USRE23258E - Single side-band system - Google Patents

Description

Patented Aug. 22, 1950 SINGLE SIDE-BAND SYSTEM Bernard E. Lenehan, Bloomfield, N. J assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Original No. 2,476,880, dated July 19, 1949, Serial No. 623,594, October 20, 1945. Application for reissue December 28, 1949, Serial No. 135,411

24 Claims. (01, 332-45) Matter enclosed in heavy brackets E 1 appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue This invention relates to systems for transmitting signals, and it has particular relation to single sideband systems.

The advantages of single side-band transmission are well known in the art. For example, reference may be made to the Radio Engineering Handbook, Third Edition, by Keith Henney, 19%1, pages 552 and 553. This handbook is published by the MoGraw-Hill Book Company of New York city. However, the size, cost and complexity of the equipment required for single sideband systems designed in accordance with the prior art have discouraged extensive commercial utilization of the system.

In accordance with the invention, an improved and simplified single side-band system is provided for transmitting Signals. Transmission may be effected over any desired channel. For example, the transmission may be eifected by the propagation of radiation through space, that is, radio, or it may be effected over conductors.

A signal transmitted by systems embodying the invention may be employed for any desired function. For example, the signals may be employed for communication purposes, for relaying purposes, for telemetering purposes or for supervisory control of any desired equipment.

The invention contemplates the provision of a polyphase carrier wherein the phase components of the carrier are displaced by a substantial angle. In addition, the signal-to-be-transmitted is also produced in polyphase form wherein the phase components are displaced by substantially the same angle present between the phase components of the carrier. Each component of the signal to be transmitted modulates a separate one of the carrier components in a suitable modulator. Preferably the modulator is of the balanced type in order to suppress the carrier component. The outputs of the two modulators consist respectively of the sum and difierence of the upper and lower side bands formed by the modulation of the carrier. By suitably adding or subtracting the outputs of the two modulators, either the upper or the lower side band alone may be produced.

In systems embodying the invention, three major problems are present. The balanced modulator must be adequately balanced in order to suppress substantially the carrier supplied thereto. In addition, the outputs of the two modulators must be properly related to effect substantial cancellation of one of the side-bands produced thereby. Finally, adequate equipment must be provided for producing the phase components of the carrier and signal. If phase shifters are penoxide selenium rectifiers.

employed, the phase shifters must maintain substantially constant amplitude and phase relationships between the associated phase components throughout the required frequency range.

In accordance with an aspect of the invention, barrier-layer modulators are employed. Such modulators may employ rectifiers, such as cop- Of the barrierlayer modulator types, the copper oxide type has been found particularly satisfactory.

Since the impedance of barrier-layer modulators, such as the copper-oxide modulator, is a function of the current passing therethrough, it is possible to balance a modulator having characteristics of this type by passing direct current through appropriate rectifiers in the modulator.

Cancellation of one of the side-bands produced by the two modulators in systems embodying the invention is facilitated by the provision of adjustments for controlling the amplitudes of the outputs derived from the two modulators.

In order to provide polyphase carrier and signal quantities, phase shifters are hereinafter illustrated and described which maintain the amplitude and phase displacement of related components substantially constant over the entire frequency range desired. The phase shifters are of the static type employing only resistors, ca.- pacitors inductance elements.

It is, therefore, an object of the invention to provide an improved system for transmitting signals.

it is a further object of the invention to provide an improved single side-band transmitter.

It is a further object of the invention to provide a single side-band transmitter employing barrierlayer modulators.

It is also an object of the invention to provide means for adjusting the impedance distribution of a barrier-layer modulator.

it is an additional object of the invention to provide means for adjusting the extent of suppression of a carrier in a modulator.

It is a still further object of the invention to provide means for adjusting the extent of cancellation I of a side band in a phase-rotation, single side-bandtransmitter.

Additional objects of the invention will be apparent from the following discussion taken in conjunction with the accompanying drawing in which:

Figure 1 is a schematic view of a system emcdying the invention.

Fig. 2 is a schematic view showing a modified output unit suitable for the system of Fig. l, and

Fig. 3 is a block diagram of a receiver suitable for the system illustrated in Fig. 1.

Referring to the drawing, Fig. 1 shows equipment located at two spaced stations. A and B for the transmission and reception of signals between the two stations. Such signals may be transmitted in any suitable manner. However, for the purpose of discussion, it will be assumedthat the stations A and B are connected by elec-- tric power lines comprising conductors L1 and L2 which are to be employed for guiding signals between the two stations. A system of this type is commonly referred to as a power-line carrier system.

At the station A an oscillator I is provided for generating a suitable carrier.

have anydesired, frequency. However, for

power-line, carrier-transmission: purposes, it isconventional to employ frequencies within the range of 50 to 150 kiloc-ycles per second. It is de- "sirable that the oscillator I be extremely stable.

If desired, it may be of the piezo-electric crystal type. However, master oscillators, which employ no-crys-tals, have been designed in the prior art with sufficient stability for present purposes.

The oscillator l is coupled througha suitable transformer 3 to a phase shifter 5. The phase shifter 5 is designed to supply to the primary 1A of the transformer a quantity represented by the expression Ec cos 21rfct, wherein Ec is a constant amplitude factor, fc represents the carrier frequency, and t represents time. The phase shifter then may supply to the primary winding QAof the transformer 9 a phase component represented by the expression Ec sin 21rf'et, The

secondary winding 1B of the transformer is connected across two terminals of a modulator H. In a similar manner the secondary winding 9B of the transformer 9 is connected across two "terminals of'a modulator |3';

In order to modulate the carrier-components supplied by the transformers and 9, a modulating signal is produced in any desired manner.

This signal may be a tone for relay operation, 'or for telemetering, or for supervisory control purposes, or it may be a voice signal; For present purposes, it willbe assumed that a voice signal is provided by means of a microphone I5, the output of which may be amplified, if necessary, by means of an amplifier IT. The output of the This carrier may For example, the phase shifter '5 amplifier H is coupled through a transformer l9 to a phase shifter 2|. This phase" shifter is designed to produce two signal components which are displaced in phase by substantially the same angle of displacement present between the two carrier components supplied by the transformers land 9.

For example, let it be assumed that a signal having a frequencynfs is supplied through the transformer |9- to the phase shifter 2|. The

phase shifter then may be designed to provide an output between the conductors 23 and 25 which is represented by the expression Es sin 21rfst, wherein Es represents an amplitude factor. In addition, the phase shifter provides an output between the conductors 21' and 25 which may be represented by the expression Es cos 21rfst. By

inspection of Fig. 1, it will be observed that the modulator H has applied thereto the carrier component E0 cos 21rfct' and: the signal component Es cos 21rfst. Consequently, if the modulator II is of the balanced type, the output thereof may be represented by the expression (EC cos 21rfct) (Es cos Z'n'fst. This expression represents the sum of the two side bands produced by the modulator and may be represented by the expression [cos 21rt(f,,-f,) +cos 21rt(f +f,)]

In. a similar manner the modulator l3 has applied thereto the two components represented by the expressions --Ec sin Z'n'fet and Es sin 21rfst. If the modulator I3 is of the balanced type, itsv output may be-represented; by the. expression (E sin 21rf,t)(E, sin 21m) The output of the modulator II is applied to a load'resistor'29 through a coupling transformer 3|. Similarly, the output of the modulator |-3 is applied to a load resistor 33 through a transformer 35- From the foregoing discussion, it is clear that the voltage drops across the- resistors 29 and 33 represent respectively the sum and difference of the upper and lower side-bandsproduced by the modulators H and 3. Consequently, by adding these voltage drops, the resultant voltage represents the lower side-band. By subtracting the two voltage-drops the resultant voltage-represents the upper side-band;

The resultant Voltage-is applied to a suitable amplifier 3 1'. To this end, a conductor 39 is connected'to the resistors 29and 33 through adjustable taps 4| and 43. Each tap and its associated resistor may constitute a potentiometer. By suitable adjustment of the taps, the portions of the voltage drops across the resistors 29 and 33 which are utilized may be adjusted to eliminate substantially'one of the side-bands. This is desirable for the reason that the circuits associated with the modulators II and I3 may not be exactly similar;

One terminal of the, resistor 33 is connected to ground through a conductor 4-5 and the conductor 25. One terminal of the; resistor 2G is con-- nected through a conductor to the amplifier 31. The output of the amplifier 31' may be coupled in any suitable manner to the conductors LI and L2, as throughcapacitors 49 and El.

From the preceding discussion, it is clear that a single side-band signal is applied to the conductors LI and L2 through the capacitors 49 and 5|. This signalmay be picked up at the station B inany suitable manner. As illustrated in Fig. 1, areceiver 53' suitable for receiving single side-band signals is coupled to the conductors LI and L2 at the station B through capacitors 55 arr-d151,

The system having been described in outline, certain components-thereof now will be discussed in greater detail. As previously pointed out, the phase shifter 5 isdesigned to divide the output of the oscillator i into two components displaced by a substantial angle. To this end, a capacitor 59 and a resistor 6| are connected across the secondary winding of the transformer It will be noted that the primary winding 1A is connected substantially across the capacitor 53, whereas the primary winding 9A is connected substantially across the resistor 6|. A simple phase shifter corresponding to the capacitor 59 and the resistor 6| would produce substantially a 90 phase displacement between the components supplied to the primary windings IA and 9A with substantially equal amplitudes for only one frequency of the oscillator I.

It is desirable that the phase shifter be capable of producing a similar phase shift and maintaining substantially constant amplitude of the components for all frequencies for which the oscillator may be adjusted. As previously pointed out, the oscillator may be designed to operate at any frequency within the range of 50 to 150 kilocycles per second. In order to make the phase shifter 5 effective over this entire range of frequency, a pair of inductance windings or coils B3 and B5 are provided which are mutually coupled. By reference to Fig. 1, it will be noted that the coil 63 is connected in series with the capacitor 59 and the resistor 5| across the secondary winding of the transformer 3.

It is well known in the art that the voltage drops across inductance coils and capacitors vary as functions of frequency in opposite directions. Since the coils 53 and 55 are mutually coupled, a voltage is induced in the coil 65 which also varies as a function of frequency. However, the voltage across the coil 65 is substantially in phase with the voltage across the capacitor 59. Since the amplitudes of the voltages across the coil 55 and capacitor 59 vary in opposite directions as the frequency of the oscillator is changed, it may be observed that the components of the phase shifter 5 may be proportioned to maintain the resultant voltage across the coil 65 and the capacitor 59 substantially constant over a small range of frequency, such as a range of 50 to 150 kilocycles per second. Since the primary winding 1A is connected for energization in accordance with this resultant voltage, the voltage applied to this primary winding IA is rendered substantially constant over the desired frequency range.

Further improvement in the performance of the phase shifter 5 is effected by making the internal impedances of the two output circuits of the phase shifter substantially equal. To this end, a resistor 61 is provided which is substantially equal in value to the value of the resistor 5|. Also a capacitor 69 and an inductance win'ding or coil 1| are located in the circuit energized by the voltage across the resistor 5 I The capacitor 69 and the inductance coil correspond respectively in value to the capacitor 59 and the inductance introduced by the inductance coil 65.

As representative of suitable values for the components of the phase shifter 5, the following table is presented:

Capacitor 59 microfarad .01 Capacitor 59 do .01 Resistor 6| ohms 400 Resistor B1 do 400 Mutual coupling between coils 53 and henry .0004 Coil ll do .0004

These values are suitable for a frequency range of 50 to 150 kilocycles per second and maintain the phase displacement and amplitudes of the outputs of phase shifters substantially constant over this range.

The phase shifter 2| presents a much more serious problem for the reason that is required to handle a frequency range corresponding to the range required for voice communication. For example, in telephone conversation, it is desirable to have a frequency range of the order of 250 to 4,000 cyclesper second. For such a range, the phase shifter 2| must handle an upper limit frequency which is approximately 16 times the lower limit frequency.

Certain components of the phase shifter 2| correspond to components of the phase shifter 5. For example, the phase shifter 2| includes inductance windings or coils I3, 15 and 16, capacitors TI and '19 and resistors BI and 83 which correspond respectively to the coils 63, 65, and H, the capacitors 59 and 69 and the resistors BI and 61 of the phase shifter 5. The performances of the corresponding components are similar in the two circuits, but it is to be understood that the values thereof are selected in accordance with the frequencies to be handled thereby. It may be pointed out that the capacitance of the capacitor 59 and the inductance introduced by the coil 63 across the secondary winding of the transformer 3 preferably should be resonated approximately to the geometrical mean of the frequency range which is to be applied to the phase shifter. A similar comment holds for the capacitor 1! and the coil 13 of the phase shifter 2|.

If only the components thus far mentioned for the phase shifter 2| were employed, the amplitude of the voltage applied between the conductors 23 and 25 would be equal to the voltage applied between the conductors 21 and 25 for only two frequencies. If the amplitudes of these voltages were plotted as a function of frequency, the curves would intersect at two points. Because of the difference in amplitudes of the voltages within operating range, the cancellation of one side band in the system of Fig. 1 would not be as complete as desired.

In order to improve the performance of the phase shifter 2|, an additional resistor 85 of substantial value is connected in series with the secondary winding of the transformer I9. This resistor may have a value sufficient to maintain the current flowing through the secondary winding of the transformer l9 reasonably constant throughout the operating range of the phase shifter.

In addition, a parallel circuit containing an inductance coil 81 and a capacitor 89 is connected between one terminal of the secondary winding of the transformer l9 and the conductor 23, as clearly shown in Fig. 1. This parallel circuit is resonated to a frequency which is substantially the geometric mean frequency of the band which is to be applied to the phase shifter 2|. The effect of this parallel resonant circuit is to cause the curves representing the voltage between conductors 23 and 25 and the voltage between the conductors 21 and 25 to approach each other more closely over the desired range. For example they may intersect at four points over the frequency range applied to the phase shifter 2|. The deviation between the two curves at any point is small enough to assure excellent operation of the system. Moreover, the phase displacement between the two components is maintained substantially uniform. In a phase shifter similar to the phase shifter 2|, tests have shown that it is possible with a range of imput frequency of 250 to 4000 cycles per second to maintain the voltage outputs of the phase shifter balanced in amplitude within 4%, and constant 7 in phaseidisplacemenu within". omi degree: off 902 over the entire' ranger.

The value of the resistor: sihmayi vary: over:- a substantial range or the: resistor may? be: omitted completely: Preferably, thevalueot the: resistor w is selected to maintain thezphasesanglesbetween the two output phasex-components-of: theipiifiSQ shifter approximately 9.0?oven a Wide range of frequency. To this end; .if the pha'se shifter: is designed for a 90 phase: angle; the resistor: 85. should have a value selected tov make: the? phase angie substantially. 90 at the limitsof tl'lE-Jfl'file quen'cy range to be han'dled-byr the phase: shifter;

As examples of suitable values for the components of th'e'p'h'ase shifter 21, thefollowing table is presented:

Inductance coil 13 "henriesm .04 Inductance coil'lE do" .64 Mutual inductance between coils 13'and'15' do v(i l Coil Hi; -4" "do"--. .04 Capacitor Tl microfarad I Capacitor 19'. do' w 1 Capacitor 8 5! do .6056 C011 8.'I henries 6 Resistor8L r ohmsr Resistor 8'3 do 400 Resistor 85". do 4500.

Thecoilslmay haveiaQtof l'8::measure.d atza frequency off1600cyclesazpertseconde Them'odulator's Il'an'd l3rma ybe of: any suitabletypez. However, it is desirable: that. balanced modulators be employed. inorder" to suppress the carriers supplied: thereto". from; the phase shifter: 5. Although. electronic: tube" modulators maybeemployed, barrier-layer modulators. offer ainumber'of:advantages; Notonly. dozthey elimis nate power sources, such. as heater. transformers and. plate. voltage supply sources; hut-Ithey' have extremely good modulation characteristics;- for single sidei-b'and work. In addition,. the. balance thereof may be adjusted-by applicationv'thereto oia direct'currentas hereinafterpointediout.

The most common barrier-layer rectifiers in use todayare those known as selenium and copper oxide. rectifiers. Copper oxider rectifiers have been particularly satisfactory for the modulators l'l and' l3. They'may'b'etarranged in vari'--'" ous ways as well known in the art. However; in: Fig. 1 each modulator is: of the ringtype and employs four copper-oxide rectifiers el 92"; 93 and. 95. As. clearly" shownon the-drawing; each successive pair of copper oxide rectifier-s: has a terminal disposed therebetween for connection to associated transformers.. I-t will'be noted th'at' the four copper-oxiderectifiers are arranged substantially in a bridge. If each bridge is bal'an'cedg-no' carrier-should-appear inthe output transformers 3 l and 35. However, if a bridge is unbalanced, as by reason of inherent variations in the copper oxide rectifiers, some carrier may appear.

In" order to balanceor unbalance the modulators as desired; direct current issupplie'd' thereto from axsource: represented by conductors L3 and L4,. These-conductors havextwopotentiom'eters 91- and". 99 connected thereacross; The; potentiormeters respectively have adjustable. taps Ill'l' and? 33 associated therewith: The; tap. I021." is:.connect'ed: through a resistor liilaanditheconductor' 23 to a center-tap 90' onthe secondary" winding: 9B5of the transformer 91. Similarlmthe tap H132 is conn'ectechthrough a resistor: I05 and the conduc tor 21 to acenter-tap 1C on the secondary winds in; 13'. In. addition, a. resistor I 01: having a 8 grounded centerr-tapsis connected between" the conductors L3 and L4.v Center-taps. on. the primary: windings. oi: the transformersiiii and: 35 are OOIlIl'GCtBdLtO ground.

A rectifi'entof the: oopper oxide type has are;- sistan'ce which: varies inversely 'withzthe current flowing therethrough. Letit beassumedthat thetap [113 is; adjustedto make the center-tap: 1C positive: with respect? to the: center-tap 316; Under these circumstances;-curre nt 'flowsthrouglr the copper-oxide rectifiers Biz-and 95. Since the resistanceotsuch rectifiers'varies asla functionof current therethrough, this: tends: tounbalance the: bridge represented: by. the; modulator; M5,. and the extent. of: unbalance. depends. on? magnitude of the voltage applied between thetermin'a'lss. 1C. and 3| C. The value of the voltage; in turn;..is dependent on the. adjustment of? the tap: 103..

Let it be assumed. nextthat the tap N13. is adjusted tomakethe t'ap lcnegativewithirespect to the tap 3IC. Undersuch circumstances, direct current williflow through the c'opper oXide-rectia fiers-93 and Si. This: tends? to? unbalance the bridge, represented by. the. modulator Iii in: the opposite direction, and the extent. of: unbalanceis adjustedby-adjustment of the tap H131 Fromzth'e foregoing: discussiomit will be clearly. apparent that if no audioisignal is supplied to -the modulator H3 thecarrier out'putof. the modulator may be substantially suppressed: by" proper: manipulation ofithe tapi 03.. In a similarmanner; the tap [9| may be manipulated tov adjust the" modulator: l3.

In certain cases; it may. be. desirable to. unbal ance deliberately" one of the modulators. For. example, in relaying work, operatiomof: a relay may be employedi'forplacin'g a: carrier seamen the conductors: LI,. L2. Such a carrier signal: may be produced by deliberately unbalancin'gone. of thei'modulators': He or I3; For example, operation of; a relay may belemployed to l closemontacts H3; Closure. of the contacts: connects the tap: [B3 to: the conductor-hm. The-resulting applioa tion of a direct voltage to the modulator LI! un balances the modulator and produces a carrier output therefrom. Thisi carrier: output'is sup plied to the conductors LE and LL.

As previously. pointed. out, either: the=upper on lower'si'de'i-band may. be selected sis-desired For example, the. voltage drops across: the resistors ZQand 3Smay be connected either additively or subtractively; depending upon the speoific si'deband desired. The side-band: employed also may be changed? by interchanginglth'e connections of one of the transformers 3! or. 35; or by interchanging the conductorsZ-S and 21 or by inter-- changing the inputs to the transformers F and 9:from the phase shifter 5'.

If." desired, the: transformers 3i and 35- may' be combined, thereby, providing a single secondary winding for. connection to the amplifierv Int this case, the inputs to the two modulators. may be adjusted. for balancing purposes. Howevea, the preferred embodiment is that illustrated in- Fig. 1.

If it is desired to employ the upper and lower side-bands for separate purposes, connections. similar to those illustrated in Fig. 2 may be employed. Referring to Fig. 2, it will be observed that two transformers 31A and 35A are shown: These transformers correspond; respectively, to thetransformersi i andj3'5 of- Fig. 1', and thepri mary connections thereof may be. exactly the same as'thoseillustrated for the'transiormers-M and 35; It will be observed, however; that-the 9 transformer [3 I] 31A has two secondary windings I I5 and 5 I1. The transformer [35d1 35A also has two secondary windings H9 and I2I. By tracing the connections it will be observed that the windings l I5 and I is are connected in series opposition across the input of an amplifier I23. The same windings are connected in aiding series relationship across the input of an amplifier I25. In a somewhat similar manner, the windings H1 and I2I are connected in series opposition across the input to the amplifier I23, and in series aidin relation across the input to the amplifier I25. By a consideration of Fig. 2, it will be clear that each of the amplifiers I23 and I25 supplies a separate one of the side-bands produced by the system of Fig. 1.

For amplifying purposes, any suitable amplifier may be employed. As shown in Fig. 1, the amplifier 31 includes two pentode tubes I21 and I29 having plate electrodes I21a and I29a respectively, connected through a suitable plate resistor I29b and an inductance coil I21b to a common conductor I3I. This conductor I3l is connected to the positive terminal of a source of direct voltage. The negative terminal of the source is connected to ground. The screen grids I21c and I29c of the pentodes also are connected to the conductor I3I. The suppressor grid I21d and cathode I2'Ie of the pentode I21 are connected through a cathode resistor I21f and a bypass capacitor I21g to ground. In an analogous manner, the suppressor grid I29d and the cathode I29e of the pentode I29 areconnected to ground through a cathode resistor I291 and a bypass capacitor I29g.

The input to the amplifier 31 is derived through the conductor 41 which is connected to the control grid I29h of the pentode I29. The output of the pentode I29 is coupled through a coupling capacitor I33 to the control grid I21h, and a grid resistor I21j associated with the pentode I,21. The output of the pentode I21 is applied through a coupling capacitor I35 to a parallel tuned circuit comprising an inductance coil I31 and a capacitor I39. This parallel tuned circuit is tuned substantially to the output frequency of the amplifier. The voltage across this circuit is coupled through the capacitors 4| and 5| to the conductors LI and L2. As shown in Fig. l, a center tap on the coil I31 is connected to ground.

It is believed that the operation of the system is clear from the foregoing discussion. If it is desired to communicate by voice signal between the stations A and B, the voice signal is converted into a corresponding audio signal by means of the microphone I5. This audio signal is passed through the phase shifter 2| to provide two components differing in phase by approximately 90. Similarly the output of an oscillator I is passed through the phase shifter 5 to provide two components also differing in phase by substantially 90. These components are supplied to the ring modulators I I and I3 to produce carrier-suppressed, double side-band outputs in the secondary windings of the transformers 3| and 35. The outputs of the transformers 3I and 35 are supplied to load resistors 29 and 33 and are combined to eliminate one of the side-bands. The resulting single side-band signal is amplified in the amplifier 31 and applied to the conductors LI and L2 through the coupling capacitors 49 and BI. This single side-band signal is received by means of a suitable receive 53 at station E. Similar equipment may be located at each stai of the superheterodyne receiver.

tion, if desired, to permit transmission of voice communication from the station B to the station A.

If it is desired to transmit a carrier signal between the stations A and B, the contacts I I3 are actuated to unbalance the modulator II. This results in the generation of a carrier signal which is amplified in the amplifier 31 and applied across the conductors LI and L2 for reception at the station E.

Sometimes it is desirable to employ a standard superheterodyne receiver for reception of single side-band signals. A receiver of this type is illustrated in block form in Fig. 3.

Referring to Fig. 3, it will be noted that the incoming single side-band signal is represented by the sum or difference of two frequencies fc and is which are, respectively, the carrier and signal frequencies.

on whether the upper or the lower side-band is to be employed. The incoming signal is mixed in a suitable mixer I4I with a frequency equal to fc-l-fi, wherein fl is the intermediate frequency Consequently. the output of the mixer includes a frequency fifs or fi+fs, depending on the specific sideband being received. This intermediate frequency is amplified in an intermediate frequency amplifier I43 and supplied to a suitable [modulator] demodulator I45 which may be in effect a second mixer stage. If automatic volume control is provided, the controlling signal for the automatic volume control should be derived from the intermediate frequency amplifier prior to demod-' signal IE5 is obtained from the demodulator I45 and may be employed to actuate any suitable translating device I41 such as a loudspeaker.

It will be noted that two specific frequencies fc+fi and f1 are injected, respectively, into the mixer MI and the demodulator I45. If separate sources are provided for these two oscillations, extreme stability would be required for both sources.

In order to simplify the stability problems of the receiver, a local oscillator I49 is provided which is capable of generating a frequency fc. This local oscillator is designed to operate with great stability. Since an oscillator simila to the oscillator I of Fig. 1 normally will be available at each station for transmitting purposes,

a portion of the output of the oscillator I may be employed in place of the local oscillator I49.

The output of the local oscillator I49 is supplied to a mixer I5I. This mixer also receives an output from an oscillator I53 which generates an oscillation having substantially the frequency f1. Consequently, the output of the mixer I5I provides the frequency fe-l-fi which is required for the mixer I4I. In addition, the oscillator I53 provides an oscillation of the frequency f1 required for the demodulator I45.

In the system of Fig. 3, the oscillator I53 need not be as stable as the oscillator I49. It will be recalled that the intermediate frequency amplifier I43 supplies to the demodulator a quantity represented by the frequency fimfs. In the demodulator I45. a quantity is produced which is represented by the expression (fizfs) fl=:Ffs. Consequently, if the frequency f1 varies, such variation does not affect the quantity represented by the frequency f; which is supplied to the trans- Either the sum or difference of these frequencies will be received depending J 11 lating device MT. Preferably-the oscillator 153 should have a stability suflicient to maintain the signal fiIlIfs within the pass band of the inter mediate frequency amplifier M3. Consequently, the receiver of Fig. 3 requires great stability only in the local oscillator H9.

Although the invention has been described with reference to certain specific embodiments thereof, numerous modifications are possible. Therefore, the invention is broadly set forth in the appended, claims.

I claim as my invention:

1. In a system for producing a single sideband quantity, means for producing a first carrier a1- ternating quantity and a second carrier alterhating quantity having a substantial phase displacement therebetween, said first and second carrier quantities being of the same frequency, means for producing a first modulating quantity and a second modulating quantity having substantially said phase displacement therebetween,

means for modulating said first carrier quantity by said modulating quantity to produce a first double sideband output, means for modulating saidsecond carrier quantity by said second modulating quantity to produce a second double sideband output, said modulating means comprising barrier-layer modulator means, and means for combining said outputs to eliminate one of saidsidebands.

2. In a system for producing a single sideband quantity, means for producing a first carrier alternating quantity and a second carrier alternating quantity having a substantial phase displacement therebetween, said first and second 1 carrier quantities being :of the same frequency, means for producing a first modulating quantity and a second modulating quantity having substantially said phase displacement therebetween, means for modulating said first'carrier quantity by said firstmodulating quantity to produce a first double sideband output, means for modulating said second carrier quantity by said second modulating quantity .to produce a second double sideband output, said modulating means comprising balanced barrier-layer modulator means wherein the carrier quantities are substantially suppressed, and means for combining said outputs to eliminate one ofsaid sidebands.

3. In a system for producing a single sideband quantity, means for producing a first carrier alternating quantity and a second carrier alternating quantity having a substantial phase displacement therebetween, said first and second carrier quantities being of the same frequency, means vfor producing a first modulating quantity and a second modulating quantity having substantially said phase displacement therebetvveen, means for modulating said first carrier quantity by said first first double sideband output,,means for modulating said second carrier quantity by said second modulating quantity to produce a second double sideband output, said modulating means com-v prising balanced barrier-layer modulator means, means for adjusting the modulator means to eliminate substantially the carrier quantities, and means for combining said outputs-to eliminate one of said sidebands.

4. In a system for producing a single sideband quantity, means for producing a first carrier alternating quantity and a. second carrier alternating quantit having a substantial phase displacement therebetween, said first and second carrier quantities being of the same frequency,

modulating quantity to produce a means for producing a first modulating quantity and a second modulating quantity having substantially said phase. displacement therebetween, means for modulating said first carrier quantity by said first modulating quantity to produce a first double sideband output, means for modulating said second carrier quantity by said second modulating quantity to produce a second double sideband output, said modulating means comprising balanced barrier-layer modulator means, means for adjusting the modulator means to eliminate substantially the carrier quantities, said last-named means comprising means for introducing direct current into the barrier-layer modulator means, and means for combining said outputs to eliminate one of said sidebands.

5. In an electrical system, a, plurality of barrier layer rectifiers, means connecting said barrierlayer rectifiers in a circuit having parallel paths, and means for adjusting the relative impedances of said paths, said last-named means comprising means forpassing biasing current through part of the barrier-layer rectifiers.

6. In an. alternating-current electrical system, a plurality of barrier-layer rectifiers, means connecting said barrier-layer rectifiers in a circuit having parallel paths each containing one of the barrier-layer rectifiers, and means for adjusting the relative impedances of said paths, said lastnamed means comprising means for passing direct current through a preselected part of the barrierlayer. rectifiers. I

7. In a system for producing a modulated quantity, a barrier-layer modulator having a plurality of paths each containing'a barrier-layer rectifier, and means for passing direct current through at least part of said paths for modifying the impedance characteristics thereof.

I 8, In a system for producing a modulated quantity, a bridge modulator having four paths related to form a bridge circuit, each of said --minal intermediate each adjacent pair of rectifiers to form a resultant bridge circuit, and means for applying a direct voltage between a first pair of the terminals which are disposed on a diagonal of the bridge circuit and the remaining terminals for modifying the balance of the bridge circuit, said means including means for adjusting the magnitude and reversing the polarity of the direct voltage.

'11., In a system for producing a single sideband quantity, means for producing a first quantity quantities to effect substantially complete elimination of one of the sidebands.

12. In a system for producing a single sideband quantity, means for producing a first carrier alternating quantity and a second carrier alternating quantity having a substantial phase displacement therebetween, said first and second carrier quantities being of the same frequency, means for producing a first modulating quantity and a second modulating quantity having substantially said phase displacement therebetween, means for modulating said first carrier quantity by said first modulating quantity to produce a first double sideband output, an adjustable potentiometer connected for energization by said output to provide an adjustable output, means for modulating said second carrier quantity by said second modulating quantity to produce a second double sideband output, said modulating means comprising balanced barrier-layer modulator means, means for adjusting the modulator means to eliminate substantially the carrier quantities, said last-named adjusting means comprising means for introducing direct current into the barrierlayer modulator means, and means for combining the adjustable output and the second double sideband output, whereby the outputs may be adjusted relative to each other to eliminate substantially one of the sidebands.

13. In a system for shifting the phase of an alternating quantity, means responsive to an alternating input for producing a first quantity and a second quantity having a substantial phase displacement therebetween, said first quantity varying as a function of frequency of the alternating input, means for producing a third quantity varying with the frequency of the alternating input in a manner substantially opposite to the variation of the first quantity, whereby the first and third quantities may be combined to produce a fourth quantity which is substantially [less] more immune to variation in frequency of the alternating input than said first and third quantities, means associated with said first and second-named means for producing output circuits for said second and fourth quantities, and means for balancing the internal impedances of the output circuits to which the second and fourth quantities are applied.

14. In a system for shifting the phase of an alternating quantity over a substantial frequency range, means responsive to an alternating input for producing a first quantity and a second quantity having a substantial phase displacement therebetween, said first quantity varying as a function of frequency of the alternating input, means for producing a third quantity varying with the frequency of the alternating input in a manner substantially opposite to the variation of the first quantity, whereby the first and third quantities may be combined to produce a fourth quantity which is substantially [less] more immune to variation in frequency of the alternating input than said first and third quantities, means associated with said first and secondnamed means for producing output circuits for said second and fourth quantities, and means for modifying the phase displacement of the second and fourth quantities adjacent the limits of the frequency range for which the phase shifter is designed.

15. In a system for shifting the phase of an alternating quantity, means responsive to an alternating input for producing a first quantity and a second quantity having a substantial phase displacement therebetween, said first quantity varying as a function of frequency of the alternating input, means for producing a third quantity varying with the frequency of the alternating input in a manner substantially opposite to the variation of the first quantity whereby the first and third quantities may be combined to produce a fourth quantity which is substantially Eless] more immune to variation in frequency of the alternating input, means associated with said first and second-named means for providing output circuits for said second and fourth quantities, said second and fourth quantities having amplitudes which when plotted as a function of frequency have two points of intersection, and means for varying the amplitude of the second quantity relative to the amplitude of the fourth quantity as a function of frequency to provide more than two points of intersection thereof, whereby the difference in the amplitudes of the second and fourth quantities is small over a substantial range of frequency variation of the alternating input.

*16. A system as defined in claim 15 wherein the last-named means comprises a resonant circuit tuned to a frequency within the range of the alternating input, said resonant circuit being connected to alter the amplitude relationships of the second and fourth quantities as a function of frequency.

1'7. In a system for shifting the phase of an alternating quantity over the range of frequency required for intelligible speech, a circuit comprising a capacitor, a resistor and an inductance winding connected in series for energization from a suitable source in accordance with the alternating quantity, a first output circuit including an inductance winding connected for energization in accordance with the voltage across said capacitor, said inductance windings being mutually coupled to induce in the second-named inductance winding a voltage which adds to the voltage across the capacitor to provide a resultant voltage which is substantially constant over a substantial range of variation in the frequency of the alternating quantity, and a second output circuit connected for energization in accordance with the voltage across said resistor, said capacitor and the inductance introduced by the firstnamed inductance winding being resonated substantially to the geometrical mean of the frequency range for which the phase shifter is designed.

18. In a system for shifting the phase of an alternating quantity over the range of frequency required for intelligible speech, a circuit comprising a capacitor, a resistor and an inductance winding connected in series for energization from a suitable source in accordance with the alternating quantity, a first output circuit including an inductance winding connected for energization in accordance with the voltage across said capacitor, said inductance windings being mutually coupled to induce in the second-named inductance winding a voltage which adds to the voltage across the capacitor to provide a resultant voltage which is substantially constant over a substantial range of variation in the frequency of the alternating quantity, and a second output circuit connected for energization in accordance with the voltage across said resistor, said output circuits having substantially equal internal impedances.

19. In a system for shifting the phase of an alternating quantity over the range of frequency required for intelligible speech, a circuit comprising a capacitor, a resistor and an inductance winding connected in series for energization from a suitable source in accordance with the alternating quantity, a first output circuit including an inductance winding connected for energization in accordance with the voltage across said capacitor,

said inductance windings being mutually coupled to induce in the second-named inductance winding a voltage which adds to the voltage across the capacitor 'to provide a resultant voltage which is substantially constant over a substantial range of variation in the frequency of the alternating quantity, and a second output circuit connected for energization in accordance with the voltage across said resistor, the series circuit connected for energization from the suitable source including sufiicient resistance to maintain current supplied thereto from the source substantially independent of frequency throughout the operating range of the phase shifting system.

20 In a system for shifting the phase of an alternating quantity over the range of frequency required for intelligible speech, a circuit comprising a capacitor-,a resistorand an inductancewinding connected in-series for energization from a suitable source in accordance with the alternating quantity, a first output circuit including an inductance winding connected for energization in accordance with the voltage across said capacitor, said inductance windings being mutually coupled to induce in the second-named inductance winding a voltage which adds to the voltage across the capacitor to provide a resultant voltage which is substantially constant over a substantial range of variation in the frequency of the alternating quantity, and a second output circuit connected for energization in accordance with the voltage across said resistor, and a parallel resonant circuit connected for energization from said -suit-' able source.

21. A system as claimed in claim 20 wherein the parallel resonant circuit is tuned to resonate substantially at the geometrical mean of the frequency range for which the system is designed, and .wherein the output circuits have substantially equal internal impedances, said capacitor and the inductance introduced by the first-named induct- :means for applying the first carrier phase component and the firstsignal phase component .to the first modulator to produce a first modulated output therefrom, means for applying the second carrier phase component and the second signal .phase component to the second modulator to produce a second modulated output therefrom, and means for combining the outputs to eliminate one sideband therefrom.

23. A system as defined in claim 22 in combination with baianoe-control-means for introducing with one of said components to the associated one of the modulators a direct-current quantity for modifying the balance of the last-named modulator.

24. A system asdefined in claim 23 wherein the balance-control-means is adjustable for varying the magnitude and polarity of the direct-current quantity.

' BERNARD E. LENEHAN.

REFERENCES CITED The following references are of record in the file of this or in the original patent:

UNITED STATES PATENTS Number Name Date 1,666,206 Hartley Apr. 17, 1928 2,163,680 Hansell June 27, 1939 2,173,145 Wirkler Sept. 19, 1939 2,191,315 Guanella Feb. 20, 1940 2,400,133 Pray May 14, 1946 2,414,317 Middel Jan. 14, 1947

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