REFERENCE '1'0 RELATED APPLICATIONS
The present application is related to U.S. Patent 3,925,726 issued Dece~;ber 9, 1975, to I~ A. Whyte and is assigned to the assignee of this invention.
BACKGROUND OF THE INVENTION
This invention relates to power line carrier com-munication systems of the type transmitting carrier communi-cation signals through the power lines of a distribution net-work directly to customers of an electric utility and more particularly, to a distribution network power line carrier communication system including terminating impedance net-wor~s connected to the power line conductors serving the electric loads of customers receiving power line communication signals otherwise subject to adverse signal impedance ter-minations due to short circuited and varying customer load --1-- ~ i~
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~49~ 5 conditions and interfering signals occurring in the customer loads.
A distribution network power line carrier communi-cation system is disclosed in U.S. Patent No. 3,911,415 issued October 7, 1975 to I. A. Whyte and assigned to the assignee of this invention, in which transmitters, receivers and frequency translating and signal reconditioning re-peaters are described for transmitting communication sig-nals between a substation and residential customers of an electric utility company through the power line conductors of a power distribution network. This power line communi- -cation link with the residential electric power customers provides remote meter readings and/or selective load control of the customer loads A communication terminal is provided at each cus-tomer including the transmitter and the receiver described in the aforementioned patent which are coupled to the customer's service conductors. As is known, service conduc- ~;
tors interconnect the secondary power lines of a distribution ~-20 network and the customer wiring and electric loads. Accord- ; -ingly, the communication signals transmitted to the customer premises have signal impedance terminations including the `'-combined impedances of the service conductors, a watthour meter usually connected thereto and the customer wiring and --~
electric loads. These customer impedances present widely varying and often adversely low impedance values to the high frequency communication signals. Therefore, efficient and `~
suitable impedance matching of the customer's transmitter ~' and receiver is difficult to accomplish. For example, the impedance at the communication signal frequencies of a watt-
-2-`~
.
~6~44~;05 hour meter ls usually ln the order of one to two ohms. The customer electrlc loads often vary from a maximum lmpedance in the order Or 50 ohms to a minimum impedance at the signal ~;, frequencies in the order of 0.5 ohm, however, the lmpedance variatlons are quite random and unpredlctable.
With the power line communication system transmit-tlng carrier slgnals to large numbers of customers, efficlent operatlon of the system requlres that signal impedance ter- -~mlnations at each customer be relatlvely hlgh and substantlal-ly constantO This aldq in accompllshing more efficient and proper lmpedance matchlng at the customer's receivers and - -transmitters to avoid substantlal losses and attenuation of the slgnal power levels. Such impedance matching is diffi~
cult when the combined customer;impedance variations at the `
communication slgnal frequencies have a ratio of approximate- -ly thlrty to one wlth impedance values indicated above.
Also, lt is also required to isolate the power line communi- `
cation slgnais from low and virtually short circult lmpedance conditlons whlch can occur in the customer electrlc loads.
20 The customer low lmpedance condltions requlre substantially , higher signal power so that the received communication sig- `
nals have acceptable voltage levels at the input Or a cus-tomer receiver.
A further adverse conditlon to be protected agalnst -at the customer terminal end of a power line communication system is lnterferi'ng hlgh frequency signals which may ori-glnate ln customer loads including home inter-com systems, or , hlgh frequency nol~e generating sources, or unauthorized signal generators intentionally connected at the customer premlse~ to dlsrupt the reception or transmission of the com-
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; ' ,: . . ' ' '' ~, ~'' .' . ' . ' ' ~044605 munication signals being transmitted over the distribution network serving the customer. Connection of a frequency responsive signal bypass in parallel with the customer loads provides low impedance paths to ground at the communication signal frequencies to confine the interfering signals.
In UOS. Patent No. 3,925,728 issued December 9, 1975 to Io A. Whyte and assigned to the assignee of this invention, an improved watthour metering circuit is described and claimed having integral terminating impedance networks which provide high impedance elements at high frequencies for suitable impedance termination of power line communi- -cation signals transmitted to a power customer and the net- .
work also includes low impedance elements at high frequencies . :
to bypass interfering signals originating in customer loadsO :
SUMMARY OF THE INVENTION .
In accordance with the present invention a distri-bution network power line carrier communication system in~
cludes an improved terminating impedance network connected to ...
the powér line conductors of a utility company customer be- '! ' : 20 tween the customer electric loads and the customer's commun ication terminal. The terminating impedance network includes .
high series impedance elements including an inductance or ;~ :
inductance-capacitance parallel resonant connected circuit ~ .
elements tuned to the power line communication signal fre- .~-quencies so as to provide a predetermined increased value of :
signal impedance O Higher and more constant terminating im-pedance values are presented to the communication signals received at a customer's power line signal termination without substantial voltage drops or power losses at the electrical
-4- `.:
43,844 . ~ ' 05 ,.
':-power frequencies. The lnductance elements are formed by rerrlte magnetlc core members secured around a customer's power line service conductors interconnecting the customer electrlc loads and a distrlbutlon power line network. The parallel resonant clrcults are provided by ferrlte magnetlc core members havlng a power line conductor extending through the center opening of the core member with a capacitor being transrormer coupled in parallel so that the parallel resonant circuit ls tuned to the communication slgnal frequencies. `
The improved terminating impedance network further lncludes low protectlve lmpedance elements at the communi-catlon signal frequencies which are formed by capacitance ;~
elements connected across the the customer power llne servlce conductors so as to be connected in parallel wlth the cus-tomer's electric loads. A predetermined value of capacitance conflnes interfering high frequency slgnals origlnating in the customer loads when the interferlng signals have frequen-cies ln the same frequency range as the communlcatlon slg-nals~ The capacitance elements have small or negllgible current drain at the electrlc power frequency. When a lower predetermined value of lmpedance is desired, an lnductance-capacltance serles resonant clrcuit is connected across the .. ~ . .
customer's power llne servlce conductors wlth the series ,~
resonant clrcuit belng tuned to the frequencies Or the com-munlcatlon slgnals.
It is a ~eneral feature of this lnventlon to provlde a terminatlng lmpedance network for properly termlnatlng and protectlng communicatlon slgnals transmltted to a customer's premises from a power distrlbution network without substantial voltage and power losses ln the electrlcal power supplying `:
- - . ~ - . . . , : : , . .
. : . . .. , " ~ . .. . .
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43,844 ~0446(~5 the customer's electrlc loads. Another feature Or this invention ls to provide an lncreased serles lmpedance path at the frequencies of power line carrier communlcatlon slg-nals whereln an lnductor device formed by a magnetlc core member is assembled around a customer's power llne service .
conductor extending between the customer electric loads and "t' a power line distribution network transmittlng carrler com- .
munlcation signals and wherein the magnetlc core member can , also form a tuned inductance element of a parallel resonant circult when a tuned capacltance element ls coupled to lt to .; . . .
provide higher resonant circult lmpedance values. A stlll ~
: . .
further feature of this invention is to provide a parallel ;~
low protectlve impedance path in combinatlon wlth a high series impedance path in a termlnatlng lmpedance network in ;! ~
which the parallel signal path includes a capacitance ele- .. :
... . ..
ment which bypasses interfering slgnals havlng frequencies ' in the same range as a communication signal when the inter- ~.:: :-~ering slgnals are generated at a source at the customer premisesO Other ~eatures and advantages of this lnvention - -;
will be apparent from the detailed description of the embodi-ment of this invention as shown in the drawlngs.
BRIEF DESCRIPTION OF THE DRAWINGS ``
Figure 1 is an electrical schematic dlagram of a utillty company customer's power llne terminating connec~
tions which are connected to a distribution network included ..
in a power line ca~rier communication system and which in- ; .
clude a terminating impedance network made in aGcordance with this inventlon;
Flg. 2 is an electrical schematic diagram of an 30 alternative embodiment of the terminating impedance network ~ :
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.
~044605 ~
shown in Figure l;
Fig. 3 is a perspective view of a magnetic core inductor device included in the terminating impedance network illustrated in Figure l; and Fig. 4 is a perspective view of a magnetic core inductor device forming a tuned element of a parallel reson- ~
ant circuit including a capacitor coupled to the inductor -device in the terminating impedance network illustrated in Figure 2. - -DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, wherein the same numeral designates like or corresponding elements in the several figures, and more particularly to Fig. 1 wherein there is shown an electrical schematic diagram of a utility customer connections 10 forming terminating loads of an electric power transmission and distribution system. The customer connections 10 represent many terminating or load end connections of an electric power system occurring, for example, at a large number of residential customers. It is noted, however, that the present invention, as described in dètail hereinbelow, is of general application and is not llmited to use at only residential type power customers. A
fragmentary portion of a distribution network 11 is shown ^~
in a section 12 of Fig. 1 that is designated at the left-hand side of the dashed line 130 The distribution network 11 transmits sixty Hzo electric power 14 and is included in a power line carrier communication system of a type, for exam-ple, as disclosed and claimed aforementioned UOS. Patent 3,911,415 issued October 7, 1975 to Io A~ Whyte and assigned to the assignee of this inventionO Accordingly, modulated 43 ,844 1044f~QS
,:
hlgh rrequency carrier communication signals 16 are trans-mitted from a central interrogating statlon, not shown, over the prlmary distrlbutlon conductors 17A and 17B. The com- ~;
munlcation slgnals 16 are of a type having a frequency range ln the order of twenty kHz. to four hundred kHz., of a sult- -~
. .: .
able bandwidth, and sultably modulated by digitAl data base- ; . `;
band slgnals such as by frequency shlft keying, as descrlbed ;.- .
in the a~orementloned ~pplic~tion. The dlstrlbution network .,~
.. ~.;.. . .
11 includes a voltage step-down dlstribution transformer 19 . ::
.,": , .,:
to supply the electric power 14 at appropriate power voltages, usually 120 and 240 volts, to a three-wire secondary portion of the network 11 that also transmits the signals 16.. ~
An interconnectlng section 21 is deslgnated between .~-..:
the dashed line 13 and dashed line 22 ln the schematlc dia~
gram of Fig. lo The sectlon 21 typically includes service conductors 23A, 23B and 23C connectlng the secondary of the distrlbution network 11 to a customer's premises includlng a section 24 in Fig. 1 at the right-hand slde of the dashed ..
llne 220 A customer electrlc load 26 ls lncluded ln the :~.,~;,- .
section 24 and includes wiring conductors 27A, 27B and 27C :
connected, as shown, to the substantially all resistance load .~: .
devlces 28A, 28B, 28C and 29A, 29B, 29C and 30. Service en- ~:-.
trance equlpment, not shown, lncluding a main swltch and -..
fuses typlcally connect the customer wlring conductors. 27A, 27B and 27C to the servlce conductors 23A, 23B and 23C, res- `;~
pectivelyO The cdnductors 23B and 27B are grounded ln normal `..
practice by belng connected to earth grounds as shown in .j~. .....
Flg. 1. g ;
A hlgh frequency slgnallng devlce 32 ls shown con-nected ln the customer load 26 and the devlce 32 can be formed /` .:-~.. .
- :. .. .
. . . .......... ~ ' ~ . . ' . . ' .' , .................. ~
, ~ . , . ~ .
iQ44~(~5 by a home inter-com system or other slgnal generating source lncluding high frequency electrical noise. The signaling device 32 is to be understood to be capable of generatlng a ~
signal 33 which lncludes frequencies which are in the fre- -quency bandwidth of the carrier communication signal 16 and is capable of interfering or ~amming the signal 16 or other carrier communication signals associated with a power line carrier communication system connected to the distribution network 11. T~e load devices 28A, 28B and 28C, and 29A, 29B
and 29C are rated at 120 volts and are connected between the conductors 27A and 27B and between conductors 27C and 27B, respectively, and the device 30, which may be optionally included in the customer load 26, is rated at 240 volts and -is connected between the line conductors 27A and 27C. These load devices typically include switches, not shown, for use at randomly different times so that the customer electric load 26 has impedance variations from a condition of low impedance in the order of 0.5 ohm or a virtually short cir- -~
cuited condition, to a condition of maximum impedance in the ~--20 order of fifty ohms. `~
Referring further to the customer interconnecting -power line section 21, an induction watthour meter 34 of a conventional design is normally connected to the service con-ductors 23A and 23C for measuring the consumption of electri-cal energy by the customer electric load 26. A voltage mea-_suring coil 35 is connected across the conductors 23A and 23C and two current measuring coils 36 and 37 are connected in series with the llne conductors 23A and 23C as shown.
The current coils 36 and 37 have very low impedances in the order of approximately one ohm at the communication signal _9_ ., .
.~: .: . , : . , ~0446~5 : ~:
frequencies while the voltage coil is formed by a winding ~ .
having a large number of turns of a small conductor so as to .. ~:
present a high impedance across the power line conductors~
The watthour meter coils are effective to drive a rotating :.:,: .
disc at a rate corresponding to the consumption of elec~
trical energy as is well understood.
The section 21 also includes a power line communi~
cation terminal 38 of a type located at a remote customer , ~ -location, also referred to as a response communication termi- ~ .
10 nal as described in the aforementioned U.S0 Patent No. , ;
3,911,4150 The communication terminal 38 includes a logic .~ ~ :
circuit 39 which may include a pulse accumulating co~nter and encoder circuit as disclosed in U.S. Patent No. 3,820,073 issued June 25, 1974 to L. C. Vercellotti et al. A trans- : .
mitter 41 and a receiver 42 of the communication terminal . .
38 are coupled to a pair of the service conductors typically including a line conductor such as 23A and the grounded conductor ~.
23B by a coupler 43. The logic circuit 39 receives pulses from a pulse generator 44 in the watthour meter 34 for .
. ~ .
transmitting remote meter reading information to the central interrogation communication terminal, not shown, associated .
with the power line communication system connected to the .
distribution network 11. Accordingly, the communication `
signals 16 represent a bandwidth of signals transmitted .~ -and received at the communication terminal 38 and coupled by the coupler 43 for transmission to and from the distri-bution network power line carrier communication system . . ~. .
associated with the network conductors 17A and 17B. :. .
Referring now to an improved terminating impedance .:
43,844 ,-~V~ 5 network 46 which ls made in ~ccordance wlth a principal fea-ture of this invention, the network 46 protects and improves the impedance termlnations of the communlcation signals 16 of the power line communication system transmitted to the customer as described hereinafter. The terminatlng impedance network 46 is preferably connected ln the sectlon 21 and to the servlce conductors 23A, 23B and 23C at the distribu-tion secondary ~ide of the connections to the watthour meter 34, so that any power losses of the networ~ 46 are not mea-sured by the customer's meter. The network is further lo-cated on the customer load slde of the coupler 43 and the communication terminal 38 to more suitably termlnate the sig-nals 16 and protect them from the load 26. The network 46 ~-presents hlgh signal impedance ln the series path of the signal~ 16O Inductor devices 47, 48 and 49 present predeter~
mined inductance valued circuit elements connected in series with the service conductors 23A, 23B and 23C, respectively.
The network 46 also includes a low protective im- `
pedance in a paral~el path to ground for the signals 160 Capacitors 51 and 52 provlde predetermined valued capacitance circuit elements connected between the grounded service con-ductor 23B and the service conductors 23B and 23c , respec- ~;
tively, as shownO It is important that the capacitors 51 and 52 are connected between the inductors 47, 48 and 49 and the customer load 2~ and in the parallel relationshlp with the customer load devices as shown~ The lnductance values of the inductor devices 47, 48 and 49 and the capacltance values of the capacitors 51 and 52 are selected, as illus-trated by the exemplary embodiments described in detail here- `~
inbelow, so as to provide the desired termination and protec-43,844 . . ..
3~. . .
10~4~;Q5 ,; ~ ~
tlve lmpedances values at the frequencle~ of the ¢ommunl-catlon signal 16 while havln~ mlnlmal or substantlally negll-gible power current draln, power losses and voltage drops by the electric power 14.
Fig. 2 illustrates an electrical schematlc dlagram ~- ~
of another preferred embodlment of a protectlve terminatlng ` ~ -impedance network 46A made ln accordance wlth the present -:: -lnventlon to accommodate a wlder varlety of slgnal lmpedancetermlnatlon values. The network 46A is lntended to replace the network 46 at the same aforementloned connection to the servlce conductors 23A,-23B and 23C. Three lnductance- ',,:
capacltance (L-C) parallel resonant circults 54, 55 and 56 ~: -lncludlng lnductor devlces 58, 59 and 60 and capacitors 62, ~ .
63 and 64 as shown whlch are tuned to the mid-frequency of the bandwidth of the communciation slgnals 16. The clrcults 54, 55 and 56 are connected ln serles with the conductors ` -23A, 23B and 23C, respectively. The parallel resonant clr-~;~ cults form the hlgh serles lmpedances of the network 46A to present higher values of lmpedance than are presented at a glven frequency of lnterest for the signals 16 than by the single inductor devices 47, 48 and 49.
The terminating lmpedance network 46A further has two lnductance-capacitance (L-C) series resonant circuits 65 and 66 for replacing the single capacltors 51 and 52 in the -~
network 46 and, accordingly, they are connected across the service conductors 23A and 23B and across the conductors 23C ~;
and 23B, respectively. The series resonant circuits 65 and 66 include capacitors 67 and 68 and inductors 69 and 70, re-spectlvely, having predetermined capacitance and inductance values series tuned to the mid-frequencles of the bandwidth , ~ .
S ~:
, , ~
of the communication signals 16. Lower values of signal impedance are presented in the signal path of the network 46A at a given frequency of interest than ls possible with ~
the single capacitors 51 and 52 of the network 46. The ~-capacitors 67 and 68 and the inductors 69 and 70 can be provided by discrete conventional capacitance andiinductance elements connected at a convenient point such as at the ser-vice entrance éguipment directly across the service conduc-tors 23A, 23~ and 23C as described hereinabove. ~ ~
Referring now to Figs. 3 and 4 wherein there is ~ ;
illustrated further important features of the present inven-tion wherein hollow tubular magnetic core members 47A, 48A and 49A shown in Fig. 3 are made of a high frequency magnetic core material such as powdered iron or more preferably a ferrite magnetic material. The conductors 23A, 23B and 23C extend through the hollow center portions of the tubular magnetic core members 47A, 48A and 49A, respectively, to form the inductor devices 47, 48 and 49 shown in the network 46 in Fig. 1. The magnetic core members 47A, 48A and 49A have a hollow cylin-2Q drical shape formed by two identical semi-cylindrical halves, æuch as designated 47A-1 and 47A-2 at the core member 47A. ~
The dimensions of the core members are made to have a pre- ~ -determined value of inductance to form the desired high serles impedance for the signals 16 which if formed by conventlonal electrical circuit lnductance elements would be quité large ^-and often qulte expensive. Suitable thicknesses of the -magnetic cores 47A, 48A and 49A have been found to be in the order of a fraction of an inch, for example about 0.25 inch ~;
and sultable lengths are provlded ln an approximate range of 1-1~2 to 4 inches to have the predetermined inductance impedance ~;
. ~
1~4~S
to be provided at a frequency of interest which may be established as described ~urther hereinbelow.
Fig. 4 illustrates a preferred embodiment of the ;
resonant circuits 54, 55 and 56 lncluded in the terminating impedance network 46A shown in Fig. 2. Hollow rectangular cross-sectionally shaped tubular magnetic core members 58A, 59A
and 60A are made of a suitable high frequency magnetic core `~
material such as powdered iron or preferably a ferrite magnetic material for surrounding the conductors 23A, 23B and 23C. The ;
members 58A, 59A and 60A correspondingly form the predetermined value of inductance for the tuned inductor devices 58, 59 `
and 60, respectively, described in connection with the description of Fig. 2. The rectangular shape of the magnetic core members 58A, 59A and 60A is preferably made into two halv~es as indicated by the halves 58A-1 and 58A-2 designated at the magnetic core member 58A in Fig. 4. The thickness, as described for the magnetic core mem~ers of Fig. 3 may be in the order of a fraction of an inch, for example in the order of 0.25 inch, and have combined height, width and length dimensions to provide the predetermined tuned inductance impedance characteristics for the resonant circuits 54, 55 and 56 as also noted further hereinbelow. It is apparent to those skilled in the art that the same or similar hollow rec~angular cross-sectional configuration of the tubular mag-netic core members 58A, 59A and 60A or other elongated, hollow ;
non-circular cross-sectional form may be used to provlde the tubular magnetic core members 47A, 48A and 49A shown in Fig. 3 having a hollow circular conflguration and vice versa. 1`~
Accordingly, the term tubular as used herein and in the claims ;
is to include a configuration that is hollow and has a sub--14- ;
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;
- . ~ . ! ` ` , ...... . . ...
stantially constant cross-section along an elongated length.
The capacitors 62, ~3 and 64 shown in Fig. 4 form the resonant circuits 54, 55 and 56 by being transformer coupled with the magnetic core members 58A, 59A and 60A by conductor loops 72, 73 and 74 connected to the opposite ends ~-of the capacitors and extending in magnetically coupled rela-tionship through the hollow centers of the magnetic core -membersO
It is an important advantage of this invention that ;
the magnetic core members shown in ~igs. 3 and 4 form the inductor devices included in the terminating impedance net-works 46 and 46A in Figs. 1 and 2 in an inexpensive manner to provide the desired values of inductancesO It is a fur-ther important advantage that the magnetic core members are installed to the service conductors 23A, 23B and 23C without having to disconnect or cut the conductors, accordingly they ., . i .. ~
can be installed while these conductors are energized. This ~-is particularly advantageous since it is desirable that the inductor devices folrming the high series impedances for the signals 16 in the networks 46 and 46A be provided toward the distribution network 11 relative to the watthour meter con-, .
nections where often it is not convenient to break a conduc-tor so as to have a separate inductance element connected in series therewith.
In practicing the present invention there are a number of variable conditions at the customer terminating ~`
connections 10 which determine the different values to be used for the inductance and capacitance elements of the dif-ferent embodiments in the circuits of the terminating impedance networks 46 and 46A illustrated in FigsO 1 and 2. As noted ; . . , . ?
. .
lV4~605 hereinabove, either of the high series impedances for the signals 16 being formed by the single inductors in the net-work 46 or the parallel resonant circuits included in the network 46A may be used in combination with either of the low protective impedances for confining the interfering sig- ~
nals 33 as provided by either the single capacitors or the ~ :
series resonant circuits of the networks 46 and 46A, respective-lyO An initial consideration in determining the value of the magnetic core inductor devices 47, 48 and 49 is made by the ~.
10 limitation of permissible voltage drop of the electric power ~ -~
14 conducted through the inductor devices at the power fre-quency. Also, in the initial consideration of the inductance ~-;
values L47, L48 and L49 of the inductor devices 47, 48 and 49 respectively, the inductances L47 and L49 are assumed to , -~
equal each other and to twice the inductance L48 since under '~
balanced load conditions the grounded service conductor 23B
will conduct twice the current of the conductors 23A and 23C .-since it is a return path for both of the line service con- ;:
ductors. Further, only the impedances of the terminating .
20 impedance networks 46 and 46A which are connected with the service conductors 23A and 23B will be considered since it is these conductors which are conducting the communication ~
signals 16 in Fig. 1. With the aforementioned conditions in -mind, the equation Vd = 2ff6o(imax)(L47 + L48) i determine the inductances L47 and L48 at the permissible voltage drop Vd.across the inductor devices 47 and 48, where imaX is the maximum power current flowing through the induc- . :
tors and f60 is the sixty HZo frequency of the electric .`~
power 140 ` :
Upon determining the inductance values for the .
.. ., ~ ... , . , ". "........... .. . . .
~044605 inductor devices 47 and 48 at the desired value of Vd, the desired value of impedance to be presented by the high series .
impedances of the network 46 of 46A is then determined. It ~ -is desirable that the series signal termination impedance at ~ ' a frequency of interest of the communication signal 16 is somewhat higher than the maximum impedance of the customer ~ -load 26. Typically, the load 26 is in the order of fifty ohms and is subject to random variations to low impedance values in the order of 005 ohms, as noted hereinabove. The series 10 impedance equation Zs = 2~fS(L47 + L48) is used for inductors 47 and 48 where Zs is equal to the desired impedance at the communication signal frequency, fs is the frequency of interest of the communication signal 16, and the values of the induc-tances L47 and L48 have been established in accordance with :
the permissible voltage drops described immediately herein- `
aboveO If the value obtained for Zs is greater than the desired series signal impedance then the inductor devices 47 and 48 r_ . .
would be used as shown in the network 460 However, if the value of Zs is less than the desired series signal impedance 20 desired, then the parallel resonant circuits 54 and 55 must .^
be usedO It is found that more commonly it is necessary to utilize the parallel resonant circuits 54 and 55 included in the network 46A~ ~To determine the quality or Q factor of the inductor devices which are used in the parallel resonant circuits 54 and 55 the equation Z = 2~f5 Q (L58 + L59) wherein Z is the desired parallel resonant circuit signal impedance to be provided and the inductances L58 and L59 would be equal to the inductances determined for the inductances L47 and L48, respectively~ It is preferable to have a series impedance in an approximate range of fifty to six hundred ohms at the :~
.. .
~, . .: ,.
1~46~S
communication signal frequenciesO
Referring now to the low protective impedances provided by the capacitors 51 and 52 and the terminating impedance network 46 and the series resonant circuits 65 and 66 included in the network 46A. Only the capacitance C51 is considered since it is connected across both of the conduc~
tors 23A and 23B transmitting the communication signals 16 - -~
and the capacitances are to be equal. Usually a single ca-pacitor rather than the series resonant circuit will provide a desired low impedance value to the signal frequencies of :-,~
interest. The desired low protective impedance value Zp is determined by the equation Zp = 1/(2~fs C51) where the -frequency fs is equal to the carrier signal frequency and i ~
C51 is the capacitance of the capacitor 51. If the desired r. ,' ;' ' .
low protective impedance is to be lower than provided by the ~, -above equation at the frequencies of interest then the series resonant circuit 65 of the network 46A must be utilized. The , - . .. ..
use of the capacitor 51 or the capacitor 67 of the series .
resonant circuit 65 assures that there is only small current ~i, . . . .
20 drain of the sixty Hz. electric power. It is preferable ~7.'. ~ ~'.1 ' :' '' that the parallel protective impedance be one ohm or less at the communication signal frequencies.
Illustrative values for the inductances and capa-citance of the circuits forming the terminating impedance `,~ `
networks 46 and 46A are now set forth hereinafter for purposes -of explaining the present invention and are not to be con-sidered as limitations since many other alternative values -~
~ ! .
and arrangements are possible due to the varying frequencies ~.
of the communication signal 16 and customer terminating im- ~-pedance conditions occurring in various distribution line '. :
.
. .. . , ., ,, , ., . .. . . , ~ .. .. , . ,. . ... - . ~ . .:
1044~i0S ~ ~
arrangements at a customer's premises.
If initially it is determined that the maximum permissible voltage dro~ Vd is to be 105 volts and the maxi-mum current imaX is equal to 200 amperes, then at the elec-tric power frequency of sixty Hz~ ~ the inductance L47 of the inductor 47 is approximately equal to seventeen micro-henrys and inductance L48 of the inductor 48 is approximate-ly equal to 8.5 microhenrys. The series impedance for these values of inductors 47 and 48 at the power frequency will be approximately 0.01 ohm~
If the desired series signal impedance is to be 630 ohms and the communication signal of interest f is equal s to 100 kHzo ~ it is found from the above equation for Zs that the impedance present by the inductors 47 and 48 at the carrier signal frequency of 100 kHz is substantially less than 630 ohms. Consequently, when the parallel tuned circuits ~
54 and 55 of the network 46A are used and the inductor devices 58 and 59 have the values of inductances found for the inductors 47 and 48 i.e. 17 and 8.5 microhenrys noted above and the inductor devices 58 and 59 have a quality or Q factor of 40 and the capacitors 62 and 63 have values of approximately 0.14 microfarads, the required impedance of 630 ohms will be provided at the carrier signal requency -of 100 kHzD The signal series lmpedance variations are then ;
between essentially 630 and 650 ohms with the random changes in the customer loads.
If it is assumed that the desired low protective impedance of for bypassing interfering signals is to be in the order of 0~58 ohm, then the capacitor 51 may be used having a capacitance of approximately 3 microfarads. At '' ' : : ' .
1~) 4 4 6 ~) 5 ` . ! :
the power frequency~of sixty Hz. the current drain of the electric power i4~is in the order 12 milliamps rms~which is sufficiently:small to be considered negligible. , ;
In accordance with the above explanation, a ~
working embodiment of the parallel resonant circuit 54, ` `
shown in Fig. 4 includes the magnetic core member 58A made of a ferrite material with dimensions including a height in -~ -the order of approximately 1-3/4 inch, a width of approximately ,~; .,~ -2-1/8 inches and a length of approximately 2-1/2 inches.
10 These dimensions provide an inductance of 30 microhenrys. -The capacitor 62 coupled to the magnetic core 58A has a value , of o.o85 microtf,arads for tuning to a communication signal frequency o~ 120 kHz. - .r~
~ Accordingly, it is seen that an improvement is made for~the termination of power line carrier signals transmitted -to a customer's premises which are protected from very low and widely varying impedance customer load conditions and inte;rfering signals originating in the customer loads. Other modifications and embodiments will be apparent to those `~:
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skilled in the art without departing from the spirit and scope of this invention. , ~, ~'.,~; .
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