U.S. patent number 3,872,408 [Application Number 05/466,664] was granted by the patent office on 1975-03-18 for signal directional tap.
This patent grant is currently assigned to Lindsay Specialty Products Limited. Invention is credited to James Park Reilly.
United States Patent |
3,872,408 |
Reilly |
March 18, 1975 |
Signal directional tap
Abstract
A directional tap for tapping electronic signals from a
distribution line is shown herein. The tap includes a directional
coupler having a current sensing transformer and a voltage sensing
transformer. The current sensing transformer has a primary winding
connected in series with the distribution line, and the voltage
sensing transformer has a primary winding connected in shunt to the
distribution line. The secondary windings of the two sensing
transformers are connected in parallel with each other. The current
sensing transformer has a turns ratio of ##SPC1## And the voltage
transformer has a turns ratio of ##SPC2## In which N is a variable
representing the number of selected windings and r is the ratio Z/z
in which z is the characteristic impedance of the distribution
line, Z is the selected output impedance of the coupler, and Z is
not equal to z.
Inventors: |
Reilly; James Park (Lindsay,
Ontario, CA) |
Assignee: |
Lindsay Specialty Products
Limited (Lindsay, Ontario, CA)
|
Family
ID: |
23852639 |
Appl.
No.: |
05/466,664 |
Filed: |
May 3, 1974 |
Current U.S.
Class: |
333/112 |
Current CPC
Class: |
H03H
7/482 (20130101) |
Current International
Class: |
H03H
7/48 (20060101); H03H 7/00 (20060101); H03h
007/48 () |
Field of
Search: |
;333/6,8,10
;325/308 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gensler; Paul L.
Attorney, Agent or Firm: Fors; Arne I. Piper; Frank I. Wong;
David W.
Claims
What I claim as new and desire to protect by Letters Patent of the
United
1. In a directional tap for tapping electronic signals from a
distribution or transmission line and supplying the tapped signals
to at least one subscriber, said directional tap including a
directional coupler means having a current sensing transformer and
a voltage sensing transformer wherein said current transformer has
a primary winding connected in said distribution line and said
voltage sensing transformer has a primary winding connected in
shunt across said distribution line, the secondary windings of the
sensing transformers being connected in parallel, the improvement
comprising said current sensing transformer having a turns ratio of
##SPC7##
and said voltage sensing transformer having a turns ratio of
##SPC8##
wherein N is the number of windings and r is the ratio Z/z in
which
z is the characteristic impedance of said distribution or
transmission line and
Z is a selected output impedance of said coupler means, and Z
.noteq. z.
2. A directional tap according to claim 1 including a resistor R
connected in parallel with the secondary winding of the current
sensing transformer, said resistor having a resistance equal to
3. A directional tap according to claim 1 including a resistor R
connected in series with the secondary winding of the voltage
sensing transformer, said resistor having a resistance equal to
4. A directional tap according to claim 2 including a splitter
means connected to said coupler means for receiving the tapped
signals from an output port of said coupler means and for dividing
said tapped signals into two equal feed signals for servicing two
subscribers, said splitter means comprising an auto-transformer
having a center tap connected to said output port of said coupler
means and two output terminals for supplying
5. A directional tap according to claim 3 wherein the resistance of
said
6. A directional tap according to claim 3 including a splitter
means connected to said coupler means for receiving the tapped
signals from an output port of said coupler means and for dividing
said tapped signals into two equal feed signals for servicing two
subscribers, said splitter means comprising an auto-transformer
having a center tap connected to said output port of said coupler
means and two output terminals for supplying
7. A directional tap according to claim 6 including a compensation
capacitor connected to said center tap for compensating leakage
inductances in said autotransformer, and an output resistor
connected
8. A directional tap according to claim 7 including two output
autotransformers, each of said output autotransformers having a
center tap connected to one of said two output terminals for
dividing said feed signals into further equal feed signals, and
having four output ports for supplying said further feed signals
through feed lines operative for
9. A directional tap according to claim 8 wherein each of said
center taps of said output autotransformers has a compensation
capacitor connected thereto for compensating leakage inductances in
said output autotransformers, and each output autotransformer
having an output resistor connected across its output ports.
Description
BACKGROUND OF THE INVENTION
This invention relates to directional taps and particularly relates
to directional couplers and splitters in such taps for retrieving
electronic signals from a transmission trunk line or distribution
line and supplying the tapped signals to a plurality of
subscribers.
In a community antenna television system (CATV system) the
electronic signals including the video and audio signals are
transmitted by coaxial trunk lines over long distances. Amplifiers
are located at periodic intervals to maintain the level of the
transmitted signals. The signals in the trunk lines are fed to
distribution lines to selected areas and a plurality of directional
taps are located in the distribution lines to tap a portion of the
signals transmitted therein. The tapped signals are fed through
feed lines to the subscribers. The directional taps also provide
the required impedance matching between the distribution line and
its feed lines.
A directional tap conventionally comprises a directional coupler
circuit connected to the distribution line for routing a portion of
the signals therefrom. The retrieved signals are then divided into
a plurality of equal portions by a splitter circuit for feeding to
the subscribers. The splitter circuit also provides a buffer for
isolating the feed lines from one another.
The directional coupler circuit comprises mainly a current sensing
transformer and a voltage sensing transformer which are identical
in construction and have the same turns ratio but the current
sensing transformer has a relatively small primary winding and a
large secondary winding. On the other hand, the voltage sensing
transformer has a large primary winding and a relatively small
secondary winding. The primary winding of the current sensing
transformer is connected in the distribution line such that the
current of the distribution line flows through this winding. The
primary winding of the voltage sensing transformer is connected in
shunt across the distribution line such that the voltage of the
line appears across this winding. The secondary windings of the two
transformers are connected in parallel aiding such that the signals
retrieved from the distribution line input are in phase and
additive at the coupler output; whereas, voltages induced in these
secondary windings due to any reflected signals on the distribution
line are 180.degree. out of phase at the coupler output and cancel
each other. In this manner, the directional coupler prevents any
reflected signals to reach the tap output and permits only signals
coming from one direction to be routed therethrough.
The signals at the output of the coupler circuit are fed to a
splitter circuit which comprises an input shunt step-down
transformer for stepping the output impedance of the directional
coupler to a desirable lower value to match with the impedance of
the output transformers of the splitter which divide the signals
into equal portions to be supplied to the subscribers.
The main drawback in the conventional directional taps is the high
insertion loss in the retrieved signals due to losses in the stray
impedances in the various transformers and particularly in the
shunt stepdown transformer in the splitter circuit. This drawback
may be somewhat alleviated by incorporating more amplifiers in the
distribution line to compensate for the losses. However, amplifiers
cause noise and distortion to be injected into the signals. Such
amplifiers are very expensive to make and maintain and they
contribute to the major sources of unreliability in the CATV
system.
Furthermore, the frequency response of such conventional taps is
poorer in the low frequency bands than in the mid-frequency and
high frequency bands. This is mainly due to the difficulty involved
in obtaining enough shunt inductance in the transformers. Also, the
stepdown transformer in the splitter circuit contributes to the
bulkiness of the tap and the design of such stepdown transformer is
very unwieldy.
PURPOSE OF THE INVENTION
The principal object of this invention is to provide a directional
coupler which is easily adaptable to distribution lines of elected
impedances to tap the signals therefrom.
It is an object of the present invention to provide a directional
tap in which no stepdown input transformer is required in the
splitter circuit.
It is another object of the present invention to provide a
directional tap which has an even frequency response for all bands
of the signals.
It is another object of the present invention to provide a
directional tap having a low insertion loss.
It is yet another object of the present invention to provide a
directional tap which is simple and flexible in structure.
SUMMARY OF THE INVENTION
In the directional tap of the present invention, the coupler
circuit comprises a current sensing transformer and a voltage
sensing transformer. The primary winding of the current sensing
transformer is connected in series with the distribution line such
that the current of the distribution line flows through this
winding. The primary winding of the voltage sensing transformer is
connected across the distribution line such that the voltage in the
distribution line appears across this winding. The secondary
windings of the two transformers are connected in parallel aiding
and a resistor is connected in series with the secondary winding of
the voltage sensing transformer. The turns ratio of the current
transformer is equal to ##SPC3##
wherein N is the number of windings and r = Z/z in which
z is the characteristic impedance of the distribution line, and
Z is the selected output impedance of the coupler circuit and Z
.noteq. z.
The turns ratio of the voltage transformer is equal to ##SPC4##
The variable N determines the amount of the voltage attenuation
between the coupler input and the tap output.
BRIEF DESCRIPTION OF DRAWING
The foregoing and other objects and features of this invention will
be more fully understood from the following description of an
illustrative embodiment thereof taken in conjunction with the
accompanying drawing, the single FIGURE of which shows in schematic
form, the details of a directional tap in accordance with this
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
Referring now to the drawing, the directional tap has a coupler
circuit which includes a current sensing transformer 10 which has a
primary winding 11 and a secondary winding 12. The primary winding
11 is connected in series in a distribution line 13 to form part of
the line such that the current in the line flows through this
winding. The signal in the input end of this distribution line is
schematically represented by a variable voltage source 14 having a
voltage Vs and the impedance of the line is represented by resistor
15 which has a value z. The output end of the distribution line may
be connected to a load 18 which normally also has an impedance
equal to z.
The directional coupler circuit also includes a voltage sensing
transformer 20 which has a primary winding 21 and a secondary
winding 22. The primary winding 21 is connected across the
distribution line such that the voltage of the distribution line
appears across this winding.
The secondary windings 12 and 22 are connected in parallel aiding
and a resistor 24 is connected in series with the secondary winding
22 of the voltage sensing transformer. A compensation capacitor 23
is connected in parallel with the resistor 24. The node 26 is the
output node of the coupler circuit.
The turns ratio of the voltage sensing transformer is ##SPC5##
in which N is the number of windings in the primary winding in
relation to each winding of the secondary.
The turns ratio of the current sensing transformer is ##SPC6##
in which N is equal to the value N derived from the voltage sensing
transformer, and r = Z/z in which z is the characteristic impedance
of the distribution line and Z is the selected output impedance
appearing at the output node 26 of the coupler circuit and Z is not
equal to z.
The voltage induced in the secondary winding 22 of the voltage
sensing transformer is
Vs/N
and the current produced in the secondary winding 12 of the current
sensing transformer is
I/N.sup.. r
in which I is the current in the distribution line.
The current in the secondary winding 12 due to a signal coming from
the input end of the distribution line is in phase with the voltage
Vs/N induced in the secondary winding 22 of the voltage sensing
transformer. Therefore, the signal having a voltage Vs/N will
appear at the output node 26 of the coupler circuit, and the
impedance seen looking into node 26 is z.sub.. r.
If a reflected signal exists in the distribution line such that
this reflected signal is flowing in the opposite direction from the
output end of the line to the input end, the current produced by
this reflected signal in the secondary winding 12 of the current
sensing transformer will produce a voltage across resistor 24 (and
capacitor 23) which is equal and opposite to the voltage in the
secondary winding of the voltage sensing transformer. Thus, the
reflected signal in the distribution line will not produce any
voltage at the output node 26 of the coupler circuit.
In this manner, only the signals flowing from the input end to the
output end of the distribution line are routed to the output node
of the coupler circuit. The total output impedance Z of the coupler
circuit as appearing at the output node 26 is
Z = R + (z/N.sup.2)
in which R is the resistance of the resistor 24. Normally, Z is
approximately equal to R since the value of z/N.sup.2 is relatively
much smaller than the value of R.
The ratio of the turns ratios of the two sensing transformers may
be selected such that the output of the coupler circuit may be
connected to a distribution line of any selected impedance. Also,
the ratio of the turns ratios of the two sensing transformers may
be selected such that the output impedance matches that of the
splitter circuit or the impedance of the feed lines directly so
that a stepdown transformer is not required to match the impedances
of the coupler circuit and the splitter circuit as in known
directional taps.
The output impedance Z of the coupler is predetermined by z.sup..
r, where r is the preselected or desired ratio between the output
impedance of the coupler and the characteristic impedance z of the
distribution line. The resistor R may then be chosen equal to
R = Z - (z/N.sup.2)
Then, the turns ratio of the current sensing transformer is made
equal to 1:N.r to obtain the directional characteristics of the
tap.
In the above manner, it is able to select any desired impedance at
the output of the coupler, thus alleviating the necessity for any
impedance changing transformer at the input of the splitter circuit
as in known directional taps.
The turns ratio of the voltage sensing transformer N:1 is selected
to provide the desired attenuation between the input of the coupler
and the tap output. This attenuation expressed as a ratio is equal
to 1/N.
It will be appreciated that the capacitor 23 and resistor 24 may be
connected to the node 26 instead and the output of the coupler
taken from the secondary winding of the voltage sensing transformer
to achieve the same effective result. In this case, the total
output impedance of the coupler is
Z = [R.sup.. z(N.sup.. r).sup.2 ] /[R+z(N.sup.. r).sup.2 ]
or R = [Z.sup.. z(N.sup.. r).sup.2 ]/[z(N.sup.. r).sup.2 -Z]
The tapped signals at the output node 26 of the coupler circuit may
be divided into two equal portions by a splitter comprising an
autotransformer 27 having output terminals 36 and 37. Each output
terminal 36 or 37 may be connected to a feed line to supply the
feed signals to two subscribers. A compensation capacitor 28 is
connected to the input of the autotransformer 27 to compensate for
the leakage inductances in the autotransformer. An output resistor
31 may be connected across the terminals 36 and 37 to provide the
impedance match with the impedance of the feed lines and to provide
electrical isolation between the feed lines. Additional
autotransformers 29 and 30 may be connected to the output terminals
36 and 37 to divide the feed signals into further equal portions to
supply the further feed signals to four subscribers as shown in the
drawing. A compensation capacitor 34 connected to the input of the
autotransformer 29 to compensate for the leakage inductances in the
autotransformer, and an output resistor 32 is connected across the
output terminals to provide the required impedance matching with
and electrical isolation between the feed lines. Similarly, a
compensation capacitor 35 is connected to the input of
autotransformer 30 to compensate for the leakage inductances in
this autotransformer and an output resistor 33 is connected across
the output terminals to provide the proper impedance matching with
and electrical isolation between the feed lines. In this manner, a
selected number of autotransformers may be incorporated to make up
the splitter circuit to provide signals to two, four or even eight
subscribers if required. It will be appreciated that the output
impedance of the coupler, and therefore the ratio r, changes with
the number of tap outputs provided for the subscribers.
In known directional taps, the splitter circuit must necessarily
and basically include the stepdown transformer as well as at least
three autotransformers to provide four tap outputs, or at least one
autotransformer when only two subscribers are serviced by the tap.
On the other hand, the coupler circuit of the present invention may
be directly connected to a selected number of subscribers by
incorporating a splitter circuit comprising of a suitable number of
autotransformers without any stepdown transformer. Hence, the
design of the directional tap of the present invention is extremely
flexible such that it can match any requirements without the
presence of any unused component parts. The number of component
parts is therefore greatly reduced. This reduces the manufacturing
time and costs. And, due to the elimination of the stepdown
transformer, the insertion loss of the directional tap is greatly
reduced.
In a typical example, if a directional tap is required to be
connected to a 75 ohms distribution line to provide a 20 dB signal
output and the output impedance of the coupler is desirable to be
18.75 ohms as is the case when four tap outputs are required, the
ratio r of the coupler of the present invention is
r = Z/z = 18.75/75 = 1/4
The turns ratio N/1 = 10.
The insertion loss from the input to the output ports of the
directional tap is
insertion loss (dB) = 20 log (1- 4/N.sup.2)
= 20 log (1- 4/100)
= -0.36 dB
or equal to -0.4 dB maximum on the average, taking into
consideration other imperfect component leakages and losses.
For a conventional directional tap to meet the same requirements,
the turns ratio of both sensing transformers is 4:1. The insertion
loss is
20 log (1- 1/N.sup.2)
= 20 log (15/16) = -0.56 dB
The maximum insertion loss on the average in a conventional tap,
taking into consideration the imperfect component leakages, is
about -0.8 dB.
Therefore, by using the directional tap of the present invention a
saving of about 0.4 dB can be achieved.
For a distribution line having ten directional taps cascaded
between line amplifiers, the present directional tap provides a
maximum reduction of about 4 dB in insertion loss. After a
five-amplifier cascade, there is a saving of 20 dB. The gain of an
amplifier in a distribution line is normally about 20 dB, so that
by the use of the present directional tap, one amplifier may be
eliminated in a conventional five-amplifier cascaded distribution
line to achieve a saving of 20% of the number of amplifiers
required. This will result in a substantial improvement in noise,
distortion and reliability for the system.
Since no stepdown transformer is required in the present
directional tap and a relatively large turns ratio may be
incorporated in the sensing transformers, the present directional
tap has an even response in all frequency bands of the transmitted
signals.
Although a specific embodiment of this invention has been shown and
described, it will be understood that various modifications may be
made without departing from the spirit of this invention and within
the scope of the appended claims.
* * * * *