|
The stick type is the
lowest cost delay line available if its electrical and physical
specifications are adequate.
The Spiradel® line consists of an inductor up to
several feet long, combined with a capacitance that is distributed
uniformly along its length. This can be compared to a length of coaxial
cable, where the inductance along the center conductor has been increased
and sufficient capacitance added per unit length to provide the desired
characteristic impedance. Size reduction is accomplished by winding the
inductor on a flat thin strip and reducing the coaxial sheath to a
multiple flat strip conductor, placed parallel to the strip inductor.
These long lengths are then wound into a spiral and encapsulated.
The inductor is formed by winding a copper conductor on a continuous thin
strip of magnetically permeable material. This inductor is designed to
give a fixed inductance per unit of length. The distributed capacitance
component is fabricated by placing conductive foil strips between two
pieces of dielectric sheathing. The thickness of the dielectric strip is
determined by the amount of capacitance per unit length required. |
Due to the uniform
values of inductance and capacitance, a predetermined length of inductor
strip may be cut to provide a given delay before fabrication. As a result
of being able to use extremely long inductive and capacitance components
in the Spiradel®, the delay to rise time ratio
is greatly improved over that of the stick type, reaching a figure of
merit of up to 30. Spiradel delays range from 2 nsec to 6 μsec
and impedance ranges from 50 ohms to 1000 ohms. The temperature
coefficient from -55° to +
105° C is less than 150 ppm. The Spiradel®
runs from 1/4” to 7/16" high and from 0.7” to 3.75” in diameter.
High figure of merit, small size, large delay range, and low cost make it
popular.
Lumped Constant Delay Lines
The lumped constant line is the most widely used type. Like the stick and
Spiradel® types, its passband extends from d-c
to its 3 dB cut-off frequency and phase linearity over this range is good.
The lumped constant line consists of a number of inductors and capacitors
similar in value. The inductors are connected in series and the capacitors
are connected from the junctions between inductors to the ground lead.
Schematically, the circuit is shown in Fig. 3. The total inductance and
capacitance is determined by the following equations: Lt = Td
x Z, Ct = Td / Z. The number of “lumps” (coils and
capacitors) required for a given delay line can be determined, directly,
from the time delay to rise time ratio: N = R1.36 (with N
= number of sections, R =Td /Tr), see Fig. 4. Since
the basic cost of a lumped line is related to the number of sections used,
it is apparent that over-specification of time delay to rise time ratio
results in a higher cost.
Once the number of sections has been determined, the inductance and
capacitance of each cell or lump can be found by dividing the total
inductance and total capacitance by the number of lumps required. When the
total delay is small and the ratio relatively high, the values of the
individual inductors and capacitors become so diminutive that the stray
values of circuit capacitance and inductance become significant. This can
prevent the realization of a delay line design. Another problem is the
expense of high Q inductors. This is especially important when a large
number of sections is necessary. Powdered iron bobbins, toroidal cores,
universal winds on magnetic and non-magnetic forms, as well as ferrite pot
cores, are all employed (where applicable) to achieve the highest possible
inductor Q at the right frequency. Very seldom is a problem encountered in
the Q of the capacitor.
continued
 |
|
This insulated conductive screen (ground plane) provides one plate of
the distributed capacitance, while the individual turns of the strip
inductor form the other. The ground plane, when combined with the
inductor, also provides shielding between successive turns of the spiral
when wound as shown in Fig. 2. |
