U.S. patent number RE31,477 [Application Number 06/017,491] was granted by the patent office on 1983-12-27 for flat multi-signal transmission line cable with plural insulation.
This patent grant is currently assigned to Thomas & Betts Corporation. Invention is credited to Joseph Marshall.
United States Patent |
RE31,477 |
Marshall |
December 27, 1983 |
Flat multi-signal transmission line cable with plural
insulation
Abstract
A multi-signal transmission line is formed of a flat cable
having a plurality of generally parallel conductors embedded in a
dielectric core material, with an insulator jacket encasing the
flat cable and being made of a dielectric material having a higher
dielectric constant than the dielectric core material of the flat
cable. The resulting composite transmission line cable insures that
substantially all of the transverse electromagnetic propagation
field created by the passage of a fast rise time pulse in a signal
conductor is confined to the geometric area of the cable and the
cable functions in a manner to greatly reduce the far end
line-to-line interference (crosstalk) between the signal conductor
and adjacent quiet lines.
Inventors: |
Marshall; Joseph (Toyonaka,
JP) |
Assignee: |
Thomas & Betts Corporation
(Raritan, NJ)
|
Family
ID: |
26689945 |
Appl.
No.: |
06/017,491 |
Filed: |
April 16, 1979 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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163199 |
Jul 16, 1971 |
|
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Reissue of: |
235723 |
Mar 17, 1972 |
03763306 |
Oct 2, 1973 |
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Current U.S.
Class: |
174/115;
174/117F; 174/117FF; 333/12; 333/243 |
Current CPC
Class: |
H01B
11/00 (20130101); H01B 7/0838 (20130101) |
Current International
Class: |
H01B
11/00 (20060101); H01B 7/08 (20060101); H01B
007/08 () |
Field of
Search: |
;174/117F,117FF,112,115
;333/243,12 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Blodgett et al. "Insulation & Jackets for Polyethylene Cables"
in IEE Transactions, Dec. 1963, pp. 971-979. .
Burndy "Flat Cable-The Modern Cable System For Electronic
Applications" 10/69, Roch. NY. .
APC Application of Beckett S.N. 218848, Published
6/8/1943..
|
Primary Examiner: Goldberg; Elliot A.
Attorney, Agent or Firm: Rodrick; Robert M. Abbruzzese;
Salvatore J. Woldman; Jesse
Parent Case Text
This is .Iadd.an application for reissue of U.S. Pat. 3,763,306,
issued October 2, 1973, which was .Iaddend.a continuation-in-part
application of application Ser. No. 163,199 filed July 16, 1971 by
Joseph Marshall entitled "IMPROVED FLAT MULTI-CONDUCTOR CABLE," now
abandoned.
Claims
What is claimed is:
1. A composite multi-signal transmission line cable for
transmitting fast rise time electrical pulses with minimum far-end
crosstalk comprising:
a flat multi-conductor cable including a plurality of generally
parallel conductors embedded in a planar sheet of insulation
material having a dielectric constant, .Iadd.adjacent conductors
being spaced apart at a predetermined pitch .Iaddend.with selected
conductors adapted to be connected to ground, while remaining
conductors are used as signal-carrying conductors .Iadd.the ratio
of the thickness of the insulation material to said pitch being at
least 2.0 such that at least about 98 percent of the transverse
electromagnetic (TEM) field propagates within said insulation
material.Iaddend.; and
an insulator jacket surrounding the flat multi-conductor cable and
in intimate contact with said insulation material; said insulator
jacket being made of a dielectric material having a dielectric
constant greater than the dielectric constant of said insulation
material whereby the electrical effect of the composite of the
insulation material and the insulator jacket is to establish a
balance between the inductive and capacitive coupling coefficients
between adjacent signal-carrying conductors thereby minimizing the
far-end cross-talk.
2. A composite multi-signal transmission line cable as in claim 1
wherein the insulation material is polyethylene, and the insulator
jacket is made of vinyl.
3. A composite multi-signal transmission line cable as in claim 1
wherein the flat cable includes alternating flat and round
conductors, with the round conductors being the signal carrying
conductors, and wherein the characteristic impedance of the cable
(Z.sub.d) is expressed by the following relationship:
where
d=diameter of round conductors;
p=spacing between a round conductor and an imaginary round
conductor falling within the confines of the adjacent flat
conductor;
x=p/d; and .epsilon.=relative dielectric constant of insulation
dielectric material to air dielectric.
4. A composite multi-signal transmission line cable as in claim 1
wherein the signal conductors are round in cross-section, while the
ground conductors are rectangular in cross-section.
5. A composite multi-signal transmission line cable as in claim 1
wherein the insulator jacket is extruded over the flat cable. .[.6.
A composite multi-signal transmission line cable as in claim 1
wherein the thickness of the jacket is approximately two-thirds
that thickness of a theoretical cable core capable of confining
approximately 98 percent of the TEM
field..]. 7. A composite multi-signal transmission line cable as in
claim
4 wherein the insulator jacket is bonded to the flat cable. 8. A
composite multi-signal transmission line cable as in claim 1
wherein the total thickness of the insulation material of the flat
cable is sufficient to contain substantially all of the transverse
electromagnetic field area
generated by a signal passing through a conductor. 9. A composite
multi-signal transmission line cable as in claim 1 wherein the
insulator jacket is made of a material which is self-extinguishing
when exposed to a
flame. 10. A composite multi-signal transmission line cable for
transmitting fast rise time electrical pulses with minimum far-end
crosstalk comprising:
a flat multi-conductor cable including a plurality of generally
parallel conductors alternating in cross-section between round and
rectangular, and embedded in a planar sheet of insulation material
having a dielectric constant and a low dissipation factor.Iadd.,
the center to center distance between a round conductor and the
center of an imaginary round conductor tangent with the facing wall
of an adjacent rectangular conductor defining a pitch therebetween,
the ratio of the thickness of the insulation material to said pitch
being at least 2.0 such that at least about 98 percent of the
transverse electromagnetic (TEM) field propagates within said
insulation material.Iaddend.; and
an insulator jacket extruded over said flat cable, and completely
surrounding and in direct contact with said flat cable, said
insulator jacket being made of a material having a dielectric
constant greater than the dielectric constant of said insulation
material, whereby the electrical effect of the composite of the
insulation material and the insulator jacket is to establish a
balance between the inductive and capacitive coupling coefficients
between adjacent signal-carrying
conductors thereby minimizing the far-end cross-talk. 11. A
composite multi-signal transmission line cable as in claim 10
wherein the insulation material is made of polyethylene, and the
insulator jacket is formed of
vinyl. 12. A composite multi-signal transmission line cable as in
claim 10 wherein the characteristic impedance (Z.sub.d) of the
cable is expressed in the following relationship:
where
d=diameter of round conductors;
p=spacing between a round conductor and an imaginary round
conductor falling within the confines of the adjacent flat
conductor;
x=p/d; and
.epsilon.=relative dielectric constant of insulation dielectric
material to air dielectric. .[.13. A composite multi-signal
transmission line cable as in claim 10 wherein the thickness of the
jacket is approximately two-thirds that thickness of a theoretical
cable core capable of confining
approximately 98 percent of the TEM field..]. 14. A composite
multi-signal transmission line cable as in claim 10 wherein the
insulator jacket is
formed of a material which is self-extinguishing. 15. A composite
multi-signal transmission line cable as in claim 10 wherein the
insulator
jacket is bonded to the insulation material. 16. A composite
multi-signal transmission line cable as in claim 10 wherein the
dissipation factor of the dielectric material of the insulator
jacket is greater than the dissipation factor of the insulation
material of the flat cable.
Description
The present invention relates to transmission line cables, and more
particularly to a composite multi-signal transmission line cable
including a multi-conductor flat cable.
For applications where a plurality of signals have to be
transmitted through cables in small spaces, flat cable transmission
lines have been found to be the best solution. In conventional flat
cables alternate ground-signal-ground conductors are positioned
side by side in one plane to establish the required transmission
parameters: characteristic impedance; velocity of propagation;
attenuation; and adequate control of line-to-line interference
(crosstalk). Flat cables of the conventional type are being used to
a great extent today transmitting signals with rise times of a few
nanoseconds only, and performing quite satisfactory at these
frequency levels. However, with advancements in the state of the
art of sophisticated electronics, conventional flat cables are
being pushed to the limit of their capability. More particularly,
as an example, the crosstalk from one signal line to the other is
often of a magnitude whereby the crosstalk is sufficient to
inadvertently trigger or actuate adjacent circuits, as in the case
where the cable is employed in a computer. Of course, the
interference between the adjacent signal lines increases as pulse
rise times become faster. This crosstalk effect is mainly due to
the fact that only the central part of the electromagnetic field is
propagating within the cable insulation, while the outskirt of the
field extends into the immediately surrounding air. As is well
known, the ideal medium for signal transmission in a
multi-conductor flat cable is a homogeneous dielectric. When the
field of propogation is confined within the boundaries of a uniform
dielectric material, there is only one propagation velocity in the
cable. However, when this field extends beyond the solid dielectric
of the cable into the surrounding air, this particular composite
dielectric effect introduces several undesirable phenomenon at high
transmission line frequencies or fast rise time pulses: (1)
distortion of the transmitted signal itself; (2) excessive ringing
above the fast crosstalk level at the near end of the adjacent
quiet line; and (3) differential crosstalk at the far end of the
interconnection. It should be noted that the term "air effect" is a
misnomer, and should be more definitively stated as being a
"fringing field effect" in that it is the phenomenon of the
transverse electromagnetic propogation field extending beyond or
infringing upon the field surrounding the dielectric of the core
material of the flat cable.
The concept of using a flexible, metallic all-around shield over a
flat cable was an early effort in eliminating the fringing field
effect and confining the field of propogation for each signal
between the adjacent ground conductor and the shield to a
homogeneous dielectric. In order to establish a flexible all-around
shield over a transmission line flat cable, it has been known to
provide a tight fitting, braided metallic shield around the flat
cable without slackening at the wide sides. The wire braided cable
was then covered with a yarn braided jacket. The resulting cable
thus required both a shield and a covering, thereby increasing the
physical dimensions of the cable and the cost of manufacture. To
achieve control of the fringing field-effect with less increase in
the cable dimensions through a less expensive method is the primary
objective of the present invention.
Besides the desirable characteristics of electrical transmission
efficiency and flexibility, it is particularly desirable that
cables employed in sophisticated electronics also be rugged and
flammably self-extinguishing.
Thus, it is an object of this invention to provide a multi-signal
transmission line cable exhibiting low crosstalk at fast rise time
pulses.
It is a further object of this invention to extend the advantages
of the flat cable concept into a field where miniature coaxial
cables were previously employed because conventional flat cables
were electrically inefficient due to high crosstalk, noisiness, and
leakiness.
It is another object of this invention to provide a low cost,
multi-signal transmission line cable which is characterized by
being self-extinguishing and rugged.
It is still another object of this invention to provide an
extremely flexible multi-signal transmission line cable which is
also characterized by being self-extinguishing and rugged.
It is still a further object of the invention to provide a low cost
multi-signal transmission line structure having desirable
electrical and physical characteristics for use in sophisticated
electronic equipment.
The composite multi-signal transmission line cable of the subject
invention comprises: (1) a central flat cable insulation core
having a low dielectric constant, and including a plurality of
generally parallel conductors embedded within the insulation; and
(2) a surrounding insulator jacket having a higher dielectric
constant than the core material. More particularly, the jacket has
distinctly different electrical and physical properties, whereby
for electrical reasons it has a higher dielectric constant and a
higher dissipation factor, whereas for physical advantages, it is
self-extinguishing and has a quality for wear and abrasion
resistance and, additionally, seals the composite cable for
protection against the environment.
A prototype cable having a polyethylene insulation for the core and
a vinyl compound for the jacket has been designed, and consists of
20 signal and 21 ground conductors, the signal conductors being
round, whereas the ground conductors are rectangular in
crossection. Polyethylene was chosen for the insulator material of
the flat cable in that it is relatively cheap in cost, and may be
readily formed for use in the manufacture of multi-conductor flat
cable. Subsequent to the formation of the polyethylene
multi-conductor flat cable, the cable structure is covered,
preferably through an extrusion process, with the surrounding
jacket of vinyl material which includes the desirable
characteristic of being self-extinguishing when exposed to the
flame.
The conductor arrangement of the prototype cable has been found to
have many advantages: (1) the width of the rectangular conductors
is chosen so that the signals may be located in alignment with
given conductor terminals, yet establishing the necessary spacing
for the characteristic impedance between signals and grounds; (2)
the insulation removal (stripping) is easier at the cable ends
because both the round and rectangular conductors have uniform
height; (3) there is only one ground to terminate between adjacent
signals (in conventional round wire flat cables two or more grounds
have to be used, partially because of crosstalk control and
matching the cable signals to connector terminal locations); and
(4) the flat-round arrangement offers an easy identification for
signals and grounds, this feature being quite helpful, particularly
when signals and grounds will be terminated in two different planes
(e.g., to printed circuit boards used as microstrip).
The main electrical parameters for any multi-signal transmission
line cable are: (1) characteristic impedance; (2) propogation
velocity; (3) attenuation; and (4) line-to-line interference
(crosstalk). In the composite multi-signal transmission line cable
of the subject invention, the first three parameters are controlled
by the cable core which may be made of a polyethylene material,
while the last parameter, crosstalk, is limited by the outer
insulator jacket, which may be an extruded vinyl jacket. It is
noted that the vinyl jacket has a higher dielectric constant than
the polyethylene core material. The vinyl jacket substantially
eliminates the fringing field or air effect from the surface of the
polyethylene core. The basic cable core without the jacket looks
and acts very similar to the conventional signal transmission line
flat cables. The latter have limitations in handling pulse type
signals with 1 nanosecond and faster rise times, the crosstalk
between lines becoming prohibitive. In thin cables the pulse edge
is also distorted. The reason for these undesirable effects is that
the outskirt of the propogation of the transverse electromagnetic
field (created by the passage of a signal through a signal
conductor) passes through the air or fringing field surrounding the
basic cable insulation and, accordingly, conventional flat cables
have extremely undesirable limitations in the transmission of high
frequency, sine wave signals.
In a properly designed composite multi-signal transmission line
cable of the subject invention, this harmful fringing field effect
is substantially eliminated. Consequently, the pulse edge is not
degraded beyond the attenuation characteristics inherent in the
selected conductors and insulation. The near end crosstalk is
limited practically to the level of fast crosstalk, while at the
far end the differential crosstalk is nearly eliminated, and at
least is reduced to a minimum, so as to not inadvertently trigger
circuits to which the adjacent lines are connected. For the proper
design of the improved composite multi-signal transmission line
cable of the subject invention, a formula has been empirically
developed for the calculation of the characteristic impedance of
the cable, based on the size and spacing or pitch of the
conductors, the thickness of the cable core, the thickness of the
surrounding jacket, and the relationship between the dielectric
constants of the core insulation and the surrounding jacket.
The outer insulator jacket of the multi-signal transmission line
cable of the subject invention also contributes to the overall
physical features in that it can be designed to establish
self-extinguishing properties despite the flammable core material.
With a jacket made of vinyl material, the cable becomes more rugged
and resists wear and abrasion. For that reason, the multi-signal
transmission line cable of the subject invention is suitable not
only for interior use, but also for exterior interconnections in
communication installations. It has been found that when thin
polyethylene cable is embedded into a vinyl jacket, the higher
dielectric constant of the vinyl jacket material will materially
contribute to the composite dielectric constant (of the jacket and
core) by the percentage of the square root of its dielectric
constant weighed by its field effect. Consequently, the propagation
velocity of the subject cable will be somewhat lower; however,
crosstalk still will be confined at higher frequencies where
conventional flat cables fail.
In a prototype cable made according to the teaching of the subject
invention, the maximum operating temperature of the prototype cable
was found to be 60.degree. C., however, the subject cable may be
designed with higher temperature insulation materials and offer the
same advantages of the prototype polyethylene core cable. The
prototype structure was found to be extremely flexible and capable
of being bent into a radius equal to the cable thickness and
straightened out without any noticeable change; it also may be
folded straight back upon itself or in a 45.degree. angle without
causing any physical damage, with no detrimental effect on the
electrical performance. The prototype cable also passed standard
flame tests for self-extinguishing properties.
The above objects and advantages, along with other inherent
advantages, will become more apparent upon a reading of the
following detailed description of the invention taken in
conjunction with the following drawings in which:
FIG. 1 is a perspective view of the multi-signal transmission line
cable of the subject invention, with the outer jacket partially
removed from a portion of the cable;
FIG. 2 is a cross-section taken along line 2--2 in FIG. 1;
FIG. 3 schematically illustrates apparatus for making the composite
multi-signal transmission line cable of the subject invention;
FIG. 4 is a table of calculations for determining the rise time
attenuation (T.sub.o) of a prototype cable for several input
pulses;
FIG. 5 is a tabulation of different amplitude levels of an input
pulse to a prototype cable of the type of the subject
invention;
FIG. 6 is a tabulation of calculated and measured values of pulse
edge attenuation utilizing a prototype cable made according to the
teachings of this invention;
FIG. 7 is a graph depicting the sine wave attenuation of the
prototype cable in dB/100 feet as plotted against the frequency of
the input signals in megaherz (MHz);
FIG. 8 are two oscilloscope tracings depicting the far end
crosstalk measurements on the vinyl jacketed prototype cable and on
a standard polyethylene cable without a jacket;
FIG. 9 is an oscilloscope tracing of the far end crosstalk measured
on a polyethylene jacketed cable, encapsulating a basic
polyethylene flat cable core; and
FIG. 10 is a graph depicting the distribution of the transverse
electromagnetic field in the cross-sectional geometry of the flat
cable in terms of the ratio (b/p) of the thickness of the cable
core (b) to the center distance between signal and ground
conductors.
Referring to FIGS. 1 and 2, the multi-signal transmission line
cable 10 of the subject invention is shown as including alternating
flat (rectangular) 11 and round 12 wires embedded in a body or core
of dielectric material 13, such a polyethylene. Polyethylene is
relatively inexpensive, and has a dielectric constant of
approximately 2.3. The conductors 11 and 12 are spaced uniformly
and generally parallel to each other, with the center-to-center
spacing between the conductors being designated as the pitch. As
shown in the cross-section of FIG. 2, the round conductors are
spaced at a distance "D," with the diameter of the round conductors
being designated by the letter "d." The width of each flat
conductor 11 is designated by the letter "w," with the spacing
between the flat conductors and the round conductors being
designated by the letter "s." Assuming that each end of the
cross-section of each flat conductor 11 is defined by an imaginary
round conductor, (designated by dotted line 15), the
center-to-center distance between the adjacent round conductor 12
and the imaginary round conductor 15 is designated by the letter
"p." Also, the thickness of the dielectric material in which the
conductors are embedded is designated by the letter "b." As
indicated previously, the thickness of the dielectric material is
of primary importance in order to insure that substantially the
majority or a designed part of the field of propagation is
contained within the solid dielectric of the cable core. The
remaining part is contained within the jacket and little or none is
in the surrounding air or fringing field. FIG. 10 shows the field
distribution in the cross-sectional geometry of the flat cable in
terms of b/p.
Forming a portion of the subject multi-signal transmission line
cable 10, and preferably extruded about the entire periphery of
said cable, is an insulator jacket or covering 16, preferably made
of self-extinguishing material, such as vinyl. The jacket material
has a higher dielectric constant than the body of dielectric
material 13 in which the conductors are embedded, with the higher
dielectric constant jacket material contributing to the composite
dielectric constant (i.e., of both the jacket and insulation
material 13) by the percentage of the square root of its dielectric
constant weighted by its field effect. As a result of this
arrangement, the propogation velocity of the multi-signal
transmission line cable of the subject invention will be slightly
slower; however, this structure will materially contribute to the
reduction of crosstalk at higher frequencies.
In a preferred method of making the subject multi-signal
transmission line cable, as schematically illustrated in FIG. 3, a
plurality of alternating flat and round conductors are unwound from
reel 21 and passed through a set of tension rollers 22 through an
alignment device 23 into the nip of rollers 24. Also passed into
the nip of rollers 24 are upper and lower sheets of dielectric
material such as polyethylene, such sheets being designated by
numerals 30 and 31, which are stored on rolls 32 and 33. Heat and
pressure are applied resulting in the cable structure which is then
passed through an extruding apparatus 40 where the vinyl covering
16 is extruded over the cable structure. The composite cable 10 is
then passed over roller 41 and stored on storage roll 42. The
polyethylene cable core may also be manufactured by extrusion.
The geometry of the resulting cable is of primary importance, with
the specific arrangement of the conductors, the thickness of the
insulation material in which the conductors are embedded, the
relative thickness and dielectric constants of the jacket and
embedding material, being of a critical nature to the efficient
operation of the subject cable.
The cable 10 embodies a multitude of individual signal transmission
lines which are the round conductors 12 that alternate with the
flat conductors 11 that are connected to ground. The overall
construction of the cable thus provides high density packaging in a
single plane, and results in a thin multi-signal transmission line
cable structure which is extremely flexible. The insulator jacket
16 is maintained in close proximity to the insulation 13 to insure
uniform electrical performance by preventing the presence of air
between the jacket and the core insulation 13. If desired, the
jacket 16 may be bonded by a suitable adhesive to the core as a
further aid in eliminating air from the composite cable.
The electrical parameters of the cable 10 may be controlled
individually and in different proportions by the interspersed
ground conductors 11 and by the insulator jacket 16. The
characteristic impedance will be established depending mainly on
the location and shape of the conductors and on the dielectric
constant of the cable core insulation and, to a much lesser degree,
by the jacket 16, while crosstalk and overall isolation properties
are greatly influenced by the insulator jacket 16.
Following is a detailed discussion relative to the electrical
features of the subject composite multi-signal transmission line
cable.
1. ELECTRICAL FEATURES IN GENERAL
The geometry of a multi-signal transmission line cable made
according to the teachings of the invention, wherein the core is
made of polyethylene while the jacket is made of vinyl, insures a
uniform polyethylene dielectric practically for the total area of
the transverse-electromagnetic propagation field. It is noted that
the transverse-electromagnetic propogation field is the mode most
commonly excited in coaxial and open wire lines.
The main electrical parameters for multi-signal transmission cables
in general are:
1. Characteristic impedance;
2. Propagation velocity;
3. Attenuation; and
4. Line-to-line interference (crosstalk)
In a prototype cable, hereinafter designated as prototype cable
CA-490, the first three parameters are substantially controlled by
the polyethylene cable core, while crosstalk is limited by the
special vinyl jacket.
Multi-signal transmission line flat cables of conventional types
have a major drawback at fast rise time pulses, i.e., to keep
line-to-line interference or crosstalk at an acceptable minimum
level. The surrounding air at the outer periphery of conventional
cables raises crosstalk and, in particular, far end crosstalk to
such levels that loads, such as adjacent circuits, may be triggered
unintentionally. The main electrical feature of the multi-signal
transmission line cable of the subject invention is to replace this
undesirable air or fringing field effect with a layer of dielectric
material, and more particularly, a jacket of dielectric material
having a higher dielectric constant than the dielectric constant of
the core material. The jacket which has a higher dielectric
constant and may have a higher dissipation factor than the core
material, confines the field of propagation of the individual
signals to such a degree that crosstalk will be reduced to an
acceptable level for high frequency applications.
It is a criteria of the design of the subject composite
multi-signal transmission line cable to select.Iadd., for example,
by reference to FIG. 10 .Iaddend.a geometry for the thickness of
the polyethylene core in coordination with the conductor
arrangement where the majority of the electromagnetic field will be
caused to propagate within the low loss dielectric, and attenuation
or propagation velocity will be affected very little by the
surrounding jacket. In the prototype design, CA-490, approximately
98 percent of the field propagates in the polyethylene cable core
.Iadd.wherein in accordance with FIG. 10, the ratio of the core
thickness to the conductor pitch is about 2.0.Iaddend..
However, the composite multi-signal transmission line cables of the
subject invention may be designed with thinner cable cores with
certain trade-offs on the propagation velocity and attenuation. In
these designs the higher dielectric constant material of the
insulator jacket wil contribute more to the composite dielectric
constant by the percentage of the square root of its dielectric
constant weighed by its field effect. Consequently, the propagation
velocity in such cable will be somewhat slower; still the
beneficial electrical effects achieved by the cable of the subject
invention will apply and crosstalk will be limited to acceptable
levels.
The prototype cable CA-490 was designed for unbalanced systems;
however, the basic concept of the subject invention applies as well
for balance pair designs and improves crosstalk in both cases.
CHARACTERISTIC IMPEDANCE
The Characteristic Impedance of signal transmission lines depends
on the size, shape and location of the conductors and on the
dielectric constant of the insulation material. In order to
establish a formula with measurable properties, the following basic
relationship was utilized:
where
Z.sub.o =characteristic impedance, ohms, in air
c=velocity of propagation, meter per second, in air
C=capacitance, Farad per meter, in air
A simplified form of this with practical units:
where:
C=capacitance, picofarad per foot, in air
To account for the dielectric material:
where:
Z.sub.d =characteristic impedance in cable
Z.sub.o =characteristic impedance in air
.epsilon.=the effective dielectric constant, relative to air
To ease the prediction and calculation of the prototype cable's
characteristic impedance a formula was established based upon the
dimensions of the cross-sectional geometry:
where:
x=pd
p=pitch, signal to ground
d=diameter of conductor.
The above formula was found to be in close agreement with actual
measurements. It gives direct applicability for multi-conductor
cables where both signal and ground conductors are round wires.
Therefore, further explanation is needed for the characteristic
impedance of flat-round-flat conductor arrangements, the design
used for the prototype cable and illustrated in FIG. 1. Since the
thickness of the flat conductors is equal to the signal wire
diameter and the corners are radial, the following consideration
was accepted:
where:
s=the separation between the round and rectangular conductors
Recognizing the dimensions for the conductor arrangement in the
prototype cable:
d=.Badd.0.0113 inch
s=0.00685 inch
p=0.01815 inch
the calculation gives Z.sub.o =76.2 ohm characteristic impedance in
air.
Having 98 percent of the field propogating in the polyethylene and
2 percent in the insulator jacket, results in a square root of the
composite dielectric constant equal to 1.525; consequently, the
characteristic impedance of the prototype cable:
PROPOGATION VELOCITY
The electromagnetic energy propogates in free space with a velocity
of .Badd.3(10).sup.8 meters per second. Expressing this in more
practical units on a time delay base:
TP.sub.o =1.016 nanosecond per foot, in air and
TP.sub.d =TP.sub.o (.epsilon.).sup.1/2 nanosecond per foot, in
cable where:
TP.sub.d =propogration time in cable.
The signal propogation time in the prototype cable CA-.Badd.490 is
1.55 nsec./foot. The polythylene cable core without the jacket
yields 1.53
nsec./foot propogation delay. These measured results are comparable
to calculated values based upon the assumption that 98 percent of
the TEM (transverse electromagnetic) field propogates within the
polyethylene. These readings were taken at the 10 percent level of
the input pulse rise time, t.sub.r =0.18 nanoseconds.
ATTENUATION
Signals transmitted through flat cables are attenuated along the
lines. This attenuation is due to conductor losses and insulator
losses; both are frequency dependent. Copper losses are affected by
the square root of frequency, and insulation losses by the
frequency. The measure of attenuation is expressed in decibel/foot
at sine wave frequencies and by the slope change of the rise time
at pulse type signals.
The shape of a selected pulse rise time (1,5, or 10 nanoseconds,
etc.) may be matched by the ascending half of an equivalent sine
wave frequency through a two channel oscilloscope. Such
measurements showed good agreement with the following formula:
where
t.sub.r =pulse rise time 10% to 90%, seconds
f.sub.o =corresponding frequency, in Hz
SINE WAVE ATTENUATION
Conductor losses may be expressed by the following formula:
where:
A.sub.c =attenuation of the copper conductor, dB/100 ft.
Z.sub.d =characteristic impedance of cable, ohms
d=diameter of copper conductors, inches The formula for insulation
losses:
where:
A.sub.d =attenuation of the insulation, decibel/100 ft.
.epsilon.=dielectric constant, relative to air
D.sub.f =dissipation factor
The calculated attenuation for the prototype cable in decibel/100
ft. units:
______________________________________ Frequency in MHz 10 100 1000
______________________________________ A.sub.c = (1.327)
(f.sub.MHz) 4.20 13.27 42.0 A.sub.d = (0.008) f.sub.MHz 0.08 0.08
8.0 A.sub.f = 4.28 14.07 50.0
______________________________________
FIG. 7 shows the calculated and measured Sine Wave attenuation
values on the prototype cable (CA-490).
PULSE RISE TIME ATTENUATION
Both the edge and magnitude of the pulse are attenuated through a
length of signal transmission line. These losses are due to the
cable conductors and insulator, and will become evident by
comparing the input and output shape of the pulse. In a given cable
these losses are affected by the input rise time of the pulse and
also by the length of cable.
To calculate the rise time attenuation a formula was developed for
coaxial cables:
where:
T.sub.o =time in seconds, needed for the output pulse to reach the
50 percent reference level of the input rise time
A.sub.o =A.sub.c +A.sub.d in decibel/100 Ft. units
f.sub.o =0.295/t.sub.v ; frequency in Hz
l=length of cable in feet
t.sub.r =input pulse rise time in seconds.
This formula applies to a theoretically vertical pulse edge or at
least to a very fast pulse rise time. In practice, however, it is
necessary to deal with 1 or 5 nanosecond rise time pulses. When the
actual input rise time is comparable or slower than this calculated
rise time attenuation of the cable, both should be considered.
Referring to the tabulation of FIG. 6, with t.sub.r =5 nanosecond
pulse input, the output rise time based on the T.sub.o calculation
is only 2.515 nanoseconds. It is obvious, however, that when the
cable is input with a pulse edge of 5 nanoseconds, the output
should be at least 5 nanoseconds, but cannot be less. Consequently,
it seems necessary that the actual input rise time would be
accounted for in the calculations. Using this method for
predictions, a reasonable agreement can be found to the actual
measurements. FIG. 4 is a tabulation showing the calculation of
T.sub.o. FIG. 5 is a tabulation showing the schedule of the
different amplitude levels of the actual input pulse, normalizing
the 10-90% rise time for the tabulation of FIG. 4.
The prototype cable CA-490 was tested for rise time attenuation
with three different pulse rise times:
0.18 nanoseconds
1.0 nanoseconds
5.0 nanoseconds
Results are shown in the tabulation of FIG. 6 in comparision with
the calculated values.
LINE-TO-LINE INTERFERENCE (CROSSTALK)
Bundled twisted pairs, triplets and conventional multi-signal flat
cables generally give no particular difficulties with certain
transmission line parameters, such as characteristic impedance,
propagation velocity or attenuation; however, line-to-line
interference or crosstalk becomes a problem with fast rise time
pulses or high frequency signals; for such signals coaxial cables
are used at present for adequate overall performance.
In describing crosstalk between closely located signal transmission
lines, the generally used terminology throughout the industry
is:
1. Signal line: consists of signal and ground conductors;
2. Active line: conducting signal line;
3. Quiet line: nonconducting signal line;
4. Near End crosstalk: interference measured in
Quiet line at the end where signals enter the Active line;
4a. with pulsed signals: Fast crosstalk and peak crosstalk;
4b. with sinusoidal signals: the maximum level;
5. Far End crosstalk: interference measured in the
Quiet line at the load end of the Active line;
5a. with pulsed signals: Differential crosstalk; peak crosstalk
5b. with sinusoidal signals: the maximum level;
6. Fast crosstalk; reaches maximum magnitude when twice the
propagation time of Quiet line is greater than input pulse rise
time: this is a miniature replica of the input pulse; the width is
equal to twice the propagation time of the Quiet line; same
polarity as the input signal;
7. Peak Near End crosstalk: develops generally at the end of the
Fast crosstalk; may be caused by fringing field effect or
termination mismatch;
8. Differential crosstalk: spike shaped, opposite polarity than the
input pulse; magnitude depends on fringing field effect at the
cable's outskirt and on the length of cable;
9. Peak Far End crosstalk: either polarity;
10. Matched-terminated: the output impedance of the generator
(pulse or signal) is matched to the characteristic impedance of the
Active line (lines);
The input impedance of the measuring instrument (oscilloscope) is
matched to the characteristic impedance of the signal line being
tested (Active or Quiet); all other signal lines are terminated to
loads equal to their characteristic impedances.
In conventional multi-signal transmission line cables the
Differential Far End crosstalk causes the highest level of
interference; at the same time this is the area where the composite
multi-signal transmission line cable of the subject invention is
most effective. To study the differences, test results on the
prototype cable (CA-490) were compared to those measured on a basic
polyethylene cable core (CA-489); the latter may be considered a
good quality conventional flat cable. FIG. 8 shows oscilloscope
traces of the crosstalk at the Quiet line output measured by
utilizing the following pulse rise times: 0.18, 1, 2, 5 and 10
nanoseconds consecutively for the Active line input signal. For
this test one Active line was driven adjacent to the Quiet line.
This condition offers an optimum fidelity and clarity for obtaining
and studying shapes of different crosstalk pictures because the
characteristic impedance of the cable specimen can be matched to
the impedances of the Pulse Generator and Oscilloscope (50 ohms)
without special network means. The improvement exhibited by the
jacketed, composite multi-signal transmission line cable of the
subject invention may be concluded by the following tabulation:
______________________________________ Crosstalk in Percent Basic
Prototype t.sub.r Polyethylene Cable nsec. CA-489 CA-490
Improvement ______________________________________ 10 0.3 0.05 6
times 5 0.6 0.1 6 times 2 1.5 0.1 6 times 1 3.0 0.5 6 times 0.18
8.6 1.1 7.8 times ______________________________________
FIG. 8 is a representation of actual photographs showing far end
crosstalk with each of the five different pulse rise times on both
pictures. On CA-489, the base lines are aligned for easier reading.
The crosstalk control is clearly visible in the reduction of the
spikes and are most evident at the fast rise time pulses.
The reason for this crosstalk reduction is the physical difference
between the basic polyethylene cable CA-489 and the prototype cable
CA-490; i.e., the jacket. The latter makes the cable thicker and it
also has a higher dielectric constant and higher dissipation factor
than the polyethylene cable core.
A reasonable question arises; was the crosstalk reduced by the
thicker cable or by the different dielectric? For an answer to this
query, another cable (CA-580) was built with a polyethylene jacket
around the basic polyethylene cable core with dimensions identical
to the prototype cable CA-490. The Far End crosstalk measured on
this specimen is shown in FIG. 9.
Comparing these measurements to the previous two cables, the
following conclusions may be reached:
1. Far End crosstalk improved with the thicker polyethylene cable
(CA-580) compared to the thinner one (CA-489). However, the
composite multi-signal transmission line cable of the subject
invention utilizing different dielectric materials confined the
crosstalk far greater.
2. The differential spike (opposite polarity than the signal pulse)
at fast rise time pulses still characterizes the thicker
polyethylene cable, while the same is diminishing in the composite
multi-signal transmission line cable of the subject invention (with
t.sub.r =0.18 nsec., it is 2.6 percent in cable CA-580, while only
0.5 percent in prototype cable CA-490).
For the 10 foot prototype cable, the Near End crosstalk was
measured with one Active line adjacent to the Quiet Line.
The above tests clearly demonstrate the crosstalk controlling
features of the multi-signal transmission line cable of the subject
invention. However, for quantitatively worst case results, tests
were made driving four Active lines (two at each side of the Quiet
line) on 10-foot and 50-foot long prototype cables. In each
instance, the performance of the cable made according to the
teachings of the subject invention were substantially better than
conventional flat cable transmission lines.
It is the inventor's conclusion that the construction of the
subject multi-signal transmission line cable, and in particular,
the difference in the dielectric constants of the insulated jacket
and the core material results in the extremely desirable electrical
characteristics. The inventor is not apprised of a definitive
explanation at this time as to why the provision of an outer jacket
having a higher dielectric constant provides the surprising and
unusual, and extremely desirable result, of reducing the
differential far end crosstalk on adjacent quiet lines, when a fast
rise time pulse is applied to an active signal line. It is believed
that the basic TEM mode in its ideal form is affected adversely by
the surrounding air at the boundaries of conventional multi-signal
flat cables causing "Differential" cross-talk in the adjacent
signal lines at the far end of the cable. Differential crosstalk is
created by the transients (leading edge, trailing edge) of a pulse
and always has a polarity opposite to the direction of the swing in
the Active line pulse. The magnitude of the Differential crosstalk
is increased by both: faster transient times; and length of cable.
The term "first mode" of propogation may be used to describe this
air-affected fringing mode.
The jacket of the invention alters this harmful "first mode" of
propogation by changing the character and magnitude of the
"Differential" crosstalk. The term "Second mode" of propogation may
be used to describe the propogation of the subject jacketed cable.
It is theorized that the higher dielectric constant of the
insulator jacket may effectively prevent or effectively the nature
of the "first mode" propogating fringing field of the input pulse
signal so as to substantially cause attenuation of the far end
crosstalk in the adjacent signal line.
It is believed that the provision of the higher dielectric constant
outer jacket may excite a "second mode" of propogation in the
Active signal line, and that may be of an opposite polarity to the
interference or crosstalk created by the "first mode"
electromagnetic field, whereby the resulting effect on the adjacent
Quiet line at the far end is a substantially reduced far end
crosstalk level.
Stated differently, the construction of the subject composite
multi-signal transmission line cable, and specifically the
arrangement of the higher dielectric outer jacket, may excite a
second mode of propogation. The effect of this "second mode" on the
adjacent quiet line may be of a positive interference or crosstalk
effect, whereas the "first mode" crosstalk may be of a negative
value, whereby the total of the impositions of the first and second
modes on the adjacent quiet line is either a cancellation or a
substantially reduced crosstalk signal which is below the allowable
limits for operation of the circuitry.
It is anticipated that further experimentation may yield a
definative answer as to the operation of the system. Nonetheless,
it has been positively determined, that this specific construction
of a flat cable encased in an insulation of a higher dielectric
material than the core material provides extremely beneficial
electrical characteristics and, when the outer jacket is made of
certain desirable materials such as vinyl, it additionally provides
mechanical properties which greatly enhance and increase the value
of the resulting transmission line cable.
It is noted that the desirable characteristics of the subject
composite cable appear to be further enhanced by the selection of
insulation materials which provides that the lossiness or
dissipation factor (i.e., the property of a dielectric material to
dissipate energy) of the inner core material 13 is less than the
lossiness or dissipation factor of the insulator jacket 16.
It is understood that the present invention is susceptible to
various modifications, changes and adaptations, and the same are
intended to be comprehended within the meaning and range of
equivalence of the appended claims. For example, it is readily
apparent that other dielectric materials may be employed in the
subject invention, with the one limitation being that the other
insulator jacket have a higher dielectric constant than the inner
core material in which the conductors are embedded. Furthermore,
although the invention has been described with respect to
alternating flat and round conductors, it is also readily apparent
that other conductor configurations, for example, two
rectangular-shaped conductors interposed between adjacent round
conductors, may also be employed, in which case the parameters "w"
"s" and "p" will be adjusted accordingly. The parameter "w" would
be the combined width of the several flat conductors disposed
between the round signal conductors, whereas the parameters "s"
would be the spacing between the signal conductor and the nearest
rectangular conductor. Furthermore, the parameter "p" would
likewise be the spacing between a signal conductor and the adjacent
imaginery ground conductor disposed withih the adjacent flat
conductor. Also, the composite multi-signal transmission line cable
of the subject invention may be constructed so as to include only
round conductors. Likewise, the conductors of the subject
multi-signal transmission line cable may take the form of a
plurality of generally parallel conductors made up of twisted pairs
or twisted triplets, in which case the multi-component conductors
will be embedded in a core having a lower dielectric constant,
about which is disposed an insulator jacket made of a material
having a dielectric constant greater than the dielectric constant
of said inner core.
It has been empirically determined that the combined thickness of
the jacket is preferably approximately two-thirds of the thickness
of the cable core which is capable of confining within its
cross-sectional area approximately 98 percent of the TEM field.
Stated differently, the combined thickness (i.e. above and below
the cable core) of the jacket should be 2/3 of the thickness of the
hypothetical cable core capable of confining 98 percent of the TEM
field, in those instances where the thickness of the cable core is
insufficient to confine 98 percent of the TEM field.
* * * * *