Note: Descriptions are shown in the official language in which they were submitted.
-- 1067188 ~ ::
The invention relates to connection networks for time-
division multiplex systems, in particular for PAM telephone ex- `~
changes. -~
According to the time-division multiplex technique, the `~
communication line betwçen various pairs of subscribers is periodi-
cally allocated to a conversation for a short sampling time period.
In the time interval between two successive sampling periods relat-
ing to the same conversation, the line remains available for further
communications. The modern PAM technique transmission ~ystems are
capable of establishing one hundred simultaneous communications at
a sampling frequency of about 8 KHz along a single communication
line.
The large volume of traffic which can take place along
the same line presents various problems the solution of which is
essential in order to obtain good quality communication. For exam-
ple, during the time period allocated to a conversation the connec-
tion network tends to store energy which causes alterations to
occur in the signal samples from communications allocated to sub-
sequent channels, thus causing cross-talk therebetween. As a
result, information is transferred from one transmission channel
:. .
; to another, thereby affecting the signal associated with each.
One way of at least partially eliminating this drawback
~ is by oonnecting the transmission line to earth during the time
;~ periods separating contiguous channels.
A second known solution which is described in the Cana-
~- dian Patent No. 939,087 issued on December 25, 1973 to Formenti
et al provides a three-shaped structure for the communication
line; the energy stored in each single length or section is dis-
` charged via the characteristic impedance of the same section during
`~ 30 the separation periods between contiguous channels.
; Further known solutions, described in the Canadian Pa-
tent Nos. 1,010,518 and 1,010,519, both issued on May 17, 1977 to
;~ ~. . , - , . : -
,i~ 1067188
; Erculiani, and No. 1,013,442 issued on July 5, 1977 to Erculiani,
involve the use of co-axial telephone lines having a specially
designed structure in order to store very little energy.
Althou~h the above-described techniques improve the quali-
.
.:
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~067. 8~3
ty oE ~lle ~ral~smi-;sion, ~l~c abovc-men-tioned disadvantages are not
completely eliminated since persistence of cross-talk may still
be observed.
An object of the present invention is to provide a trans-
mission line which is substantially free from cross-talk and other
-~ related effects.
The present invention concerns a connection network in a
transmission system for a telecommunication network with time-
':'
division-multiplex exchanges, said system including trunk.and
branch lines with coaxial inner and outer conductors separated
by solid insulation and a group switch for controlling signal
transmission to a multiplicity of subscriber stations connected
to a common branch line, said connection network being characteri-
~ zed in that said group switch comprises semiconductor means
encased in dielectric material and leads connecting said semi-
conductor means between two inner condu~tors, said insulation for-
.~ ming a cavity receiving said group switch within an outer conduc-
~, tor and in line with at least one of said inner conductors sur-
i~ rounded by said outer conductor,said insulation forming a substan-
. 20 tially continuous sheath around said cavity and said group switch.
. A preferred embodiment according to the present invention
~. will be described with reference to the accompanying drawings, in
.~ which:
.~ Figure 1 shows a connection network arrangement according
- to the present invention;
Figure 2a is a diagram illustrating the law according to
. which the magnitude of the current density vector varies through-
. out the conductor cross-section;. - . ...... . .. - ._._.. ___ .. .. _
1'''
~067188
~ igure 2b $s a diagram illustrating the law according to
v~ich the phase of the current density vector varies throughout the
conductor cross-section;
~ igure 3 diagrammatically illustrates the phenomenon deter-
- mining the current distribution in a conductor;
Figure 4 i8 an equivalent electrical circuit diagram of
a conductor length for cross-tal~ purposes;
: Figure 5 diagrammatically illustrates examples of group
switches incorporated in the same line;
Figure 6a is an RC damping circuit located between the
, subscriber circuit and the respective telephone s~itch;
.. Figure 6b i~ an electri¢ circuit diagrammatically showing
the behavior of the open telephone swhtch during the transient sta- -:
::
te;
Figure 6c ~hows the loop formed by a subscriber circuit
and the rsspective telephone awitch when considered in an open con-
: dition;
~igure 7a shows the configuration of the voltages occuring ~ :
along the sound lines relative to two generic speech channels; and
- ~igure 7b shows the configuration of the oscillations
induced in a subscriber circuit by the sudden variations o$ the vol-
: tages occuring along the telephone lines.
.: The improvement~ mainly characterising the connection
network according to the present invention, an embodiment of which
is given in Figure 1~ have been ~ound from the results obtained
after a thorough study made on the phenomena go~rning the propaga-
tion of a time-varying current.
It is already know~ that such a current is distributed in
Pn irregular way throughout the cross-sectional area of the conduc-
tor along which it flows.
An analytical ex~m;nation of this phenomenon~ which i~
.. known a~ "skin effectn~ in connection with conductors of circular
. . .
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~ 3 -
:. .. . ~, .' , ~ .' .,
1067188
¢ross-section leads to the current density J expressed a~ a function
of the frequency and the di9tance r from the axis of the conductor.
The results of such examination are illustrated by thedia-
grams~ of Figure 2_ and Pigure 2b.
In the diagrams:
J/JS Z ratio between the current density at a generic point (indi-
cated by J) and the current density at the surface of the
conduotor (indicated by JS);
~ (J/JS) = phase angle of vector J/JS;
r/rO = ratio bet~een the distance from the axis of the conductor
of the generic point to which J refers (indicated by r) and
the radius Or the conductor (indicated by rO);
. E e ~ where S ~ ~ where ~J is the frequency o~ the
; current and G and J4 the electrical conducti~ity and
magnetic permeability respectively~ of the conductor.
From the dimensional point of view, ~ is~a length and
is called "penetratlon depthn~ For example~ in the case of copper
conductors and alternating currents at a frequency of 0.6 ~Hz (0.6
~Hz being the order of magnitude of the freguency used at present
for the resonant transfer in time-division telephone ~ystems)~
S = ~' 90 ~m and thus~ si~ce usually k >> 1~ the current is
distributed in a very irregular ~ay throughout the conduotor cross-
;~ section.
- - A pos~ible physical illustration of this phenomenon i6
to consider the generation of c~rculating edd~ currents induced in
the conductor by the magnetic f~eld accompanying the slgnal current.
~y indicating the density of the circulating eddy currents
~- by Jc, the density of the signal current assumed to be 1n~formely
distributed by Ju~ the actual current flowing along the c~nductor~
indicated by the density vector J, i9 obtained as superimposition
of the two current fields Jc and ~u.
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1067188
Figure 3 illustrates the above~ in the case of a co-axial
line.
~ igure 4 (in particular part ~T) shows a circuit which
simulates the electri¢al behavior of the transmission line~ obtained
as a phy~ical simplification of the above illu~trated phenomenon.
In this Figure~ where only four subscribers ~A~ ~B~ U~ and UD are ;-
considered for purposes of clarity~ the following notations have
been used:
Va = signal voltage.
Vd - cross-talk voltage.
= signal current associated with the resonant transfer Va.
I2 = takes into account parasitic effects of the circularing- -~
; currents.
~12 = takes into a¢count the magnetic coupling between ~1 and ~2.
2~ R2 = equivalent parameters which depend upon the particular
transmission line.
C = inductance and capa¢itance of the subscriber circuit.
~ssu~e that~for the possible connectio~s bet~een the subs-
cribers of ~ieure 4, two channels ~l and ~2 are available. ~oreover~
assume that UA is speaking to ~ during 01~ while UB is speaking to
UD during ~2-
Upon recuring of the time phase ~l ~ the signal current I
associated with the transfer of the ~ignal voltage Va induces a
flowing current I2 into the circuit ~2~ R2 through the mutual in-
f ductance Ll2.
hen pha~e ~2 occurs, the current I2 which would self~es-
~ tinguish in a much loDger time than the time spacing bet~een the
; ~ two channels~ induces an interferen¢e current Iu into the line,
which current produces cro~s-talk voltage~ at the capacitors of
the ~ubscribers ~B a~d UD.
Since the capacit~n~e of the subscr~ber capacitors are
~- all substantiallg the same, the cro~s-talk voltages ~d occur ~th
... ...
.. .
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1067188
amplitudes equal to one another and op~osite phases.
The circuit of Figure 4 establishes the following
relationship between Va and Vd:
__ 12 2 ~ 2 2 (1 -dr)2 -a /~ c
Va 4L *L2 (a ~0) e (*)
where besides the circuit parameters already mentioned above
there appears
L* = L + 21 : semi-inductance of the resonant circuit:
~ 0= 1 : frequency of the resonant transfer
L C
~ L2
= ~Y \/ L*C half-period of the resonant wave:
~t= guard time between channels.
The results of tests made on a transmission line the
parameters of which were known confirm to a good approximation
the law expressed in the relationship (*).
Furthermore, it was found, in accordance with the
' theory referred to above, that the cross-talk applied to su~-
scribers UB and UD are equal to one another and opposite in sign.
~,
The experimental data thus conform the validity of
the model suggested and show that a high percentage of cross-
talk has to be attributed to the storing of magnetic energy
along the circuit through w'nich the signal current flows. In
.
the above Canadian patent Ns. 1,010,518, 1,010,519 and
1,013,442, there are already illustrated telephone lines
especially designed for providing a substantial decrease in
the storing of magnetic energy.
- According to the present invention, further details of
the connection network have been improved in order to obtain a
drastic decrease in all factors responsible for causing cross-talk.-
In view of the above described theory, a source of cross-
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~067188
td k is to be attributed to the intrinsic construction of the group
switches ( Sl,...., Si,....~ Sm) shown in Figure 1 and in particular
, . .
to the materials commonly usea for manufacturing the same.
A multiplicity of speech channels relating to various con-
versations can flow through the same group switch during an exch~nee
frame. ~he small time spacing between tuo successive channels~ which
at the worst are allocated to two cont~guou~ time phases~ makes these
elements of the connection network particularly critical as far as
cross-talk is concerned. It is kno~n that in the conventional tech-
nique of time division~ the switching funct~on is performed by a
; trAn~istor circuit.
: Tests carried out by us have proved that a ma~or part of
cro~s-talk caused by the group switch is to be ascribed to the
; transistors and in parti¢ular to some of the materials used for pro-
ducing the same.
First of all the connecting means have to be considered.
As known~ the connecti~g leads of a transi~tor must have
a ¢oefficient of thermal expansion substantially egual to that of
the capping insulating materlal ( e.g. glass or ceramic material)
to avoid breaking due to thermal stresses. ~n alloy widely used for
manufacturing connectors is ueova~". Although covar has all the re-
quired thermal characteri~tics, its u~e should be avoided because it
has a high magnetic permeability.
Referring to the suggested simplified theory of the skin
effect~ it should be noted that flowing eddy current~ J¢ in ferroma-
gnetic materiale are of high value, ~ince the magnetic field associa-
ted with the signal current i~ enhanced by the characteristics of
the material.
This is in agreeme~t with the diagrams of Figure 2a and
Figure 2b , when k = ~ i~ considered high)because
= ~ in the case of ferromagnetic materials (high~i )
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f ~ 1067188
becomes very small (the relationships referred to above are those
already mentioned). By simplifying the behaviour of a ferromagne-
tic conductor according to the electric circuit or model of Figure
4 and taking into account the above formula (*) which describes its
behaviour (it should be noted that L12 takes a high value because
a very close relationship is established between Il and I2) the
cross-talk voltage Vd becomes of high value.
Although only a small length of lead is used, the high
cross-talk level caused by the covar cannot be tolerated in a
:.
; 10 connection network of high performance.
In the manufacture of group switches used in a connec-
- tion network according to the present invention, transistors having
leads consisting of non-ferromagnetic alloy (gold-plated, nickel,
silver) which causes very low cross-talk are employed. Should a
multiplicity of transistors be used to perform the switching, (see
U.S. Patent No. 3,567,962 delivered on March 2, 1971, to Formenti,
:... .
for example), some advantages can be attained by arranging them
all in a single encapsulation or by forming them on a single chip
of semi-conductor material.
These measures are designed to reduce the leads both in
length and in number in order to minimize the areas of possible
cross-talk.
`~ Still with referejnce to the characteristics of the mate-
rials to be used in the manufacture of the group switches, it was
found that even the encapsulation of the transistors used can be
a source of cross-talk.
As a matter of fact, the envelope of the conventional
transistors available on the market consists of a sheet of common
; ferrous material. The ferro-magneti~ behaviour of such material
and the presence of a variable magnetic field associated with the
` speech current flowing through the switch cause eddy currents to
occur in the metal of the container. These eddy currents, in turn,
,
. .. - . ~ .. . ~
` 1067188
generate a magnetic field whereof the interaction with the speech
currents causes alterations in the transmitted signals, i.e. it
causes cross-talk.
In order to prevent the phenomenon referred to above -
from occurring,the group switches according to the present inven-
tion are produced by using transistors having a containcr consist-
ing of a non-magnetic material, such as plastics capping materials.
The validity of the innovation made to attain a drastic
decrease in the cross-talking effects referred to above, has been
fully confirmed by the results of experimental tests.
Another measure adopted in the connection network accord-
ing to the present invention is to substantially decrease the num-
ber of welding spots along the paths of the signal currents. As a
matter of fact, the increase in the cross-section of the conductors
due to the weld material allows circulating eddy currents to be
formed whidh in accordance with the suggested theory are a source
of cross-talk (further details on this matter are found in the
Canadian Patent Nos. 1,010,518 - 1,010,519 and 1,013,442).
The reduction in the number of welding spots (and the
elimination of a substantial source of cross-talk thereby) is
attained by incorporating the electronic group switches Sl,....
Si,..., S directly in the telephone lines HWl,... HWi,... HWm.
In the conventional art, said switches are supported on
suitable plates and each of them is electrically connected to the
remainder Gf the telephone line by means of a circuit comprising:
a printed circuit section which connects the switch to a connection
; plug of the plate, a socket for receiving the plate, a conductor
connecting the plate to the telephone line.
It should be noted that the connections required by the
: ~ .
welding of very thick conductors or wires are numerous and result
in a high level of cross-talk due to the magnetic interaction of
; the signal with the mass or with other circuits because of the
speech current flowing along non-coaxial conductors.
. ~ ~ 9 -
r- ~ 1067~88
.
By directly incorporating the switches into the tele-
phone lines (and in any case applicable to said iines) the
` welding spots are reduced to a large extent since the connectors
- of the transistor forming the group switch (Sl,... ,Si,... ,Sm)
can be directly connected to the two conductor lengths to be
connected without interposition of other circuits.
Moreover, by incorporating the group switches in the
telephone lines, it is possible to obtain,a connection network
which is almost fully coaxial in structure (all circuits of
non-coaxial structure, which connect the group switches mounted -
,~ on separated plates together with the telephone lines, are
eliminated) w}lich greatly supresses cross-talk due to eddy
magnetic fields.
Figure 5 diagrammatically illustrates two examples of
~'~ sw~tches applied according to the invention.
In Figure Sa switch S is incorporated in the telephone
line formed by two lengths of a coaxial cable. In particular,
,~ ~
S is connected bstween the internal conductors CI of the two
lengths, while continuity of the outer conductors CE is provided
2~ with a perforated sleeve M in order to allow the control signals
B,Pl and P2 to be applied to the switch S.
; In one of its embodim2nts, the sleeve M comprises two
jaws of insulating material which are metallized on their inner
surface in order to establish an electric contact between
. . .
; conductors CE of the two lengths to be connected.
The thicknéss of the conducting layer is of the same
. . (
order of magnitude as the penetration depth "~" of the signal
currents.
In Figure 5b the switch S is used in a main telephone
line (SHW in Figure 1, for example) a* a branching point of a
secondary telephone line (KWj in Figure 1, for example). By
adopting the technique already illustrated in the Canadian
10 -
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1067~88
Patent N. 1,010,518 of May 17, 1977, the connection between
the switch S and the inner conductor CI of the main telephone
line is obtained by means _ _ _
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~067188
of a conducting plug P, while the outer conductors CE~ accessible
through apertures of suitable shape and dimensions formed in the
insulating layers IE, are connected to one another by a cable (such
as a ~itze wire). ~he tests carried out by use ha~e definitely
proved the ef~iciency of these measures.
In order to eliminate another source of cross-talk, a damp--
i~g cir¢uit R-C may be applied to the subscriber swdtches Ill,....
Ilm,..., Imk, one embodime~t of which is shown in ~igure 6a.
In ~igure 6a there are indicated: a first bloc~ 1~ compris-
ing the subs¢riber circuit (subscriber capacitor Cu and inducta~ce~ù); a second bloc~ 2 ¢omprising the damping circuit RC; a third
block 3, comprising a circuit forming a telephone switch, a control
~ignal being applied to the terminals Pl and P2, and a negative vol-
tage for blocking the switoh during the off period thereof being
applied to terminal B.
With regard to signals passing along the lines of the con-
nection network, the blocked telephone switch may be represented by
the circuit of ~igure 6b in which the parasitic capacitance of
transistors ~1 and ~2 have been taken into account by means of C~
C"~ cn~, the capacity to earth due to the control transformer ~RP
by means of Cm and the resistance of the blocking circuit by Rb.
;~ Figure 6c illustrates a subscriber circuit directlg connec-- ted to the respective telephone sw~tch wqthout u~ing a dampiLg cir~
cuit RC (in the block 3 from capacitors C~ C", C"~ of ~igure 6b
one proceeds to the ~ivalent capacitors C~ C2, C3 through a
transformation ~ -i~ y ).
~ hen a resonant transfer between any pair of subscribers
occurs along the telephone line~ point ~ receives a ~oltage step
. ,~.,
equal to the average voltage V~ among the voltages across the 5ubs-
criber capacitors before the transfer. Similarly, when the resonant
transfer is term nated~ the transmission line is reset by means of
a ~ery rapid transient.
~ ~067~88 ,
The diagram of ~igure 7_ illustrates the waveform of volta-
ge V~ of any two speech phases ( n and n + 1) arranged in ~uccession.~ `
It should be apparent (see circuit of Figure 6c) that steep
voltage fronts applied to ~ ¢auses disturbances at U (such distur-
bances having amplitude proportional to the line voltage V~) which
glve ri~e to damped oscillations in the loop ~, indicated in the Pi-
gure~ due to the presence of reacti~e components. `
Figure 7b shows the wave*orm of the os¢illations generated
at the rising and falling edges of the pulseg VL relative to the
speech phase n.
These os¢illations have also bee~ experimentally detected ~ -
by placing at U a probe having a very low capacity.
If the subscriber being tested is not connected to other
subscriber~ the transients mentioned above have no effect as far as
cross-talk is conoerned. Howe~er, if, in the speech phase n + 1, the
subscriber i9 ¢onnected to another subs¢riber ( by swQtchi ng on the
respecti~e telephone switch)~ it may happen that a certain amount of -~
the e~ergy still prese~t in the loop is picked up by the capacitor - -
Cu and a random ¢ross-talk can occur.
- The cross-talk depends upon the exact operating time of
the telephone switch, the nature of the transistors of the co~nec-
;! tio~ network which vary to a large extent from one point to another,
the various ~witching speeds of the tran~istors in the telephone
switch, an~ other causes whose e~fects 2re uncertain.
An evaluation based on oscilloscope tests of voltages at U -
indicat~ a ~ost unfavourable cross-talk value of ~ 80 dB in connec-
tion with this phenomenon.
I~ our connection networ~, the cross-talk contribution re-
~- ferred to abo~e is praotically eliminated b~ the damping circuit
~-C (block 2, ~igure 68) which damps the oscillation~ in a time
period shorter t~n the interphase guard interval.
. ,.
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