US2301023A

US2301023A - Coupling network

Info

Publication number: US2301023A
Application number: US403959A
Authority: US
Inventors: Darlington Sidney
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1941-07-25
Filing date: 1941-07-25
Publication date: 1942-11-03
Anticipated expiration: 1959-11-03
Also published as: GB554329A

Description

NGV 3, 1942- s. DARLINGTON COUPLING NETWORK Filed July 25, 1941 Patented Nov. 3, 1/942 COUPLING NETWORK Sidney Darlington, New York, N; Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 25, 1941, Serial No. 403,959 l18 Claims. '(Cl. 17d-44) This invention relates to Wave transmission networks and more particularly to a network for coupling two unequal load impedances.

An object of the invention is to connect two loads of unequal impedance.

Other objects are to widen the band transmitted by a network for coupling unequal` load impedances, reduce the transmission loss in the higher frequency range and minimize the phase shift introduced by such a network and reduce the reflection eiects at the point of juncture between the network and the load.

In a. wave transmission system, it is often required to connect together two unequal load impedances while introducing the minimum of high frequency transmission loss, phase shift and impedance mismatch. Heretofore, it has been diflicult to accomplish these objectives, especially if the system is to transmit a wide band of frequencies suri, for example, as is employed in television.

In accordance with the invention, there is provided a coupling network of comparatively simple structure which will meet all of the requirements set forth. The network comprises two inductivelyA coupled inductors, connected in the series-opposing relationship in an electrical path between the two unequal load impedances, and an impedance branch including a capacitor and a resistor in parallel, connected from a point between the inductors to a point in a second electrical path between 4the load impedances. In the lower frequency range, where the impedance of the inductors is low and the impedance of the capacitor is high, the network functions as a resistance coupling. In the higher frequency range, where the impedance of the capacitor is low compared to that of the resistor, the network acts as a transformer coupling. The elements may be so proportioned that one of the loads is terminated in an impedance which is substantially matching over the entire frequency range.

As compared to a simple resistance coupling the network provides a transmission gain which increases with frequency, at first more rapidly and later less rapidly. The frequency range covered depends upon the choice of values for the inductors and the capacitor.

The network may be constructed in either the balanced orv the unbalanced form. Blocking capacitors may be added between the coupled inductors to prevent the passage of direct current. In order to annui the effects of the leakage inductance and the stray capacitances, a shunt lter section. To annui the effect of a negative reactive component which may be associated with one of the load impedances a series inductor may be added to form another low-pass filter section.

The knature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawing in which like reference characters represent like or similar partsl and in which:

Fig. 1 is a schematic circuit of a coupling network in accordance with the invention;

Fig. 2 shows a family of curves representing the transmission gain introduced by the network, as compared to a resistance coupling; and

Fig. 3`is a schematic circuit of the coupling network in balanced form.

Taking up the gures in more detail, Fig. 1

shows one embodiment of the coupling network y of the invention in its unbalanced form. The impedances Ri'and Rz connected respectively to the two pairs of terminals i, 2 and 3, 4 represent the unequal loads which are to be connected capacitor may be added to provide a low-pass together. The load R1 may, for example, be a transmission line or other utilization circuit and R2 may be an amplifier or other terminal appa.- ratus, the impedance of which is higher than that of the load R1. In an electrical path between the terminals l and 3 are two closely coupled inductors L1 and L2 connected in the series-opposing relationship. Connected from a pointv 5 between the inductors L1' and L2 to a point 6 in an electrical path between the terminals 2 and 4 is an impedance branch comprising a resistor R and a capacitor C in parallel.

In order to avoid reilection eiiects at the junction of the line or other load R1 and the network, it is desirable that the load R1 be terminated in a matching impedance. At low fre,- quencies the impedance of the inductors L1 and L2 Will be small and the impedance of the capacitor C will be large, leaving the shunt resistor R asr the only effective impedance. The resistor R is, therefore, given ,such a value that, in parallel with the load Rz, the combination will have an impedance equal to R1. Assuming that the impedances R1 and R2 are non-reactive, the required value of the resistance of R is found from the equation:

RIRZ

R-R2R1 When R has this'value, the impedance of the network at the terminals l and 2 will be equal to R1 and, therefore, the load R1 will be terminated in its own impedance in the low frequency range.

At high frequencies the impedance of the capacitor C is small and the resistor R is effectively shunted out, leaving only"th\ '\V4 transformer action of the inductors L1 and I a. ,For-proper impedance matching the' inductances of L1 and Lz should, therefore, have the same ratio as that of the loads R1 and R1. Assuming perfect coupling between the inductors,

impedance of the network at thetgrminals I and 2 will be equal to R1 over ,diiglilfrequency range.

If the capacitance of h'icapacitprac is propeny chosen, the network will also have' an impedance at the terminals I and 2 which is approximately `equal to R1 over the intermediate frequency range. The required value of C is given-by the equa tion s, l i 3 4 RlRg rf the inductors L1 and Le have ,thif'sfrand the i The only factor yet to be `determined is the inductance oi' the inductor L1, the choice of which depends upon the frequency range to be covered. 'Ihe curves of Fig. 2 show on a logarithmic frequency scale, the transmission gain in decibels resulting from the insertion of the coupling network of Fig. l, as compared with the insertion of only the shunt resistor R. Curve 8 represents this gain if L1 has a certain value. The curve rises with frequency, at first more and then less rapidly. If L1 is doubled, for example, the gain characteristic will be as shown by curve 9, and

if L1 is halved curve III will result.,A It is seen, therefore, that by properly choosing the value of L1 any one of an inilnlte family of gain characteristics may be obtained In practice' L1 `may often be so chosen that the resulting gain characteristic serves to equalize to a large extent for attenuation distortion in other parts of the system such, for example, as an associated transmission line. It should be pointed out, further, that the coupling network oi' Fig. 1 introduces the minimum phase 1 shift that it is possible to associate with a given insertion loss characteristic.

The schematic circuit of Fig. 3 shows a balanced form of the coupling network of the invention.

thermionic tubes connected in series and working plate impedances or the grid irnpedances of,.two

push-pull. These tube lrnpedances are ordinarily to the'tubes included in the load Rz.'

resistance of value R tapped at its mid-point M and grounded as shown at I5. sistors Ra and R4, of comparatively highA resistance,` vare connected between the inner terminaisV I6 and I1 of the two halves of Le;4 Tl`se resistors have a common terminal Ii'which may be used, for example, in supplying plate current Terminals I2 and I6 are connected through a capacitor C1 and terminals I3and I'l are connected through a capacitor Cz. These capacitors have large capacitances and their function4 `is to provide a low impedance path at high frequencies but to prevent the flow of direct current through the network.

The shunt capacitor C: is proportioned with 'respect to the leakage inductanceiofnthe coupled inductors and the associated strayfpapacitances to form a low-pass filter section with-"cut-otf above thehighest frequency to be transmitted by the network. The two equal series inductors L: and 'L4 are proportioned with respect to the negative reactive component associated with the load Rz tol provide 4a second low-pass filter section also having a cut-oli.' above the highest frequency of interest. These filter sections are so designed that their image impedances approximate the impedance Rz over the transmitted range.

These added elements Ca, In and L4 serve to make the impedance of the network at terminals work of Fig. 3 may be designed to transmit satisfactorily the broad band of frequencies used in television, extending, for example, from 30 cycles or lower to 5 megacycles or higher. When used in television circuits the minimum phase shift property of the network, already mentioned, is of 'considerable importance as it materially simplifies the problem of phase correction.

What is claimed is:

1. A network for coupling two unequal load impedances comprising two electrical paths adapted to interconnect said impedances, two inductively coupled inductors connected in the series-opposing relationship in one of said paths and an impedance branch including a capacitor and a resistor in parallel connected from a point in said one path between said inductors to a point in said other path, the inductance ratio o'f said inductors being approximately equal to the ratio of said load impedances.

2. A network for coupling two unequal load impedances comprising two electrical paths adapted to interconnect said impedances, two inductivelylcupled conductors connected in the series-opposing relationship in one of said paths and an impedance branch including a capacitor and a resistorin parallel connected from a point iiisaid-"one path between said inductors to a point in said. other path, said resistor having a resistance approximately equal to the product of said lead impedances divided by the difference shunted by a resistance to reduce and sta'giilize ""f Said:impedanes the impedance R2. As shown in Fig. 3 each of the windings L1 Lz of the transformer is dividedf'into two 4equal parts. Between the inner terminals I2 and'l3 uf. the two halves off` L1 is connected the impedance branch comprising the capacitor C shurrfed by a `z 3. A', networkzfor coupling two unequal load impedances comprising tufo electricalj' paths adapted to interconnect saidlimpedarrces, two inductively coupled conductors connected in the series-opposing relationship-in one of, said paths and an; impedance branch'mcluding a capacitor Two equal re4 and a resistor in parallel connected from a point in said one path between said inductors to a point in said other path, the capacitance C of said capacitor having approximately the value and an impedance branch including a capacitor and a resistor in parallel connected from a point in said one path between said inductors to a point in said other path, the capacitance of said capacitor being so proportioned with respect to the inductance of one of said inductors and the values of said load impedances that the network has at one end an impedance which, over n wide 'range offrequencies, is approximately equal to the load impedance at that end.

5. A network for coupling two unequal load impedances comprising two electrical' paths adapted to interconnect said impedances, two inductively coupled conductors connected in the series-opposing relationship in one of said paths andan impedance branch including a capacitor and a resistor in parallel connected from a point in said one path between said inductors to a point in said other path, said resistor having a resistance approximately equal to the product of said load impedances divided by the difference of said impedances and the inductance ratio of said inductors being approximately equal to the ratio of said impedances.

6. A network for coupling two unequal load impedances comprising two electrical paths adapted to interconnect said impedances, two inductlvely coupled conductors connected in the series-opposing relationship in one of said paths and an impedance branch including a capacitor and a resistor in parallel connected from a point in said one path between said inductors to a point in said other path, said resistor having a resistance approximately equal to the product of said load impedances divided by the difference of said impedances and the capacitance of said capacitor being so proportioned with respect to the inductance of one of said inductors and the values of said load impedances that the network has at one end an impedance which is approximately equal to the load impedance at that end.

7. A network for coupling two unequal load impedances comprising two electrical paths adapted to interconnect said impedances, two inductively coupled conductors connected in the series-opposing relationship in one of said paths and an impedance branch including a capacitor and a resistor in parallel connected from a point in said one path between said inductors to a 4point in Said other path. the inductance ratio of said inductors being approximately equal to the ratio of said load impedances and the capacitance of said capacitor being .so proportioned with respect to the inductance of one of said inductors and the values of said load impedances that the network has at one end an impedance which is approximately equal to the load impedance at that end.

8. A network for coupling two unequal load transmitted by the network.

impedances comprising two electrical paths adapted t'o interconnect said impedances, two inductively coupledl-.conductors connected in the series-opposing'elationship in one of said paths and an impedance branch including a capacitor and a resistor inparallel connected from a point in said-fr path between said inductors to a point in said otherl path, said resistor having a resistance approximately equal to the product of said load impedances divided by the difference of said impedances, the inductance ratio of said inductors being approximately equal to the ratio oi said impedances and the capacitance of said capacitor being so proportioned with respect to the inductance of one of said inductors and the values of said load impedances that the network has at one end an impedance approximately equal to the load impedance at that end.

9. A network in accordance with claim 1 and an added capacitor connected in shunt at one end of the network, the capacitance of said added capacitor being proportioned with respect to the leakage inductance 'of said inductors and the stray capacitances to provide a low-pass `filter having a cut-olf above the highest frequency to be transmitted by the network.-

10. A network in accordance' with claim 1 in which inductance is added in serieswith one o1' said induotors, the value of said inductance being proportioned with respect to a negative reactive component associated with one of said.

11. A network inA accordance with claim 1 which includes an added capacitor connected in shunt at one end of said network and added inductance in series with one of said inductors, the capacitance of said Aadded capacitor being proportioned with respect to the leakage inductance oi said coupled inductors and the stray capacitances, and the value of said added inductance being proportioned with respect to a negative reactive component associated with one of said load impedances, to provide a low-.pass lter having a. cut-oi! above the highest frequency to be transmitted by the network.

12. A network for coupling two unequal load impedances comprising two electrical paths adapted to interconnect said impedances, a pair of inductively coupled inductors connected in the series-opposing relationship in one of said paths, a second similar pair of inductors similarly connected in the other of said paths and an' impedance branch including a capacitor and aresistor in parallel connected from a point in said one path between said first pair of inductors to a point in said other path between said second pair of inductors, the inductance ratio of the inductors forming each of said pairs being approximately equal to the ratio of said load imped- BfnCeS.

13. A network for coupling two unequal load impedances comprising two electrical paths adapted to interconnect said impedances, a pair of inductively coupled inductors connected in the series-opposing relationship in one of said paths, a second similar pair of inductors similarly connected in the other of said paths andan impedance branch including a capacitor and a resistor in parallel connected from a point in said one path between said first pair of inductors to a point in said other path between said second pair of inductors, said resistor having a resistance approximately equal to the product of said load iin-- pedances divided by the diierence of said impedances.

14. A network for coupling two unequal load impedances comprising two electrical paths adapted to interconnect said impedances, a pair ance branch including a capacitor and a resistor;y

in parallel connected from a point -in---s'aid one path between said rst pair of inductors to a. point in said other path between said second pair of inductors, the capacitance of said capacitor being so proportioned with respect to the inductance of one of said inductors and the values of said load impedances that the network has at one end an impedance which, over a wide range of frequencies, is approximately equal to the load impedance at that end.

15. A network for coupling two unequal load impedances comprising two electrical paths adapted to interconnect said impedances, a pair of inductively coupled inductors connected in the series-opposing relationship in one of saidpaths, a second similar pair of inductors similarly connected in the other of said paths and an impedance branch including a capacitor and a resistor in' parallel connected from a point in said one path between said first pair of inductors to a 4point in said other path between said second pair of inductors, the two inductors in each of said pairs being connected through an added capacitor.

16. A network for coupling two unequal load impedances comprising two electrical paths adapted to interconnect said impedances. a pair of inductively coupled inductors connected in the series-opposing relationship in one of said paths, a second similar pair of inductors similarly connected in the other of said paths and an impedance branch including a capacitor and a resistor in parallel connected from a point in said one patin between said rst pair of inductors to a point in said other path between said second pair of inductors, an inductor in one of said pairs being connected to an inductor in the other of said pairs through an added resistance.

17. A network in accordance with claim 12 and an added capacitor. connected in shunt at one end of the network, the capacitance ofsaid added capacitor being proportioned with respect to the leakage inductance of said inductors and the stray capacitances to provide a low-pass fllter having a cut-oil.' above the highest frequency to be transmitted by the network.

18. A network in accordance with claim 12 in which inductance is added in series with one of said inductors in each of said pairs, the value of said lnductance being proportioned with respect to a negative reactive component associated with one of said load impedances to provide a low'- pass lter having a cut-off above the highest frequency to be transmittedl by the network.

SIDNEY DARLINGTON.

Certilcate of Correction Patent No. 2,301,023. l November 3, 1942. SIDNEY DARLINGTON It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Page 2, first col-umn, lines 24, 25, and .26, and page 3, first column,

lines

5, 6; and 7, for that portion of the equation readmg h I IF F 2 (JE-1) ad (x/-l) and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Offce.l

Signed and sealed this 11th day of January, A. D. 1944.

[SEAL] HENRY VAN ARSDALE,

Acting Uommiss'iouar of Patents.