US2293720A - Suppression of crosstalk - Google Patents

Description

Aug. 25, 1942. J. o. EDSON SUPPRESSION OF CROSSTALK Filed Aug. 14, 1941 3 Sheets-Sheet 1 ,INVENTOP J 0. EDSON BY ATTORNEY Aug. 25, 1942. J. o. EDSON SUPPRESSION OF CROSSTALK Filed Aug. 14, 1941 3 Sheets-Sheet 2 INVENTOR J 0. E DSON A TTOR/VEY 5, 9 J. OMEDSON Y 2,293,720

- SUPPRESSION 0F CROSSTALK Filed Aug. 14, 1941 s Sheets-Sheet 5 llllllll uvmvron J. 0. EDSON ATTORNEY PatentedAug. 25, 1942 SUPPRESSION or CROSSTALK James 0. Edson, Great Kills, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application August 14, 1941, Serial No. 406,799

'JClaims- Cl.17978) I The present invention relates particularly to reduction of crosstalk between multistage amplifiers such as are used in telephone repeaterstations where all the amplifiers receive their space current supply from a common source.

Suppression of crosstalk between amplifiers arising from the use of a common battery requires the use of some type of filtering that'will permit the supply of direct current from the battery to the various amplifier elements but will not introduce sufiicient impedance n the common leads at the signal frequency to produce intolerable amounts of crosstalk.

' It is common practice to use a certain amount? of filtering per amplifier per stage, such filtering in each case being kept to a minimum for reasons of economy, because of the large number of such filters required. I

In accordance with the present invention,

where a number or similar multistage amplifiers are installed at a common point, the corresponding stages of the amplifiers are grouped for purscheme in a case where it is desired to use the poses of current supply and filtering, thereby filtering in the filters that are common to;the

reducing the number of filter units required and making economical a more complete degree of -It is found that when the this basis, there is also an economy'cf space requirements on the amplifier panels.

filtering is done on In order for one circuit to produce noticeable crosstalk into another, the former must give rise to currents in the latter of sumcientf magnitude to be disturbing. It the level of the currents of the former is higher than those of the latter and ii the two circuits are coupled by high impedance, the higher level circuit is bound to produce disturbing crosstalk in the circuit of lower level.

If the stages of like levelof each of the several amplifiers are grouped together and their space currents supplied through one common filter,

only one such filter is required forall stages of.

the same level. With the required'number of filters thus reduced, one can afford to make high the inductance of the coil and the capacity-of the condenser oieach filter and thus achieve a very low shunt coupling impedance between all stages of like level and a very high series impedansce between stage groups of unlike level. Since the signal currents inrstages oi the same level are of the same magnitude there will be but little tendency for one tocrosstalk into another. with stages grouped according to level, each :group having its own low-pass filter with a very low cut-ofi, the tendency for a group of higher level to crosstalk into a group of lower level is also reduced by the separation effect of the filters. Fig. 1 shows the most general application of the filtering idea applied to the case where each stage is supplied with plate current in the usual Fig. 2 shows a modified form of the filtering combined voltage of plate and filament to provide a higher voltage to the amplifier than the plate alone can supplyrand Fig. 3 shows another modification of this filtering scheme to a case in which the available space current battery voltage is correct for the output stage but must be supplied to the other two stages in series.

Fig. 1 shows schematically a group of amplifiers A1, A: to As, each amplifier being inserted in the path of a pair of leads which form part of a large group of communication circuits such as a telephone cable. Signal currents on line 5| are. introduced into amplifier A1 through input transformer 52 and then pass through the various stages of amplification 3|,32 and 33 and finally pass out through output transformer 53 to output pair 55 thereby being raised to a level sufficient to withstand the attenuation they will ex- .perience between this and the next repeater point. The electron discharge devices M and 42 shown in stages 3| and 32 may be voltage pentodes such as the Western Electric 3l0-A and the one,

that is, device 43,'in the output stage 33 may be a power pentode such as the 311-A.

The amplifier A1 of itself forms no part of the present invention; it can have any number of stages and may or may not have stabilized feedback. Its space current may be taken from the output terminals of any direct current source such as battery P.

In Fig. 1 the battery filter F is composed of several low-pass filters connected to the battery I in parallel with each other. Since all the component filters are, alike in configuration and since all stages of amplification of this amplifier are substantially alike in configuration an explanation of the relationship between section 2| 0! filter F to stage 3| of amplifier A1 will sufiice as typical of all other stages. The various terminals of the filter are connected to their corresponding terminals on all amplifiers as shown. The negative terminal 6 of the battery P is run to ground [0. Then the path of the direct cur rent for all the first stages, since they are all connected in parallel, is from the positive'terminal l of the space current power supply 1? through the first stage filter retard coil 2 to the screen grid of tube 4| and through retard coil 3 to its plate, thence through the space of tube 4| to its cathode, and suppressor grid, through cathode resistance 4 and thence back to the negative terminal 6 of power supply P. Alternating current is generated in tube 4| by the variable control action of its signal grid on the direct current flowing through its space. The path of this alternating current around the first stage of the amplifier is the same as that of the direct current (except that most of it is shunted from bias resistor 4 by the cathode condenser but in passing through the filter almost all of it is shunted by the first stage filter condenser Only that portion of the alternating current that passes through the battery can produce crosstalk in the other stage groups. If the capacity of condenser is made very high, its impedance will be very low and since this advantage is further enhanced by the fact that the currents of all first stages are of the same level, the voltage drop levels.

Sometimes it is desirable to make available the combined voltage of plate and filament to supply space current for the tubes, as when it is desired to increase the power output of repeater amplifiers. In such a case it might be necessary in a repeater ofilce to put a ground on the battery at a point between its terminals. The circuit shown in Fig. 2 provides for that condition. The principle, as far as space current supply is concerned, is the same as for Fig. 1, the battery filter F being modified as shown. The cathode of the first tube 4| is maintained at a negative potential by connecting it to battery through coil 8 which is bypassed to ground through condenser 9. The junction point between condensers 9 and I is brought to the amplifier panel ground. A similar filter supplies power to the second stage and still another supplies power to the third stage.

The power supply system of Fig. 2 is connected to all amplifiers as shown. This particular power supply comprises the customary plate voltage source included between terminal I and ground and the filament voltage battery included between ground and terminal 6. Thebattery connected to the other side of terminal 6 is a C battery, the function of which will be discussed later. The connections to the amplifiers are such that the filament heating battery and plate battery are employed in series to supply space voltage to the tubes with the filament battery below ground potential. When it is desired to operate the filament and plate batteries of a repeater ofilce in series in order to get a higher space current voltage the direct space current must be returned to the negative end of the filament battery which is not grounded while at the same time the alternating current generated in the space of thetube must be prevented from passing through the battery. To accomplish this requires anotherlow-pass battery filter made up of inductance 9 and condenser 8, which acts as a filter around the filament battery to supplement the similar inductance condenser combination 1 and 2 around the plate battery. Thus under the conditions of this circuit the path of the direct current for all the first stages, is from the positive terminal of the space current power supply P through the first stage filter retard coil 2 to the screen grid of tube 4| and through retard coil 3 to its plate, thence through the space of tube 4| to its cathode and suppressor grid, through cathode resistance 4, thence through the first stage filter retard coil 8 and back to the negativeterminal 6 of power supply P. The path of the alternating current, generated in the tube, around the first stage of the amplifier is the same as that of the direct current except that a great deal of it is bypassed to ground by condenser 5 and substantially all the rest is shunted around the space battery through condensers 9 and 1. The use of combined filament and plate battery voltage to realize increased space current is disclosed inthe United States patent to J. O. Edson et al. No. 2,191,167, granted February 20, 1940 to which reference may be made for more detailed information.

The additional circuit elements shown in Fig. 2, namely resistance 62, condensers BI and 65 and inductance 64, have to do with the feedback circuit of this amplifier and are not related to the invention. The additional wire between each section of the filter F and all amplifiers is furnished to connect the mid-points with the amplifier panel ground I 2 and the battery ground l0 which is intermediate the battery in order to get the higher voltage discussed above. Additional wire 14 between all amplifiers and filter F is provided to connect the negative terminal of grid bias battery 1| and its associated resistance 1.2 common to the grid circuits of the output stages of all amplifiers. Condenser 63 provides an alternating current shunt circuit around grid bias battery I I.

Fig. 3 illustrates an application of the present invention to an amplifier having directly heated filament type cathodes each filament being supplied by a different winding of a filament transformer. The amplifier receives its space current from a power source having the correct space voltage for the output .tube but being equal to the sum of the space voltages required for the tubes of the first two stages which are therefore operated in series, the power source being grounded at about its mid-point. Since the direct current potential of the amplifier panel which is connected to ground is correct for supplying the plate voltage of the first tube and for connecting the cathode of the second tube, just one battery filter of .the type used by the circuit of Fig. 2 is required for .both the first two stages.

Since the initial stages of the amplifier of Fig. 3 have their space current circuits connected in series, the anode of device 4| is-directly connected to the input control grid of device 42 and the resistance -I3 is suflicient to provide the desired biasing potential for the control grid of device 42. The path of the direct current for the first two stages in series is from the positive terminal I of the space current power supply P through the filter retard coilrz for the first two stages to the screen grid of tube 42 and through retard coil 30 to its plate, then through the space of tube 42 to its cathode and suppressor grid, thence to the plate of tube 4| through retard coil 3 and resistance l3 to its screen grid, then through the space of tube 4| to its cathode and suppressor grid, through cathode resistance 4, thence through the filter retard coil 8 and back to the negative terminal 6 of power supply P. Most of the alternating current, generated in tube 4| by condenser 5, and that which is not is shunted around the filament battery by condenser 9. The alternating current generated in tube 42 is bypassed around the plate battery by condenser E. For a more detailed discussion of an amplifier having its first two tubes in series, reference may be made to United States patent to J. B. Maggio, No. 2,254,377, issued Sept. 2, 1941, for Repeater station.

The additional circuit elements 62, 64, 65, 66 and those contained in 68 are components of a feedback circuit used in the amplifier of Fig. 3 and form no part of this invention. The additional wiring ll between the filter and amplifiers is necessary to connect the mid-point of this filter with amplifier ground l2 and battery ground it! because of the special space current voltage requirements of this amplifier as discussed above. Additional wiring i4 is provided to connect resistance 12 which is common to all output stages to supply them with a grid bias. Resistance 61 is also associated with the feedback circuit of this amplifier.

The fact that this invention has been described with reference to amplifiers having only three stages as found in some telephone repeater stations in no way limits its application. The scope of this invention is defined by the following claims.

What is claimed is:

1. In a repeater station having a multiplicity of lines passing through it, each line conveying electrical signals, an amplifier interposed in each line, said amplifier having a plurality of stages,.

each stage comprising an electron discharge device for raising the level of said signals, means connecting the space paths of all said electron discharge devices which raise said signals to the same level in parallel with each other as a group, one common source of space current for all of said groups, and a wave filter individual to each of said groups, each such filter interposed between its group and said common source whereby crosstalk between said amplifiers is minimized.

2. In combination, a plurality of means for altering in a desired way signal currents passing through them, said means composed of several connected circuits arranged in stages, the various stages having electron discharge devices, the level of the signal currents diiIering from stage to stage, all stages wi' 1. signal currents at the same level having their space current circuits connected in parallel as a group, a lowpass filter for each such group, and a common source of space current supply, each said filter inserted between its group and said space current source whereby amplifier crosstalk is appreciably reduced.

is by-passed 1 circuits of all stages having signal currents of the same intensity being connected in parallel as a group, a common source of space current supply, and a low-pass filter comprising a series coil terminated in a shunt condenser inserted between each such group and said source of space current, each said shunt condenser connected directly across each said group to suppress intra stage to stage, all stages of said amplifiers in which signal currents are at the same level having their space current circuits connected in parallel, a single source of space current supply, and a low-pass wave filter inserted between each parallel circuit so formed and said space current source, said low-pass filter having a high inductance series arm and a high capacity shunt arm, said shunt arm connected adjacent to said amplifier, said series arm preventing signal cur- I rents from passing through the battery thereby preventing crosstalk between stages of different level and said shunt arm presenting a low impedance to signal currents thereby preventing crosstalk between stages of the same level.

5. In combination, a group of amplifiers having a plurality of stages, signal currents passing through all such amplifiers simultaneously, the space current circuits of all stages with signal currents of the same level b'eing connectedin parallel, a single source of space current supply, and a separate low-pass filter inserted between said source and each said parallel circuit, said filters preventing the currents generated in any stage of any amplifier from crosstalking into any stage of any amplifier.

6. In a telephone repeater station, a multiplicity of lines passing therethrough, all said lines carrying signal currents simultaneously, a multistage amplifier in tandem with each line for raising the level of said signal currents in stages, each amplifier being mounted on a separate panel and having its individual panel ground return circuit, a single space current battery for all of said amplifiers, said battery having its ground at a point intermediate its positive and negative terminals, all space current circuits of stages at the same level being connected in parallel as a group, and a filter with a balanced to ground configuration inserted between each of said groups and the positive and negative ends of said battery, the mid-point of each of said filters '7. The combination of claim 6 in which at leasttwo of the stages have theirspace current circuits connected in series, all series circuits so formed being connected inparallel as a group and each such group connected to space current battery through a respective one of said filters.

JAMESO. EDSON.

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