CA1104222A - Transmission line filter - Google Patents
Transmission line filterInfo
- Publication number
- CA1104222A CA1104222A CA256,444A CA256444A CA1104222A CA 1104222 A CA1104222 A CA 1104222A CA 256444 A CA256444 A CA 256444A CA 1104222 A CA1104222 A CA 1104222A
- Authority
- CA
- Canada
- Prior art keywords
- filter
- conductive
- conductive line
- line
- section
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
- H01P1/00—Auxiliary devices
- H01P1/10—Auxiliary devices for switching or interrupting
- H01P1/15—Auxiliary devices for switching or interrupting by semiconductor devices
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
- H01P1/00—Auxiliary devices
- H01P1/20—Frequency-selective devices, e.g. filters
- H01P1/201—Filters for transverse electromagnetic waves
- H01P1/202—Coaxial filters
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
- H01P5/00—Coupling devices of the waveguide type
- H01P5/12—Coupling devices having more than two ports
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
- Y10T29/00—Metal working
- Y10T29/49—Method of mechanical manufacture
- Y10T29/49002—Electrical device making
- Y10T29/49016—Antenna or wave energy "plumbing" making
Landscapes
- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Control Of Motors That Do Not Use Commutators (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A transmisson line filter having two conductors with insulation therebetween. A third conductor of a length related to the wavelength of the signal being filtered is spaced from one of the conductors but electrically coupled thereto. Terminal connectors are located at each end of the filter to interconnect t? filter into the regular transmission line of the system in which the filter is being used. The filter can be formed of coaxial cable and can be a single frequency filter, a multiple frequency filter or a bandpass filter arrangement, In a filter trap arrangement, one end of the third conductor is conductively connected to the other of the main conductors.
A method for making a coaxial embodiment of the filter is achieved by initially placing the main line inner conductor together with the third conductor within the dielectric and subsequently chopping out sections of the third conductor line to adjust its length to the wavelength of the signal being filtered and to provide electrical connection to the outer shield which is subsequently placed over the dielectric. The filter provides unique application in controlling programs sent to subscribers in cable TV systems.
The aforementioned Abstract is neither intended to define the invention of the application which, of course, is measured by the Claims nor is it intended to be limiting as to the scope of the invention in any way.
A transmisson line filter having two conductors with insulation therebetween. A third conductor of a length related to the wavelength of the signal being filtered is spaced from one of the conductors but electrically coupled thereto. Terminal connectors are located at each end of the filter to interconnect t? filter into the regular transmission line of the system in which the filter is being used. The filter can be formed of coaxial cable and can be a single frequency filter, a multiple frequency filter or a bandpass filter arrangement, In a filter trap arrangement, one end of the third conductor is conductively connected to the other of the main conductors.
A method for making a coaxial embodiment of the filter is achieved by initially placing the main line inner conductor together with the third conductor within the dielectric and subsequently chopping out sections of the third conductor line to adjust its length to the wavelength of the signal being filtered and to provide electrical connection to the outer shield which is subsequently placed over the dielectric. The filter provides unique application in controlling programs sent to subscribers in cable TV systems.
The aforementioned Abstract is neither intended to define the invention of the application which, of course, is measured by the Claims nor is it intended to be limiting as to the scope of the invention in any way.
Description
1~34~2z BACKGROUND OF TIIE INVENTION
Field of the Invention This invention relates in general to filter devices and more particularly to a transmission line filter which can be incorporated within a main line transmission system, and also describes a method for making a particular embodiment of the tranqmission line filter.
Description qf the Prior Art tn various types of high frequency systems, such as mic~owave systems and television systems, it is necessary to incorporate a filter within the system. Numerous filte~
deyices are pxesently available. ~oweve~, most of these filters are complex and difficult ~o fabricate. Those filters which a~e produced in large quantities at inexpensive and ~educed cost have restrictive limitations and usually do nDt provide narrow bandwidth or deep nulls. To achieve such narro~
bandwidth and deep nu~l$ req~ires more complex filtering devices.
Aqditionally~ the filters ge~erally used in the art ~equire separate coupling to the transmission system and cannot be directly interconnected in s~ries with the transmission ~ine.
One specific area which finds great use and need for a transmission line filter is in connection ~ith cable televiaion, In cable TV syste~s the p~ograms are sent out along a main cable and various taps are positioned along the main cable which interconnect the vario~s subscriber lines, It is necessary, however ! to be able to control the p~ograming to each subscriber so that only the subscribe~ paying for a particular p~ogram will receive it. Each subscriber generally has a discrete address on his li~e to direct the program along ~hat . ~
Z~
subscriber line. However, various means are needed to insure that only a paid-for program will be sent along a particular ~ubscriber line. Security systems are therefore needed within the cable TV system. Maximum security systems involve scrambling the video along with the audio as well as the color carrier.
~lowever, such security systems add cost to the TV prog~ams.
At the other extreme are some cable ~V systems Which do not have a~y ~ecurity and only rely on automatic pqlling after the prog~am has bee~ on for a considurabl¢ length o~ ti~e.
An in between compromlae is to ut~lize ilter~ at the tap p~ints of the feeder line. The coSt and complexity of the filter~, as well a~ the ~bility to cQ~trollably switch ~u~h ~ilters is ~herefo~c an imp~rtant part of a ~ucce~ful cable ~V syStem~
SUMMARY OF T~ INVENTION
It is accordingly an object of the present invention t~ ~rovide a transmission line filter which avoids the afore-men~ioned problems of prio~ art devicçs.
A further object of the present invention is to provide ;a transmission line filter which has two main conductor linçs and includes a third conduc~ive line of a length related to the wavelength of the signal to be filtered, wherein the third conductive line is spaced apart from but electrically coupled to one of the main line cond~ctors.
A further object of the present invention is to proyide a coaxial filter which inclu~es a conductive strip spaced apart f~om but electrically coupled to the inner conductor and ha~ing one end thereof conductively connected to the outer conductor.
1~39~X
Still a further object of the present invention i~ to provide a coaxial filter trap having sections of conductive line length ~ located within the dielectric material, separated from the inner conductor by a ixed distance.
Yet a further object of the present invention is to provide a coaxial filter having a~ add~tional electrical con-ductive line located within the dielectric of a length ~, having one end of the conductive line electrically connected to the outer shield, and the other end capacitively coupled to ground.
A further object of the present invention is to proyide a coaxial filter trap wherein a third conductive line is positioned within the dielectric material and spaced apart a distance fram the inne~ c~nductor, such that the distance bet~een the inne~ conductor and the third conductive line cont~ols the Q and band~idth of the filte~, Still another object of the prese~t invention is to proyide a coaxial bandpass filter having additional condu,ctive ' ' line~ located within the dielectric, ~herein the length of ~ach-conductive line is related to the wave].ength of a si~nal forming the limits of the f~equency pand.
Yet a further object of the present invention is to provide a coaxial cable filter incl~ding a folded ove~ section of ~ransmission line on the ou~er shield ~hich forms a sh4rted length of t~ansmission line.
Still another object o~ the present inVention is ~o provide a transmission line filter formed of triaxial cable having the two shields shorted together to simulate a folded over section o~ transmission line forming a shorted length of line onto the inner shield.
_4_ ~ 1 0 ~ ~ 2~
yet 4nother ob~ect of ~he pre~ent lnventi~n is ~o provlde a transmis~ion linq ~ilter utilizing quadraxial cable including an inner conductor and three ghields, wherein the th~ee shields ~re ~horted together to p~ovide a double folded aection to thereby create two re~ona~o~s on top of each other~
A~other object of the present inve~tion is to pro~ide a t~ansmis~io~ line ~ilter includlnq a contr411able switch to permlt opening and cl~sing of the filter.
~ ~urther obi~ct of the pre~ent lnvention i~ to provide a t~ansmiasion l~ne ~llter i~cluding a controllable ~witch whlc~ ~an be arranged to pe~it failure o~ the filte~ in eithe~ the trapped o~ the pa~a mode.
Still ~ ~u~ther object of the p~e~ent invention i~ to prov~de a tr~n~mi~slon line fllte~ havln~ a plurality of ~ectlo~s oS re~onato~ and inpludiny te~ipal connecto~s a~
either end o~ the ~ilte~ to permit ln~ertion a~ the ilt~
di~ect~y in a main t~ns~i~sicn line.
A u~ther ob~ect ~f the ~esen~ invention i8 to provlde a ~Fansmis~ian line ~lter for use ln a cable ~y ~y~te~.
yet anothe~ abject of the p~e~ent inYention i~ ~o provide a swltchable co~xial ~ilte~ whic~ c~n be l~erted ln a subscribe~ line of a cable ~V ~y~te~.
_5_ ~ nother object of the present invention is to provide a method for making a coaxial filter.
A further o~ect of the present invention is to provide a method for makin~ a coaxial filter trap which includes the placing of an additional conductive line in the dielectric and utili~in~ the additional conductor line to ~orm spaced apart resonators.
These and other objects, features and advantage~ of the invention will, in part, be pointed out with particula~ity and will, in part, become obvious from the follo~ing moxe detailed description of the invention taken in conjunction with the accompanying drawings which form an integ~al part thereof.
Briefly, the invention describes a ~ransmission line filter compr$sin~ a fi~st conductor~ a second conductor an~
a first insulation means separating t~e first anq second conquctors. A thi~d conductor, of a len~th related to the wavelength of the signal being filtered, is located in the first insulatio~ means in a spa~e~ apart relationship with the first co~ductor b~t being electFi~ally coupled thereto. Ter~inal connectors are placed at èither end of the filter and serve as the filter input and filter outpu~ thereby permittin~ ~he filter ~o be included directly within a main trans~ission lin~. In one embodiment one end of ,he third conductor is conductively ~nnec~ea/
e.~owcr=~ to the second conductor. The transmission line filter .~. .. .
c~ be ~or~ed ln a ~oaxlal embo~iment. ~he ~lte~ can e~ther be a bandpass filter or a filter t~ap. By includin~ a controllable switch in the ~ransmi~sion line ~llter, the filter ~ind~ particular application ln a cable TV sy~tem wherein the ~witchable filter can be included in one o~ the ~ubscrib~r line~.
Z~ ' ' :
A method is provided for making a coaxial filter, by firstextruding a dielectric over two axially spaced apart longitudinal conductive lines. A small section of the dielectric i8 then chopped out at spaced intervals and one of the conducti~e lines is cut at each of the chopped out sections, ~he po~tion of the cut conductive line is then bent upward to protrude above the dielectric. The dielectric is covered with a conductive shield which ~ermits the protruding portion of the conductive line to extend through the conductive shield. ~he protruding po~tion is then folded onto the conductive shield and electrically connected thereto. ~n insulating covering is then placed over the conductive shiel~.
In anot~.er embodiment of the invention, a multiaxial cable is provided with an inner con~uctor, and at least two outer conductlve sheaths ~ith insulating means separa~ing each o~ the conductors. The outer sheaths are coupled together at two spàced apart locations to effectively form a folded over sçction of a shorted conductor onto, o~e of the conductive sheaths.
BRIEF D~SCRIPTION OF ~HE DRAWING
In the drawing;
FIG. 1 is a schematic drawing of a section of transmission line including a parallel resonant circuit in series with its center conductor;
FIG. 2 is a schematic dr~winq of a transmission li~e with a parallel resonant circuit in series with its outer conductor;
FIG, 3 is a schematic drawing of a transmission line having a series resonant circuit in shunt between its inner and outer conductors;
~10~
FIG, 4a is a schematic drawing of a parallel Fesonant circuit and FIG. 4b is the transmission line equivalent thereof;
FIG. 5a is a schematic drawing of a series resopapt circuit and FIG. 5b is the transmission line equivalent thereofi FIGS. 6a and 6b show a section of coaxial cable includinq a folded over resonator on the inner conductor;
FIGS. 7a and 7b show a section of coaxial cable including a folded over section on the outer conductor;
FIGS. 8a and 8b show a section of triaxial cable si~ulating a folded over section on the outer conductor;
FIGS. ~a and ~b show a section of ~uadraxial cable simulating a double folded ~ver Section on the outer conductor;
FIG. 10 is a schematic drawing of a f ilter including an elPctrically coupled parallel r~sonan~ circuit;
FIGS. 11 and 1~ show a coaxial embodiment of a filter trap;
FIGS. 13 and 14 show a cpaxial qmbodiment of a filter trap for f ilterin~ out two frequencies;
FIGS. 15a and 15b show multiple sections of a coaxial filter trap;
FIG. 16 shows a coaxial switchable filter which fails in the trappin~ mode;
FIG. 17 shows a coaxial switchable filter which fails in the pass mode;
FIG. 18 shows a switcha~le coaxial f ilter which can disconnect the entire flow of the signal;
_~_ FIG, 19 is a schematic drawing of a switchable filter used in a cable TV system;
FIG. 20 shows another em~odiment of a coaxial filter trap;
FIGS. 21a-21e show va~ious steps in a method for making a coaxial filter trap;
~ FIG. 22 shows another embodiment of a coaxial filter trap;
FIG, 23 shows one step in the method of making a coaxial ilter trap; and FIGS. 24a and 24b show a coaxial bandpass filte~.
DESCRIPTIO~ OF TUE PREFERRED EMBODIMENT
I~ forming a filter, a resonant circuit is generally utili~ed, having a resonant frequency the same as the f~equency ~o,f the signal WhiCh is t~ be filtered. In forming a transmission line filter, there are various WayS of i~cluding this resonant circuit i~to the transmission line. Referring to Fig, 1, there is shown a tra~s~issio~ line generally at 1~ which include~
an inner conducto~ 12 and an outer conductive sheath 14. For purposes of simplification, the dielectric between the inner ,a~d oute,r conductors is not shown and simila~ly, the insulating coating usually surrounding the outer conductive sheath 14 is also not shown. ~oweVer r for those skilled i~ the ar~ these items would normally be part of a ~oaxial cable.
As shown in Fig. 1, the resonant circuit, including the parallel combination of an ~nductor 16 a~d a capacito~
'is placed in series ~ith the i~ner conductor 12.
Another way o$iorming a t~ansmis~ionl~ne filter is as shown in Fig. 2, wherein the transmission line 1~ now has the parallel resonant circuit, including the inductor 16 ~nd the capacitor 18, placed in series with the outer conductive sheath 14, _g_ A third way of forming the filter is to utilize a series resonant circuit in4tead of the parallel resonant circuit.
Such arrangement is shown in Fig. 3 wherein a series re~o~ant circuit including the inductor 16 and the capacitor 18 i~ placed " .
in shunt arrangelne~t between the inner conductor 12 and ~he outer conductive sheath 19.
.:
Figs. 1-3, are shown as electrical schematic drawings including the inductor and capacitor as discrete elements.
liowever, when forming an actual transmission line~ instead of the discrete electrical components, transmission line equivalents are utilized, Referring noW to Figs. ~a and b, the e~uivalent of the p~rallel resonan~ circuit including the i~ductor 16 and the capacitor 18 is shown as a shorted length o~ tranismission line 2~ of a lengtll ~ wherein ~ iS the wavelength of~ the resonant frequency, SUch equivalent is shown i~ ~p as a trans~ission line 20 i~cluding an inner conductor 22 shorted ~y means of a sho~ting line 24 to an outer conductive sheath 26. The transmission llne equivalent of the series s~na~t circuit shown in Fig. 5a including the inductqr 16 i,n ~eries With capacitor 18~ wquld be an open circuited; length o~f transmission line of ~ length, as shown in Fig. 5b, In this case the inner conduc~or 22 and the outer cond~ctive sheath 26 are r.ot con~ected, Utilizing the transmission line equivalents shown i~
Figs. 4 and 5, one can now ~orm a practical embodiment of the sche~atic circuit shown in Fig. 1, ~hus, a parallel resonan~
circuit in series with the inner conductor aa shown i~ Fig. 1, would row appea~ as a shorted length of t~ansmission line of length in series with the inner conductor. This is shown in Fig. 6a and b wherein a sec~ion 28 of coaxial transmissiqn line is shown and including an inner conductor 30, an outer sheath 32, .
22~
and a dielectric 34 separating the inner and outer conductors.
a folded over section of the inner conductor is shown generally at 36 which is of a length ~. The folded over section would provide a shorted length of transmission line in series with the inner conductor. ~lowever, to manufacture the embodiment shown in Figs. 6a and b would be ir~practical and expensive.
i In order to provide a practical transmission line ' equivalent of the circuit shown in Fig. 2, it would be necessary to include a shorted section of transmission line of length A
i~ series with the outer conductor. This is shown in Figs. 7a and .~
b which show a section 38 of transmission line having an ' ~ inne~ conductor 30 and outer conductor 32 with a dielectric medium 34 separatin~ the conductors. A shorted length of transmission line of length ~ is included in series with the outer conductive sheath. This shorted len~th could be folded ove$ onto the outer sheath itself to fo~m the configuration shown at 42 in dotted lines. The folded over section 92 of the shorted trans~ission line lying over the quter conductive sheath ~can be achieved in a practical manner by utilizing triaxial cable as sho~n in ~igs. 8a and b. A section of such triaxi~l cable shown at 44, ~ncludes an inner conductor 46 separated fro~
a first outer conductive sheath 48 by means of a dielectric 50.
A 5econd outer conductive sheath 52 is se~arated from the first she~th 48 by means of a second dielectric 54, The two dielectrics SO, 54 can yielq a different characteristic impedance from each other. In order to form the folded back section, a first groove 56 is made in both of the outer sheaths 48, 52. A co~ductive membqr 58 is then interconnected between the first and second sheaths q8, 52 within the groove 56. At another location spaced fxom the first groove 56, a second groove 60 is made, this time only in the outer most conductive sheath S2. ~ second conductive member 62 connects the first and second conductive sheaths 48, 52 ln the groove 6~. In this manner, the section 64 forms a folded over shorted section of transmission line in series with the oute~ conductive sheath 48.
The length of the folded over section 64 can be of length a where ~ is the wavelength of the signal to be filtered. In this manner, a resonator of the desired frequency is connected in series ~ith the outer sheath. It is noted that even though triaxial cable is utilized, the filter is effectively a coaxial cable, si~ce the outer most conductive sheath 52 is only utilized to form the folded over section, but does not ta~e pa~ in the main line trans~ission System, Mul~iple sections of the filte~ can be fabricated by utilizing the tri~xial cable of Figs. ~a, b and placing the esonators of ~ length at ~djacent spacings axially along the filter. Terminal con~ectors can then be placed at the e~ds af a multip~e section filter, and the filter could then be seria~ly positio~ed within a ~ain transmiSsion line to p~ovide a ~ilter trap of the des~red fre~uqnc~, , An alternate way of providin~ a multiple section filter having folded over sections of shorted transmission line is shown in Figs. ~a, b. In these figures, quadraxial cable is utilized as shown generally at 66. The cable includes an inner conductor 6~ separ~ted from a first sheath 70 by a first dielectric 72;
a second sheath 74 separated from the fiFst sheath by a dielectric 76, and a third sheath 78 separated from the second sheath by a third dielectric 80. The three dielectrics 72, 76 and 80 can each be of a different characterlstic impedance. The m~in line conductors for the transmission signal are the inner conductor 68 and the first outer sheath 70. The other sheaths74, 78 are only utilized for a double folded over section to achieve the equivalent of two resonators placed one on top ~f the other. In order to fabricate the double folded oVer sections a first groove B2 is fonned colinearally in all three outer sheaths 70, 74, 78. A conductive member 84 interconneCts all three sheaths in the grooVe 82. At a distance spaced from the first groove 82, is a second grooye 86 which is placed in the two outer most conductive sheaths 74, 78. A conductive member B8 interconnect8 the outer most sheath 78 with the inner most sheath, and a second conductive member 90 interconnects the sheath 70 with the next adjacent conductive sheath 74. In this manner a first folded over shorted transmission section is fo~med utilizing the conductive member 84, the outer condu~tive sheath 74 and the conductive membe~ 90. At the same tim~
second shorted transmissio~ line is achieved folded over the first shoxted transmission line. The second folded transmission line section utilizes the co~ductive member 84~ the auter most sheath 78 and the conductiye`membe~ ~. In this manner, t~o resQnators are located one o~ top of the other. The length f the reSonators are each ~. Ut~lizing the arrangement shown in Figs. 9a~ b, the length fo~ two sections of ~esonato~s is'still - instead of a length of ~ whiçh would be needed utilizing the triax cable shown in Figs. 8a,,b. Multiple sections of t~e filter s~own in Figs. 9a, b could be fabricated by spacing the double folded over sections apart from each other. A usable filter could be then formed by placing connectors at opposite e~ds of the multiple section filter and interconnecting the multiple section filter within a main transmission line. ~he o~ter most two layers 74, 78 are only utilized to permit easy -13~
1~4~2~
fabrication of the double folded over section but do not actually participate in the transmission of the signal itself.
- ~ither of the techniques shown in Figs. 8a, ~b, or ~ Figs. 9a, 9b, provide for an inexpensive and easily fabricated ,!, coaxial filter trap. For certain applications, however, the embodiment shown has limitations. For example, the unloaded Q of the resonators must generally be quite high in order to ` produce deep nulls in a filter. For the highest unloaded Q, the optimum imuedance of the resonators should be about 70 ohms.
~t the same time, in o~der to achieve a narrow bandwidth or a small percentage bandwidth, it is necessary that the parallel resonant circuit should be fabricated from very low impedance ~; i lines. For the se~ies resonant circuits, high impedance lines .
~ould be utilized, Therefore~ in the embodiments heretofore shown utilizing a parallel resonant circuit, low impedance lines should be used to produce a narrow bandwidth. I~owever, the unloaded Q will therefore not be high enough to provide deep nulls. While low impedance lines of approximately 7 ohms would be adequate for t~apping out channels which are no~ closely spaced to adjacent channels, in order to trap out a chan~el with closely spaced adjacent channel$ it would be necessary to use impedance lines lower tha~ even 7 ohms to obtain such a ;na~row bandwidth, However, this would produce a Q whi~h would be too low for adequate ~ejection of a channel without ffectin~ an adjacent channrel.- ~ ~
In order to obtain these apparent conflicting requi~ements, it is possible to utilize a resonator having an impedance of 70 ohms so as to obtaip a high ~ and, nevertheless maintain a narrow bandwidth. This is achieved by not directly inter-connecting the resonant circuit to the main conductive lines 1~14222 but instead separating it from the conductive lineselectrically coupling it to t}le lines. In this manner, it is possible to both optimize the loss and control the bandwidth by varying the spacing bet~een tlle resonant circuit and the conductive line to thereby control the coupling coefficient.
Referring now to ~ig. 10 there i5 shown a schematic diag~am showing a transmission line generally at 92 having an input terminal 94 and an output terminal 96 with a first r;~ conductive line 9R and a second conductive line 100. The parallel resonant circuit 102 is shown spaced from the conductive lipe 98 but electrically coupled thereto. One end of the resonant circuit lV2 is, however, electrically connected to the other conductive line 1~0 by means of the conductor 104.
; l~eferring now to ~igs. 11 and 12there are shown the ~ -~oaxial line e~uivalent of the traPsmission line filter shown generally in Fi~, 10. In Figs. 11 and 12the coa~ial ~ine is ~ , shown at 126 and includes a center conductor 128 separated fro~
an outer conductive sheath 130 by ~eans of a dielectric medium 132. Ano~her conductive li~?e 134 is located witilin the dielectric medium 132 spaced from the inner conductor 123 by a distanc~ d. The conductive line 13~ is parallel to the inner conduct~ such that the distance d is uniform thr~ughout the t length of the conductive lipe 13~. The distance d can be preset to thereby control the coupling coefficient for determining ..
Field of the Invention This invention relates in general to filter devices and more particularly to a transmission line filter which can be incorporated within a main line transmission system, and also describes a method for making a particular embodiment of the tranqmission line filter.
Description qf the Prior Art tn various types of high frequency systems, such as mic~owave systems and television systems, it is necessary to incorporate a filter within the system. Numerous filte~
deyices are pxesently available. ~oweve~, most of these filters are complex and difficult ~o fabricate. Those filters which a~e produced in large quantities at inexpensive and ~educed cost have restrictive limitations and usually do nDt provide narrow bandwidth or deep nulls. To achieve such narro~
bandwidth and deep nu~l$ req~ires more complex filtering devices.
Aqditionally~ the filters ge~erally used in the art ~equire separate coupling to the transmission system and cannot be directly interconnected in s~ries with the transmission ~ine.
One specific area which finds great use and need for a transmission line filter is in connection ~ith cable televiaion, In cable TV syste~s the p~ograms are sent out along a main cable and various taps are positioned along the main cable which interconnect the vario~s subscriber lines, It is necessary, however ! to be able to control the p~ograming to each subscriber so that only the subscribe~ paying for a particular p~ogram will receive it. Each subscriber generally has a discrete address on his li~e to direct the program along ~hat . ~
Z~
subscriber line. However, various means are needed to insure that only a paid-for program will be sent along a particular ~ubscriber line. Security systems are therefore needed within the cable TV system. Maximum security systems involve scrambling the video along with the audio as well as the color carrier.
~lowever, such security systems add cost to the TV prog~ams.
At the other extreme are some cable ~V systems Which do not have a~y ~ecurity and only rely on automatic pqlling after the prog~am has bee~ on for a considurabl¢ length o~ ti~e.
An in between compromlae is to ut~lize ilter~ at the tap p~ints of the feeder line. The coSt and complexity of the filter~, as well a~ the ~bility to cQ~trollably switch ~u~h ~ilters is ~herefo~c an imp~rtant part of a ~ucce~ful cable ~V syStem~
SUMMARY OF T~ INVENTION
It is accordingly an object of the present invention t~ ~rovide a transmission line filter which avoids the afore-men~ioned problems of prio~ art devicçs.
A further object of the present invention is to provide ;a transmission line filter which has two main conductor linçs and includes a third conduc~ive line of a length related to the wavelength of the signal to be filtered, wherein the third conductive line is spaced apart from but electrically coupled to one of the main line cond~ctors.
A further object of the present invention is to proyide a coaxial filter which inclu~es a conductive strip spaced apart f~om but electrically coupled to the inner conductor and ha~ing one end thereof conductively connected to the outer conductor.
1~39~X
Still a further object of the present invention i~ to provide a coaxial filter trap having sections of conductive line length ~ located within the dielectric material, separated from the inner conductor by a ixed distance.
Yet a further object of the present invention is to provide a coaxial filter having a~ add~tional electrical con-ductive line located within the dielectric of a length ~, having one end of the conductive line electrically connected to the outer shield, and the other end capacitively coupled to ground.
A further object of the present invention is to proyide a coaxial filter trap wherein a third conductive line is positioned within the dielectric material and spaced apart a distance fram the inne~ c~nductor, such that the distance bet~een the inne~ conductor and the third conductive line cont~ols the Q and band~idth of the filte~, Still another object of the prese~t invention is to proyide a coaxial bandpass filter having additional condu,ctive ' ' line~ located within the dielectric, ~herein the length of ~ach-conductive line is related to the wave].ength of a si~nal forming the limits of the f~equency pand.
Yet a further object of the present invention is to provide a coaxial cable filter incl~ding a folded ove~ section of ~ransmission line on the ou~er shield ~hich forms a sh4rted length of t~ansmission line.
Still another object o~ the present inVention is ~o provide a transmission line filter formed of triaxial cable having the two shields shorted together to simulate a folded over section o~ transmission line forming a shorted length of line onto the inner shield.
_4_ ~ 1 0 ~ ~ 2~
yet 4nother ob~ect of ~he pre~ent lnventi~n is ~o provlde a transmis~ion linq ~ilter utilizing quadraxial cable including an inner conductor and three ghields, wherein the th~ee shields ~re ~horted together to p~ovide a double folded aection to thereby create two re~ona~o~s on top of each other~
A~other object of the present inve~tion is to pro~ide a t~ansmis~io~ line ~ilter includlnq a contr411able switch to permlt opening and cl~sing of the filter.
~ ~urther obi~ct of the pre~ent lnvention i~ to provide a t~ansmiasion l~ne ~llter i~cluding a controllable ~witch whlc~ ~an be arranged to pe~it failure o~ the filte~ in eithe~ the trapped o~ the pa~a mode.
Still ~ ~u~ther object of the p~e~ent invention i~ to prov~de a tr~n~mi~slon line fllte~ havln~ a plurality of ~ectlo~s oS re~onato~ and inpludiny te~ipal connecto~s a~
either end o~ the ~ilte~ to permit ln~ertion a~ the ilt~
di~ect~y in a main t~ns~i~sicn line.
A u~ther ob~ect ~f the ~esen~ invention i8 to provlde a ~Fansmis~ian line ~lter for use ln a cable ~y ~y~te~.
yet anothe~ abject of the p~e~ent inYention i~ ~o provide a swltchable co~xial ~ilte~ whic~ c~n be l~erted ln a subscribe~ line of a cable ~V ~y~te~.
_5_ ~ nother object of the present invention is to provide a method for making a coaxial filter.
A further o~ect of the present invention is to provide a method for makin~ a coaxial filter trap which includes the placing of an additional conductive line in the dielectric and utili~in~ the additional conductor line to ~orm spaced apart resonators.
These and other objects, features and advantage~ of the invention will, in part, be pointed out with particula~ity and will, in part, become obvious from the follo~ing moxe detailed description of the invention taken in conjunction with the accompanying drawings which form an integ~al part thereof.
Briefly, the invention describes a ~ransmission line filter compr$sin~ a fi~st conductor~ a second conductor an~
a first insulation means separating t~e first anq second conquctors. A thi~d conductor, of a len~th related to the wavelength of the signal being filtered, is located in the first insulatio~ means in a spa~e~ apart relationship with the first co~ductor b~t being electFi~ally coupled thereto. Ter~inal connectors are placed at èither end of the filter and serve as the filter input and filter outpu~ thereby permittin~ ~he filter ~o be included directly within a main trans~ission lin~. In one embodiment one end of ,he third conductor is conductively ~nnec~ea/
e.~owcr=~ to the second conductor. The transmission line filter .~. .. .
c~ be ~or~ed ln a ~oaxlal embo~iment. ~he ~lte~ can e~ther be a bandpass filter or a filter t~ap. By includin~ a controllable switch in the ~ransmi~sion line ~llter, the filter ~ind~ particular application ln a cable TV sy~tem wherein the ~witchable filter can be included in one o~ the ~ubscrib~r line~.
Z~ ' ' :
A method is provided for making a coaxial filter, by firstextruding a dielectric over two axially spaced apart longitudinal conductive lines. A small section of the dielectric i8 then chopped out at spaced intervals and one of the conducti~e lines is cut at each of the chopped out sections, ~he po~tion of the cut conductive line is then bent upward to protrude above the dielectric. The dielectric is covered with a conductive shield which ~ermits the protruding portion of the conductive line to extend through the conductive shield. ~he protruding po~tion is then folded onto the conductive shield and electrically connected thereto. ~n insulating covering is then placed over the conductive shiel~.
In anot~.er embodiment of the invention, a multiaxial cable is provided with an inner con~uctor, and at least two outer conductlve sheaths ~ith insulating means separa~ing each o~ the conductors. The outer sheaths are coupled together at two spàced apart locations to effectively form a folded over sçction of a shorted conductor onto, o~e of the conductive sheaths.
BRIEF D~SCRIPTION OF ~HE DRAWING
In the drawing;
FIG. 1 is a schematic drawing of a section of transmission line including a parallel resonant circuit in series with its center conductor;
FIG. 2 is a schematic dr~winq of a transmission li~e with a parallel resonant circuit in series with its outer conductor;
FIG, 3 is a schematic drawing of a transmission line having a series resonant circuit in shunt between its inner and outer conductors;
~10~
FIG, 4a is a schematic drawing of a parallel Fesonant circuit and FIG. 4b is the transmission line equivalent thereof;
FIG. 5a is a schematic drawing of a series resopapt circuit and FIG. 5b is the transmission line equivalent thereofi FIGS. 6a and 6b show a section of coaxial cable includinq a folded over resonator on the inner conductor;
FIGS. 7a and 7b show a section of coaxial cable including a folded over section on the outer conductor;
FIGS. 8a and 8b show a section of triaxial cable si~ulating a folded over section on the outer conductor;
FIGS. ~a and ~b show a section of ~uadraxial cable simulating a double folded ~ver Section on the outer conductor;
FIG. 10 is a schematic drawing of a f ilter including an elPctrically coupled parallel r~sonan~ circuit;
FIGS. 11 and 1~ show a coaxial embodiment of a filter trap;
FIGS. 13 and 14 show a cpaxial qmbodiment of a filter trap for f ilterin~ out two frequencies;
FIGS. 15a and 15b show multiple sections of a coaxial filter trap;
FIG. 16 shows a coaxial switchable filter which fails in the trappin~ mode;
FIG. 17 shows a coaxial switchable filter which fails in the pass mode;
FIG. 18 shows a switcha~le coaxial f ilter which can disconnect the entire flow of the signal;
_~_ FIG, 19 is a schematic drawing of a switchable filter used in a cable TV system;
FIG. 20 shows another em~odiment of a coaxial filter trap;
FIGS. 21a-21e show va~ious steps in a method for making a coaxial filter trap;
~ FIG. 22 shows another embodiment of a coaxial filter trap;
FIG, 23 shows one step in the method of making a coaxial ilter trap; and FIGS. 24a and 24b show a coaxial bandpass filte~.
DESCRIPTIO~ OF TUE PREFERRED EMBODIMENT
I~ forming a filter, a resonant circuit is generally utili~ed, having a resonant frequency the same as the f~equency ~o,f the signal WhiCh is t~ be filtered. In forming a transmission line filter, there are various WayS of i~cluding this resonant circuit i~to the transmission line. Referring to Fig, 1, there is shown a tra~s~issio~ line generally at 1~ which include~
an inner conducto~ 12 and an outer conductive sheath 14. For purposes of simplification, the dielectric between the inner ,a~d oute,r conductors is not shown and simila~ly, the insulating coating usually surrounding the outer conductive sheath 14 is also not shown. ~oweVer r for those skilled i~ the ar~ these items would normally be part of a ~oaxial cable.
As shown in Fig. 1, the resonant circuit, including the parallel combination of an ~nductor 16 a~d a capacito~
'is placed in series ~ith the i~ner conductor 12.
Another way o$iorming a t~ansmis~ionl~ne filter is as shown in Fig. 2, wherein the transmission line 1~ now has the parallel resonant circuit, including the inductor 16 ~nd the capacitor 18, placed in series with the outer conductive sheath 14, _g_ A third way of forming the filter is to utilize a series resonant circuit in4tead of the parallel resonant circuit.
Such arrangement is shown in Fig. 3 wherein a series re~o~ant circuit including the inductor 16 and the capacitor 18 i~ placed " .
in shunt arrangelne~t between the inner conductor 12 and ~he outer conductive sheath 19.
.:
Figs. 1-3, are shown as electrical schematic drawings including the inductor and capacitor as discrete elements.
liowever, when forming an actual transmission line~ instead of the discrete electrical components, transmission line equivalents are utilized, Referring noW to Figs. ~a and b, the e~uivalent of the p~rallel resonan~ circuit including the i~ductor 16 and the capacitor 18 is shown as a shorted length o~ tranismission line 2~ of a lengtll ~ wherein ~ iS the wavelength of~ the resonant frequency, SUch equivalent is shown i~ ~p as a trans~ission line 20 i~cluding an inner conductor 22 shorted ~y means of a sho~ting line 24 to an outer conductive sheath 26. The transmission llne equivalent of the series s~na~t circuit shown in Fig. 5a including the inductqr 16 i,n ~eries With capacitor 18~ wquld be an open circuited; length o~f transmission line of ~ length, as shown in Fig. 5b, In this case the inner conduc~or 22 and the outer cond~ctive sheath 26 are r.ot con~ected, Utilizing the transmission line equivalents shown i~
Figs. 4 and 5, one can now ~orm a practical embodiment of the sche~atic circuit shown in Fig. 1, ~hus, a parallel resonan~
circuit in series with the inner conductor aa shown i~ Fig. 1, would row appea~ as a shorted length of t~ansmission line of length in series with the inner conductor. This is shown in Fig. 6a and b wherein a sec~ion 28 of coaxial transmissiqn line is shown and including an inner conductor 30, an outer sheath 32, .
22~
and a dielectric 34 separating the inner and outer conductors.
a folded over section of the inner conductor is shown generally at 36 which is of a length ~. The folded over section would provide a shorted length of transmission line in series with the inner conductor. ~lowever, to manufacture the embodiment shown in Figs. 6a and b would be ir~practical and expensive.
i In order to provide a practical transmission line ' equivalent of the circuit shown in Fig. 2, it would be necessary to include a shorted section of transmission line of length A
i~ series with the outer conductor. This is shown in Figs. 7a and .~
b which show a section 38 of transmission line having an ' ~ inne~ conductor 30 and outer conductor 32 with a dielectric medium 34 separatin~ the conductors. A shorted length of transmission line of length ~ is included in series with the outer conductive sheath. This shorted len~th could be folded ove$ onto the outer sheath itself to fo~m the configuration shown at 42 in dotted lines. The folded over section 92 of the shorted trans~ission line lying over the quter conductive sheath ~can be achieved in a practical manner by utilizing triaxial cable as sho~n in ~igs. 8a and b. A section of such triaxi~l cable shown at 44, ~ncludes an inner conductor 46 separated fro~
a first outer conductive sheath 48 by means of a dielectric 50.
A 5econd outer conductive sheath 52 is se~arated from the first she~th 48 by means of a second dielectric 54, The two dielectrics SO, 54 can yielq a different characteristic impedance from each other. In order to form the folded back section, a first groove 56 is made in both of the outer sheaths 48, 52. A co~ductive membqr 58 is then interconnected between the first and second sheaths q8, 52 within the groove 56. At another location spaced fxom the first groove 56, a second groove 60 is made, this time only in the outer most conductive sheath S2. ~ second conductive member 62 connects the first and second conductive sheaths 48, 52 ln the groove 6~. In this manner, the section 64 forms a folded over shorted section of transmission line in series with the oute~ conductive sheath 48.
The length of the folded over section 64 can be of length a where ~ is the wavelength of the signal to be filtered. In this manner, a resonator of the desired frequency is connected in series ~ith the outer sheath. It is noted that even though triaxial cable is utilized, the filter is effectively a coaxial cable, si~ce the outer most conductive sheath 52 is only utilized to form the folded over section, but does not ta~e pa~ in the main line trans~ission System, Mul~iple sections of the filte~ can be fabricated by utilizing the tri~xial cable of Figs. ~a, b and placing the esonators of ~ length at ~djacent spacings axially along the filter. Terminal con~ectors can then be placed at the e~ds af a multip~e section filter, and the filter could then be seria~ly positio~ed within a ~ain transmiSsion line to p~ovide a ~ilter trap of the des~red fre~uqnc~, , An alternate way of providin~ a multiple section filter having folded over sections of shorted transmission line is shown in Figs. ~a, b. In these figures, quadraxial cable is utilized as shown generally at 66. The cable includes an inner conductor 6~ separ~ted from a first sheath 70 by a first dielectric 72;
a second sheath 74 separated from the fiFst sheath by a dielectric 76, and a third sheath 78 separated from the second sheath by a third dielectric 80. The three dielectrics 72, 76 and 80 can each be of a different characterlstic impedance. The m~in line conductors for the transmission signal are the inner conductor 68 and the first outer sheath 70. The other sheaths74, 78 are only utilized for a double folded over section to achieve the equivalent of two resonators placed one on top ~f the other. In order to fabricate the double folded oVer sections a first groove B2 is fonned colinearally in all three outer sheaths 70, 74, 78. A conductive member 84 interconneCts all three sheaths in the grooVe 82. At a distance spaced from the first groove 82, is a second grooye 86 which is placed in the two outer most conductive sheaths 74, 78. A conductive member B8 interconnect8 the outer most sheath 78 with the inner most sheath, and a second conductive member 90 interconnects the sheath 70 with the next adjacent conductive sheath 74. In this manner a first folded over shorted transmission section is fo~med utilizing the conductive member 84, the outer condu~tive sheath 74 and the conductive membe~ 90. At the same tim~
second shorted transmissio~ line is achieved folded over the first shoxted transmission line. The second folded transmission line section utilizes the co~ductive member 84~ the auter most sheath 78 and the conductiye`membe~ ~. In this manner, t~o resQnators are located one o~ top of the other. The length f the reSonators are each ~. Ut~lizing the arrangement shown in Figs. 9a~ b, the length fo~ two sections of ~esonato~s is'still - instead of a length of ~ whiçh would be needed utilizing the triax cable shown in Figs. 8a,,b. Multiple sections of t~e filter s~own in Figs. 9a, b could be fabricated by spacing the double folded over sections apart from each other. A usable filter could be then formed by placing connectors at opposite e~ds of the multiple section filter and interconnecting the multiple section filter within a main transmission line. ~he o~ter most two layers 74, 78 are only utilized to permit easy -13~
1~4~2~
fabrication of the double folded over section but do not actually participate in the transmission of the signal itself.
- ~ither of the techniques shown in Figs. 8a, ~b, or ~ Figs. 9a, 9b, provide for an inexpensive and easily fabricated ,!, coaxial filter trap. For certain applications, however, the embodiment shown has limitations. For example, the unloaded Q of the resonators must generally be quite high in order to ` produce deep nulls in a filter. For the highest unloaded Q, the optimum imuedance of the resonators should be about 70 ohms.
~t the same time, in o~der to achieve a narrow bandwidth or a small percentage bandwidth, it is necessary that the parallel resonant circuit should be fabricated from very low impedance ~; i lines. For the se~ies resonant circuits, high impedance lines .
~ould be utilized, Therefore~ in the embodiments heretofore shown utilizing a parallel resonant circuit, low impedance lines should be used to produce a narrow bandwidth. I~owever, the unloaded Q will therefore not be high enough to provide deep nulls. While low impedance lines of approximately 7 ohms would be adequate for t~apping out channels which are no~ closely spaced to adjacent channels, in order to trap out a chan~el with closely spaced adjacent channel$ it would be necessary to use impedance lines lower tha~ even 7 ohms to obtain such a ;na~row bandwidth, However, this would produce a Q whi~h would be too low for adequate ~ejection of a channel without ffectin~ an adjacent channrel.- ~ ~
In order to obtain these apparent conflicting requi~ements, it is possible to utilize a resonator having an impedance of 70 ohms so as to obtaip a high ~ and, nevertheless maintain a narrow bandwidth. This is achieved by not directly inter-connecting the resonant circuit to the main conductive lines 1~14222 but instead separating it from the conductive lineselectrically coupling it to t}le lines. In this manner, it is possible to both optimize the loss and control the bandwidth by varying the spacing bet~een tlle resonant circuit and the conductive line to thereby control the coupling coefficient.
Referring now to ~ig. 10 there i5 shown a schematic diag~am showing a transmission line generally at 92 having an input terminal 94 and an output terminal 96 with a first r;~ conductive line 9R and a second conductive line 100. The parallel resonant circuit 102 is shown spaced from the conductive lipe 98 but electrically coupled thereto. One end of the resonant circuit lV2 is, however, electrically connected to the other conductive line 1~0 by means of the conductor 104.
; l~eferring now to ~igs. 11 and 12there are shown the ~ -~oaxial line e~uivalent of the traPsmission line filter shown generally in Fi~, 10. In Figs. 11 and 12the coa~ial ~ine is ~ , shown at 126 and includes a center conductor 128 separated fro~
an outer conductive sheath 130 by ~eans of a dielectric medium 132. Ano~her conductive li~?e 134 is located witilin the dielectric medium 132 spaced from the inner conductor 123 by a distanc~ d. The conductive line 13~ is parallel to the inner conduct~ such that the distance d is uniform thr~ughout the t length of the conductive lipe 13~. The distance d can be preset to thereby control the coupling coefficient for determining ..
2~2 the ~andwidth of the fllter. 'rhe conductive line 134 is electrically connected to the outer sheath 130 ~y means of t~e conductive member 136. rrhe lengt}l of the conductive ~ember 134 is ~ wherein /~ is the wavelength of the signal to be filtered.
l~eferring now to Figs. 13 and 1~ there is shown how the same inventive approach can be utilized to filter out more than one freque~cy. Thus, in addition to the conductive line 134 of a len~th ~Iwhich filters out frequencies having a wavelength ~, a second conductive line 140 can be included of a length '~
which would filter out frequencies having a wavelength ~7 The conductive line 140 is also spaced from the inner conductor 128 and is electrically coupled thereto. It is also conductively connected to the oute~ conductive sheath 130 by means of the conductive member 142.
Multiple sections of the filter can be formed as shown in Figs. 15a and b which shows multiple sections of the coaxial filter ~f the type shown in Fiqs. 11 andl2, wherein each of the conductive line sections 134 a~e o~ a length ~ and are slight~y spaced apa~t from each other. The multiple section filter is shown to include ter~inal connectors at either end .
thereof to permit the insertion of the coaxial filte~ di~ectly in serles with a transmission line.
A bandpass coaxial filter could also be formed, as shown in Figs. 24a and b wherein the coaxial line includes an outer conductive sheath 196 sepa~ated from an inner conductor 198 by the dielectric 200. '~wo additional conductive lines 202, 204 are also located in the dielectric, each spaced from the inner and outer conductors- Each Of the addltional conductive lines 2~2, 204 are of a ~en~th ~! wherein ~ is the wavelength at the center f~equency and the coupling coefficient determinea the bandwidt~ and variation in bandpass insertion lo~.
Conductive line 202 gerves as the filt~r input and concl~ctiveline 204 ~erVe~ as the filter output.
It is also possible to make the filter heretofore descri~ed as a switchable filter by including a controllable switch in the filter section. One such well known type of controllable switch is a diode; however, relays or other well known switches could also be utilized. The switch is included to open and close the shorted end or open end of the resonator, depending upon whether a series or parallel resonator is utilized. Also, whether the switch opens or closes the end will depend upon which mode the filter fails in, i.e., the trapped mode or the pass mode (Aasumlnc~ excess current will have the diode fail as an open circuit).
Referring now to Fig 16 there is shown an embodiment of the s~itcllable filter of the type heretofore described whe~ein the filter fails in the trapped mode. 'rhe coaxial filter includes an inner conductor 128 separate~ from the outer conductor 13~
by means of the dielectric 132 and including the conductive }ine 134 electrically coupled to the inner conductor 12~ and conductively conpected to the outer sheath by means of conductive member 136 at one end thereof. ~he controllable diode switch 144 in~erconnects the other end of the conductive line 134 to a batte~y source 147 through a controllable switch circuit 145.
~her. the diode 144 is conduçting, the conductive line 134 will not haye a~y effect on the coaxial line section and will not filter out any si~nal. On ~he other hand, when the diode 144 is nonconducting, the conductive li~e 134 will serve to t~ap out the signal being filtere~. In this manner, should the diode 144 fail to operate, the filter shown in Fi~. 16 will re~ain in the trapped mode and will filter out the frec~uency of the signal.
4Z~
Referrlng now to Fig. 17 there is shown another method of utilizing the diode switch to form a switchable filter. In this embodiment, the diode switch 144 forms part of the conductive interconnection between the conductive line 134 and the outer conductor sheath 130. ~he diode is controlled by a circuit connecting the diode to a switch 145 and to a battery 147. When the diode 144 is conducting the conductive line 134 is operative to trap out the signal being filtered. On the other hand, when the diode switch 144 is nonconducting, the line 13~ will not operate to filter out the frequency. Therefore, if the diode 144 fails the coaxial section shown in Fig, 17 will remain in the pass mode a~d will not filter out any sigpals.
It is also possible to include the diode 144 directly in series with the inner conductor 128 as shown in Fig. 18, In this way when the diode is not conducting, no sig~al at all will pass through the enti~e filter section.
The switchable filter he~etofore described in Figs. 16-~8 fi~ds particular use in connection with cable television sy6tems. In such systems it is necessary to have cont~ol over the program being sent to a subscriber. When the subscriber has paid for the program, the program will be se~t to the su~scriber line. However, if not paid fo~, it is necessary to restrict the program signal fro~ being sent to the subsc~iber ll~e, A switchable filter is af conVenient use for such pu~poses.
Referring now to Fig. 19 the~e is shown how such a switchable filter of the type heretofore described could find use i~ ~ cable television system. The main line of the cable televisian system is shown at 146. A directional tap is shown gene~ally at 14~
and includes control circuit~y 150 for controlling the ~dd~ess location of each of a plurality of taps 152. Each of t~e taps z controls a different subscriber line 154. The switchable trap156 is interconnected in the subscriber line. The switchable trap can be utilized to connect or disconnect a particular pro~ram from the subscriber line. Utilizillg the control address circuitry 150, it is possible to control the switchable trap directly from the main office utilizing the co~trol address of the particular subscriber line.
Using the filter trap as shown in Fig. 16, the ilter would fail in the trapped mode while utillzing the switchab~e filter in Fig. 17 it ~ould fail in the pass mode. Probably the embodiment shown in Fig, 16 would be more advantageous since in thi~ way the subsc~iber would notify you if the switchable filter failed. Also, in the embodiment of Fig, 16 the diode has les~ effect on the Q of the resonator. On the other han~, utilizing the embodiment iP Fi~. 17 should a failure occu~ in the SWitch the proqram would still come to the subscriber and chançes are the subscriber would not notify the station if a failure o~ccurred in the pa~s mode. If the embodi~ent of Fig. 18 were u~ed, there ~ould be qn additional control in that the entire service would be disconnected from the subscriber line. The e~bod~ment shown in Fig, 18 coulj of course be utilized in ~
c~njunction with the switching control of the filter thereby obtaining the dual control of both a switchable trap as well as complete disconnectipg of se~vice.
Referring now to Fig. 20 there is shown an additional ~
embodiment of the coaxial filter heretofore described. In Fig.
20, the coaxial cable includes an inne~ conductor 158, an outer conductive sheath 164, a first dielectric 162 located around the inner conductor and a spiral conductive winqing 164 wound around the dielectric 162. In this manner the conductive wlndln~ 164 will be ~oaitioned at A ~ixed dist~nce from ~he inne~
conductor 158 and Will be electrically coupled thereto. One end 166 o the spir~l conducto~ is conductively connected t~ the outeX conductlve ~heath 160 by li~e 166~ A 80cond dlelect~ic ~68 s~parateg the spiral windiPg 164 from the outer sheath 1~0.
Th~ length of the ~piral conducto~ would be less than ~because o~
mutual coupling between turns. ~he outer conductive uheath 160 i~ shown a~ a bralded conductor. A pl~8tic ~uter ~oating 170 i~ ~ho~n oVe~ the con~uctive bsald 160.
The embodiment shown in Fig, 20 can be made by con-tinuously forming the variou~ layers and then notching out and isolating the ~ spiral sections. After that the spiral sections could be conductively connected to the outer conductor sheath.
This method will become better understood hereinafter, in connection with the methQd for making the coaxial filter e~bodiment, Refe~ring now to ~igs. 21a-e there ls shown,a method of making a coaxial filter of the type heretofore described in connection with Fig. lS. Initially, two inner conductors 172, 174 are spaced apart, and a dielectric 176 is extruded over the accurately spaced apart conduators. Using a tool and die, sections 178 are chopped out of the dielectric 176. At the same time, the conductor 172 is c~t and a portion of 180 is kent up-wa~rdly to protrude above the dielectric 176. Additional notches 1~2 are also chopped out of the dielectric 176 as well as from the conductor 172. The spacin~ ~etween the p~otruding section 180 and the chopped out section 182 i8 approximately ~.
The dielectric covered conductors are now sent to a braiding machine which adds the outer conductive sheath 184, typically a braided conductor. When the braiding is pu~ on, P1~4~
the portions 180 which protrude above the dielectric also extend through the braiding. ~he protruding portions 180 are then bent over the conductive sheath 184 so as to be substantially flush with the braid, and are then electrically connected t the conductive braiding by either soldering, welding or the like.
If the protruding sections 180 are welded o~to the braided conductive sheath 18q, it iS possible to both bend and weld the member 180 during the same processing step. This can be seen by referring to Fig. 23, wherein electrodes 186 are shown connected to an energy source 188. The braided cable, shown generally at 190, passes through the electrodes ~hereby the electrodes will both bend the protruding section 180 and at the same time weld them onto the conductive b~aided sheath 184. The fi~ished product is shown at 192 ~hich shows the weld located above the bent member 194.
Refe~ring back to Fig, 21d it is noted tha~ after the prot~uding sections have beenj bent and welded onto the con-ductive sheath 184, the cablq is now passed through an extruder whrich places a plastic jacke~ 196 over the entire cable. Qne or mqre sections may now be cut off and terminal connectors 1~8, 2~Q can then be placed on eithe~ end of the coaxial filter, s~ch that one of the connecto~rs serVes as the input and the other serves as the output.
Although the device shown in Figs. 21a-21e is for a filte~ trap, it is understood that the bandpass filter could also be made in a similar manner.
~ eferring now to Fig. 22 there is shown another embodiment whereby the length of the co~ductive line can be reduced. In addition to having the one end 180 protruding from the dielectric 176, the other end 202 is also made to protrude above the 1~34~
dielectrlc, While the end 180 will then be conductively connectedto the outer electrical braided ~heath, the end 2~2 will be terminated in a capacito~ 200 which wlll be connected ~o g~ou~d.
This ~ill r¢duce thq length o~ the fllter to le~s than ~, the length depe~ding ~n the value of C, and increage the frequency at Which the trapping effect repeat# ~tsel~, This also avoid~ the well known three time~ the frequency problem experie~ced with 1/4 ~Pvelen~th ~ections.
~ here has beeA dlsclosed hereto~ore the best embodime~t o~ the inve~tlon p~esently contempl~ted. However, it 1~ tq be u~der~tood that yarious features and ~odiflcation~ may bq made thereto without departln~ from the spirit of the lnvent~on.
l~eferring now to Figs. 13 and 1~ there is shown how the same inventive approach can be utilized to filter out more than one freque~cy. Thus, in addition to the conductive line 134 of a len~th ~Iwhich filters out frequencies having a wavelength ~, a second conductive line 140 can be included of a length '~
which would filter out frequencies having a wavelength ~7 The conductive line 140 is also spaced from the inner conductor 128 and is electrically coupled thereto. It is also conductively connected to the oute~ conductive sheath 130 by means of the conductive member 142.
Multiple sections of the filter can be formed as shown in Figs. 15a and b which shows multiple sections of the coaxial filter ~f the type shown in Fiqs. 11 andl2, wherein each of the conductive line sections 134 a~e o~ a length ~ and are slight~y spaced apa~t from each other. The multiple section filter is shown to include ter~inal connectors at either end .
thereof to permit the insertion of the coaxial filte~ di~ectly in serles with a transmission line.
A bandpass coaxial filter could also be formed, as shown in Figs. 24a and b wherein the coaxial line includes an outer conductive sheath 196 sepa~ated from an inner conductor 198 by the dielectric 200. '~wo additional conductive lines 202, 204 are also located in the dielectric, each spaced from the inner and outer conductors- Each Of the addltional conductive lines 2~2, 204 are of a ~en~th ~! wherein ~ is the wavelength at the center f~equency and the coupling coefficient determinea the bandwidt~ and variation in bandpass insertion lo~.
Conductive line 202 gerves as the filt~r input and concl~ctiveline 204 ~erVe~ as the filter output.
It is also possible to make the filter heretofore descri~ed as a switchable filter by including a controllable switch in the filter section. One such well known type of controllable switch is a diode; however, relays or other well known switches could also be utilized. The switch is included to open and close the shorted end or open end of the resonator, depending upon whether a series or parallel resonator is utilized. Also, whether the switch opens or closes the end will depend upon which mode the filter fails in, i.e., the trapped mode or the pass mode (Aasumlnc~ excess current will have the diode fail as an open circuit).
Referring now to Fig 16 there is shown an embodiment of the s~itcllable filter of the type heretofore described whe~ein the filter fails in the trapped mode. 'rhe coaxial filter includes an inner conductor 128 separate~ from the outer conductor 13~
by means of the dielectric 132 and including the conductive }ine 134 electrically coupled to the inner conductor 12~ and conductively conpected to the outer sheath by means of conductive member 136 at one end thereof. ~he controllable diode switch 144 in~erconnects the other end of the conductive line 134 to a batte~y source 147 through a controllable switch circuit 145.
~her. the diode 144 is conduçting, the conductive line 134 will not haye a~y effect on the coaxial line section and will not filter out any si~nal. On ~he other hand, when the diode 144 is nonconducting, the conductive li~e 134 will serve to t~ap out the signal being filtere~. In this manner, should the diode 144 fail to operate, the filter shown in Fi~. 16 will re~ain in the trapped mode and will filter out the frec~uency of the signal.
4Z~
Referrlng now to Fig. 17 there is shown another method of utilizing the diode switch to form a switchable filter. In this embodiment, the diode switch 144 forms part of the conductive interconnection between the conductive line 134 and the outer conductor sheath 130. ~he diode is controlled by a circuit connecting the diode to a switch 145 and to a battery 147. When the diode 144 is conducting the conductive line 134 is operative to trap out the signal being filtered. On the other hand, when the diode switch 144 is nonconducting, the line 13~ will not operate to filter out the frequency. Therefore, if the diode 144 fails the coaxial section shown in Fig, 17 will remain in the pass mode a~d will not filter out any sigpals.
It is also possible to include the diode 144 directly in series with the inner conductor 128 as shown in Fig. 18, In this way when the diode is not conducting, no sig~al at all will pass through the enti~e filter section.
The switchable filter he~etofore described in Figs. 16-~8 fi~ds particular use in connection with cable television sy6tems. In such systems it is necessary to have cont~ol over the program being sent to a subscriber. When the subscriber has paid for the program, the program will be se~t to the su~scriber line. However, if not paid fo~, it is necessary to restrict the program signal fro~ being sent to the subsc~iber ll~e, A switchable filter is af conVenient use for such pu~poses.
Referring now to Fig. 19 the~e is shown how such a switchable filter of the type heretofore described could find use i~ ~ cable television system. The main line of the cable televisian system is shown at 146. A directional tap is shown gene~ally at 14~
and includes control circuit~y 150 for controlling the ~dd~ess location of each of a plurality of taps 152. Each of t~e taps z controls a different subscriber line 154. The switchable trap156 is interconnected in the subscriber line. The switchable trap can be utilized to connect or disconnect a particular pro~ram from the subscriber line. Utilizillg the control address circuitry 150, it is possible to control the switchable trap directly from the main office utilizing the co~trol address of the particular subscriber line.
Using the filter trap as shown in Fig. 16, the ilter would fail in the trapped mode while utillzing the switchab~e filter in Fig. 17 it ~ould fail in the pass mode. Probably the embodiment shown in Fig, 16 would be more advantageous since in thi~ way the subsc~iber would notify you if the switchable filter failed. Also, in the embodiment of Fig, 16 the diode has les~ effect on the Q of the resonator. On the other han~, utilizing the embodiment iP Fi~. 17 should a failure occu~ in the SWitch the proqram would still come to the subscriber and chançes are the subscriber would not notify the station if a failure o~ccurred in the pa~s mode. If the embodi~ent of Fig. 18 were u~ed, there ~ould be qn additional control in that the entire service would be disconnected from the subscriber line. The e~bod~ment shown in Fig, 18 coulj of course be utilized in ~
c~njunction with the switching control of the filter thereby obtaining the dual control of both a switchable trap as well as complete disconnectipg of se~vice.
Referring now to Fig. 20 there is shown an additional ~
embodiment of the coaxial filter heretofore described. In Fig.
20, the coaxial cable includes an inne~ conductor 158, an outer conductive sheath 164, a first dielectric 162 located around the inner conductor and a spiral conductive winqing 164 wound around the dielectric 162. In this manner the conductive wlndln~ 164 will be ~oaitioned at A ~ixed dist~nce from ~he inne~
conductor 158 and Will be electrically coupled thereto. One end 166 o the spir~l conducto~ is conductively connected t~ the outeX conductlve ~heath 160 by li~e 166~ A 80cond dlelect~ic ~68 s~parateg the spiral windiPg 164 from the outer sheath 1~0.
Th~ length of the ~piral conducto~ would be less than ~because o~
mutual coupling between turns. ~he outer conductive uheath 160 i~ shown a~ a bralded conductor. A pl~8tic ~uter ~oating 170 i~ ~ho~n oVe~ the con~uctive bsald 160.
The embodiment shown in Fig, 20 can be made by con-tinuously forming the variou~ layers and then notching out and isolating the ~ spiral sections. After that the spiral sections could be conductively connected to the outer conductor sheath.
This method will become better understood hereinafter, in connection with the methQd for making the coaxial filter e~bodiment, Refe~ring now to ~igs. 21a-e there ls shown,a method of making a coaxial filter of the type heretofore described in connection with Fig. lS. Initially, two inner conductors 172, 174 are spaced apart, and a dielectric 176 is extruded over the accurately spaced apart conduators. Using a tool and die, sections 178 are chopped out of the dielectric 176. At the same time, the conductor 172 is c~t and a portion of 180 is kent up-wa~rdly to protrude above the dielectric 176. Additional notches 1~2 are also chopped out of the dielectric 176 as well as from the conductor 172. The spacin~ ~etween the p~otruding section 180 and the chopped out section 182 i8 approximately ~.
The dielectric covered conductors are now sent to a braiding machine which adds the outer conductive sheath 184, typically a braided conductor. When the braiding is pu~ on, P1~4~
the portions 180 which protrude above the dielectric also extend through the braiding. ~he protruding portions 180 are then bent over the conductive sheath 184 so as to be substantially flush with the braid, and are then electrically connected t the conductive braiding by either soldering, welding or the like.
If the protruding sections 180 are welded o~to the braided conductive sheath 18q, it iS possible to both bend and weld the member 180 during the same processing step. This can be seen by referring to Fig. 23, wherein electrodes 186 are shown connected to an energy source 188. The braided cable, shown generally at 190, passes through the electrodes ~hereby the electrodes will both bend the protruding section 180 and at the same time weld them onto the conductive b~aided sheath 184. The fi~ished product is shown at 192 ~hich shows the weld located above the bent member 194.
Refe~ring back to Fig, 21d it is noted tha~ after the prot~uding sections have beenj bent and welded onto the con-ductive sheath 184, the cablq is now passed through an extruder whrich places a plastic jacke~ 196 over the entire cable. Qne or mqre sections may now be cut off and terminal connectors 1~8, 2~Q can then be placed on eithe~ end of the coaxial filter, s~ch that one of the connecto~rs serVes as the input and the other serves as the output.
Although the device shown in Figs. 21a-21e is for a filte~ trap, it is understood that the bandpass filter could also be made in a similar manner.
~ eferring now to Fig. 22 there is shown another embodiment whereby the length of the co~ductive line can be reduced. In addition to having the one end 180 protruding from the dielectric 176, the other end 202 is also made to protrude above the 1~34~
dielectrlc, While the end 180 will then be conductively connectedto the outer electrical braided ~heath, the end 2~2 will be terminated in a capacito~ 200 which wlll be connected ~o g~ou~d.
This ~ill r¢duce thq length o~ the fllter to le~s than ~, the length depe~ding ~n the value of C, and increage the frequency at Which the trapping effect repeat# ~tsel~, This also avoid~ the well known three time~ the frequency problem experie~ced with 1/4 ~Pvelen~th ~ections.
~ here has beeA dlsclosed hereto~ore the best embodime~t o~ the inve~tlon p~esently contempl~ted. However, it 1~ tq be u~der~tood that yarious features and ~odiflcation~ may bq made thereto without departln~ from the spirit of the lnvent~on.
Claims (17)
1. A coaxial filter having two ends for insertion be-tween transmission lines comprising a flexible inner conductor, at least one flexible conductive outer sheath, first insulation means embedding said inner conductor therein and separating said inner conductor from said outer conductive sheath, a section of conductive line extending longitudinally within and embedded in said first insulation means and laterally spaced apart from said inner conductor and capacitively coupled thereto, second insulating means covering said outer sheath and coaxial terminal connectors at the opposite ends of said coaxial filter to permit said coaxial filter to be interconnected between the transmission lines, one of said termi-nal connectors serving as the filter output and wherein the length of the section of conductive line is approximately .lambda./4, wherein .lambda.
is the wavelength of the signal to be filtered by the coaxial filter.
is the wavelength of the signal to be filtered by the coaxial filter.
2. The filter as in claim 1 and further comprising con-nection means for conductively interconnecting one end of said sec-tion of conductive line with said outer conductive sheath
3. The filter as in claim 2 and wherein said section of conductive line is parallel to said inner conductor and laterally spaced apart a distance ? therefrom throughout its length, wherein ? is dependent upon the capacitive coupling desired between the inner conductor and the section of conductive line.
4. The filter as in claim 2 and further comprising a second section of conductive line longitudinally extending within said first insulation means laterally spaced apart from said inner conductor and electrically coupled thereto, second connection means for conductively interconnecting one end of said second section of conductive line with said outer conductive sheath, said second sec-tion of conductive line being of a length .lambda.'/4 wherein .lambda.' is the wavelength of the frequency of a second signal passing through the coaxial filter, whereby said filter is capable of filtering out signals of two different frequencies, and wherein the lengths of said first and second sections of conductive lines longitudinally overlap.
5. The filter as in claim 2 and further comprising a plurality of said sections of conductive line, all said sections of conductive line being colinear with each other along a common line and axially spaced apart from each other along said common line.
6. The filter as in claim 2 and wherein said section of conductive line is spirally wound around the inner conductor, said first insulation means being located between said spirally wound conductive line and said inner conductor, and further including second insulation means separating said spirally wound conductive line and said outer conductive sheath.
7. The filter as in claim 2 and further comprising a controllable switch electrically connected to said section of con-ductive line, and a control circuit coupled to control said switch for selectively introducing and removing the filter from the trans-mission line.
8. The filter as in claim 2 and further comprising a controllable switch electrically connected in series with said inner conductor.
9. The filter as in claim 7 and wherein said controlla-ble switch is coupled between the other end of said conductive line and said control circuit, and wherein said control circuit further comprises a switch means in series with energy source means.
10. The filter as in claim 7 and wherein said control-lable switch is coupled in said connecting means and thereby po-sitioned between said one end of conductive line and said outer conductive sheath.
11. The filter as in claim 7 and wherein said control-lable switch is a diode.
12. The filter as in claim 1 and further comprising a second section of conductive line longitudinally positioned in said first insulation means laterally spaced from said inner con-ductor and electrically coupled thereto, said second section of conductive line being axially displaced from said first section of conductive line, one of said sections of conductive line being adapted to receive the signal to be filtered and the other section of said conductive line producing the filter output, the length of each section of conductive line being .lambda./4 wherein .lambda. is the wavelength at the center frequency and the coupling coefficient determines the bandwidth and variation in bandpass insertion loss.
13. A method of making a coaxial filter comprising the steps of:
a. extruding a dielectric over two accurately spaced apart longitudinal conductive lines;
b. chopping out small sections of the dielectric at spaced intervals;
c. cutting one conductive line at each chopped out section;
d. upwardly bending a portion of said one conductive line at each chopped out section to protrude above the dielectric e. covering the dielectric with a conductive shield permitting said portions to protrude through said conductive shield;
f. folding the protruding portions of the conductive line onto the conductive shield;
g. electrically connecting the folded over portion onto the conductive shield; and h. placing an insulating covering over the conductive shield.
a. extruding a dielectric over two accurately spaced apart longitudinal conductive lines;
b. chopping out small sections of the dielectric at spaced intervals;
c. cutting one conductive line at each chopped out section;
d. upwardly bending a portion of said one conductive line at each chopped out section to protrude above the dielectric e. covering the dielectric with a conductive shield permitting said portions to protrude through said conductive shield;
f. folding the protruding portions of the conductive line onto the conductive shield;
g. electrically connecting the folded over portion onto the conductive shield; and h. placing an insulating covering over the conductive shield.
14. The method as in claim 13 and further comprising the step of chopping out additional sections of the dielectric together with said one conductive line, such that the spacing of the one conductive line between said additional chopped out sec-tions and said bent up portions are related to the wavelength of the signal being filtered.
15. The method as in claim 14 and wherein said related spacing is .lambda. /4 wherein .lambda. is the wavelength of the signal being filtered
16. The method as in claim 13 and wherein said step of folding, as well as said step of electrically connecting, are both achieved by the step of passing the covered dielectric material through electrodes which bend and weld the protruding portions onto the conductive shield.
17. The method as in claim 14 and further comprising the step of cutting off a unit length of the product produced after step (h), said unit length including at least one of said spacings of said one conductive line, and placing electrical con-cuctors at the ends of said cut off unit.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA360,130A CA1104221A (en) | 1975-07-09 | 1980-09-11 | Transmission line filter |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US594,270 | 1975-07-09 | ||
| US05/594,270 US4004257A (en) | 1975-07-09 | 1975-07-09 | Transmission line filter |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1104222A true CA1104222A (en) | 1981-06-30 |
Family
ID=24378229
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA256,444A Expired CA1104222A (en) | 1975-07-09 | 1976-07-05 | Transmission line filter |
Country Status (4)
| Country | Link |
|---|---|
| US (1) | US4004257A (en) |
| BE (1) | BE844001A (en) |
| CA (1) | CA1104222A (en) |
| CH (1) | CH611745A5 (en) |
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| FR2479576A1 (en) * | 1980-03-25 | 1981-10-02 | Cables De Lyon Geoffroy Delore | DEVICE FOR GALVANIC DECOUPLING OF A TELEVISION OR TELEALIMENTATION ANTENNA CABLE |
| FR2495844A1 (en) * | 1980-12-10 | 1982-06-11 | Snecma | TUNABLE TAG FILTER ON A PREDETERMINED NUMBER OF DISCRETE FREQUENCIES DISTRIBUTED IN A BROADBAND OF FREQUENCIES |
| US4467296A (en) * | 1982-08-23 | 1984-08-21 | Loral Corporation | Integrated electronic controlled diode filter microwave networks |
| FR2539933A1 (en) * | 1983-01-25 | 1984-07-27 | Thomson Csf | Switchable filter for microwaves |
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| FR2628571B1 (en) * | 1988-03-11 | 1990-11-09 | Thomson Csf | MICRO-BAND MICROWAVE TECHNOLOGY BAND CUTTER FILTER |
| JPH04302501A (en) * | 1991-03-29 | 1992-10-26 | Anritsu Corp | Signal selection device |
| FR2678450B1 (en) * | 1991-06-27 | 1993-09-03 | Dassault Electronique | MICROWAVE BAND CUTTER FILTERING DEVICE. |
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| JP2866054B2 (en) * | 1996-05-28 | 1999-03-08 | 宇呂電子工業株式会社 | Line radiation prevention element |
| FR2783654B1 (en) * | 1998-09-23 | 2006-07-28 | Sagem | BIBANDE TRANSMITTER-RECEIVER WITH DOUBLE RADIATION DEVICE |
| US6284971B1 (en) | 1998-11-25 | 2001-09-04 | Johns Hopkins University School Of Medicine | Enhanced safety coaxial cables |
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| US2925565A (en) * | 1955-05-12 | 1960-02-16 | Bell Telephone Labor Inc | Coaxial couplers |
| DE1146559B (en) * | 1960-09-27 | 1963-04-04 | Siemens Ag | Directional coupler, consisting of an outer screen and two inner conductors arranged inside this screen |
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-
1975
- 1975-07-09 US US05/594,270 patent/US4004257A/en not_active Expired - Lifetime
-
1976
- 1976-06-29 CH CH830376A patent/CH611745A5/xx not_active IP Right Cessation
- 1976-07-05 CA CA256,444A patent/CA1104222A/en not_active Expired
- 1976-07-09 BE BE168805A patent/BE844001A/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| CH611745A5 (en) | 1979-06-15 |
| BE844001A (en) | 1976-11-03 |
| US4004257A (en) | 1977-01-18 |
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