US2050680A

US2050680A - Wave signal receiving system

Info

Publication number: US2050680A
Application number: US8467A
Authority: US
Inventors: Harold A Wheeler
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1935-02-27
Filing date: 1935-02-27
Publication date: 1936-08-11
Anticipated expiration: 1953-08-11

Description

Aug- 1l, 1936. H, A. WHEELER I* WAVE SIGNAL RECEIVING SYSTEM Filed Feb. 2T, 1955 4 Sheets-Sheet 1 Aug. 1l, 1936.' v H A, WHEELER 2,050,680

wAvE SIGNAL RECEIVING SYSTEM Filed Feb. 27, 1935 4 Sheets-Sheet 4 Patented Aug. ll, .1936

UNITED STATESv WAVE SIGNAL'RECEIVING SYSTEM Harold A. Wheeler, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application February 27, v1935, serial No. 8,467 f 16 claims.

This invention relates to signal-modulated carrier wave signaling systems, and more particularly to such systems designed for distortionless and selective reception of radio signals, free from interfering signals and other disturbances.

In my copending application, Serial No. 691,927, led October 3, 1933, of which the present application is a continuation-in-part, there is described a system for receiving signal-modulating carrier waves and utilizing only the carrier and either sideband of modulation. As explained at length in that application, such a system has a number of advantages, among which maybe noted that the signal selecting circuits need be designed-to pass only one-half the band of frequencies required by a conventional receiver, resulting in increased selectivity and a minimum of interference. Further, the system may beadjusted to `receive that sideband which is subject to a minimum interference from signals on adjacent channels.

. The system of my aforesaid application included as desirable features thereof, provision for adjusting the circuits so that the signal carrier is located at either one edge or the other of the narrow frequency band passed by the selecting circuits; an arrangement of selectiveadmission effective to increase the response of a receiver when it is properly tuned to receive-a single sideband; an automatic volume control of the suspended type, that is, one which operates to reduce the ampliilcation of the system only for received signals above a predetermined intensity; a reverse automatic volume control associated `with themain volume control, procuring the required ultimate variation of the bias voltage for a gain control of the system without any appreciable change inthe ampliedcarrier output; and-a quieting system leffective to maintain the "system unresponsive unless the received signal exceeds a predetermined intensity and unless the receiver is correctly tuned.

My present invention relates to signal translating, amplifying and controlling circuits and sysl5-tems which, while of general application, are particularly suitable for use in connection with signal-modulated carrier wave systems of the type disclosed and claimed in my, aforesaid copending application and. briefly described above. In- -cluded in my present invention is an arrangement for optionally operating a single sideband 4 type otreeiving system for double sideband reception, which may be found desirable when a very weak signal ismbeing received and a condi- I05 'udn of vel-y mgn selectivity vmusi; be provided,

(Ci. Z50-20) The above and other features of the invention will become apparent from the following detailed description and reference to the accompanying drawings.

In the accompanying drawings, Fig, 1 is a dia- 5 gram showing in generalized form a receiving system of the superheterodyne type, adapted for single sideband operation in accordance with this invention; Fig. 2 is a schematic circuit diagram of a receiver constructed in accordance with the 10 general diagram of Fig. 1; Fig. 3shows graphically the selectivity of the intermediate-frequency amplifier of Fig. 2; Fig. 4 shows graphically the fidelity of the audio amplifier `of Fig. Zand-illustrates the compensation for the loss of part. of 15 a sideband under the condition of single sideband reception; Fig. 5 shows the change of audio-frequency fidelity upon change of volume level; Fig. 6a illustrates the responsiveness of the intermediate-frequency trap of Fig. 2; Fig. 6b illustrates 20 the relationship between grid bias and plate current of the quieting tube; Fig. 6c illustrates the amplification characteristic of the '.uieting system; Figs. 6d and 6e illustrate, respectively, the variations in plate current and in relative ampliiication of the quieting system; and Fig. 7 shows the change of automatic volume control bias voltage with change of intermediate-frequency carrier voltage, in the receiver of Fig. 2.

The general principles of single sideband reception and their application to a superheterodyne radi receiver are discussed at length in my aforesaid copending application, to which reference is made'. In brief, the signal-selecting ciri cuits for transmitting the signal-modulated carrier frequency are designed and adjusted to pass only the carrier, one sideband, and the inner part of the other sideband, so as to include the complete audio-frequency spectrum with each audiofrequency component represented in its proper 40 relative magnitude. In case of a receiver having a tuned radio-frequency amplifier, but not the superheterodyne feature. the general principles of operation are otherwise the same as in a conventional double sideband receiver, provision vbeing made for compensating for the loss of signal intensity by selecting against the outer part of the other sideband. It is there explained, also, that in the case of superheterodyne radio receivers, to which thevpresent invention is illustrated as applied, one sideband of the signalmodulated carrier frequency, when modulated in a frequency-changing circuit by the locally generated oscillations. whose frequency is greater than the received carrier frequency, results in a single sideband of difference frequency or intermediate frequency shifted along the frequency scale but having characteristics in all other respects the same as the original sideband, save only that the high and low sidebands are reversed in position on the frequency scale. The amplification and detection of the intermediate-frequency modulated carrier through the remaining portions of the receiving system are accomplished in the same general manner as in a conventional receiver. However, certain features described briefly above may be utilized to advantage in a single sideb'and system of my invention to improve the selectivity, sensitivity and fidelity `of reproduction of the system.

Circuit arrangement and general operation Fig. l is a generalized circuit diagram illustrating the arrangement of 'a superheterodyne receiver adapted for single sdeband operation in accordance with this invention. The path traversed by signals between the antenna and the loudspeaker is shown conventionally. There are provided an antenna 30 and ground 3i for intercepting the signals, which are then supplied, in order, to a radio-frequency amplifier 32; a local oscillator and modulator 33; an intermediatefrequency amplifier 34; a diode rectifier 35; an

audio-frequency quieting amplifier 36; an audio-frequency amplifier 31; and a loudspeaker 38.

With reference to thisxnain signal path, the radio-frequency signal is received in the usual manner by the antenna 30 and selected and amplifled at the signal frequency in the radio-frequency amplifier 32, which for broadcast reception is capable of tuning over the frequency range of about 550 to 1500 kilocycles. The oscillator and modulator system 33 converts the radio-carrier frequency signal to an intermediate-carrier frequency signal in the manner well understood in the art. The intermediate-carrier frequency willbe one of two frequencies depending upon which of the two sidebandsis being selected. These two alternative intermediate-carrier frequencies will, therefore, differ by the width of a sideband, which may be in the neighborhood of 3 kilocycles. These intermediate-carrier frequencies may conveniently be taken as 173.5 or 176.5 kilocycles, depending upon which sideband is to be utilized. 'Ihe signal is' further amplified in the intermediate-carrier frequency amplifier wherein the carrier and one sideband are selected, the band width passed by this amplifier being, for the particular intermediate-carrier frequencies mentioned above, 173.5 to 176.5 kilocycles, centered at 175 kilocycles, The diode rectifier 3S derives from the carrier and the single intermediate-frequency sideband the audio frequencies of modulation, which are then amplined by the audio amplifiers 3l and 31, from whence they are supplied t the loudspeaker Il.

For the purpose of maintaining the volume output o! the receiver substantially constant in spite of wide variations of received signal intensities, there is provided an automatic volume control system comprising connections 4l from the output terminals of intermediate-frequency amplifier 34 to the following elements connected in succession: an intermediate-frequency trap 4l, an intermediate-frequency amplifier 42, a diode rectifier 43 and a diode suspender 4 4. The functions oi' the intermediate-frequency trap 4| and of the diode suspender '44 are described below. In accordance with the operation of the automatic volume control system, there is created at the diode suspender 44 a unidirectional voltage which varies withY the received signal intensity; and this unidirectional voltage is applied through the automatic volume control 4bias connection 4l to the control elements of

amplifiers

32 and 34 5 and oscillator-modulator 33.

For the purpose of insuring the proper variation of control bias voltage over a wide range of received signal intensities there is provided a connection 46 from the intermediate-frequency i0 amplifier 34 of the main signal path to a control element of the intermediate-frequency amplifier 42 in the automatic volume control system. 'I'his is a reverse control bias connection, the function and operation of which are more fully del scribedv below.

In accordance with a feature of the invention, there is created what is here termed selective admission, which is an operating characteristic whereby the user is constrained to tune the receiver so that the intermediate-carrier frequency is located at either edge of the selected intermediate-frequency band, that is, 173.5 or 176.5 kilocycles, in this case. The feature of selective admission is effected by the action of the intermediate-frequency trap 4i of the automatic volume control system, and is perfected by the cooperation of the quieting system associated with amplifier 36, as will more fully appear below. The trap 4i contains a selective circuit propor- 30 tioned to cause the automatic volume control bias of conductor 45 to become partially relaxed when the receiver is so tuned that the intermediatecarrier frequency is located exactly at the center of the selected intermediate-frequency band, that 3 is, 175 kilocycles in this case. At this tuning point, the intermediate-frequency carrier voltage present at connections 40 is maintained at a substantially higher level than in the case of any other tuning points.

There is associated with the audio-frequency amplifier 36 a quieting system whereby sideband selection and quieting between stations are obtained by the manner in which the amplifier is operated. To secure this operation, the amplifier tube is so adjusted that its gain is a maximum at a critical negative value of grid bias, and rapidly approaches zero for greater or lesser biasing voltages.

The-biasing voltage for the quieting tube is obtained from the diode rectifier and is proportional to the carrier voltage at connections 40. When no signal is present, or when the average rectified signal or noise voltage is less than the critical bias, the gain through the quieting amplifier 36 is substantially zero, and there is practically no transmission through this amplifier. Also. when a strong intermediate-frequency carrier signal is tuned to the center of the band (175 kilocycles in this case), the bias developed by the rectifier 3S becomes considerably greater than the critical value, and the gain of amplifier 3l is again reduced nearly to zero. The critical bias, which produces this operative condition, is obtained only when' the intermediate-frequency carrier is detuned'to either edge of the selected band (173.5 or 176.5 kilocycles in this case). It should be noted that the grid bias is determined by the amplitude of carrier voltage at connections 4U. This amplitude is in turn determined by the response characteristic of the intermediate-frequency discriminating circuit, or trap 4i, in cooperation with the automatic volume control system. The trap should, therefore, be made to have such a characteristic that the critical bias 75 is obtained at either edge of the intermediatebe tuned as accurately fas those of the interfrequency band.

Fig. 2 is a detailed circuit diagram illustrating a superheterodyne receiver adapted for single sideband operation, designed according to the general arrangement of Fig. 1. 'Ihe rectangular boxes of Fig. 1 are indicated in Fig. 2 in dotted lines and are similarly numbered.

In Fig. 2, the radio-frequency amplifier 32 comprises a pentode tube 6| suitably coupled to the antenna 38, and Vhas vassociated therewith three simultaneously tunable selecting circuits, two of which, designated |30 and |3|, are locatedahead of the amplier, and the other of which, |32, is coupled between the amplifier and the tube 62. These circuits are tuned by variable condensers |36, |36 and |31, respectively.y

The oscillator vand modulator arrangement 33 comprises the hexode oscillator-modulator tube 62. Such an oscillator-modulator circuit constitutes no part of this invention and consequently is not described in detail here. This type of i' oscillator-modulator is described in my Patent No. 1,958,027, issued May 8, 19,34.' 'I'his oscillatormodulator comprises a frequency-determining circuit 99 having a variable tuning condenser 2 I6.

The tuning condensers |86, |36, |31 and 2|6 are ganged together by a mechanical.unicontrol deviceindicated by the dashed lines U. I'hese condensers constitute the tuning means for tuning the signals to the desired intermediate-frequency band.

The output of the oscillator-modulator is coupled to the input of the intermediate-frequency amplifier 34 which comprises two amplifying tubes 53 and 64 and the three intermediate-fr@ quency coupling systems |60; |6| and |62 located before, between, and after, the amplifying tubes.

The signals are rectified in the diode portion of a double-diode pentode tube 66' in stage 36. Only one of the diodes is employed for signal detection,

lthis particular diode comprising the cathode |63 and one of the diode anodes |64. Thepentodel elements oi this tube are used in the audio-frequency quieting amplifier 36. For the purpose of illustration, therefore, tube 66' is shown again in the audio amplifier section 36, where it is designated as tube 65. This second illustration of the same physical elements is proper and convenient'because, insofar as circuit'operation is concerned, the action is that of ,two separate and distinct tubes. j

The rectined current flows through a resistor 83, from whence rectied voltage is impresscd'upon the control grid of the pento'de amplifier 66 by way of conductor |66 through resistors 34, 81 vand 66 .and

condensers

86, 89 and 36. The resistor 81 and the condenser 89 are proportioned to proyvide proper audio-frequency compensation, as

will be more fully described hereinafter.

The anode of pentode amplifier 66 is coupled to A .amplified in the push-pull amplifier including tubes 61 and 66, whence they arf` conveyed to the loudspeaker 38. y

It ls of primary importance that the receiving system select aband of Vfrequencies having a width about equal to the highest required 'audio frequency of-modulation. Since the selective circuits of the audio-frequency amplifier 32 cannotY mediate-frequency amplifier 34, it is preferable that they be tuned sufliciently broadly to pass a lband ofthis width witha uniformity within one tem |6| comprising a double-tuned transformer figure the intermediate-frequency spectrum of a signal is represented at 26, being centered at frequency fie of 173.5 kilocycles and having upper and lower intermediate-frequency sidebands with boundaries fie', fin", respectively (corresponding to the lower and upper carrier sidebands, re-

spectively). In this same figure the curve ID represents the .selection characteristics of the intermediate-frequency selecting circuits of -amplifier 34 centered at a frequency fie of 175 kilocycles and fiat within one decibel over a band o! -three kilocyclesiv that is, over the upper intermediate-frequency sideband. There is also shown in thisgure the intermediate-frequency spectrum of two other signals and 2, correspond- 'ing to adjacent broadcast channels, and centered S5 at ,fm-.10 and fie-F10 kilocycles, respectively; that is, separated by 10 kilocycles from the signal 26 being received.

For Athepurpose of this example, it is Vassumed that signalilll is stronger than signal H2; hence,

the receiver isshown to be properly tuned to the sideband which is farthest away from the stronger signal. 'I'he intermediate-carrier frequency of signal and also its sideband nearest'to signal 26, are subject to |6 decibels` more attenuation than the carrier frequency and nearest sideband of the signal ||2. Both of the adjacent channel signals are attenuated much more than theyv would be if the curve |-|0 were doubled in width to include both sidebands;v and it is shown how the receiver may be tuned to discriminate against countered Ain the use oi this receiver for single sideband reception are: (1) to cause the operator to tune the receiver correctly so that the inter- -the stronger of the signals in the two adjacent channels. v'Iwo of the most important problems enmediate-carrier frequency is located' at one edge of the selected frequency band. as shown in Fig. 3; and (2) to compensate for the loss offnearly all of the rejected sideband.

' 'I'he iirst of these problems is solved by a system of selective admission, which greatly lessens or entirely quiets the Vresponse of the receiver when not correctly tuned. This system,

which has been mentioned above and is described below in more detail, is made necessary or at least vdesirable because the sound at the loudspeaker is harsh and unpleasant when the carrier lfrequency is tuned too far oi! pf the edge of the transmitted sideband.

'I'he second of these problems isv solved by the' -use of resistor 31 and condenser 38 associated with the audio-frequency tube 66. In Fig. 4. which is plotted as relative gain or responsiveness against 4 rent upon the supply of intermediate-frequency audio frequency, curve ||3 indicates the equivalent audio-frequency loss resulting from the cutting out of most of one sideband. This loss is shown to be about 3 decibels at 1 kilocycle and 6 decibels at 3 ki1ocyc1es,in the case of the particular receiver under discussion. Accordingly, theelements 81 and 89 are proportioned to produce a relative audio-frequency gain which varies substantially as represented by curve I |4. This latter variation being complementary to that of curve ||3, uniform overall audio-frequency response is produced as represented by curve ||5. In producing this compensation it has been found proper to make the impedances of elements 81 and. 89 of the same order of magnitude at one kilocycle.

Automatic volume control The proper operation of the quieting system requires that the output of the intermediatefrequency amplifier 34 be maintained at a constant predetermined value, substantially independent of both the received signal intensity and the percentage of modulation. The arrangement and operation of the automaticvolume control system is described with reference to Figs. l, 2 and 7. Intermediate-frequency voltage is derived from a coil 1| tuned by a con-l denser 12 and coupled to the primary coil of the intermediate-frequency coupling'system |62. and, by means of connection coupling transformers 14 and 61 of the intermediate-frequency trap 4|, and intermediate-frequency amplifier' tube 58, it is applied between the cathode 6| and diode anodes 60 and |12, constituting the rectifying portion of a double diode pentode tube 59, hereinafterv called the A. V. C. diode since it determines the operation of the automatic volume control. Y

Associated with the diode rectifier in the tube l59 is a voltage divider comprising resistors 92 and 63 serially connected across a battery 68 through the diode 55", constituting cathode |83 and diode anode |15 of the multi-purpose tube l5 referred to above, which is referred to herein asthe suspended diode. The anodes 40 and |12 are connected to the junction 84 of resistors 62 'and 63 which are so proportioned that, in the absence of a. signal. the anodes are maintained substantially at the same voltage with respect to ground as the cathode 8|, which is biased with respect to ground due to the flow of the space current of the pentode portion 0f the tube 59 through the resistor 55. The result is that there is substantially no flow of current in the diode section of the tube 59, but it is in an incipient conductive condition and carries curvoltage to the anodes 90 and |12. Under these conditions points 39 and |01, which are connected to the grid of the pentode 59 and to the anode |15 of the diode 55", respectively,- are eii'ectively grounded through the diode 55." so

that no automatic volume control bias voltageis supplied to the conductor 45.

64. A further increase in signal intensity increases the negative voltage at these points, which is applied through the conductors and 45 to the control grids of the intermediate-frequency amplifying tubes 53 and 54, the radiofrequency ampliiier tube 5|, and the oscillatormodulator tube 52, respectively.

The control grid |13 of the tube 59 is also effective under these conditions to reduce the space current in this tube, and thus to reduce the voltage across the resistor 65. In this manner, the change of negative voltage on the cathode 5|, relative to ground, augments that of the anodes 60 and |12, with respect to the cathode 6|, thereby increasing the resultant negative voltage existing at the point 64, and therefore at the points 39 and |01.`

The operating characteristics of a typical automatic volume control circuit operating on these principles are shown in Fig. '1 of the drawings, in which the line |23 represents the voltage of point 64 with respect to ground, which voltage changes negatively in proportion to the signal intensity; In this figure the dimension |24 indicates the voltage across resistor 62 and the dimension |25 that across resistor 63. When the signal intensity increases to the point where the voltage of the junction point 64 becomes negative, .the current in the4 diode 55" is reduced to zero and the average voltage applied to the automatic volume control bias conductors 45 and 45' from the points 39 and |01 becomes equal to that at the point 64.

Asstated above, a decrease in the voltage of the points 39 and |01 decreases, by virtue of the connection of the grid |13 thereto, the space current in the tube 59, and thus effectively increases the change of negative voltage of the diode anodes 60 and |12, to eilect a resultant increase in the negative voltage of the point 64. This is represented by the increased slope 0f the curve |23 at its intersection with the zero axis. I f the effect of the pentode 59 were omitted, the curve |23 would continue at its original slope, asr shown by the dotted line |21. In this diagram4 the ordinate |26 represents the voltage of the point |01 relative to ground.

The action of such automatic volume control biasing connections for controlling the gain of amplifiers andmodulators is well understood in the-art and requires no detailed discussion here. It is sumcient to state that the connections- 45 and 45 cause the control grids of thelcontrolled tubes to become more negative when the signal strength increases above a predetermined value, thereby maintaining the intermediate-frequency output level fairly uniform. A

The desired highly uniform output of intermediate-frequency ampliiier 34 is accomplished by means of a reverse automatic volume control bias voltage provided by a connection 44 extending from the cathode of amplifier tube I4 to the cathode of amplifier tube 58 of the automatic volume control system. This c ommon cathode connection 46 is connected to ground through a resistor 68 which carries the space current of both tubes; hence, the space current of tube 54 partially controls the grid-cathode bias of tube 58 and, therefore, the gain of this latter tube. 'I'his bias on tube 58, however, varies oppositely fromthat on the controlled amplifier and modulator tubes of the main signal path. When the grid-cathode, bias of tube 54 becomes greater by automatic volume control action, its ampliilcation and space current decrease, and consequentsomewhat.

causing the gain of the latter tube to increase Therefore, the increased intermediate-frequency voltage, which is required by the A. V. C. diode in tube 59 to decrease the amplification of amplifier 34, is supplied by the f increased gain of tube and without any increase of the input to the latter tube, cr of the output of amplifier 34. This action is called reverse automatic volume control because the gain of tube 53 is automatically varied oppositely to that of

tubes

5I, 52, 53 and 54. Quantitatively, the gain of tube 58 can be just about doubled by this reverse automatic volume control connection, this being suflicient to vary the intermediate-frequency voltage on the A. V. C. diode of tube 59 over the desired wide range without any appreciable variation of the intermediate-frequency input to tube 58.

The reverse and the suspended .automatic volume control actions cooperate to maintain the output of intermediate-frequency amplifier 34 very nearly uniform during reception of large variations of signal intensity.

Selective admission 4The selective admission, mentioned above, is provided to cause the oscillator correctly `to tune the receiver for single sideband reception. This includes means for causing the audio-frequency output to reach sharply a maximum value when the intermediate-carrier frequency is tuned on either edge of the band selected in the intermediate-frequency amplifier 3 4.4 This action re case), so that this center frequency is sharply attenuated. Condenser l0 is adjustable to en- .able the trap to be readily tuned to the center frequency. Coil "il is smaller than the primary coil of coupling system 682 so as to produce a voltage step-down from the output of tube 54 to the. trap circuit. Transformer 14, which couples the `trap to the input of tube 58 is 'a step-up transformer in which the secondary coil fit is iarger than the primary coil 4S. A condenser '3% is inserted in series with the primary coil di@ of transformer i4 and serves to tune the said primary coil broadly to the center of the selected band (i75 kilocycles) The resistor 'il has the proper value to broaden the resonance ci elements iii and l5; and the resistor 76 serves to limit the current which enters the trap vfrom coil il. The ratios of the transformers at the input and output of the trap are chosen to permit coil t8 to be of the same order of magnitude as the secondary coil 88 of transformer i6, in spite oi the fact that transformers y'i3 and lli work out of, and into, respectively, circuits oi high impedance.

`'Zihe eifect of the automatic volume control system including intermediate-frequency trap 4l upon the rectified output of the signal rectifier at tube is shown in Fig. 6a, in which curve il@ represents the rectified signal voltage secured f when a signal at the input to section 34 Ais tuned within the band uniformly passed by the intermediate-frequency selectors of section 34.- The sharp peak of curve H8 is caused by the action of the trap circuit in relaxing the A. V. C. voltage, hence increasing the amplification in section 34,

vwhen the carrier frequency is tuned to the trap frequency, 175 kilocycles. The resulting signal level at the input of section 36 reaches a sharp maximum as shown by curve H8 when the car'- 'I'he effect of the l rier is tuned to 175 kilocycles. trap characteristic H8, in enabling the receiver to be properly tuned, will vbe fully brought out in the following discussion of the quieting sys` tem.

Automatic queting The automatic quieting action is the means for quieting the receiver at all times except when the receiver is properly tuned at either edge of the intermediatefrequency band. This system quiets undesired noises and distorted signals which would otherwise be reproduced when the receiver is not properly tuned to a useful signal. The quieting system cooperates with the trap 4i and the automatic volume 'control system to produce the selective.admission.

The quieting actiontis obtained by reason of the peculiar transmission, or transfer ratio, characteristic which is imparted to the pentode translating tube 55. To produce the single sideband type of reception. the switches i9 and 8l, ganged for unitary operation, are thrown to the xight so that switch 80 connects a resistor al in series between the screen 292 and the positive terminal of the direct voltage source 203. Resistor 8l has a relatively low value so that the voltage at screen 2oz is substantially independent of screen current. A relatively high resistor lo is con-y nected in series between the anode lll and the positive side of direct voltage source 205, the latter source being in series with source 203.

The relationship between the grid bias and the anode current of this pentode as thus connected, is shown by curve IIB of Fig. 6b,- wherein grid bias voltage is plotted against plate current in milliamperes. nearly to' zero by ananode current flow-of about l milliampere through resistor 1l, the remaining space current of the tube flowing to the screen 2&2. llt is observed from curve H6 that when the negative vgrid bias reduces to about 9 volts, the plate current rises to about l millampere. Hence, for values of negative grid bias less than 9 volts,

the plate voltage is substantially zero. For

values of negative grid bias greater than 9 volts',

The anode, voltage isreduced the total space current is reduced and therefore the anode current falls rapidly, approaching zero beyond l5 volts bias.

The gain, or amplification, ratio of quieting tube 55 depends on the slope of curve H6 and is, therefore, a maximum in the region between the limiting values of 10 and l2 volts negative bias. The slope in this region represents an amplification of about nity times, for the particular tube arrangement used. The relative amplification, or repeating ratio, of the pentode bias controlled repeater tube 55is represented by curve lll of Fig. 6c, in which percentage amplification is plotted against negativey grid bias. For the parseen to be a critical value of grid bias, since only those bias voltages which are at and near this tisular tubearrangement described, 11 volts is Il. Condenser 02 is so small as to have a negligible effect at audio frequencies and serves with resistor I4 and condenser 85 to remove the carrier-frequency components from the rectified voltage. The lower audio-frequency modulation components of the Vrectified voltage follow the path of

elements

86, 81 and 88 to the control grid |15 of pentode 55. The impedance ofthe input circuit of tube 55 to higher audio frequencies is decreased by condenser I8 and a relatively low resistor 90, connected across the high resistor 01. This lower impedance path 89, 90 compensates for the selection of the lower audio frequencies of both sidebands. Grid bias voltage proportional to the average rectied signal voltage is applied to control grid |16 through a path including the high resistors 8G, 8| and 92. Condenser 93 has a sufilciently large capacity to bypass all modulation components at this biasing connection.

The entire direct voltage appearing across resistor 83 is applied to grid |16 asla quieting bias. But only a small portion (about 1A3) of the audiofrequency component of rectified voltage is applied to grid |16 as a signal. This limits the grid swing to a value which will not cause the peaks oi the waves to lie beyond the straight portion of curve H6, thereby avoiding distortion.

The bias voltage derived from diode 55' varies in accordance with curve H0 of Fig. 6a. It is observed from curve H8 that the trap 0| which produces this characteristic is so proportioned as to cause the critical bias voltage (ll volts in'Fig. 6b) to be obtained when the carrier is tuned to either edge of the selected intermediate-frequency band (1.5 kilocycles ony either side of the center in this case). Because of this relationship between curves ||6 and H8, pentode tube 55 is operative to pass signals only when the intermediste-carrier frequency is properly tuned for single sideband reception at either-edge of the selected band. i

Visual tuning indicator There is connected in series Awith a resistor 18 in the anode circuit of tube 55 a visual tuning indicator in the form of a milliameter 98. When the receiver is in operation, this milliameter has a maximum deflection of l milliampere and a minimum deilection of zero, as can be observed from Fig. 6b. The milliameter indicates the proper tuning position for single sideband operation when th'e meter reads half deection at 0.5 milliampere. Fig. 6b shows this to be the anode current which flows when the critical grid bias voltage of 11 volts is reached. Curve lli oi' Fig. 6d and curve |20 of Fig. 6e show, respectively, the variations" in anode current and in relative amplification in tube 55 when the intermediatecarrier frequency is tuned in therreglon of 175 kilocycles. The two sharp peaks of curve |20 show the audibility points of correct tuning of the intermediate-carrier frequency at 1.5 kilocycles above or below the mid-point frequency oi. 175 kilocycles, and correspond to mid-scale deilection of the tuning,meter.

Double sideband reception To cause the receiver to operate as a double sideband receiver, switches 10 and 80 are thrown to the left position. Switch then causes resistor 8| to become open-circuated so that the screen voltage is furnished from the positive side of voltage source 205 through a relatively high resistor 05. Switch l0. also then closes a connection, placing in the grid bias circuit a resistory 91 which reduces the bias voltage to a lower value than that used for the single sideband adjustment. A bias voltagewhich produces a 0.5 milliampere anode current when a signal is tuned to 175 kilocycles has been found correct. The average screen voltage and screen current are, under this condition, held nearly constant for small values of grid bias. This results in an amplifier plate current-grid bias curve of normal shape, in which the slope is always positive.

The switch 19, in the left position, now shortcircuits resistor 95 which reduces the audiofrequency voltage applied to the control grid |16. At the same time, this provides a lower impedance path to ground for the higher audio frequencies through resistor 94. The increased amplification of the high audio frequencies (under single sideband reception) is thus removed, since high audio-frequency compensation is not required under double sideband reception; Resistors @t and 95 are proportioned to cause the audiofrequency voltage at the anode |-11 to be the same for both single and double sideband reception.

Under this condition of double sideband reception, the receiver is tuned to a minimum deilection on the tuning meter which will be at about mid-scale. This corresponds to tuning so that the intermediate-carrier frequency is exactly at the center of the intermediate-frequency selected band kilocycles) and is indicated by the loudest signal.

In the single sideband condition of operation, all signals of intensity greater than 'a threshold value .are received at two points on the tuning dial corresponding to one or the other of the two sidebands. Signals of greater or lower intensity than the threshold value can be received by throwing switches 19 and 80 to the double sideband position. The signal is then received at only one point corresponding to center-tuning and highly selective double sideband reception. The higher audio-frequency response, and consequently the noise which attends signals of low intensity, are thus, reduced by this highly selective double sideband reception.

It is a feature of the present invention that with switches 19, 80 in the position for single aideband reception, signals of lower than threshold intensity are received automatically at only one point corresponding to 'center tuning and double sideband reception.

The transition from the single sideband to the double sideband operation takes place automatically when the signal intensity drops only a certain amount below the threshold value. The reason for this transition can be observed from an inspection of curve |08 of Fig. 6a. This curve represents rectified voltage applied as a. bias to the grid of the quieting tube when the signal tuned in is just strong enough so that the peel: reaches the critical value of 1l volts. Under this condition. the signal is tuned in at the point on the dial corresponding to 175 kilocycles.

oscillator-modulator Briefly, the oscillatory system is associated with the

inner electrodes

201, 209 and 209. A

feedback coil 213 is connected in series withv a.

choke coil 211v and a direct voltage source 219, between the inner screen, or oscillator anode 209 and ground. Coil 219 is coupled to a coil 214 included in the oscillatory system. The oscillator frequency-determining circuit 99 is composed of coil 214, a variable condenser 215 and a fixed (though adjustable) condenser 216. .A coupling condenser 219 is connected from the junction of elements 213 and 211 to the junction of elements 214 and 216. Hence, there is capacitive as well as magnetic feedback coupling from electrode'.

209 to the oscillator circuit 99. This arrangement vof the oscillatory system lprovides a uniform oscillator voltage over the entire frequency range of the oscillator.

` audiov range.

The oscillatory circuit 99 is also coupled both capacitively and magnetically to the circuit of grid 208. The magnetic coupling exists between coil 214 and a coil 220 of the grid-cathode circuit; and the capacitive coupling is due to a deadend'turn 221- connected with coil 220 and elec-v trostatically associated'with coil 214. By reason of this arrangement, any capacity change in the circuit of grid 2&9 produces a frequency change which is very small and substantially the same for all oscillator frequencies,

The coupling elements 213, 219, 220 and 221, when adjusted to the proper values, remain permanently xecl at such desired values.

Volume level'control and ione compensation The volume level control and tone compensator comprises the adjustable voltage divider 100 at the input of audio amplifier tube 56. The voltage divider is designed to provide an approximately exponential relationship between the decibels attenuation and 4angle of rotation of the contact control knob (represented by the arrow).

Experience teaches that the normal ear desires less change of intensity at the higher and lower audio-frequencies than at the mediu'n 'audio frequencies. The volume level control is designed to satisfy this desire of the ear. At the greatest volume, the contact of potentiometer 100 islocated at the upper end thereof. In this position, the elements associated with the potentivolume level, the fidelity curve is substantially dat as shown by curve 109. In this potentiometer arrangement, elements 101 and 102 are small equal condenserswhich reduce the attenuation changes at the higher audio frequencies; and elements 104 and 105 are large equal condensers which reduce the attenuation changes at the lower audio frequencies.

It should be understood that the foregoing description applies to a preierredembodiment of '.the inventions and should not be construed as a.

limitation thereof. For example, values other than those used herein, of voltage, grid bias, frequency and band width, may be employed. Likewise, vacuum tubes of equivalent performance but having more or less electrodes per tube may be substituted. Although, in the drawings, the direct voltage sources are, for convenience, shown symbolically -in the form of separate batteries, it should be understood -that a common source of power supply of any of the conventional types may be used rather than separate sources in the several locations where batteries are represented.

What is claimed is: l. In a modulated-carrier-signal receiver, a rectier circuit for rectifying the signal to produce a, modulated unidirectional voltage, a biascontrolled modulation voltage repeater stage having input and output circuits, means for applying at least a part of said unidirectional voltage to said input circuit as bias and input modulation voltage, and means for causing the transfer ratio of said stage to be maximum for a predetermined value of bias voltage and to approach closely zero for all substantially different values, whereby the output modulation voltage is caused to be substantial only when the signal causes said bias voltage to be substantially equal to saidpre- Q i) determined value.

2. In a modulated-carrier-signal receiver, a' rectiner circuit for rectifying the signal to produce a modulated unidirectional voltage, a modulation-voltage repeater stage including a pentode -o repeater having a cathode, control grid, screen,

ysuppressor and anode situated in the order named, 'means for impressing said unidirectional voltage on said grid .as the modulation-voltage input and as a bias voltage negative relative to said cathode, means for impressing on said screen a substantially constant direct voltage positive relative to said cathode, means for maintaining said suppressor at subtantially zero voltage relative to said cathode, means for impressing on said anode ay direct Avoltage from' a source positive relative to said cathode, and aresistor between vsaid anode and said source for reducing the `anode direct voltage to approximately halt the source -voltage for a substantial negative value of said bias voltage, whereby the modulation-voltage output oi' said repeater stage is caused to be sharply maximum for the value of modulationvoltage input corresponding to said value oi' bias voltage.

3.*In a modulated-carrier-signal receiver, a-

rectifier circuit for rectifying the signal to pro- Y duce a modulated'unidirectional voltagefa modulation-voltage repeater stage including a pentode repeater having a cathode, control grid, screen, suppressor and 4anode situated in the order named,.means for impressing said unidirectional voltage on said grid as the modulation-voltage input and as a 'bias voltage negative relative to said cathode, means for impressing on said screen a substantially constant direct voltage positive relative to said cathode, means for maintaining said suppressor at substantially zero voltage relative to said cathode, means for impressing on said anode a direct voltage from a source positive relative to said cathode, and a resistor between said anode and said source for reducing the anode direct voltage to a value not exceeding half the source voltage for a substantial negative value of said bias voltage, whereby the amplification in said repeater is caused to increase substantially with increasing input amplitude for input amplitudes corresponding to voltages less than said substantial value.

4. In a modulated-carrier-signal receiver including a carrier-frequency ampliler, a rectien TID and a modulation-frequency amplier having cathode, control grid, screen and anode, means for automatically regulating the output voltage of said carrier-frequency amplier, means for coupling said output voltage to said rectifier for rectification, means for coupling to the gridcathode section of said modulation-frequency amplifier the resulting modulated unidirectional voltage, means forimpressing on said screen a substantially constant direct voltage positive relative to cathode, and means for impressing on said anode through a. high resistor a direct voltage positive relative to cathode, saidresistor and both said direct voltages being proportioned to cause the modulation-frequency amplification to .be sharply maximum at a predetermined value of grid-bias voltage substantially negative relative to cathode, whereby a predetermined value of carrier-frequency output voltage is caused to produce said grid-bias voltage and thereby to produce maximum modulation-frequency voltage across said resistor.

5. In a modulated-carrier-signal receiver, a carrier-frequency amplifier including .a band selector, a control circuit including a control rectifier and a,` trap circuit sharply tuned to the center frequency of said band for coupling the output of said amplifier to said control rectifier, means for reducing the amplification in said amplifler with increasing rectified voltage from said control rectifier, a main rectifier for rectifying the'output of said amplifier to produce a modulated unidirectional voltage, a modulation-frequency amplifier for amplifying the modulationfrequency component of said unidirectional voltage, means for impressing on said modulationfrequency amplifier a bias voltage which varies in accordance with said unidirectional fvoltage, said modulation-frequency amplifier having an amplification ratio which decreases abruptly as said bias voltage departs either way from a predetermined value, said control circuit being proo portioned to provide a rectified voltage of an amplitude which maintains the carrier output of said amplifier at a value which causes said bias voltage to assume said predetermined value when the carrier frequency equals either of the two frequenciesI differing from said center frequency by a predetermined difference not exceeding half the band width of said band selector, and at a substantially greater value when the carrier frequency equals said center frequency.

6. In a modulated-carrier-signal receiver, a band selector, a carrier-frequency amplifier, means for causing the output of said receiver to be maximum only at a predetermined criticalV value of amplified carrier voltage, and auxiliary means for automatically maintaining the amplitude of said amplified carrier voltage nearly independent of the received signal intensity, said auxiliary means including a sharply selective circuit tuned to the center frequency of said band for causing said amplified carrier voltage'to exceed substantially said'critical value only whenthe carrier is tuned substantially to said center frequency, and said auxiliary means being proportioned to maintain said amplified carrier voltage at said critical value, and thereby to cause the output of said receiver to be maximum, only when the carrier frequency equals either of the two frequencies differing from said center frequency by a predetermined difference not exceeding half the band width of said band selector.

7. In a modulated-carrier-signai receiver according to claim 6. adjustable means for enabling said sharply selective circuit to be tuned to said center frequency in the band of said selector, whereby thetwo frequencies of maximum output may be adjusted relative to said center frequency without altering the difference between said two 5 frequencies, which differ from said center frequency by a predetermined dierence not exceeding half the band width of said band selector.

8. In a modulated-carrier-signal,- receiver, a band selector for selecting a frequency band equal in width to one of the modulation sidebands of a modulated-carrier signal, tunable means for adjusting the carrier to any frequency within said band, means'for greatly attenuating the output of said system except when the carrier is adjusted to eithex edge of said band, and a switch for disabling said output-attenuating means to permit the operation of said system when the carrier is adjusted to the center of said band.

9. In a. modulated-carrier-signal receiver, a 20 band selector for selecting a frequency band equal in width to one of the modulation sidebands of a modulated-carrier signal. tunable means for adjusting the carrier frequency to any frequency within said band, selective automatic control 25 means for causing input signals of an intensity greater than a predetermined threshold value to produce maximum output when the carrier frequency is adjusted to either of the edges of said band, said control means being proportioned 30 automatically to cause signals of an intensity lesa than said threshold value to produce maximum output when the carrier frequency is adjusted to the' center of said band.

lo. In'amcdulated-carrer-signal receiver, a 35 band selector for selecting a frequency band equal in width to one of the modulation sidebands of a modulated-carriersignal, tunable means for adjusting`the carrier frequency tovany frequency within said band, selective automatic control means for causing signals of an intensity greater' than a predetermined threshold value to produce maximum output from said receiver when the carrier frequency is adjusted to either edge of said band, said control means comprising auxiliary means for maintaining the output voltage of seid band selector substantially independent of the intensity of said signals above said threshold value, a rectifier producing a unidirectional voltage dependenton said output voltage, means for impressing the modulation components of said unidirectional voltage on the control grid of an amplifier tube adjusted to amplify only at or near a critical value of grid bias, a connection from said rectifier to said grid for controlling said 55 grid according to the average value of said unidirectional voltage, a trap circuit associated with said auxiliary means for causing said output voltage to reach a sharp maximum when said carrier` frequency ls adjusted to the center of said band, whereby said critical value of grid bias is substantially exceeded when said carrier frequency is so adjusted. and said critical value is obtained only when the carrier frequency is adjusted to either of the edges of said band.

1l. In a modulated-carrier-signal receiver for receiving either one of the modulation sidebands of a modulated-carrier signal, a band selector for selecting a frequency band equal in width to one of said sidebands, tunable means for adjusting the carrier frequencyto any frequency within said band, a tuning meter for indicating the space current of a vacuum tube in'said system, means for limiting said space current to a maximum meter deflection in the absence of a 75 signal, selective automatic control meansl for reducing said space current to half maximum deflection when said carrier frequency is adjusted to either of the edges of said band, and to a very small deflection when said carrier frequency is adjusted to the center of said band.`

l2: In a wave signaling system designed lto transmit a band of frequencies comprising a carrier frequency and at least one modulation sideband, a signal-translating .stage comprising means for producing a unidirectional voltage which varies with variations in frequency,v with-` in said band, of signal input to said stage, a bias-controlled repeater having a repeating ratio which decreases abruptly as the bias departs either way 'from a predetermined value, and

means for applying said unidirectional voltage to said repeater as a bias voltage whereby the amplitude of the signal output from said repeater is caused to be sharply maximum for a critical value of frequency,`within said band, of signal input. v

13. In a wave signaling system designed to transmit a band of frequencies comprising a carrier frequency and at least one modulation sideband, a signal-translating stage Acomprising means for producing a unidirectional voltagea selective circuit excited from'the signal input of said stage and eective to vary the unidirectional A 14. In a wave signaling vsystem designed tor transmit a band of frequencies comprising a carrier frequency and at least one modulation sideband, a signal-translating stage4 comprising a rectifier for producing a unidirectional voltage proportional to the input carrier amplitude of said stage, a selective circuit for varying the carrier amplitude ofthe input to said stage between predetermined minimum and maximum values with variations in frequency, within said band, of signal input to said stage, a bias-controlled repeater having a repeating ratio which decreases abruptly as the bias departs either way from a predetermined value, and means for applying said unidirectional voltage to said repeater as a bias voltage,v whereby the amplitude of the signal output` from said repeater is caused to be sharply maximum for a critical value of 'frequency withinv said band -of the signal input to said stage.

15. In a wave signaling system designed to transmit va band of frequencies comprising a carrier frequency and at least one modulation sideband, a signal-translating f stage, control means for automatically maintaining the carrier amplitude of the input to said lstage within a relatively narrow range for a wide range of inp ut carrier amplitudes to said system, said stage comprising' a rectifier for producing a unidirectional voltage proportional to the input carrier amplitude of said stage, a selective circuit for modifyingv the action of said control means t0 vary the carrier amplitude of the' input to said stage between predetermlnedminimum and maximum values with -variations in frequency, within said band, of signal input to said stage, a biascontrolled repeater having a repeating ratio which decreases abruptly as the bias departs either way from a' predetermined value, and means for applying said unidirectional voltage to said repeater as a bias voltage, whereby the amplitude of the signal .output from said repeater is caused to be sharply maximum for a critical value offrequency within said band of the signal network substantially independent of the carrier 3 amplitude of the input thereto for 'a wide range of input carrier amplitudes on carrier frequencies sov within said band, a rectifier coupled to the output of said network for producing a unidirectional voltage proportional to the carrier amplitude thereof,\a selective circuit for causing the out- -put carrier amplitude and therefore said unidirectional voltage .to vary between predetermined minimum and maximum values as the frequency' changer is tuned to change signal carrier frequency to different frequencies within said band, and a bias-controlled signal repeater coupled to the out-.-4

put of said network and having applied thereto said unidirectional voltage as a bias voltage, the

circuit of said repeater beingproportioned to se-A cure maximum repeating ratio for an intermediate value of said unidirectional voltage and greatly reduced repeating ratio for said minimumfand'