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When a step function, consisting of many
different frequencies, is introduced to a lumped line, the phase shift
should be a constant independent of frequency. When a delayed pulse is
distorted, it is the result of poor phase linearity. With present coupling
techniques, however, it is usual for lumped lines to display excellent
phase linearity over all the frequencies within its passband. For most
lumped delay lines, a ±2% phase variation can be readily maintained.
The impedance of a lumped constant delay line
(Z =
√L/C) and the time delay (Td =
√LC) determine the total values of
inductance and capacitance. These values determine the type of components
from which the line must be made and, therefore, are important in
determining the realizability, as well as the size and cost. Therefore,
the system designer should, if possible, specify an impedance range to
permit the manufacturer to take advantage of this specification. The
result could be a smaller, less costly delay line.
The attenuation of a lumped delay line is
related to the resistance of the inductors and the impedance of the delay
line. Attenuation of a delay line decreases with both an increase in size
and a decrease in time delay to rise time ratio. Generally, the
attenuation of a lumped line does not present a severe problem, unless
size is extremely limited or time delay to rise time ratios are very high.
Most lumped lines will exhibit about 0.5 db to 3 dB attenuation. Even time
delay to rise time ratios of 100:1 rarely attenuate more than 8 dB. When
this ratio reaches 250:1, however, one can expect an attenuation of about
20 dB.
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Temperature stability in most lumped lines is
generally good; typically 50 ppm/° C to
100 ppm/over a -55° to +
105° C range. Tighter temperature
coefficient requirements can be manufactured with an increase in cost.
Lumped lines having an operating voltage range of from 0 to 50 volts d-c
are always available, and can be supplied to handle up to 300 working
volts d-c.
Lumped lines are available in many packages.
Epoxy encapsulated shells are becoming more universally used. Larger
units, in hermetically sealed cans, have the delay line foamed in place.
For ease of electrical connections to test equipment cable, BNC
connectors are now commonly used. With the advent of the active and data
processing components, delay lines have become available in dual-inline
(DIP) packages that allow them to be used by high-speed automatic
insertion equipment.
Are there any rules of thumb for choosing delay lines?
Ultrasonic lines are limited by their narrow bandwidth and poor pulse
fidelity and are rarely used. Of the electromagnetic lines, the stick is
the cheapest but has the smallest figure of merit and shortest maximum
delay time. The Spiradel, for a slight increase in cost, obtains a higher
figure of merit and a greater maximum delay time. Lumped constant lines
can achieve marked improvements in both figure of merit and delay time.
However, this can be accompanied by a significant increase in cost. Since
cost increases with performance, choose the least expensive delay line
that supplies adequate performance. — H/G •
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