US2122775A

US2122775A - Radio receiving circuits

Info

Publication number: US2122775A
Application number: US745880A
Authority: US
Inventors: Kenneth W Jarvis
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1934-09-28
Filing date: 1934-09-28
Publication date: 1938-07-05
Anticipated expiration: 1955-07-05

Description

July 5; 1938.

K. W. JARVIS RADIO RECEIVING CIRCUITS 3 Sheets-Sheet l Filed Sept. 28, 1934 3 Sheets-Sheet 2 K. W. JARVIS RADIO RECEIVING CIRCUITS ETN AMS Filed Sept. 28, 1954 H. 1 n @x MJ m |l j ,WQ www o W W O m max s bm v m Q m am NK It 8 Nxx I ll.. mkl' ww T.. A6 bn N .MN W mx July 5,4-1938.

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RADIO RECEIVING CIRCUITS Filed Sept. 28, 1954 5 Sheets-Sheet 3 mm N o WJ.

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5 @sA um a m M z W. im d og 2 H 4 m 5 7M H In. W w Il. H A 87 L+ v H .W5 L E wm@ i I d|=l w Patented July 5, 1938 UNITED STATES PATENT OFFICE RADIO RECEIVING CIRCUITS Kenneth W. Jarvis, Chicago, Ill., assignor to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application September 28, 1934, Serial No. 745,880

Claims.

My invention relates in general to radio circuits and more particularly to radio receiving circuits.

An object of my invention is to improve the A5 quality of radio reception.

t15 It is also an object of my invention to provide for varying the coupling between a plurality of resonant circuits and for modifying the resonant circuits to maintain the over all mean resonant frequency at substantially the same value,

.20 as the coupling is varied.

Another object of my invention is the provision of a plurality of resonant circuits coupled together andarranged to have a certain over all mean resonant frequency and a certain over all response with respect to frequency, taken in combination with controllable means for modifying the resonant circuits to give a different over all response with respect to frequency, while maintaining substantially the same over all mean I resonant frequency.

A still further object of my invention is the provision of a plurality of resonant circuits coupled together and arranged to receive a band of incoming frequencies having a mid-frequency value, said circuits being also arranged to have a certain over all mean resonant frequency of substantially the same value as the mid-frequency value and a certain over all response with respect to incomingfrequencies, taken in combination with controllable means for modifying the resonant circuits to give a different over all response with respect to incoming frequencies While maintaining the over all mean resonant frequency at substantially the same value as the mid-frequency value.

55 lnation with controllable means by varying the coupling and for modifying the resonant circuits to give a different over all response with respect to incoming frequencies while maintaining the over all mean resonant frequency at substantially the same value as the mid-frequency 5 value.

Another object of my invention is the provision of variable coupling for resonant circuits, which coupling modifies the resonant circuits, as the coupling value is varied. 10

A still further object of my invention is to improve the response with respect to frequency of an intermediate frequency amplifier of a radio receiving circuit.

A further object of my invention is to provide 15 for receiving the modulation side band frequencies at substantially equal amplitude.

Other objects, and a fuller understanding of my invention may be had by referring to the following description, taken in conjunction with 20 the accompanying drawings in which;

Figure 1 is a diagrammatic illustration of the circuit connections, embodying the features of my invention.

Figure 2 is an illustration of a critical re- 25 sponse curve with respect to frequency, and illustrates the general form of the response with respect to frequency, as given by my invention, when operated in its sharp" tuning position.

Figure 3 is a broad response curve with 30 v respect to frequency, and illustrates the ideal or theoretical type.

Figure 4 is a broad response curve with respect to frequency, and illustrates in general the shape of a response curve with respect to 35 frequency, as given by my invention and being an approximation to the ideal response curve as shown in Figure 3.

Figure 5 is a broad response curve with respect to frequency, and illustrates in general the 40 shape of a response curve with respect to frequency as given by the preferred form of my invention and being an approximation to the ideal response curve as shown in Figure 3.

Figure 6 illustrates the variations in the value 45 of the two resonant frequencies, and also the shifting of the mean resonant frequency from the mid-frequency value as the coefficient of coupling is varied between the resonant circuits.

Figure 7 illustrates an intermediate frequency amplier of a radio receiving circuit, embodying features of my invention.

Figure 8 illustrates a fragmentary and a modied view of the switches that are adapted to control the circuits of Figure 7. H545 Figure 9 a modified form of my invention, wherein a tube is employed as a coupling between two resonant circuits.

Figure 10 is a further modified View of my invention, wherein a tube is utilized as a coupling between two resonant circuits, and

Figure l1 is an illustration of a coupling device between two resonant circuits, which coupling device modies the condition of the two resonant circuits as the coupling is varied.

In the present state of the art, a radio trans mitter produces, when no modulation is taking place, an undamped current of a constant ainpli tude and of a single frequency, except for any harmonics that might be present. This undamped current is normally referred to as the carrier current. However, when modulation takes place there are two groups of modulation frequencies extending one on either side of the carrier frequency. These two groups of frequencies, which have a mean frequency value equal to the frequency of the carrier current, are generally referred to as side band frequencies. The group with a frequency higher than the carrier frequency is called the upper side band and the group with a frequency iower than the carrier frequency is called the lower side band. It is to be noted that the side bands extending on either side of the carrier frequency have a number of cycles equal to that of the microphone modulating circuit.

Inasmuch as the current in the receiving sys tem is to be a reproduction of that in the transrnitting antenna, it follows that the receiving system circuits must not be sharply tuned to any one frequency, to the partial or entire exclusion of other frequencies, but must be so designed as to be able to pick up all of the various side band frequencies with a uniform response, and to exclude frequencies differing from the carrier frequency by more than the frequency of modulation. This means that if the difference between the maximumand minimum frequencies, expressed as a percentage of the carrier wave frequency, is very large the tuning of the receiving circuit must be broad in order for it to have a substantially uniform response throughout a wide range of side band frequencies. In Figure 2 of the drawings, I illustrate gen erally, by the full line 3l, the shape of a response curve when the receiving circuits are sharply tuned. The shape of the frequency response curve 3|, it will be seen, is such that a comparatively short range of the lower modulation side band frequencies on either side of the carrier frequency pass through the tuned circuit with uniform response. The higher modulation side band frequencies will be attenuated by reason of the sloping sides of the response curve, so that these frequencies are not passed through with uniform response. It will be clear therefore that good audio quality reproduction will not result with a tunable circuit or circuits having a peaked frequency response curve such as 3|.

In Figure 3 I illustrate an ideal response curve. It is manifest, in this connection, that the response is uniformly received throughout the entire range of the lower and the upper side band frequencies. The width of the ideal response curve in Figure 3 may be of any desired value. In the present state of the art, a satisfactory value may be about 15 kilocycies. This makes the width of each of the side band frequencies 'l1/2 kilocycles, and thus gives a very excellent quality of reproduction. Therefore, in Figures 2 and 3 of the drawings, the outer ranges of frequencies which fall outside of the desirable frequency range are indicated as being the undesirable frequencyl ranges.

When listening to close or strong transmitting stations it is preferable to have the radio receiving circuits broadly tuned, so as to give good reception, but when listening to distant or weak stations, it is often desirable to have the receiving circuits sharply tuned, in order to be able to eliminate interference andto hear the station, even though the reception is not so good as it is with broad tuning. Therefore, in the practice of my invention, I provide a receiving circuit having two or more degrees of selectivity, so that the user may selectively operate the receiving circuit to accommodate best his requirements. Such a receiving circuit comprising the usual inductance and capacity is diagrammatically illustrated. in Figure 1 of the drawings. In this arrangement, the resonant circuit No. 1 is impressed with and adapted to receive the incoming upper and lower modulation side band frequencies of a transmitted signal. As the incoming current passes through the resonant circuits No. 1, No. 2, No. 3 and No. there is produced a certain over all response with respect tov frequency, which is transmitted upon leaving the resonant circuit No. 4, to the audio system and then translated into sound. The couplings No. 1, No. 2 and No. 3 may comprise mutual capacity or inductance, or external capacity or inductance, or their combination, or any other circuit element or elements, or relay devices, such as tubes to give the desired effect. Each of the modifiers may comprise capacity or inductance, or any other circuit element or elements connected in any desirable circuit relation with the elements of the resonant circuits.

As illustrated, the coupling No. 1 and. the modii fiers No. 1 and No. 2 may be simultaneously operated as indicated by the dash line 20, and similarly the coupling No. 3 and the modifiers No. 3 and No. 4 may be simultaneously operated as indicated by the dash line 2 I cated by the dash lines 22, 23 and 24 and by the interconnecting arrows 25 and 2B, the coupiing No. and the modiers No 1 and No. 2 may be operated either independently of, or simultaneously with, the coupling No. 3 and the modifierslv No.3 and No.4.

For the purpose of discussion, let it be assumed rst that the coefficient of coupling of the coupling No. 1 and the coupling No. 3, is relatively low, that is, giving what is commonly called loose coupling. Under this condition, as is well known in the art, the over all response for the As further indic resonant circuits No. 1, No. 2, No. 3 and No. 4

is relatively sharp, having a shape similar tothe full line response curve 3l shown in Figure 2.;

Under the condition of reiatively loose coupling, the over aii mean resonant frequency, which is the correct tuning position, is substan tially the same as the mid-frequency value of the modulation side band frequencies of the in;

circuits No, 3 and No. 4 is materially increased, ""75 yin the received signal.

thus giving what is usually termed close coupling. Under this assumed condition, as is well known in .the art, there are produced two resonant peaks, such as is shown by the dash line 33 in Figure 2, instead of the single resonant peak, as shown by the full line 3l in Figure 2. The amplitude of these two resonant peaks and their respective frequencies, and the dip between them, all depends on the various circuit constants. In practice, the various circuit constants are so proportioned that the width of the broad response is suflciently wide to include all of the incoming modulation side band frequencies. Accordingly this method provides a means of obtaining a more uniform response over the desired frequency band than the sharply tuned circuit. However, the over coupling method has at least one major disadvantage, in that, as the coefficient of coupling is increased, the over all mean resonant frequency of the resonant circuits, which is the correct tuning position, shifts from the mid-frequency value of the modulation side band frequencies of the incoming signal. This is shown by the curves of Figure 6. The curves indicated by the reference characters f1 and f2y show where the resonant frequencies occur; that is, where the peaks of the coupled resonant circuits appear.

Asillustrated, when the value of the coefficient of coupling is increased to unity the curve f1 approaches infinity, while the curve f2 Vdecreased to a yvalue somewhat less than its original frequency.

The net result of this over coupling is to make one resonant peak shift very rapidly from the mid-frequency value, while the other resonant peak shifts very slowly in the opposite direction from the mid-frequency value. In other words, the over all mean resonant frequency, which for a particular coeiiicient of coupling is the arithmetical mean or average frequency between the resonant peak frequencies and is indicated by the dash line 32 in Figure 6, deviates from the midfrequency value. As is manifest, when the unequal shift in the resonant peaks occurs by reason of theV over coupling, theY individual modulation side band frequencies are not received with equal amplitude, nor is the prase shift of the equal plus and minus modulation side bands frequencies the same. Both of these asymmetrical effects give distortion in the received signal. However, in my invention by the utilization of the modifiers No. 1, No. 2, No. 3, and No. 4, as indicated generally in Figure 1, I provide for maintaining the over all mean resonant frequency at substantially the same value as the mid-frequency value, regardless of the value of the coefficient of coupling. This means that there is no distortion In actual practice, I find that the modifiers may conveniently comprise the adding, or the subtracting, external inductance or capacitance, or any other circuit constants, with the elements of the resonant circuits. But it is to be understood that the modifiers of my invention are not limited to the addition or subtraction of external inductance or capacitance to the-elements of the resonant circuits. The modifiers may comprise other equivalent means.

In the operation of my invention, let it be assumed that it is desirable to employ a broad response, such as indicated by the substantially rectangular curve 34 of Figure 4. This may be accomplished by simultaneously actuating the coupling No. 1 and the modifiers No. 1 and No. 2 to such position that the coefficient of lcoupling is relatively large, and by allowing the coupling No.

3 and the modifiers No. 3 and No. 4 to remain in a loosely coupled position. Under this condition, it is manifest that the combined action of the closely coupled resonant circuits No. l and No. 2, together with the modified or reshifting action of the modifiers No. 1 and No. 2 produce a broad response curve having two peaks and having a mean resonant frequency value substantially equal to the mid-frequency value, such as that shown generally by the dash line curve 21, of Figure 4. It is also observed that, since there is a loose coupling between the two resonant circuits No. 3 and No.4, a relatively sharp single resonant peak is produced, such as the one indicated by the dash line curve 28 of Figure 4. The over all response of the resonant circuits No. 1, No. 2, No. 3 and No. 4 is given by multiplying the amplitudes of the two

curves

21 and 28 of Figure 4. This gives a relatively broad response curve 34, similar in shape to the ideal curve as shown in Figure 3. In Figure 4, it is pointed out that the broad response curve 21, as produced by the two resonant circuits No. 1 and No. 2, has a mean resonant frequency value that coincides substantially with the mid-frequency value, as indicated by the centrally positioned vertical line. In this connection, it is to be noted that, if the modifiers No. 1 and No. 2 were not simultaneously actuated with the actuation of the coupling No. 1, the broad response curve 21 would be shifted to such a position that its mean resonant frequency value would not coincide with the mid-frequency value. This shifted condition, as pointed out before, if allowed to exist, would cause distortion in the received signal.

A complete operating circuit embodying the preferred features of my invention, which produce a broad band, fiat top response, is shown in Figure 7. This embodiment of my invention is based upon the form, illustrated diagrammatically in Figure 1 of the drawings, and is incorporated inl the intermediate frequency amplifier of a radio receiving circuit.

The receiving4 antenna and the ground connection therefore, are indicated by the reference characters 40 and 4l, respectively. The reference character 42 designates the radio frequency am,- plied and the reference character 43 indicates the heterodyne device. As is well known in the art, the combined action of the radio frequency amplifier and the heterodyne device produces an intermediate frequency, having a mid-frequency value with the modulation side band frequencies extending on either side. The first detector tube is indicated by the reference character 44. As il- ,g

lustrated, this detector tube comprises a control grid 45, a cathode 46, a screen grid 41, a suppressor grid 48 and a plate 4. The cathode 46 of the tube has the customary by-pass condenser 54 and a self biasing resistor 55, connected to a ground 55. The batteries 51 and 5B, or other suitable source of supply, furnish voltage to the plate 49 and the screen grid 41 of the detector tube 44. The plate 49 is connected to the high sides of a resistor 5l, a tuning inductance 52 and a normal tuning, adjustable condenser 53. As illustrated, the lower ends of the resistor 5| and the tuning inductance 52 are connected through a by-pass condenser 59 to a ground GE). The condenser 63 is a modifier condenser, and corresponds to the modifier No. 1 of Figure 1. The condenser 64 is the normal capacity coupling, and when used gives a relatively sharp response curve similar to the dash curve 28 of Figure 4 of the drawings. The condenser 65 is the mutual capacity to over couple the resonant circuits and when used gives a relatively broad response curve, similar to the dash curve 29 of Figure 5; assuming, of course, that the circuit constants are such that they produce this shape of response. The. shifting from the coupling condenser 64 to the coupling condenser 65, and vice versa, corresponds to the action aiforded by the variable coupling No. 1 of Figure 1. The adjustable condenser 61 is the grid o'r input side tuning condenser, and the condenser 68 is a modifier condenser and corresponds to the modifier No. 2

of Figure 1. The reference character 69 indicates the grid tuning inductance and the. low potential end thereof is by-passed through the large condenser 12 to the ground 13. As illustrated, the inductance 52 is shielded by a shield 6| that is connected to a ground 62, and the inductance 69 is likewise shielded by a shield 10 that is connected to a ground 1|. This serves to isolate the two inductances 52 and 69. The point |35 of the low potential end of the inductance 69 is connected to the automatic volume control system |33 by means of a conductor |34. This automatically controls the grid bias of the tube 86, which may be similar to the rst detector tube 44. As shown, the tube 86 comprises a cathode 81, a control grid 83, a screen grid 89, a suppressor grid 90 and a plate 9|. The tube 86 serves to amplify the voltage across'the inductance 69 and impress it upon the next selective unit. The cathode 81 of the tube 86 is connected directly to a ground 92, as this tube receives its bias voltage through the automatic volume control system |33. The batteries 93 and 94 furnish voltage to the plate 9| and the screen grid 89, and they may be in common with the batteries 51 an-d 58. The plate 9| of the tube 86 is connected to the high sides of the tuning inductance 95 and the adjustable condenser |06. As illustrated the low potential end of the tuning inductance 95 is connected through a by-pass condenser 98 to a ground 99, and is shielded by a shield 96 which is connected to a ground 91. The condenser |6| is the modier condenser in the plate tube circuit and corresponds to the modier No. 3 of Figure 1. The reference character |02 indicates a resistor. The reference character |05 designates a coupling inductance which serves to over couple the two resonant circuits. In` this case, I utilize an inductance in order to reverse the resonant peaks from that of the first selective unit; see the reverse resonant peaks of curves 29 and 30 of Figure 5.

The condenser |06 is the normal capacity coupling and when used gives the sharpselective response, as illustrated generally by the dash curve 28 of Figure 4. This condenser |06 is connected to a ground |01. The shifting from the coupling condenser |06 to the coupling inductance |05, and vice versa, corresponds to the action alforded by the variable coupling No. 3 of Figure 1. The adjustable condenser |03 is the grid tuning condenser, and the condenser |64 is the modier condenser and corresponds to modier No. 4 of Figure 1. The reference character ||3 designates the grid tuning inductance and is connected to a ground I6, and is shielded by shield ||4 which is connected to a ground I5. The voltage across the grid tuning inductance ||3 is fed to the amplier tube ||1, which comprises a cathode H8, a control'grid ||9, a screen grid |20, a suppressor grid-|2I and a plate |22.

The switches 11 and serve to give two degrees of selectivity and correspond tothe arrows 25 and 26 of Figure 1. Their action will no-w be described. When the switch 11 is turned so that the center contact 19 is connected vto the two conductors 14 and 16, the circuit is in the broad tuning position. the normal coupling condenser 64 and the modifier condensers 63 and 68 are shorted or excluded from the resonant circuits. The resonant circuit, which may be designated as the one corresponding to the resonant circuit No. 1 of Figure 1, may be traced as follows: Beginning with the ground 66, the circuit extends through the condenser 59, and in parallel through the inductance 52 and the resistor 5|, the adjustable tuning conl denser 53, the conductor 14, the switch 11 to the center contact 19, the conductor 15, the condenser 65, to the ground 66. The resonant circuit which may be designated as the one that corresponds to the resonant circuit No. 2 of Figure 1, may be the circuit extends through the coupling condenser 65, the conductor 15, the switch 11, the conductor 16, the adjustable condenser 61, the inductance 69, the by-pass condenser 12 to the ground 13. The condenser 65, which is relatively small in capacity, serves to over couple the resonant circuits and thus give a response similar to that indicated by the dash curve 29 of Figure 5. The relative amplitude of the two peaks is affected by the various impedance conditions in the system. That is, the inductance-capacity ratio and the eifective resistance of each resonant'circuit each have a modifying effect on the relative peak amplitude and shape of the response curves. The resistor 5| is used to lower the input impedance even more than is accomplished by the L-C ratios in the resonant circuits, and makes one of the resonant peaks of the coupled resonant circuits lower thanthe other, see the dash curve 29 of Figure 5. Y

When the switch 11 is turned so that its center contact 19 is connected to the ground 18 the resonant circuits are in their sharp tuning position. In this position the small condenser 65 y is shorted, and the relatively large condenser 64 constitutes the coupling between the two resonant circuits. As the condenser 64 is relatively large, only one resonant peak results, being smilar to the response shown by the dash curve 28 in Figure 4. It is also notedv that, when the center contact 19 is grounded, the modifier condensers 63 and 68 rare added to the resonant circuits, The action of these added modier condensers 63 and 68 is such that they modify the resonance frequency of the circuits, and thus maintain the mean resonant frequency at substantially the same value as the mid-frequency value of the modulated side band frequencies. This means that the correct tuning position is always maintained. This would not be the case, if the mean resonant frequency were shifted from the mid-frequency of the modulated side band frequencies. In practice the condensers 63, 64, 65, and 68, since they are not so critical, are made In this position of the switch,

.30 traced as follows: vBeginning with the ground 66,

fixed units, the values thereof being determined by the design of the receiving circuits.V The adjustable condensers 53 and 61 are used to tune the resonant circuits when in the sharp resonant position. This is done during the process of manufacturing, and when once tuned they remain fixed. The simplicity of tuning and the non-criticalness of the values of the various circuit elements of my resonant circuits, insures a good quality of reproduction throughout a relatively wide range of manufacturing tolerances on the condensers 53, 64, 85, and 58.

The action of the switch is essentially the same as that of switch 11, but of a reverse order. As illustrated by the dash line 8|, the switches 11 and are ganged together, but may be made to work independently of each other, or in sequence in case more than two degrees of selectivity are desired. When the switch is turned such that the center contact is connected to the conductors |08 and I |0 the resonant circuits are placed in their sharp tuning position and give a response similar to the dash curve 28 of Figure 4. In this position, the modifier condensers |0| and |04 and the resistor |02, and the over coupling impedance |05 are shorted, and only the normal tuning inductance and the normal tuning capacity |00, and the normal tuning inductance I3 and the normal tuning condenser |03, and the normal coupling condenser |06 are effective. The resonant circuits are tuned in this selective or sharp resonant condition during the manufacturing process by adjusting the tuning condensers |00 and |03, and when once adjusted they remain xed. It is to be noted that in this position the mean resonant frequency of these circuits substantially coincides with the mid-frequency value of the modulated side band frequencies.

When the switch is turned so that the center contact 80 is connected to the ground ||2, the circuits are broadly tuned, and the normal coupling condenser |06 is shorted, and the over coupling inductance |05 is connected in circuit relation with the two resonant circuits. In over coupling, the tendency normally is to push the mean resonant frequency of the two resonant circuits to a higher value, but in this case, by reason of the large value of the added inductance |05, the mean resonant frequency is pushed below the'mean resonant frequency of the normally critically tuned resonant circuit. Therefore, it is necessary under this condition to add the modifier condensers |0| and |04 tothe resonant circuits in order to raise the mean resonant frequency value to substantially the same as the mid-frequency value of the modulated side band frequencies, It is to be noted that, by utilizing an inductance, instead of a capacitance in over coupling the resonant circuits, the resonant frequency peaks are just reversed. Compare curve 29 and 30 of Figure 5. The resistor |02 is used to lower the input impedance even more than is accomplished by the use of the L-C ratios in the resonant peaks of the dash curve 30 of Figure 5 to be lower than the other. The resistor |02 serves substantially the same function with reference to its own resonant circuits as the resistor 5| served with its own resonant circuits, and therefore could be placed in parallel circuit connection with the tuning inductance 95. However, in the series position as shown, the resistor |02 is effective only in the broad tuning position, and thus ineffective in the sharp tuning position, with the result that in the sharp tuntube.

ing position the resistor |02 has no resistive action on the resonant circuits.

It is noted from the function of the switches 11 and that, when both of them are turned in their broad position, a response curve similar to that shown in Figure 5 is given by the entire circuit, and that, when the switches are turned to their sharply tuned position, a response is given similar to the full line 3| curve in Figure 2. Therefore, my invention is such that it gives two degrees of selectivity, and that when positioned in the broad tuning position the response is substantially uniform through the modulated side band frequencies,

In Figure 8, I illustrate a fragmentary view of a modified set of switches which `give three selective positions. In the modified arrangement, the two switches |35 and |42 are ganged together as indicated by the dash line |48. In the switch |36, the Contact segments |39 and |40 are relatively long and the contact |4| is relatively short, and in the'switch |42 the contact |45 is relatively short and the contacts |46 and |41 are relatively long. Therefore, by the arrangement of these contact segments, the switches are so designed that, in the position shown, the two sets of resonant circuits are broadly tuned. In an intermediate position, the set of resonant circuits associated with the switch |36 is broadly tuned as the contact segments |39 and |40 are still bridged, but the set of resonant circuits associated with the switch |42 is sharply tuned as the center Contact |44 is connected to the two conductors |538 and H0. In the third position, the sets of resonant circuits are both sharply tuned, as the center contact of the switch |36 is connected to the ground |91 and as the center contact of the switch |42 is still connected to the conductors |08 and H0. In this manner thereare three selective tuning positions. However, if more than three selective tuning positions are desired, this may be accomplished by utilizing a stepped coupling unit and a stepped system of modifiers. Also, while the physical size may be somewhat large, a continuous change from one degree of tuning may be made to another degree of tuning by providing a continuous system of variable couplings and variable modifiers.

In Figure l it is noted that the resonant circuit N o. 2 and the resonant circuit No. 3 are coupled by a coupling No. 2. As before mentioned, this coupling No. 2 may comprise an amplifying The embodiment of a circuit utilizing the relation that exists between the resonant circuit No. 2 and the resonant circuit No. 3, together with the coupling No. 2 is shown in Figure 9. In this figure, the antenna and the ground therefore are designated by the reference characters |51 and |58, respectively. The radio frequency amplifier |59 and the heterodyne device |60 jointly act to impress an intermediate frequency having a mid-frequency value, with modulated side bands on either side, upon the rst detector tube |61. The cathode |91 has the customary by. pass condenser |98 and a self biasing resistor |63 connected to a ground |82. The batteries |95 and |96 supply voltage to the plate |55 and the screen grid |54. The condenser |64 is a by-pass condenser. The tube |10 corresponds to the coupling No. 2 of Figure 1. The cathode |99 has the usual by-pass condenser |14 and the self biasing resistor |13 connected to the ground |15. The illustrated batteries |52 and |53 supply voltage to the plate |12 and the screen grid |5|. The condensers |16` and |83 arey by-pass condensers.

The resonant circuit of Figure 9: is designed to have two degrees of selectivity, and is controlled by the switch |84 having movable bridge member |89 adapted to contact the three contacts |86, |81 and |86! In the open position of the switch as shown,the circuits areesharplyetuned, and give a response similar to the full line curve 3| of Figure 2. In the sharply tuned position, the modifier condenser |68 and the modiereinductance |11 are included in the resonant circuit. rlhese resonant circuits are tuned to give a Vsharp response when the switch |84 is open, during the course of the manufacturing process by the adjustable condensers |61 and H88, and when they are once set they remain fixed. In the sharp position, the mean resonant frequency value of the two,Y resonant circuits coincides substantially with the mid-frequency value of the modulated side band frequencies.

When the switch |84 is turned so that the bridgeV member |89 contacts the three contactsnl, |81 and |88, the over all circuit is broadly tuned. This is effected by shunting the modifier condenser |68 and., the modifier inductance |111. It is noted that the shunting of the modifier condenserY |68 andthe modier inductance |11, in effect, performstwo functions, in that the shunt.= ing not only produces a broad response which may be similar to the dash curve 21 of Figure 4,

but also maintains the mean resonant frequency value of the resonant circuits at substantially the same value as the mid-frequency of the modulated side band frequencies, for the reason that the exclusion ofthe modifier condenser |68 causes one ofgthe resonant peaks to shift in one direction from the mid-frequency value the same amount as the exclusion f of the modifier inductance |11 causes the other resonant peak to shiftfin the opposite direction. The two equally andY opposite shifted resonant peaks produce an over all response similar to the giash curve 21 of Figure 4. This means that there is no asymmetrical distortion in the reeeived signal. Also, it is noted Vthat uponthe closing of the switch |84 a resistor |94, isv connected in parallel with the self biasing resistor |13. *This causes the amplitude of the ffbroad response curve Yto be elevated, for the reason that the tube |18 passes more current resulting from the changed self biasing eifectfof the cathode |99.

In Figure 10, I illustrate a modified form of Figure 9. In this form the tube 2|2 feeds the tube 2| 3 through means of loosely coupled resonant circuits 285 and 286, which comprise, respectively, the inductance 288 and adjustable condenser Y281, andgthe inductance 2H! and an adjustable condenser 289, and the tube 2|3 feeds the second detector andaudio system through means of the loosely coupled resonant circuits 214 and '2|5,

which, respectively, comprise the inductance 2|1 and the adjustable condenser 2 6, and the inductance 2|9,and the adjustable condenser 228. The cathode of the tube 2|3 is connected directly to the ground 223, as the grid of the Vtube 2 lis automatically biased by the automatic volumegcontrol system through a conductor 238. The tube 2|3 is the coupling tube, and corresponds to the coupling No. 2 of Figure 1.,.: The condenser 2|| is a modifier, and corresponds to the modifier No. 2 of Figure 1. The inductance 2|8 is'a modifier, and corresponds to the modifier No. 3 of Figure 1. The

condensers

224, 222, and 22| are blocking con?i densersL This ircuit has two degrees :of selecn tivity similar to that of the circuit in Figure 9. Both the sharp response and the broad response are controlled by the switch 225 which comprises two

contacts

226 and 221 bridged by the contact member 228 which is connected to a ground 229. In the position shown in Figure 10, the circuits are sharply tuned to give a response curve similar to that shown by the full line 3| in Figureg2. When in this position it is noted that the modier condenser 2|| and the modifier inductance 2|8 are included in the resonant circuit, and it is in this position that the adjustable condensers 281, 289, 2|6 and 228 are tuned during the process of their manufacture. YThey are so acljusted, when in the sharply tuned position, that the mean resonant frequency value coincides substantially with the mid-frequency value of th modulated side band frequencies. Y

When the switch 225. is turned so that the bridge member 228 bridges the two

contacts

226 and 221, the circuits are broadly tuned, and produce a response curve similar to the dash curve 21 of Figure 4. In this position the modier condenser 2 and the modifierinductance 2|8 are shorted. The exclusion of the modifier condenser 2|| and the modifier inductance 2|8 performs two functions-f in a manner similar to the two functions performed by the circuit in Figure 9, in that the shunting of these elements not only produces a broad response, but also maintains the mean resonant frequency value of the resonant circuits at substantially the same vaiue as the mid-frequency of the modulated side band frequencies. It should be noted that the same effect can be obtained by turning in the sharp position with themodiers out, and then getting the broad position by inserting the modifiers. 'I'his is just the-reverse of the above description. Y

In Figure 11, I illustrate a coupling device which performs two functions, inthat it not only varies the coupling between the resonantl circuits but also simultaneously produces the modifying actionY upon the resonant circuits to maintain the mean resonant frequency value of the two resonant circuits substantially the same as', the mid-frequency value of the modulated side band frequencies. This coupling may Ycomprise two movable coils 239 and 258 placed between two current conducting plates 249YV and 244. The adjustable condensers 24| and 242 are adjusted during the course of their manufacturing to give sa sharp tuning, when the coils 239 and 248 are separated to -give a loose coupling condition. As the coils 239 and 248 are moved closer together the coupling between them is increased to produce a broad response; but at the same time the coils themselves move farther away from the two spaced plates. 243 and 244, which causes their self Yinductance to be greater, thereby producing a modifying action upon the resonantu circuits which maintains the aix:

mean resonant frequency value substantially the same as the mid-frequency value of the modulated side-band frequencies. The operation of the two

tubes

235 and 235, together with their associated circuits vis substantially the same as that previously described with reference to the other circuits. It is noted that this coupling device produces as the

coils

239 and 240 are moved toward each other, a gradual change from a sharply tuned circuit to a broadly tuned circuit, while at the same time maintains the mean resonant frequency value at substantially the same value as the mid-frequency of the modulated side band frequencies.

Although I have described my invention with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example and that numerous changes in the details of vconstruction and the combination and arrangement of parts may be resorted to Without departing from the spirit and the scope of the invention as hereinafter claimed.

I claim as my invention:

1. In combination, a pair of resonant circuits, means including a common coupling reactance for over coupling the pair of resonant circuits so that they have a certain over all response with respect to frequency in which the higher resonance pea-k predominates, and have a certain over all mean resonant frequency, a second pair of resonant circuits, means including a common coupling reactance for over coupling the second pair of resonant circuits so that they have a certain over all mean resonant frequency and have a different over all response with respect to frequency from the response of the first pair of resonant circuits in which the lower resonance peak predominates, a vacuum tube amplifier and circuit connections for connecting the two pairs'of resonant circuits thereto, and circuit modifying means associated with the several resonant circuits for maintaining the over all mean resonant frequency of the rst pair of resonant circuits substantially the same as the over all mean resonant frequency of the second pair of resonant circuits.

2. In combination, a first pair of coupled tuned circuits, common coupling reactance between said circuits, a second pair of coupled tuned circuits and common coupling reactance therebetween, an auxiliary reactance included in each of the tuned circuits, a coupling device interconnected betweensaid two pairs of circuits, and switch means associated with the auxiliary and coupling reactances, said switch means in one position operating to short-circuit the auxiliary reactance in each tuned circuit of the first pair only and to Vary the common coupling reactances of both pairs of circuits, and in a secon-d position operating to short-circuit the auxiliary reactance in each tun-ed circuit of the second pair only and to Vary the common coupling reactances of both pairs of circuits, said mentioned means being operated in unison to compensate for a shift in the mean resonant frequency caused by overcoupling in each pair of circuits.

3. The combination defined in the preceding claim wherein in a third position of the switch means the auxiliary reactances of all the tuned circuits are short-circuited and the common coupling reactances of both pairs of coupled oircuits are varied.

4. In combination, a pair of coupled tuned circuits, each having an inductance, said inductances arranged to be moved in an axial direction closer together or farther apart whereby the degree of coupling between the tuned circuits is varied, and a pair of spaced but irnmovable current conducting plates extending perpendicularly to' the direction in which the inductances move, the said inductances being positioned between the plates.

5. The combination dened in the preceding claim wherein the circuits are tuned to a desired frequency when their inductances are spaced to provide loose coupling, so that when the inductances are moved closer together to provide close coupling, the circuits are slightly de-tuned due to the varying values of self-inductance of the tuned circuit inductances with different spacings of the inductances from their respective conducting plates.

6. A high frequency coupling system comprising a pair of resonant circuits each including reactance elements of opposite types and tuned to a selected frequency, a reactance common to and coupling said circuits, means for substituting for the coupling reactance between said circuits a common coupling reactance of different value to vary the resonance band width of said system and for simultaneously inserting a supplemental reactance element in each of said circuits to prevent shifting of the resonance band in the frequency scale with variation in the band width.

7. A high frequency coupling system comprising a pair of resonant circuits each including inductance and a capacity, a second capacity common to and coupling said circuits, means for substituting for the coupling capacity between said circuits a common coupling capacity of different value to vary the resonance band width of said system and for simultaneously inserting a supplemental capacity in each of said circuits to prevent shifting of the resonance band in the frequency scale with variation in the band width.

8. A high frequency coupling system comprising a pair of resonant circuits each including inductance and a capacity, a second capacity common to and coupling said circuits, means for substituting for the coupling capacity between said circuits a common coupling inductance to vary the resonance band width of said system and for simultaneously inserting a supplemental capacity in each of said circuits to prevent shifting of the resonance band in the frequency scale with variation in the band width.

9. A high frequency coupling system comprising a pair of resonant circuits each containing inductance and capacitance and tuned to a selected frequency, said inductances having substantially zero mutual inductive coupling between them, a reactance common to and serving as the sole coupling means between said circuits, means for adjusting the resonance band Width of said system comprising means for substituting for the common coupling reactance a second common coupling reactance of different Value, and means for simultaneously inserting a supplemental reactance in each of said circuits, whereby shifting of the resonance band in the frequency scale co-incident with change of coupling is prevented.

10. A high frequency coupling system according to claim 9 wherein the common coupling reactance and the substituted coupling reactance are condensers.

l1. A high frequency coupling system according to claim 9 wherein the common coupling re- Iii actance is a condenser and the substituted coupling reactance is an inductance coil. n

12. In combination, a rst pair of coupled tuned circuits, selectively operable common coupling reactances between said circuits, a second pair of coupled tuned circuits and selectively operable common coupling reactances therebetween, an auxiliary reactance included in each of the tuned circuits, means for simultaneously short-circuiting the auxiliary reactances and one of the common coupling reactances of the first pair of tuned circuits, means for simultaneously short-circuiting the auxiliary reactances and one ofthe common coupling reactances of the second pair of tuned circuits, and means interconnecting said short-circuiting means for operation in unison.

13. The combination according to claim 12, wherein the short-circuiting means are interconnected for unicontrol operation in a manner so that in one position, the auxiliary reactances of the first pair of tuned circuits will be shortcircuited while those of the second pair will be operative, and vice versa in another position of the interconnected short-circuiting means.

14.1n combination, a rst pair of coupled tuned circuits, selectively operable common coupling reactances between said circuits, one p-roviding less than critical shape coupling and the other more than critical shape coupling, a second pair of coupled tuned circuits and selectively operable common coupling reactances therebetween, one providing less than critical shape coupling and the other more than critical shape coupling, an auxiliary reactance included in each of the tuned circuits, means for short-circuiting the auxiliary reactances of the rst pair of circuits when its operative common coupling reactance is such as to provide more than critical shape coupling, means for short-circuiting the auxiliary reactances of the second pair of circuits when its operative common coupling reactance is such as to provide less than critical shape coupling, and means interconnecting said short-circuiting means for operation in unison.

15. The combination dened in claim 14 wherein the selectively operable coupling reactances of the iirst pair of tuned circuits comprise a pair of condensers of diierent values, and those of the second pair of tuned circuits comprise a condenser and an inductance.

KENNETH W. J ARVIS.