
economic standpoint, but it can also contribute to the ultimate success or
failure of a system. In order to understand and compare the different
types of delay lines, it is first necessary to define certain parameters.
These definitions, with pulse diagrams, are shown in the box on page 3.
Basically, there are two types
of delay lines: electromagnetic lines and sonic delay lines. There are
important differences between these two types. The electromagnetic line is
limited in its time delay to rise time ratio to 250 :1. This limits the
upper frequency response but the passband starts at d-c and extends to its
3 dB cut-off point. For long delays, or where the required frequency
response is in the Megahertz range, this 3 dB cut-off point imposes a
severe limitation and makes the choice of a sonic line mandatory at these
higher frequencies.
The sonic delay lines can be broken down into two basic types:
magnetostrictive and solid (ultrasonic) lines (glass and quartz). Both
types feature very high frequency response and excellent temperature
stability. However, their attenuation (40 to 70 dB), narrow bandwidth, and
poor pulse fidelity has lead to the popularity of electromagnetic delay
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Electromagnetic Lines
The electromagnetic lines are
divided into two groups: distributed and the lumped constant delay lines.
The distributed delay line is further broken down into two categories, the
stick type and the Spiradel®, manufactured by
Allen Avionics. A list of electromagnetic delay lines and parameters are
shown in Chart 1.
The stick line closely
approximates a transmission line. It is fabricated by winding a coil
(either a solenoid or a multilayer) on a rod (glass, ceramic or phenolic)
that has been covered with a silver or copper coating. This conductive
coating is the ground conductor. Between the coil and the ground plane is
a thin dielectric layer. The coil provides a continuous and uniform
inductance along the rod. The coil of wire and the ground plane act as a
capacitor. The higher the dielectric constant of the dielectric layer, the
greater the capacity. Figure 1 shows the construction of a stick line and
the schematic representation of a distributed delay (both stick and
Spiradel®).
The delay of the line, Td,
is a function of the total inductance and capacitance (Td =
√LC), as is the impedance (Z =
√L/C). The stick lines generally do not
exceed 2 μsec of delay and are limited in their
figure of merit, rarely exceeding a value of 10, which requires a rod six
inches long. Not all impedances are possible for every delay, due to a
limit on obtainable capacity. The attenuation of the small delays is
generally very low since these are wound with heavy wire. When the delays
approach 2 μsec, however, the attenuation
increases greatly and reaches a maximum of about 3 dB. The temperature
coefficient is generally about 150 ppm from -55° to +
105° C. Sizes of the stick lines usually range
from about 3/8" x 3/8" x 2" to 1/2" x 1/2" x 6".
continued
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