US2296920A - Signal transmission circuit - Google Patents

Description

Sept. 2 ,1942. ($05 M 2,29 ,92

smuu. TRANSMISSION cmcbz'r 'F ned Jan. 24, 1940 SOURCE OF BALANC'ED VOLTAGE 0 U TILIZATION\ NETWORK gzvvszvroR. E. DUDLEY GOODALE BY 7+ 4W I ATTORNEY.

Patented Sept. 29, 1942 UNITED STATES." PATENT oFrlcE/ 2,296,920 a SIGNAL TRANSMISSION CIRCUIT E. Dudley Goodale, Bayside, N. Y., assignor to Radio Corporation of American, corporation of Delaware Application January 24, 1940, Serial No. 315,313

3 Claims, (Cl. 178-44) My present invention relates to signal transmission circuits of the balanced line type, and

more particularly to coupling networks em-.

ployed in connection with balanced lines.

One of the main objects of my invention may be stated to reside in the provision of means for properly terminating a balanced signal transmission line, efiectively suppressing longitudinal voltages and securing a limited amount of equalization.

Another important object of the invention is to provide a coupling network for a pair of balanced transmission lines, wherein the coupling network comprises a pair of tubeshaving output electrodes in push-pull relation, the lines being coupled into the cathodes of the tubes, and input electrodes of the tubes being additionally. crossconnected to the lines -thereby providing cancellation of push-push voltages.

Still another object of the invention is to provide a. coupling network between a pair of balanced signal transmission lines and an unbalanced line, wherein the network comprises a pair of electron discharge devices arranged to deliver balanced voltages with minimum of push-push voltage, and a third electron discharge device has input electrodes coupled to the output of the pair of devices to provide an unbalanced voltage output. 3

Still other objects of my invention are to'improv'e generally the simplicity and efllciency of coupling networks for balanced line systems, and

-more especially to provide such networks in an economical and reliable manner.

The novel features which I believe to'be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have in-'- dicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing: Fig. 1 shows one embodiment of the Fig. 2 illustrates a modification, Fig. 3 shows another modification wherein balanced voltages are converted to an unbalanced voltage,

Fig. 4 shows a modification of the circuit of Fig. 3. l

Referring now totheaccompanying drawing, wherein like reference characters in the various invention,

is shown in Fig. 1 a pair of electron discharge devices I and 2 which are included in the coupling network adapted to provide the coupling circuit between input'and output balanced lines.

The lines 3 and 4 are shown as feeding voltage.

from a source of balanced voltage. The source 5 may produce alternating voltage in a'frequency range of cycles to about 8 megacycles. The source may be a signal source operating either in the television video range or in the standard sound broadcast range. Regardless of the frequency of the signal voltage, or the frequency band width of the signal energy, the lead 3 is shown as having, merely by way of illustration, a positive polarity relative to ground, while the lead 4 has a negative polarity, The term balanced, therefore, indicates the voltage state when the two leads at any given instant are of opposite polarity relative'to ground. Tmuistribution line 3-4, for example, may be a twisted pair of leads adapted to transmit a wide band of television signal energy through an apartment house to one or more television receivers.

Regardless of the specific use of the circuits shown, the coupling tube I has its cathode. 6

Coupling condensers l2 and I3 couple the tubes I and 2 respectively to output lines 3' and 4. The grids I and 9 are returned to ground by grid leak resistors l5 and i6 respectively, the direct voltage developed across each of R1 and Rzbiasing grids I and 2 respectively. If desired, a common bias resistor may be connected between the resistors R1 and R2 and ground. I

Push-push and push-pull alternating voltages are applied to leads 3 and 4 from source I; only the polarities of the push-pull voltages are shown. These voltages. are applied directly to the cathode ends of resistors R1 and R2. The parallel combination of R1 and'the reciprocal of tube l transconductance (Gnu) acts as the termination for the lead 3 side of the balanced lines, while the combination of R: and the reciprocal of tube 2 transconductance (Gm) acts as the figures designate similar circuit elements, there termination for the other side of the system.

The grid circuit of the tubes is a high impedance bridge circuit. Terming the impedance between cathode and ground ofeach tube as R0, it follows that:

R i. L

The applied signal voltages produce changes in the plate currents of the tubes, and thus in the signal voltages developed across the load resistors R: and R4. ,The signal input voltages are also cross-connected to grids 8 and 9. This has the effect of adding to the push-pull voltages produced across R3 and R4. Push-push voltages are cancelled out in the grid circuits. The

amount of cancellation can be made very largeor small depending upon the degree of balance between the two sides of the system. It is desirable. to make the amount of cancellation large.

' It will, therefore, be seen that the tubes land 2 are connected in push-pull voltage reinforcing relation. while for push-push voltages they have the grid circuits in cancellation relation. Each of load resistors R3 and R4 are bypassed for high frequencies to ground by a condenser connected to an intermediate point. These condensers are designated as I and 2' respectively.

The modification in Fig. 2 differs "from the arrangement of Fig. 1 only in that networks R5 -l0 and Rs 1 1 are used to secure frequency discrimination. These networks should, of course, present a relatively high impedance in order not to upset the line termination. Otherwise their effeet on the termination would have to be considered. Proper choice of the constants of Rs -10 and Rs -11 can produce more uniform response curves, where high frequency response is It is possible to feed the balanced output voltages of tubes I and 2 tea device for converting the balanced voltages to an unbalanced voltage. For this purpose there may be employed a coupling device of the typee disclosed in application Serial No. 279,686, filed June 17, 1939 by E. D. Goodale and R. A. Monfort, now-Patent No. 2,256,084, issued Sept. 16, 1941. 'As shown in Fig. 3, the coupling device comprises a tube 20 having at least a cathode 2!, a signal grid 22 and a plate '23. The signal grid 22 is connected to condenser l2, while cathode 2| is grounded through resistor R. The cathode end of resistor R is connected to'condenser l3. The plate 23 is connected to a positive voltage point through a resistor R1, a part of which is bypassed to ground. The alternating voltage across resistor R1 is unbalanced" relative to ground, because the output lead -3llxhas a negative polarity to ground. The lead 3' may be connected to an amplifier, transmission cable .(such as a co-axial line), or any other desirable utilization apparatus.

The signal grid 22 of coupling tube 20 is maintained at a negative bias by connecting the grid to ground through a grid leak resistor 3|. In this type of circuit the opposite polarity signal voltage effects on the grid. and cathode of tube 20 are additive in the plate circuit. A considerable increase in overall gain and improvement in ratio of push-pull to push-push voltages is effected over other known systems. Some frequency discrimination would be introduced by virtue of the cathode resistor R and the coupling condenser 13. v To avoid the last-named objection the circuit arrangement of Fig. ,4 can be utilized. In this circuit arrangement the cathode of .tube 20 is established at the positive direct current potential of the plate of tube 2, and for this purpose the lead 32 connects the cathode end of resistor R to the plate end of load resistor R4. In order to obtain proper operating conditions it is, there- -.tive potential. This means that the plate voltage supply of tube 20 would have to be greater than the plate voltage of tube 2. The circuit of Fig. 4 will give uniform amplification down to very low frequencies, and can be used up to several megacycles without any noticeable effect. In the circuits of both Figs. 3 and 4 longitudinal voltages existing in the balanced output leads of tubes I and 2 would be cancelled out in the input circuit of tube 20. This is a decided advantage in cases where longitudinal voltages still exist in the outputs of tubes l and 2. In other words, in the arrangements of Figs. 3 and 4 the undesirable push-push voltages would be minimized because the cancellation action of tubes 1 and 2 would be supplemented by the input circuit cancellation effect of tube 20.-

While I have indicated and described several systems for carrying my invention into effect, it will lie-apparent to one skilled in the art that my'invention is by no meanslimited to the particular organizations shown and described, but that many modifications maybe made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a pair of electron discharge devices having input and output electrodes, an aperiodic wide band output circuit connecting the output electrodes in push-pull relationship, a pair of untuned high frequency wide band voltage transmission lines which are at opposite polarities with respect to a point of relatively fixed potential, means coupling each of said lines to one of the input electrodes of an associated one of said devices, additional means coupling each of said lines to the other of the input electrodes of each device in reversed rela- .tion with respect to said first coupling means,

an impedance connected between said one input electrodes of said devices, means connecting substantially the midpoint of said impedance to said fixed potential point, an additional electron discharge device-havin'g at least a grid, cathode and anode, means impressing high frequency voltages of opposite polarity developed across said output circuit upon the grid and cathode respectively of said additional tube, and means for deriving from the anode circuit of said additional tube a voltage which is unbalanced with respect to said fixed potential point.-

2. In combination with a pair of electron discharge devices having 'input and output electrodes, an aperiodic wide band output circuit connecting the output electrodes in push-pull relationship, a pair of untuned high frequency wide band voltage transmission lines which are at opposite polarities with respect to a point of relatively fixed potential, means coupling each of saidlines to one of the input electrodes of an associated one of said devices, additional means coupling each of said lines to the other of the input electrodes of each device in reversed relabalanced voltages appearing across said output circuit into an unbalanced voltage with respect to said fixed potential point, said last means a comprising an electron discharge tube havingat least a arid, cathode and anode, a resistive impedance in the space current path of said last tube, means impressing the balanced high frequency voltages of opposite polarity appearing across said output circuit upon the .grid and cathode respectively or said last tube, and said last tube having said unbalanced voltage appears across said impedance.

3. In combination, an electron discharge tube provided with at least a control grid, a cathode and a plate, a resistor connected between the cathode and a point of relatively fixed potential and being disposed in the space current path of said tube, a second tube provided with at least a control grid, a cathode and a plate, a. second voltage coupled to the cathode ends of said two resistors; a common high irequency voltage mitput circuit connected to the plates of both tubes in push pull relation, and additional means coupling said high frequencyvoltage source to the. control grids of said two tubes in a manner such I that'push-push voltage components cancel out, a third electron discharge tube having at least a cathode, a control grid and an output electrode, aresistive impedance connected between the third tube cathode and a point or" relatively fixed I potential and being disposed in the space current path of the third tube, means for impressing the push-pull voltage components appearing across said output circuit between the grid and cath- E. DUDLEY GOODALE.

resistor connected between the cathode of the second tube and said fixed potential point and .being disposed in the space current path of the second tube, a source of balanced high frequency

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