US2210738A

US2210738A - Method and apparatus for receiving electrical communications

Info

Publication number: US2210738A
Application number: US156489A
Authority: US
Inventors: Ernest A Tubbs
Original assignee: NAT TELEVISION CORP
Current assignee: NAT TELEVISION CORP; NATIONAL TELEVISION Corp
Priority date: 1937-07-30
Filing date: 1937-07-30
Publication date: 1940-08-06
Anticipated expiration: 1957-08-06

Description

E. A. TUBBS Aug. 6, 1940.

METHOD AND APPARATUS FOR RECEIVING ELECTRICAL COMMUNICATIONS 2 Sheets-Sheet 1 Filed July 30, 1937 Rm O 5 Y .E m N E R V 7 O W & m NW3 g- 6, 1940. v E. A. TUBBS 2.210.738

METHOD AND APPARATUS FOR RECEIVING ELECTRICAL COMMUNICATIONS Filed Ju ly so, 1937 '2 Sheets-Sheet 2 c o l0 0 0 I 2 3 4 5 6 7 8 9 10 ll 0 l 2 3 4 5 G 7 8 9 l0 ll Kc INVENTOR fE/VEST A. 70555 BY M1 WM ATTORNEY Patented Aug. 6, 1940 UNETED STATES PATENT orrice METHOD AND APPARATUS FOR RECEIVING ELECTRICAL COMMUNICATIONS Application July 30, 1937, Serial No. 156,489

11 Claims.

This invention relates to communication apparatus, and especially to a method and apparatus for receiving electrical communicationsas,

' for instance, carrier currents modulated by sound or other intelligence.

One of the objects of the invention is to provide a receiver in which the fidelity of reproduction of the received signals is greatly improved over apparatus of the prior art, and at the same '10 time interfering noises are greatly decreased.

Another object of the invention is to provide a radio receiver for very accurately reproducing the signals and incorporating means to control the frequency band width of the received signal.

Still another object of the invention is to provide a high fidelity radio receiver with means to eliminate the beat note which may be produced by an adjacent unwanted carrier beating with the carrier of the wanted signal.

Still another object of the invention is to provide an improved type of automatic volume control for a radio receiver.

Another object of the invention is to provide an improved arrangement for a visual tuning '2 indicator for a radio receiver;

A further object of the invention is to provide a radio receiver in which the received signal is separated into two channels, including a broad channel and a sharp channel, and which the original signal is produced by beating the signals in the two channels together.

Another object of the invention is to provide a radio receiving apparatus in which unequal efiects upon different frequencies of the signal may be wholly or partially compensated for.

Other objects of the invention and objects relating particularly to the arrangement and connections of the various parts of the circuit will be apparent as the description of the invention proceeds.

One embodiment of the invention has been illustrated in the accompanying drawings, in which:

1 represents a circuit'diagram of a superheterodyne radio receiver for sound reproduction;

Fig. 2' represents certain phase curves and audio response curves produced by the two channels; and ac Fig. 3 represents an audio output curve for the demodulation stage ofthe set.

The invention comprises a radio receiver in which the selected signal is separated with two channels, one of which is broad enough to pass all of the frequencies of the signal and the other is quite sharp. The broad channel feeds into a balanced demodulator circuit in such a manner that substantially all of the signal is normally balanced out. The signal in the sharp circuit is, however, also fed into the demodulator cir- 5 cuit in such a manner that beating takes place between the two signals, thus producing demodulation. Certain advantages are obtained in this manner which will be subsequently explained. A device for controlling the frequency band width of the broad channel is also a feature of the invention; and automatic volume control and visual tuning indication are arranged in a novel manner to produce advantageous results.

In the circuit diagram of Fig. 1 a unit It] of conventional design may include the tuning circuit for selecting the desired incoming signal, an oscillator for producing the auxiliary frequency to beat with the incoming signal and thus form the intermediate frequency, and a mixer tube where this beating action takes place. An antenna II may be connected to the unit II] for picking up the signal and the unit III may be grounded at I2, as indicated. The output circuit from the unit II], and thus from the mixer tube therein, may contain the coil I3 which may be shunted by the condenser I4, forming a resonant circuit which is tuned to the intermediate frequency. The plate of the mixer tube may be supplied with a high positive potential from a source to be later described, through a resistance I5, which may be connected to the coil I3. The lower end of the coil I3 may be connected to ground through a by-pass condenser I6.

The coil I3 may form the primary coil of a g transformer II, the secondary I8 of which may be shunted by a variable condenser I9 to tune it to the intermediate frequency, and may have one end connected to the grid 20 of an amplifier tube 2 I, while the other end may be connected through a resistance 22 to a suitable source of potential, in the present instance shown associated with the automatic volume control, to be described hereinafter. The lower end of the coil I8 may also be by-passed to ground through a condenser 23.

For reasons to be hereinafter explained, I prefer to keep the plate to grid capacity as low as possible in the amplifier tube 2|, and hence I prefer to use a pentagrid tube having an anode 24, a suppressor grid 25, a second control grid '50 26, a screen grid 21 around the second control grid 26, and a cathode 29 which may be independently heated by a suitable heating filament, not shown but well understood in the art. In using this type as an amplifier I prefer'to conmay connect the screen grid to ground, while the cathode 29 may be connected to ground through a resistor 32 which may be by-passed by a condenser 33.

The plate 24 of the tube 2| may be connected to the primary coil 34 of a transformer 35 which couples the amplifier tube 2| to a second amplifier tube 36. The coil 34 may be tuned by a variable condenser 31 to the intermediate frequency, this condenser being in series with a fixed auxiliary condenser 38 which is used for a purpose to be later described. The lower end of the coil 34 may be given a suitable high potential from a source to be hereinafter described, through a resistance 39 with the end of the coil bypassed to ground by means of a condenser 40.

The transformer 35 may have a secondary winding 4| which may be shunted by a variable condenser 42 in order to tune the coil to the intermediate frequency, and one end of the coil may be connected to the control grid 53 of the tube 36. This tube may be a pentagrid tube with its elements connected exactly the same as those of the tube 2|, and these individual elements need not be described in detail. The plate 44 of the tube may be connected to one end of a coil 45 which forms the primary winding of a transformer 46. This coil may also be tuned. to the intermediate frequency by a variable condenser 41 shunted across it and in series with a fixed condenser 48, the latter being for a purpose to be later described. The lower end of the coil 45 may be given a high positive potential from a source to be later described, through a resistance 49, while a by-pass condenser 5|] may be connected between the lower end of the coil and ground.

The transformer 46 may have a secondary winding 5| which forms the input circuit for a balanced demodulator comprising two

tubes

52 and 53, connected in push-pull, one end of the winding 5| being connected to the second control grid 54 of the tube 52, and the other end to the second control grid 55 of the tube 53. A variable condenser 56 may be connected across the coil 5| to tune it to the intermediate frequency, The coil 5| may have a center tap to which the wire 51 may be connected which leads to a source of potential for the second control grids of the

tubes

52 and 53, and which will be later described. This center tap may be connected to ground through a condenser 58.

The

plates

59 and 60 of the

tubes

52 and 53 may be connected to opposite ends of a coil 6| which forms the primary of an iron core transformer 52. A tap at the midpoint of the primary 6| may be given a positive potential in a manner to be hereinafter described. The

tubes

52 and 53 may be similar, if desired, to the tubes 2| and 36, already described, having, in addition to the second control grid and plate, mentioned above, suppressor grids, cathodes and inner control grids 33 and 64, which may be connected together and which form the means of combining the sharp channel with the broad channel.

Suitable screen grids are provided between the inner and outer control grids to shield these grids from each other. However, I have found that in most instances this shielding is not sufficient to completely prevent one grid from affecting the nect the suppressor grid 25 and the second conother, and I therefore provide a condenser 65 having two separate stators, one of which may be connected to the control grid 54 and the other to the control grid 55, and a single rotor which may be connected to both the control grids B3 and 64. By adjusting this condenser I am able to equalize the circuits including the two halves of the coil 5|. The cathode, screen grid, and suppressor grid connections for the tubes may be similar to those described in connection with the tubes 2| and 36.

With the arrangement just described, a signal picked up on the antenna and selected by the tuner in the unit I0, may be converted to the intermediate frequency by the mixer tube and amplified by the tubes 2| and 35. The combined effect of all of the circuits preceding the

tubes

52 and 53 may be such as to receive a wide channel of frequencies, which will include all of the side bands to produce the complete audio signal, and these circuits may be similar to standard circuits incorporated with a superheterodyne receiver, with the exception that they may be somewhat broader, because in the average superheterodyne receiver the circuits must be made narrow enough to eliminate any interference from unwanted stations, while with the present invention I may make these circuits broad enough to insure the complete reproduction of the wanted signal, and I need not be greatly concerned with any interference which may come in adjacent the outer side bands of the wanted signal.

The intermediate frequency with the signal impressed thereon is introduced to the

tubes

52 and 53, which form the balanced demodulator, by means of the

second control grids

54 and 55 respectively, of these tubes. It will be noted that these grids are connected in push-pull, and, excluding the effect of the other control grids, if

the circuits are properly balanced and the characteristics of the tubes are the same, the tubes will act to produce amplification of the intermediate frequencies applied to the grids without any detector action. This is because any detector action in one tube will be balanced out in the primary 5| of the output transformer 52 by the same detector action in the other tube. However, if a frequency corresponding to the carrier frequency is applied to the two control grids 63 and 54 of the

tubes

52 and 53, and this frequency is something other than degrees out of phase with the carrier frequency in the broad channel, (preferably in phase with it or degrees out of phase) beating will take place in the

tubes

52 and 53 between this frequency and the side bands included in the broad channel to produce detector action and cause signal currents to flow in the primary 6|,

In order to obtain this additional frequency, which corresponds to the carrier frequency, I may use another oscillator with means to keep it in step with the carrier frequency, but I may prefer to take some of the incoming signal and filter out or otherwise remove a considerable portion of the outer side bands and apply the remaining frequencies to the first control grids of the tubes. In the drawing, I have shown one arrangement for carrying out the latter procedure. A tube 66 may have its control grid 61 connected to the control grid 43 of the tube 35, thus connecting the control grids of the tubes 66 and 36 in parallel. By this means the modulated intermediate frequency in the output circuit of the tube 2| is impressed equally on the grids of tubes 35 and 66. The tube 66 may be a pentagrid tube with the suppressor grid 68 and a second control grid 69 connected together and to the cathode 19. which may be connected to ground through a re sistance H shunted by a condenser E2. The screen grids I3 may be given suitable potential through a resistance I4 from a source to be later described, and may be also connected to ground through a condenser 15,

The plate I6 of the tube 66 may be connected to one end of a coil 11, which may be shunted by a variable condenser I8, and the other end of which may be connected through a resistance 19 to a source of potential to be hereinafter described. The lower end of the coil Il may be connected to ground through a condenser 89.

In order to narrow the band width of the sharp channel and remove the desired amount of outer side bands and secure a suitable phase rotation before these frequencies are introduced to the control grids 63 and 64 of the

tubes

52 and 53, I may couple three resonant circuits BI, 62, and 93 in cascade to the coil TI. The resonant circuit BI may comprise a coil 64, which may be shunted by a variable condenser 95 and another condenser 86 in series with it, while the lower end of the coil may be connected to ground as shown.

The resonant circuit 82 may comprise a coil 81 which may be shunted by a variable condenser 86 and another condenser 89 in series with it, the lower end of the coil being connected to ground. The juncture of the variable condenser 85 and the other condenser 86 may be connected by means of the wire 99 to the juncture of the condenser 88 and condenser 89, thus coupling the resonant circuit 82 to the resonant circuit 8!.

The resonant circuit 63 may comprise a coil 9! which may be shunted by a variable condenser 92, one end of this coil being connected to the grids 63 and 64 of the

tubes

52 and 53 respectively. The other end of this coil may be given a suitable negative potential in a manner to be described. The coil 9 I' may be inductively coupled to the coil 8'I, there being provided a core 93 between the two coils, which is preferably made of small particles of magnetic material insulated from each other, so that the efficiency of energy transfer may be high without substantial attenuation of the frequencies to be carried.

By the use of these three resonant circuits 8!, 82 and 83, coupled to the resonant circuit containing the coil 11, all of which are preferably loosely coupled together, I succeed in considerably reducing the band width of the signal in this sharp channel, so that the carrier frequency applied to the grids 63 and 64 of the

tubes

52 and 53 may be without any sidebands at all, or at least accompanied by limited side bands, the outer ones of which are greatly attenuated. I am also able by means of these coupled circuits to obtain the desired phase rotation of the frequencies in the sharp channel so that maximum detection may be obtained.

The beating action between the sharp signal applied to the control grids 63 and 64 and the broad signal applied to the

second control grids

54 and 55 produces, as has already been stated, an audio signal in the primary 6| of the transformer 62. This signal may be amplified by any suitable audio amplifier and reproduced by any suitable translating device. To this end the secondary 94 of the transformer 62' may be shunted by a resistance 95, having a movable contact which may be connected to the control grid 96 of an amplifier tube 91. One end of the coil 94 may be grounded, as indicated, and the resistance may be tapped a short distance from the grounded end, and this tap may be connected through a condenser 98 and a variable resistor 99 to ground. The movable contact on the resistance 95 may be adjusted to control the volume of the set, while the tone may be controlled by variation of the resistance 99.

The suppressor grid I90 of, the tube 91 may be connected to the cathode I9I, which may be connected through a resistance I02 to ground, the resistance being shunted by a condenser I93. The screen grid I04 of this tube may be given a suitable potential through a resistance I95, the ends of which may be connected to ground through condensers I96 and I91. The manner of applying the potential will be hereinafter described.

The plate I98 of the tube 91 may be connected through a tuned trap I09 and a resistance '9 to a source of positive potential to be hereinafter described. A condenser I I I may connect the juncture of the resistance H9 and the tuned trap I09 to the control grid H2 of an amplifier tube 3, this grid being provided with a high resistance leak II4 to ground. This tube may be similar to the tube 91, having a suppressor grid H5 which may be connected to the cathode I I6, the later being connected, to ground. through a resistance II'I shunted by a condenser H8.

I may connect the screen grid H9 and the plate I29 together and through a primary winding I'2I of a transformer I22 to the source of I plate potential. The secondary winding I23 of the transformer I22 may have its ends connected to the grids I24 and of the power tubes I26 and I21, respectively, which are thus connected in push-pull. The filamentary cathodes I28 and I29 of these tubes may be connected in parallel and to a source of filament potential not shown. The mid-point of the secondary I23 may be connected to a source of grid biasing potential, to be hereinafter described, and the plates I39 and I3I of the tubes I26 and I2! may be connected to the ends of a primary winding I32 of a transformer I33, the mid-point of the primary I32 being connected to the source of positive potential. The secondary I34 for the transformer I33 may be connected to the loud speaker I35 or other translating device.

A suitable power supply I36 may be provided for the circuit which may be connected by means of the cable I 31 to the source of household alternating or direct current, as will be well understood. This power supply may be of any well known type for providing a plurality of different positive potentials for the plate and screen grid circuits of the various tubes, as well as negative potentials for biasing the control grids. Thus a high potential terminal I38 on the power supply may be connected to the mid-point of the primary I32 of the loud speaker transformer I33 and through a resistance I39 to the resistors H9 and I05 in the plate circuit and screen grid circuit of the tube 91.

A lower potential terminal I49 may be connected to the primary coil I'2I in the plate circuit of the tube H3, to the primary coil 6| in the plate circuits of

tubes

52 and 53, to the resistance 49 in the plate circuit of the tube 36, to the resistance 39 in the plate circuit of the tube 2!, to the resistance I5 in the plate circuit of the mixer tube in the unit I9, and to the resi'stance '19 in the plate circuit of the tube 66. Also the terminal M0 may be connected to a resistance MI in the screen grid circuit of the

tubes

52 and 53, and through a resistance I42 to lower the potential, and then through resistance Ell in the screen grid circuit of the tube 2! and through resistance M in the screen grid circuit of tube 66.

Grid potentials for the various tubes may be provided by a grounded resistance I43 which may be connected to the negative terminal M4 of the power supply unit. This terminal may also be connected to ground through a condenser M5. The midpoint of the secondary I23 of the transformer I22 may be given a suitable potential for the grids Q24 and I25 of the tubes I26 and E21 b connecting it directly to the terminal EM. A point I46 on the resistance M3 may be connected to the mid-point of the coil 5| to give a suitable potential to the second control grids 5 and tI- of the

tubes

52 and 53, and this point is preferably made adjustable to facilitate obtaining the proper grid bias after the grid circuits have been balanced.

In order to provide a sensitivity control for the receiver, I may provide a resistance 5620; which may be connected between ground and the screen grid potential supply resistance i i-ii, so that current always flows through the resistance. A movable tap M3a on this resistance may be connected to the cathodes of the radio frequency amplifier tubes in the unit It, to thus control the bias on these tubes and therefore the gain thereof.

With the arrangement so far described, a broad band of signal frequencies, picked up by the antenna l and suitably selected by the unit ill and converted into intermediate frequencies, is amplified by the amplifier ill and then divided into two channels, the tube 36 amplifying the broad channel and transferring this broad band of frequencies to the

balanced demodulator tubes

52 and 53. The band of frequencies of the other channel is amplified by the tube 66 and then narrowed down by the resonant circuit of the coil 1'! and the three resonant circuits 3!, 82 and 83, which are coupled to it. The resultant narrow band of frequencies is then applied to the other control grids and M of the

tubes

52 and 53. The beating action takes place in these tubes and the audio signal flows through the coil 6! and is thereupon amplified by the tubes 5?, i it and E26 and I21, and translated by the loud speaker I35 into sound.

I have referred above to the broad channel and the sharp channel and the frequency band Widths thereof. While it is generally sufficient in considering these two channels to compare the resonance curves thereof, the measurements of the frequency band widths are made at the useful operating levels. Wherever the expression frequency band width is used in this specification or claims, therefore, it is intended to mean the Width of the resonance curve at some predetermined operating level.

Any suitable method may be employed to narrow the band of frequencies in the sharp circuit. I may, in some instances, use a crystal to eiiminate all of the side band frequencies and leave just the carrier alone. However, I have discovered that if the sharp channel, the voltage of which is applied to the control grids tit and G l of the tubes 5?. and 53, is made too sharp, it is not only difficult to tune the receiver, but extremely difficult to maintain the receiver in tune.

This is because a slight drifting of the resonant point of the sharp circuit will reduce the signal voltage to such a point where its effect on the

tubes

52 and 53 will be very small. This is especially true where a crystal is used in the sharp circuit to filter out all of the side bands and leave the carrier alone. A slight drift in the resonant point of the crystal will change the strength of the voltage of the sharp circuit from its maximum to substantially zero, and thus cause the signal in the loud speaker to disappear entirely.

This sharp selectivity may also give rise to a motorboating action or low frequency howl, caused by the mechanical vibrations from the loud speaker affecting the oscillator tuning condenser, thus changing the tuning so as to move the carrier on and off of the resonant point of the crystal.

When a crystal is used in this sharp circuit or the equivalent sharpness of the circuit is produced, it is difficult to locate a station in tuning because the operator may pass directly through the station Without hearing it at all, unless he is moving the tuning apparatus at an exceedingly slow speed.

In view of these disadvantages of too sharp a circuit, I may prefer to give the sharp channel a sufficient band width so that slight drifting of the resonance point of this channel will not cause the defects mentioned. proper width of this channel I may prefer to gauge it by its effect on the audio output curve of the set.

The audio output of the balanced demodulator may be considered to be made up of two components: one produced by the carrier introduced by the sharp circuit beating with the side bands included in the broad circuit, and one produced by the carrier in the broad circuit beating with the side bands included in the sharp circuit. The amplitude of a given frequency in each component will depend on the amplitude of the side bands in that component and the phase relation between the side bands and the carrier with which they are beating. the vector sum of these two components' It is well known that the phase of the current in a resonant circuit rotates as the frequency of the current is shifted through the resonant point of the circuit. the rotation of the phase against frequency is never more than degrees. If the circuit is a complex one having more than one resonant circuit, the rotation of phase is more rapid, the

rapidity increasing with the number of resonant circuits used, and as the coupling between the circuits is decreased. Thus where several resonant circuits are used coupled together, the phase may make several complete rotations as the frequency is shifted from one side of the combined resonance curve to the other.

Thus a plurality of frequencies, including the wanted carrier and its associated side bands and perhaps an unwanted carrier with its associated side bands, passing through a given circuit, will In determining the The total audio output then If the circuit is a single circuit a.

of the broadchannel is represented by the curve A which is made by plotting kilocycles for the abscissa and angle of phase rotation for the ordinates. The scale of phase rotationis shown at the right. The phase curve for the sharp channel is represented by the curve B. Both curves have been plotted for the audio phase rotation from zero to 11 kilocycles. These curves will correspond to the phase rotation curves of the side band frequencies plotted on one side of the res onance point, which may then be taken as zero frequency, and the scale of kilocycles will then represent the number of kilocycles from resonance.

From an inspection of these curves it will be evident that the audio phase produced by the broad channel component rotates rather slowly so that a 90 degree rotation is first reached at a frequency slightly under 10 kilocycles. The phase of the component of the sharp channel, with its plurality of coupled tuned circuits, however, reaches 90 degrees at a little under Zkilocycles, 180 degrees at about.3.5 kilocycles, and 270 degrees at a little under. 7 kilocycles.

If any audio note in the broad channel component is 180 degrees out of phase from the same note in the sharp channel component, one will cancel the otherprovided the amplitudes are the same. If the amplitudes are not the same the result will be the difference between the amplitudes. If these audio notes have the same phase, they will add together. For all other phase differences the effect will be the vector sumof these two components.

InFig. 2 I have also shown the audio amplitude curves for the components already referred to of the broad and sharp channels, the curves C and D representing respectively the audio amplitude curves for these channels. The former is sub-' stantially fiat to beyond 10 kilocycles and the latter falls off rapidly in amplitude beyondabout 1.5 kilocycles. I

It willbe seen from an inspection of the. phase curves, that, as the frequency increases from zero, the phase difference between the audio notes of the two audio components increases rapidly until at about 2 kilocycles it reaches 90 degrees, while at about 3.8 kilocycles it reaches 180 degrees, and at about '7 kilocycles it has approached 240 degrees, whereupon it falls again sov that at. 11 kilocycles it is again in the neighborhood of 180 degrees. As the audio frequency increases from zero these two components are at first adding;

and then as their phase difference increases they finally begin to subtract until at 180 degrees the audio produced is simply the result, of arithmetically subtracting one. component from the other.

' Taking into account the varying amplitudes of the two circuits as well as the phase differences, the audio amplitude curve in the output of the balanced demodulator appears as indicated in Fig. 3. By various methods, as, forexample, by means of a suitable. by-pass condenser in the cathode. circuit of the audio amplifier tubes, the low frequency peak of the curve may be brought down in a manner well understood phase curve for both circuits, certain sound frequencies may be increased or decreased, and in this way I may not only compensate for the tendency of the speaker or audio amplifier to reproduce or amplify certain sound frequencies more than others, but I may create an output curve having a certain desired shape.

Under certain conditions I may arrange the phase relation between the circuits so that the irequency at which the modulation cancels out corresponds to the frequency of an unwanted carrier. If one of these odd multiples of 90 degree phase difference points he made to fall at kilocycles above resonance, then there will be another such point substantially 10 kilocycles below resonance, thus assuring an added protection against unwanted interference for the pres ent day standard of separation. I

I prefer to use automatic volume control with the receiver and the manner of applying such control is one feature of the invention. I obtain a control from a circuit of the sharp channel which, because it is considerably sharper than the broad channel, is free from much of the interference which is found on the broad channel. Therefore the signal may be maintained at substantially constantvolume without being greatly influenced by the interference from adjacent stations. I prefer to take the voltage for theautomatic control from the resonant circuit 8i, and to do this I connect the juncture of the coil 34 and condenser 85 through a condenser M7 to the anodes M8 of a rectifier tube M9, the cathodes I58 of which are grounded.

These anodes may also be connected through a series of resistances IBI, I52 and 53 to ground, and I then connect the grids of the various tubes to points along these resistances. A wire 554 leading from the grids of the radio frequency amplifier tubes in the unit ill may be connected through-a resistance I55 to the juncture of the resistors 252 and IE3, this wire being also connected to ground through a condenser 856. The grid circuits of the tubes 2i and 36 and 56 may be connected by meansof a wire I57 through a resistance 58 to the juncture of the resistors iii and I52.

In this manner the drop in potential across the resistances I52 and H53, as controlled by the voltage in the resonance circuit 8 i, will determine the'potentials of the grids in the unit it and the amplifying tubes 2|, 36 and v$56. As the strength of the signal, therefore, rises in the sharp channel, including the resonant circuit 8|, the bias on the grids of the various amplifier tubes increases, thereby tending to reduce the strength of the signal, and when the signal tends to fade, the bias on these tubes is automatically changed to increase the gain, and therefore increase the signal. I

It will be evident that interfering frequencies found perhaps in the broad channel, may not be found in the resonant circuit 8! owing to the loose coupling between it and the coil "H, andtherefore such frequencieswill have no effect at all on the automatic volume control which will be governed more nearly by the strength of the wanted signal alone.

Where the greatest fidelity is desired in a radio program it is necessary to receive a channel which is suficiently broad to contain all of the side band frequencies of a Wanted station with a minimum of attenuation. To accomplish this it may be'necessary to have the circuit so broad that an adjacent station separated, for instance,

by only 10 k. c. in the ether spectrum, may be picked up, either in whole or in part. If this station has a signal strength very much greater than the wanted signal, the tubes may become overloaded, and hence it may be desirable to provide some means to change the selectivity of the set so as to eliminate the eifect of such a station.

In my application, Serial No. 151,805, fled July 3, 1937, and entitled Method and apparatus for controlling frequency band width of coupled circuits, I have shown and described a method and apparatus for controlling the band width of a receiver in such a manner that a sufficiently wide band may be transmitted for receiving all of a wanted signal, and this band may then be narrowed at will in order to eliminate interference which might come in at one side of the particular station to which the set is tuned. I have incorporated this arrangement in the receiver shown in Fig. 1.

As explained in the application, I prefer to couple an auxiliary circuit to the primary of a transformer and to vary the band width of the transformer by varying the Q (the reactance of the coil divided by the total resistance of the circuit) of the auxiliary circuit or the coupling of that circuit to the primary coil. Under the proper values for the components of the circuits I may broaden the resonance curve of the transformer into a substantially fiat-topped fairly wide curve, or I may use two or more transformers in cascade and broaden each in such a manner that the combined curve is substantially flattopped with steep sides.

In the present invention I have shown three transformers i1, 35 and 4B, for the intermediate frequencies connected by the tubes-ZI and 36, and I prefer to control the band width of the transformers 35 and 36 by coupling auxiliary circuits to them. Thus the auxiliary tuned circuit I59, comprising the coil I60 connected in series with the variable condenser I6I, auxiliary condenser I62, and the switch I63, may be coupled to the primary 3 3 of the transformer 35 by means of a coaxial cable I64 which may be connected between the juncture of the condensers I62 and NH and the juncture of condensers 31 and 38. The condensers 38 and I62, therefore, act as coupling condensers for the cable. The cable may have a suitable shield I65 which may be grounded as indicated.

The switch E63 may be provided with a plurality of contacts I66, I61, I68, I69 and I10. Of these five contacts I use only four, the first one, I66, on the left not being connected to anything. The last two contacts, I69 and I10 may be connected together and to the other side of the condenser I62, while between the connected contacts I 61 and I68 and this same point on the circuit, I may insert a resistance I1I. In the present instance the Q of the coil is preferably made substantially the same as the Q of the secondary M of the transformer 35, and the resistance I61 is made a predetermined value, such that when the switch I63 is on either of the contacts I61 or I68 the Q of the circuit I59 will be changed sufiiciently so that the resonance curve of the transformer 35 will be slightly broadened. When the switch is on the contact i66, which is open, the auxiliary circuit I59 will not affect the primary 34, and hence the resonance curve of the transformer 35 will be in its sharpest form. When the switch I63 is on either of the contacts I69 or I18, the auxiliary circuit I59 has its maximum effect on the primary-34, and the transformer 35 therefore has its broadest form of resonance curve.

In a similar manner I connect an auxiliary circuit I12 to the primary 45 of the transformer 46. The circuit I12 may comprise a coil I13 connected in series with a variable condenser I14, an auxiliary condenser I15, and a switch I16, the latter having five contacts I11, I18, I19, ISIland I8I-. The contacts I13, I and NH may be connected together and to the other side of the condenser I15, which may also be grounded. The contact I18 may be connectedthrough resistance I82 to the other side of the condenser I15. The circuit may be connected by means of a coaxial cable I83 between the juncture of condensers I14 and I15 in theauxiliary circuit and condensers 41 and 48 in the circuit of the primary 45, thus using the condensers 48 and I15 as-coupling condensers. This coaxial cable may be provided with a'suitable shield I84 which may be grounded, as indicated.

-With this arrangement of the auxiliarycircuit I12, when the switch is on the contact I11 the auxiliary circuit is open and there is substantially no effect upon the resonance curve of the-transformer 46. Upon movement of the switch to the contact I18, the circuit I12 is closed through the resistance I82 with the result that the transformer 46 has its resonance curve slightly broadened. When the switch is on any of the contacts I19, I80 and I8I, the maximum effect of the auxiliary circuit is produced on the transformer 46, which gives the widest resonance curve.

The combined effect of broadening the transe formers 35 and 46 is to produce a saddle-top curve, the valley of which may then be filled in by the single peak resonance curve of the transformer I1, which is unaltered. While any two of the three transformers I1, 35 and 46 may be acted upon to broaden the resonance curve, or all three of them may be affected, if desired, I preferably choose the last two transformers so that I will have the maximum number of sharply tuned circuits in the sharp channel .at all times.

It will be noted that in moving the switches from the right hand position towards the left, resistance is inserted in the circuit I59 before any resistance is inserted in the circuit I12; hence the band width of the transformer 35 is narrowed before that of the transformer 46. Inasmuch as the sharp circuit includes the transformer 35, the first narrowing step also narrows the sharp circuit. This is a very desirable feature of the invention, as narrowing the sharp circuit produces a marked increase in the selectivity of the receiver. Whenever the degrees of expansion of the expanded circuits are unequal I may prefer to keep the expanded coupled circuit which is in the sharp channel sharper than the one which is only in the broad channel.

I have found that most standard tubes do not have low enough plate to grid capacity to prevent interaction between their plate and grid circuits, and therefore I prefer to use pentagrid tubes for the tubes 2|, 66 and 36 and to connect them as indicated. This is particularly important where an overall broad fiat top curve is desired. I have found these pentagrid tubes to give fairly good results, although still the operation is not perfect.

Great care should preferably be used in shielding the auxiliary circuits I59 and I12 from each other and from the rest of the circuit so as to prevent feedback, which may be. extremely. detriat all times.

7 the tube 9?.

mental to the operation of the receiver. To avoid confusion this shielding has not been shown.

When the resonance curves of the transformers 35 and IS are broadened there is attenuation in the circuit so that a lower signal tends to be produced in the output. The attenuation in the transformer 35 may be compensated for by the automatic volume control, but the transformer 49 does not affect the automatic volume control. Therefore the action of this transformer has the disadvantage that in tuning the set, if an adjustment is made to broaden the circuits and thus increase the fidelity of the received station, the volume will tend to decrease. In order to prevent this I provide a plurality of resistances I95, I86, I81 and I88, which may be connected in series between ground and the low side of the resistance I 42 in the screen grid supply circuit, so that current flows through these resistances I then ground the cathode I89 of the tube 35 through a resistance I99 and some portion of the series of resistances I85 to I88, and I prefer to do this by means of a switch I9| which may sweep across a plurality of contacts I92, I93, I94, I and I99. The contact I92 may be connected to the juncture of the resistances I95 and E86. The contact I99 may be connected to the juncture of the resistance I86 and Gill. The contact I99 may be connected to the juncture of the resistances I81 and I88, and the two contacts I95 and I96 may be connected together and to ground.

Inasmuch as considerable current is flowing through the resistances I85 to I88 inclusive, the movement of the switch I9I to connect points on these resistances to the cathode I89 will vary the potential on the cathode and therefore vary the gain of the tube 36. Thus, by operating the switch I9I at the same time that the other switches for controlling the band width are operthe particular type of curve desired, and the arrangement shown merely illustrates one manner in which the desired result may be obtained.

As has already been mentioned, I preferably provide a tuned trap I99 in the plate circuit of This trap is preferably chosen so that it will absorb a ten thousand cycle note. When the set is operating with its broadest resonance curve in the broad channel to receivewith maximum fidelity the signal to which it is tuned,

. it may then be broad enough to receive a carrier of the next adjacent station which maybe spaced at 10 k. c. from the carrier of the received signal. The two carriers, the wanted and the unwanted, may then beat together producing a 10 k. c. note which is reproduced by the loud speaker as a high whistle and which may be objectionable to many people. Hence, the trap I99 may be used to filter out this 10 k. c. note. I have found that with its use-although the ob jectionable note is removed, very little difference is noticed in the quality of the wanted signal.

I prefer to have the trap in the circuit at all times but to provide means to short it out when the broad channel is adjusted to have its broadest resonance curve. For this purpose I may provide a switch I91 which may have five contacts I98, the firstfour of. which may be left without connection, and the last one of which may be connected to one side of the trap I09, while the switch arm I9'I is connected to the other side. With the switch on one of the first four contacts the trap is in the circuit, while when the switch is on its lastccntact, the trap is short-circuited.

The trap is made to absorb a 10 k. 0. note because It) hilocycles has been adopted as the standard distance in the ether spectrum for separating broadcasting stations throughout the United States. The characteristics of the trap may of course he changed to comply with any other separation standard, as will be understood.

As it is convenient to operate the band width changing device, as well as the 10 k. c. trap, with a single control, I prefer to connect the arms of the switches I63, IIII, I'IE and I91 all on the same shaft, as indicated by the dotted line I99, to which a single knob (not shown) may be applied so that all four switches may be rotated at the same time. Therefore, when all the switches are on. their farthest left hand contacts the band width controlling circuits are completely open so that the sharpest resonance curve is produced. Also the cathode biasing switch I9I has given the cathode of the tube 99 a predetermined bias and the I639 is connected in the plate circuit of the tube 97. When the switches are moved towards the right to their next position, the band width of the broad channel becomes slightly broader, while at the same time a slightly different biasis given for the cathode of the tube 98 to increase the gain of that tube. Connection forthe tunedtrap I99 remains the same. Movement of the switches to the next contact still further broadens the circuit, increases the gain of the tube iifi, but does not change the tuned trap Hi9 which is still in the circuit. If desired, then, the operator may move the switches to the last contact to the right, and inasmuch as the last two contacts are connected together in the switches III-Ii, I9! and lit, there will be no change in the band width or the gain, but the tuned trap I09 will be short-circuited. These last two contacts, therefore, merely operate to short-circuit the tuned trap I99 or cut it into the circuit so that if the operator has the switches in their farthest position to the right, and is disturbed by a 10 k. 0. note, he can turn the switches back one position and eliminate the note.

When the phase relation between the carrier in the sharp channel and the same carrier in the broad channel is equal to zero or degrees, the best phase relation for maximum demodulation obtains. When the phase difference is 90 degrees or some odd multiple of 90 degrees, the conditions are the least favorable for demodulation. Therefore when the broad and sharp channels are composed of a different number of tuned circuits it will be seen that in the process of tuning in a station it may be possible to alternately go through several phase points which may produce maximum and minimum demodulation. The operator will then pass through several points adjacent the resonant point where the signal will alternately reach a maximum and a minimum, and he therefore may have difliculty in selecting the one which corresponds to the resonance point.

If the operator stops on any point other than the resonance point the quality of the reproduced signal may be considerably impaired. There are two principal reasons for this: One is that as the carrier is nearer one side of the resonance 75,

curve than the other the uniformity of the side band amplitudes is upset, some of the side bands, perhaps, being out 01f completely on one side, and the phase relation of the side bands is altered, so that the overall audio amplitude curve may take an undesirable shape. The other is that interference from one side may be introduced.

For this reason a visual tuning indicator is highly desirable. The arrangement for connecting such an indicator to be operated on the sharp channel is a feature of the invention. Forthis purpose I may use one of the visual indicating devices now in general use, such as the No. 6G5 tube. In order to operate this device I may connect the coil 9| of the resonant circuit 83 in the sharp channel to the grid 200. of a tube 2!, which I prefer to connect between the resonant circuit 23 and rectifier for the visual device to prevent damping of the resonant circuit. The tube 2|]! may be of the combined pentode and double diode type, and the plate 202 may be connected through a resistance 203 to the source of positive potential on the lower terminal I of the power supply. The plate 202 may also be connected through a condenser 204 to the diodes 205 in the same tube, and these diodes may also be connected through a resistance 206 to the cathode 20'! of the tube. The diodes may also be connected through a resistance 208 to the grid 209 of the visual tube ZIO, which, as has already been mentioned, may be of the 6G5 type. The grid 209 may also be connected through a condenser 2 to ground, as indicated, the resistor 208 and condenser 2H forming a filter circuit to prevent radio frequencies from reaching the grid 209.

Variation of voltage on the grid 209, as produced by the direct current drop in potential across the resistor 206, will vary the visual effect on the target 2l2 of the tube Zlll in accordance with well known principles, and the operator can readily see when the set is tuned directly on the carrier of the desired station. Inasmuch as the visual indicating device is connected to the resonant circuit 83 which is the output of the sharp channel, the circuit may be tuned much more aocurately than if the visual indicating device were in the broadly tuned circuit.

From the above description it will be evident that I have provided a receiver for a modulated carrier wave which will reproduce the signal at a substantially constant volume with a high degree of fidelity and with a minimum of interference. The tuning is extremely accurate and an adjustment may be made to eliminate interference at a slight sacrifice of quality, if necessary.

It will be noted that the circuit is designed to prevent overloading so that the tubes in the sharp circuit are operated below their saturation point. I have found that if these tubes become overloaded distortion may be produced by the beating of the carrier in the broad channel with distorted side bands in the sharp channel. Under this condition also the tuning is more difiicult as there appears to be a considerable decrease in volume exactly on the resonant point from points closely positioned at either side thereof.

Many modifications may be made in the circuit connection and the number and character of the parts used without departing from the spirit of the invention, and I do not, therefore, desire to be limited except by the limitations included in the appended claims.

What I claim is:

1. An apparatus of the class described comprising, means to intercept a carrier wave and its associated side bands, means to divide the signal thus intercepted into two channels, one of said channels being narrower in frequency response than the other, means to substantially eliminate demodulation in the broad channel, means to cause the frequencies in the two channels to beat together means responsive to the beat frequencies produced by beating the oscillations of the two channels for demodulating the said beat frequencies to produce only the desired signal, and means controlled by said narrow channel to maintain the volume of the demodulated signal at a substantially constant level.

2. An apparatus of the class described comprising means to intercept a carrier wave and its associated side bands, means to divide the energy thus intercepted into two channels, means to reduce the band width of frequencies in one of said channels without eliminating all of said side band frequencies, means to substantially eliminate demodulation in the other of said channels, means to cause the frequencies in the two channels to beat together to produce demodulation, and means for demodulating the beat frequencies so produced by beating the oscillations of the two channels to produce only the desired signal.

3. An apparatus of the class described comprising means to intercept a carrier wave and its associated side bands, means to divide the energy thus intercepted into two channels, means to reduce the frequency band width of one of said channels, means operated by said last mentioned channel to maintain substantially constant the gain of both said channels, and means to beat the frequencies of said channels against each other, and means for demodulating the beat frequencies so produced to produce only the desired signal.

4. An apparatus of the class described comprising means to intercept a carrier wave and its associated side bands, means to divide the energy thus intercepted into two channels, means to reduce the frequency band width in one channel, means controlled by said last mentioned channel to automatically vary the amplitude of the intercepted carrier wave and associated side bands, and a visual indicator for tuning the apparatus operated from said last mentioned channel at a point therein having a narrower band width than the point at which the amplitude is controlled.

5. An apparatus of the class described comprising means to intercept a carrier wave and its associated side bands, means to divide the energy thus intercepted into two channels, means to reduce the frequency band width of one of said channels, means to substantially eliminate demodulation in the other of said channels, means to cause the frequencies in said two channels to beat together, means for demodulating the product of such beating to produce only the desired signal, means to predetermine the phase relation between the two channels so that the reproduction of signal frequencies is varied for different frequencies in a predetermined manner, and a visual tuning indicator operated from the output of said channel reduced in band width.

6. An apparatus of the class described comprising means to intercept a carrier wave and its associated side bands, means to divide the energy thus intercepted into two channels, means to reduce the frequency band width of one of said channels, means to substantially prevent demodulation in the other of said channels, means to cause the frequencies in said two channels to beat together, means for demodulating the beat.

,ated from said channel having the reduced frequency band width.

7. An apparatus of the class described comprising means to intercept a carrier Wave and its associated side bands, means to divide the energy thus intercepted into two channels, means to maintain the frequency band width of one of said channels sufficiently broad to transmit substantially without attenuating the side bands accompanying said carrier wave, means to reduce the frequency band width of said other channel, means to adjust within predetermined limits the band width of said broad channel, means to cause the frequencies in said two channels to beat together to produce demodulation, means to translate the frequencies produced by said demodulation, and means operated simultaneously with said band width adjusting means to vary the strength of the interceptedsignal as the frequency band width of said broad channel is varied, whereby the amplitude is increased when the frequency band width is increased and the amplitude is decreased when the frequency band width is decreased.

8. An apparatus of the class described comprising means to intercept a carrier wave and its associated side bands, means to divide the energy thus intercepted into two channels, means to maintain one of said channels with a broad enough frequency band to transmit all of the side band frequencies accompanying said carrier without attenuation, means to reduce the frequency band width of said other channel, means to substantially prevent demodulation in said broad channel, means to cause the frequencies in said two channels to beat together, means for demodulating the beat frequencies so produced to produce only the desired signal, means to amplify the frequencies produced by such demodulation, and means included in said amplifying means to absorb a frequency corresponding to the beat note produced by said carrier and an adjacent unwanted carrier.

9. A radio receiver comprising means to intercept a carrier wave and its associated side bands, means to divide the energy thus intercepted into two channels, means to maintain the frequency band width of one of said channels broad enough so that it will transmit substantially all of the side band frequencies accompanying said carrier wave without attenuation, means to reduce the frequency band width of the other of said chan nels without removing all of said side bands, a

balanced demodulator included in said broad channel, means to feed said other channel into.

said balanced demodulator in such a manner that the frequencies of said other channel beat with the frequencies introduced to said balanced demodulator by said broad channel so as to produce demodulation in said balanced demodulator, and means to maintain substantially constant the volume output of said receiver, said means operating from said reduced band width channel and acting upon said receiver at a point preceding said energy dividing means.

10. A radio receiver comprising means to intercept a carrier wave and its associated side band frequencies, means to divide the energy thus intercepted into two channels, means to maintain the frequency band width of one of said channels sufficiently broad to pass substantially all of said side band frequencies without substantial attenuation, means to reduce the frequency band width of the other of said channels but still maintaining some of the side bands, a balanced demodulator in said broad channel, means to feed the frequencies of said other channel into said balanced demodulator in such a manner that the frequencies of the two two channels beat together to produce demodulation, and means to predetermine the phase relation between the two sets of beating frequencies so as to vary the strength of different frequencies produced by demodulation in accordance with apredetermined arrangement.

11. A receiver comprising means to intercept a transmitted carrier wave and its associated side bands, a plurality of coupled tuned circuits separated by thermionic tubes and connected to said intercepting means, means between the first and last of said coupled circuits to divert some of the energy intercepted by said intercepting means into another channel, means to cause beating between the frequencies of the two channels thus formed to produce demodulation, and means to vary the frequency band width of one of said coupled tuned circuits included in both of said channels and another of said circuits included in only one of said channels, said circuit included in both of said channels always having at least as narrow a frequency band width as said circuit included in only one of said channels.

ERNEST A. TUBBS.