November 1962 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
 December 1971. All copyrights hereby acknowledged.

Delay lines are used in
electronic circuits for precisely adjusting the timing of signals. That can be to
set times between events or to adjust two or more signals so that they arrive at
some point in the circuit at a specific time with respect to each other. In a radar
system, for instance, a sample of the reflected signal might be delayed in time by
one pulse repetition period in order to compare it to the current reflected signal
so that stationary (fixed, nonchanging) signals can be cancelled out, leaving only
the signal that has changed since the last sample. That is how MTI (moving target
indication) functions. In today's world the samples are stored digitally and then
compared digitally with other signals, but previously in fully analog systems, sending
the sample along a longer (in time) path for comparison was necessary. Delay lines
can be electrical like the ones covered in this 1962 issue of Electronics World
magazine, or they can be mechanical such as with a quartz or mercury delay line.
The provided nomographs are for LC (inductorcapacitor) delay lines.
DelayLine Nomograms
Fig. 1  Basic circuit arrangement of a multisection LC delay
line.
By Donald W. Moffat
Useful graphical information to speed the design of LC delay lines with various
delays and rise times.
Delay lines are finding many applications in electronic equipment because they
are passive timing devices capable of extremely good accuracy under severe environmental
conditions. Many of these lines can be made in any laboratory and the accompanying
nomograms will enable the reader to design a delay line quickly for the desired
characteristics.
Fig. 2  Nomogram for determining the number of sections needed.
Fig. 3  Nomogram used to obtain total inductance and capacity.
Two broad classes of delay lines are the mechanical and the electromagnetic.
The first group is characterized by long delays, up to thousands of microseconds,
and large attenuation. They are made of special and expensive equipment and are
not ordinarily within the province of anyone but the specialist in their manufacture.
On the other hand, electromagnetic delay lines consist of a network of coils and
capacitors, as shown in Fig. 1, and experimental models can be constructed at any
electronic workbench.
The length of time by which such a line delays the signal is a function of just
the total inductance and capacity, in accordance with the formula: T = √LC, which uses the basic units of seconds,
henrys, and farads. If inductance and capacity are expressed in microhenrys and
microfarads, respectively, then time will be calculated in microseconds. This equation
shows that time delay can be increased by increasing either capacity or inductance
or both.
However, if the characteristic impedance of the line is to be considered, the
ratio of L to C must be watched. Characteristic impedance of a line is the impedance
which the line presents to the circuit that feeds it. For instance, if the signal
from a source with 2000ohm internal impedance drops to half its opencircuit value
when a delay line is connected across it, then the delay line also has an impedance
of 2000 ohms. When the delayed signal reaches the end of the delay line, some of
it will be reflected back unless the line is terminated in a resistance equal to
its characteristic impedance. In general, proper matching will produce the best
waveform and the maximum signal output.
The formula for characteristic impedance is: Z_{0} = √LC, where Z_{0} is in ohms when
both Land C have the same prefix, such as "micro." This equation shows that increasing
inductance will increase impedance, increasing capacity will decrease impedance,
and they can both be changed without affecting impedance if their ratio remains
unchanged.
In Fig. 1, the coils are in series and the capacitors are in parallel. Therefore,
total values as given by both the formulas are found by adding up those of each
section. Conversely, the values for one section are found by dividing the totals
by the number of sections. The number of sections is selected on the basis of the
desired quality factor, which is defined as total delay divided by output rise time.
The higher this ratio, the better the delay line because either a long delay or
a short rise time will increase the quality factor. In designing a delay line, the
procedure is to select values of total inductance and capacitance, then divide those
totals into the number of sections necessary to give the desired quality factor.·
An Example
This example will help explain the use of the nomograms. Suppose it is desired
to have a total delay of 2 μsec., a rise time of 0.2 μsec., and a characteristic
impedance of 600 ohms. First, we refer to the nomogram in Fig. 3.
On Fig. 3, locate 2 μsec. on the "Time Delay" scale and 600 on the "Impedance"
scale. Draw a straight line through these points and where that line crosses the
other scales, it will give the required values of inductance and capacitance as
1.2 millihenrys and 3200 μf., respectively.
Use Fig. 2 to determine the number of sections required. Locate 2 μsec, on
the "Time Delay" scale, 0.2 μsec. on the "Rise Time" scale, and draw a straight
line through these two points. At the middle scale the line indicates that 2.5 sections
are required, therefore each coil should have an inductance of 48 microhenrys and
each capacitor should have a value of 128 (nearest standard value of 130) μμf.
This basic section of 48 microhenrys and 130 μμf. can be used to make small
corrections to the total delay. Each section contributes a delay of 1/25 of the
total, or 0.08 μsec. and sections can be added without affecting the characteristic
impedance of the line, because the ratio of L to C will remain unchanged as sections
are added.
Posted October 13, 2022
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