US2275587A - Signal combining network - Google Patents

Description

March 10, 194:.T T M, GLUYAS, JR 2,275,587

SIGNAL GOMBINING NETWORK Fil`e`d March 15,*1939 3 Sheets-Sheet l .54 mc Sou-nel 5 Signal Source March 10, 1942. T, M, GLUYASq VJR 2,275,587

SIGNAL GOMBINING NETWORK Filed March 15, 19:59 3 sheets-sheet 2 14 z Ik l March 10, 1942.` T M, GLUYAS, JR 2,275,587

SIGNAL COMBINING NETWORK Filed March l5, 1959 3 Sheets-Sheet 3 Load.. /L

" /5 mgl 5. f4 me ..\J

Patented Mar. 10, 1942 SIGNAL COMBINING NETWORK Thomas M. Gluyas, Jr., Kansas City, Mo., assignor, by mesne assignments, to Philco Radio and Television Corporation, Philadelphia, Pa., a

corporation of Delaware Application March 1t, 1939, serial No. 262,939

(ci. 17e-44) 9 Claims.

. a saving incost and simplifying the antenna array.

In the present television practice, it is usual to transmit the sound and video intelligence by means of carrier signals which are adjacent to each other in the frequency spectrum. It has been customary in lthe art to employ separate radiating systems designed for the most eicient radiation of their respective signals. Insofar as radiation is concerned this is not justified, since in the present practice the two carriers are relatively close to one another and a single radiating system may be designed which will efficiently radiate both signals. However, if both sources of signal, i. e. video and sound, are directly connected to a single load, the common antenna, the operation will, in general, be unsatisfactory. The reason for this is that each source will act as a load upon the other and will reduce the efciency of energy transfer therefrom to the antenna or useful load, as well as tending to produce crossmodulation between the signals.

This obtains by virtue of the fact that each source inherently presents an impedance across its output terminals which is finite for frequency components produced by the other source. In the worst possible case, when the resistive component of this impedance equals that of the useful load, only one-half of the power supplied by the source will go to the useful load. If one source can be made to present a very high impedance for frequencies supplied by the other source, the energy transfer between the said other source and the load will be greatly improved. It is essential, however, that the internal impedance of the source, or that of the means whereby it is connected to the load, be not appreciably increased for those frequency components produced by it; otherwise the energy dissipation within lt will be increased, thereby reducing the overall eiiiciency of the system.

A further difficulty which presents itself in the solution of this problem for a television system resides in the fact that the video and sound carrier signals occupy frequency bands which are very close together in accordance with present standards of transmission. This means that the transition from a source ofnormal impedance for the particular band of frequencies generated by it, to pne which presents a very high impedance to frequencies within a different range, must take place very rapidly. This, as will appear to those skilled in the art of network design, presents numerous difllculties from a practical standpoint which need not be mentioned in detail.

The principal object of the present invention is to provide a system which overcomes the above-mentioned diiilcultles and makes practical the transmission of video and sound carrier signals from a common antenna. A more specific object of the invention is to Aprovide means whereby the impedance of a source of frequencies within a given range may be made very large for frequenciesoutside that range, Without adversely affecting the eciency of the source as a generator of frequencies within the said range. Still another object of the invention is to provide novel means for obtaining rapid transition in impedance of a signal source without the use of a large complement of circuit elements, thereby obviating the diiculty last-mentioned above.

Fundamentally, the invention resides in shunting' a sourcev of a signal comprising frequency components with a particular band, by an impedance whose magnitude is very great for frequencies within the band and very small for frequencies within another band which may be adjacent to said rst band. This impedance is then inverted by suitable means, such as a quarter Wave line resonant at a frequency within the said other band. For frequencies within the said first band, a high impedance shunts the source and the voltage built up across the input of the impedance inverter is a maximum. On the other hand, for frequencies within the said other range, the source is shunted by a very low impedance whereby no appreciable voltagelouilds up across the inverter input, which is effectively shortcircuited. This short circuit is inverted to produce a very high 0r infinite impedance, as desired. By connecting the output circuits of the impedance inverters of two such systems having complementary impedance characteristics and designed to generate signals confined to different portions of the frequency spectrum, a system results which is suitable for supplying both signals to the same load,

The invention may be understood more clearly by reference to the followingdescription and the accompanying drawings in which:

Fig. 1 is a schematic diagram of a specic embodiment of the invention;

Figs. 1A and 1B are alternative arrangements I Referring to Fig. 1, there is shown at T a source of a modulated television carrier wave which may, for example have a carrier frequency of 51 megacycles, and at S a source of a sound signal whose carrier may be located at 54 megacycles. At L is represented a load, which may be an antenna or any other signal utilization means, to be supplied with signal energy from both sources. The two sources are coupled to a common load junction joint I by means of a pair of coaxial transmission lines 2 and the hollow conductors 3. This point is, in turn, coupled to the load by means of another pair of coaxial lines 4. It will be noted that coaxial lines are used in pairs with their outside conductors grounded where it is desired to reduce mutual coupling which might otherwise exist between the various parts of the circuit and where it is convenient to do so, though it will be understood that openwire lines might be substituted at any part of the circuit where it may be done without unduly increasing this inductive effect. The length of hollow conductor between the point 5 and the junction point l is made equal to onequarter of the sound carrier frequency wave length, While that between the point 6 and the junction point I is equal to one-quarter wave length of the television carrier. According to the method of the invention, the point 6 is to be shunted by an impedance which is high throughout the sound frequency band and low for television frequencies.l Across the point 5 is to be connected an impedance with a complementary characteristic, namely one which is low for frequencies within the sound band and high for television frequencies. The actual behavior of these impedances over the entire frequency spectrum or portions thereof will be determined in accordance with the` widths of the bands to be separated and the separations of their respective carrier frequencies. The desired characteristics may be obtained in anyone of a number of ways familiar to those skilled in the art and, although but a few are shown here, it will be understood that the invention is not to be regarded as limited to them.

For use in the separation of two signals which are very close to vone another in the frequency spectrum, one of which covers a relatively wide band and the other a relatively narrow band of frequencies, as is the case in the example chosen for purposes of explanation, impedances are required which change very rapidly from values which Aare comparatively low throughout one of the bands to values which are exceedingly high throughout the adjacent band. Simple resonant circuits might be chosen Awere it not for the fact that a more rapid transition in impedance magnitude'is desired than can be obtained by their use. The rapidity of transition and the absolute differences between maximum and minimum values of impedance might be improved by the use of transmissionlines as circuit elements and combinations thereof, but even this may not prove sufficient to give the desired behavior. By resorting to more complicated configurations of circuit elements, and by constructing these networks of transmission lines, which give unusuallyl high values of Q, almost any desired rate of transition or difference between impedance values` at particular values of frequency may be obtained.

In the embodiments here described, these impedances are obtained by combining series resonant and parallel resonant circuits either in series or in parallel to produce therdesired impedance characteristics. Returning now to the consideration of Fig. 1 it is desired to shunt the lines 3 at the point 6 by an impedance whose magnitude is very high over the narrow band of frequencies contained in the sound signal and which falls rapidly to and maintains a low value throughout the band of frequencies contained in the television signal. lSuch an impedance characteristic is conveniently obtained by connecting in parallel two series resonant circuits, of slightly different resonant frequencies. Open ended quarter-wave transmission lines resonant at irequenoies of 53 and 55 megacycles are shown at 1 and 8 in Fig. 1, having their input terminals connected across the line 3 at the point 6 as shown. In order to show more clearly the nature ofthe impedance which is thereby put across the point 6, it will be convenient to refer to Figs. 2A and 2B. The broken line curves a and b of Fig. 2A represent respectively the input admittances of the 53 and 55 mega-cycle lines. The frequency scale along the horizontal axis has been distorted in order to indicate more clearly the behavior of the admittance functions in the vicinity of their poles at 53 and 55 megacycles. The admittances, being connected in parallel, add to give the resultant characteristic corresponding to the solid line curve c and it will be noted that the resultant has poles at 53 and 55 megacycles accompanied by a sharp transition through zero between these values.

In Fig. 2B the resultant admittance is inverted to give the corresponding impedance characteristic. This impedance characteristic, it will be observed, is solely lcapacitive or negative in sign for frequencies below 53 megacycles and solely inductive or positive in sign throughout a band of frequencies immediately above megacycles. In the portion of the spectrum-between these two frequencies there is a rapid transition, the impedance changing sign at 54 megaoycles `for which frequency it may be substantially infinite if high values of Q are available in the circuit elements. Below 54 megacycles the impedance becomes gradually more inductive after passing through zero at 53 megacycles. Above 54 megacycles it becomes less capacitive until the zero value is reached at 55 megacycles.

At the point 5 (Fig. 1) it is desired to shunt the line 3 by an impedance complementary to the one just described, which is very low in value for the sound carrier frequency and which rises rapidly to alhigh value in the adjacent television band. A series combination of two parallel res' short-circuited transmission lines in series in view of the tendency of such a combination to radiate. In certain cases, this energy may be utilized to modify the radiation pattern of the transmitter. For example one of the parallel I2 attached to the outer conductors of the linesI IU and II, these conductors will form a dipole antenna which may be so oriented as to have the desired interaction with the main antenna.

If it is not desired to make use of this property of the configuration of Fig, 1A, an Varrangement may be used such as that of Fig. 1B. Here the coaxial lines I and II are placed parallel to one another, as shown. This limits the radiation but the arrangement has another property which may or may not be desirable, namely that the external conductors of I0 and II coact to produce an open circuited, open wire line, one end of which is connected across the terminals I2. When these are connected across the point of Fig. 1 the line will produce a short circuit for the frequency at which it is a quarter wave in length. Although this might prove convenient under certain circumstances, it is not suitable for the system of Fig. l since the effect is to lower the impedance shunting thepoint 5 in the television band.

Hence, the preferred form for the shunting impedance is that shown connected across the terminals 5 in Fig. 1. This is essentially the same as that shown in Fig. 1B except that the Y external conductors of the lines I0 and II are made to form a part of the line 3 to the antenna junction point I. This practice may result in the introduction of a certain amount ofmutual impedance between the circuit comprising the short-circuited quarter wave lines and the circuit to the antenna junction point since they both include the external conductors of the lines I0 and il. This may be compensated for, if need be, by altering the physical length of the lines I0 and I I so as to keep the desired electrical lengths.

Considering now the operation of the system as a whole, the effect of shunting the point 6 by means of an impedance which varies with frequency in the manner shown in Fig. 2B is to vary the ability of the line 3 to transmit energy from the sound signal source S to the point I. In effect the line 3, in conjunction with the parallel combination comprising the' lines 'I and 8, functions `as an electric wave filter having a pass band located between the resonant frequencies of the lines 'I and 8, as shown by the plot of attenuation versus frequency in Fig. 2D. According to the invention, the center of this pass band is located at frequency of the sound carrier, and the peaks of infinite attenuation occur at the resonant frequencies of the lines I and 8. In a like manner, the effect of shunting the point 5 by means of an impedance which varies with frequency in the manner complementary to that of Fig. 2B is to vary the ability of the line to transmit energy from the television signal source T to the point I. Here the line 3, in conjunction with the lines 9, I0 and II, functions as a wave filter having a stop band located between the resonant frequencies of the lines 9 and I0 or II and pass bands above and below these frequencies, as shown by the attenuation versus frequency characteristic of Fig. 2E. A comparison of the attenuation versus frequency characteristics of Figs. 2D and 2E with the impedance characteristic of Fig.-2B will clarify another feature of the invention. When the line 3 is functioning efiicientlyto transfer energy from the television. signal source to the point I, the im` pedanceshunting the point 6 will be low andv will be rinverted by the quarter wave section of the line 3 to give a high lmpedancefat the point I which puts no appreciable load on the television transmitter. Likewise when -line 3 is functioning efficiently to transfer energy from the sound signal source to the point I, the impedance shuntends.

ing the point 5 will be low and will be inverted by the quarter wave section of the line 3 to give a high impedance at the point I. It is preferable that the lines to the antenna junction points be of high characteristic impedance by comparison with the impedance shunting their input If this obtains no appreciable load is placed on either sound or television transmitter by the other, and the two signals are supplied to a single radiating system or load as desired.

Fig. 3 shows a further embodiment of the in- VentionI whereby it is possible to avoid the use of two parallel resonant circuits connected in series. In this embodiment, the lines I3 and I4 serve respectively to connect the television and sound signal sources to the junction point I5 which, in turn, is connected by means of the line I6 to the load. Points I'I and I8 are a quarter wave length removed from the junction point for 54 and 5I megacycles respectively. The impedance shunted across the lines I4 at the point I8 comprises the lines I9 and 20, the same conguration of lines as was used in the embodiment of Fig. 1, and needs no explanation. The impedance for shunting the lines I3 at the point I1, however, is obtained by inverting the combination comprising the lines 2| and 22 through an impedance inverter resonant at the sound carrier, which is shown as the line 23. The combination of the lines 2| and 22v is the same as that shunting the junction point I8 and has the impedance characteristic of Fig. 2B which, when inverted by means of the line 23, appears across the impedance is purely inductive or positive in sign while in the range above the upper critical frequency it is purely capacitive or negative in sign. The system of Fig. 3 functions in the same manner as that of Fig. 1, the only difference being in the impedance shunting the point I1; hence there is no need for further explanation.

In the embodiments shown which were constructed specifically for use in a System in which the sound and television carriers were very close together and in which the sound channel covered but a narrow band of frequencies by comparison with that of the television, itwas found satisfactory in practice to employ simple combinations 0f pairs of transmission lines used either directly or inverted by means of a quarter wave length line to obtain the desired variation impedance. However if more severe requirements were to be laid down as to the desired characteristic of the impedance shunting the source, it might be necessary to resort to more complex arrangements of transmission lines V' which, of course, could be arrived at by applying any of the known methods of network design to obtain a suitable impedance characteristic, and

by employing transmission lines wherever possible to obtain suitably high values of Q. Thus it will be understood that Ithere is no intention to limit the invention to the particular means here shown for obtaining an impedance suitable for shunting the respective signal sources.

It will further be noted that, although transmission lines have been used as circuit elements in the embodiments shown because of their convenience and greater efficiencyl at the frequencies considered, it may be convenient to employ lumped circuit elements, as opposed to the distributed ones obtained by the use of transmission lines, when the system is to be operated at other frequencies. AMoreover, in the foregoing description where the electrical length of a transmission l line is stated to be equal to a fraction of a wavelength, as also indicated in the drawings the electrical length of the line may be equal to an odd number of such fractional parts of a wave length: The inventiony therefore, is subject only to the limits imposed by the appended claims.

I claim:

l. In a television system; a source of a carrier signal having frequency components within a certain range; signal utilization means coupled to said source; a source of a second carrier having frequency components outside said range and within a second range; an impedance shunting the output of said secondsource, said impedance comprisingr a series combination of the impedances presented by a pair of circuits parallelresonant at different frequencies respectively above and below a certain frequency within said first range; and an impedance inverter having two pairs of terminals, and having one pair of terminals coupled to the said second source and having the' other pair of terminals coupled to said signal utilization means,'said impedance inverter being resonant at a particular frequency within said first range and being adapted to form across its output circuit an impedance substantially inversely proportional to the impedance across its input for wave signals within the said first range. 4

2. In a television system; a source of a carrier signal having frequency components within a certain range; signal utilization means coupled to said source; a source of a second carrier havsignal having frequency components within a certain range; signal utilization means coupled to said source; a source of a second carrier having frequency components outside said range and within a second range; an impedance shuntiflg the output of said second source, said impedance comprising a series combination of impedances presented by two short-circuited transmission lines, each having an effective electrical length equal to an odd number of quarter wave lengths at different frequencies respectively above and below a certain frequency within said first range;

and an impedance inverter having two pairs of terminals, and having one pair of terminals coupled to the said second source and having the other pair of terminals coupled to said signal utilization means, said impedance inverter being resonant at a particular frequency within said first range and beingadapted to form across its output circuit an impedance substantially inversely proportional to the impedance across its input for Wave signals Within the said first range.

4. In a television system; a source of a carrier signal having frequency components within a certain range; signal utilization means coupled to said source; a source of a second -carrier having frequency components outside said range and within a second range; an impedance shunting the within said first range and being adapted to form ing frequency components outside said range and within a second range; an impedance shunting the output of said second source, said impedance comprising a parallel combination of the impedances presented by a pair of circuits seriesresonant at `different frequencies respectively above and below a certain frequency within said second range; and an impedance inverter having two pairs of terminals, and having one pair of terminals coupled to the said second source and having the other pair of terminals coupled to said signal utilization means, said impedance inverter being resonant at a particular frequency within said first range and being adapted to form across its output circuit an impedance substantially inversely proportional to the impedance across its input for wave signals within the said first range.

3. In a television system; a source of a carrier across its output circuit an impedance substantially inversely proportional to the impedance across its input for wave'signals within the said first range.

5. In a television system; a source of a carrier signal having frequency components within a certain range; signal utilization means coupled to said source; a source of a second carrier having frequency components outside said range and within a second'range; an impedance shunting the output of said second source, said impedance comprising a series combination of the impedances presented by three short-circuited transmission lines, two of said lines having the same electrical length which is equal to an odd number of quarter wave lengths at a certain frequency, and the third line having an electrical length equal to an odd number of quarter wave lengths at a frequency separated from said certain frequency by a particular frequency within said second range; an impedance-inverting transmission line, portions of which are common to the said shortcircuited lines, having its input circuit coupled to the said second source and having its outputl circuit coupled to said signal utilization means, said impedance inverter vbeing resonant at a particular frequency within said first range and being adapted to form across its output circuit an impedance substantially inversely propertional to the impedance across its input for wave signals within the said rst range.

6. In atelevision system; a source of a carrier signal having frequency components within a certain range; signal utilization means coupled to said source: a source of a second carrier having frequency components outside said range and within a second range; `an impedance inverter having an input circuit and an outputcircuit, and having its input circuit connected across the output of said second source, said impedance inverter being resonant at a particular frequency within said iirst range so as to form across its input circuit an impedance substantially inversely proportional to the impedance in its output circuit; an impedance shunting the output circuit of said impedance inverter, said impedance comprising a combination of the impedances presented by a pair of open circuited Vtransmission lines connected in paral1e1,` each having an "effective electrical length equal to an odd number of quarter wave lengths at different frequencies respectively above and below a certain frequency within said first range; and a second`impedance inverter having an input circuit and an output circuit, having its input circuit coupled to the said -second source and having its output circuit coupled to said signal utilization means, said second impedance inverter being resonant at a particular frequency within said first range and being adapted to form across its output circuit an impedance substantially inversely proportional to the impedance across its input for wave si within the said i'irst range.

7. In a television system; a source of wave signal energy having frequency components within acertain range; signal utilization means adapted to be supplied with wave signal energy; transmission line means connecting said source to said signal utilization means, said means comprising a pair of hollow cylindrical conductorsvdisposed parallel to one another so as to form an openwire transmission line: an impedance shunting having for their external conductors portions of 45 the said hollow cylindrical conductors, and both being resonant at a certain frequency, and a third short-circuited line resonant at a frequency separated from said certain frequency by a particular frequency within said second range.

8. In an electrical system; a source of wave .signal energy having frequency components within a certain range; means for utilizing at least a portion of said wave energy; a transmission line connecting said source to said utilization means; filter means associated with said transmission line for substantially preventing the transmission thereby of frequency components within a given range, said filter means comprisingaan impedance inverter having input terminals and output terminals and being resonant at a frequency within said last-mentioned range, said impedance inverter having its input terminals connected across the transmission line; and a pair of open circuited transmission line sections, each having one end connected to the output terminals of said impedance inverter and each i thereby of frequency components outside a given range, said illter means comprising three serially connected short circuited transmission line sec- 4 tions, two of said sections having an effective electrical length equal to an odd number of quarter wave lengths at `the same frequency and the third having an eifective electrical length equal to an odd number of quarter wave lengths at a different frequency, said frequencies being separated by said second frequency range.

THOMAS M..,GLUYAB, Ja.

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