US2001146A - Pretuned radioreceiver and method of producing same - Google Patents

Description

May 14, 1935. I H, M. LEWIS PRETUNED RADIORECEIVER AND METHOD OF PRODUCING SAME Original Filed April 14 192 2 Sheets-Sheet l 1F llk gllllll INVENTOR HAROLD M. LEWIS mm. scrrmos ATTORNEY y 1935- H. M. LEWIS 2,001,146

PRETUNED RADIORECEIVER AND METHOD OF PRODUCING SAME Original Filed April 14 92 2 Sheets-Sheet 2 INVENTOR HAROLD M. LEWIS ATT'ORNEY Patented May 14, 1935 UNITED STATES PATENT OFFICE PRETUNED RADIOBECEIVER AND METHOD OF PRODUCING SAME Original application April 14, 1926, Serial No.

Divided and this application March 4, 1927, Serial No. 172,603

10 Claims.

This invention relates to radio receiving apparatus and particularly to radio receiving apparatus embodying a plurality of circuits comprising inductance and capacity, and tunable to resonance to select the desired signal; more particularly it relates to apparatus such as that described in which more than one of such circuits are arranged to be tuned simultaneously by the operation of a single controlling or indicating means, and is a division of my copendlng application Serial No. 101,848, filed April 14, 1926, for Pretuned radio receiver and method of producing same". My invention has particular utility in, and is particularly well adapted to use in, broadcast receiving apparatus, where it is particularly desirable to produce a receiver which will operate by means of a single control to select signals of desired frequency, and reject those of other frequencies and which will be fully as sensitive and selective under the action of said single control as it would be were separate controls provided for each tuning circuit.

It has been attempted in the past to produce such receiving apparatus, but without success, so far as I know. Most of these attempts have consisted simply of equalizing, as far as pwsible, the inductance and capacity of the circuits desired to be tuned by the single control and arranging the variable controlling elements of each circuit to be mounted upon or driven by a single control or indicator,- whereby it was hoped that the desired characteristics would be achieved.

This has not proved to be the case, however, for the reason that such a receiver requires tuning circuits which all tune precisely to the frequency required, at the proper setting of the indicator or control, and heretofore, it has been impossible to obtain such a result or a result suillciently close thereto to be successful. No matter howcare- 40 fully the coils and condensers have been selected in commercial receivers of this type, it has'been found that such receivers are relatively insensitive, particularly to weak signals, and relatively inselective to strong signal; as compared with similar receivers with separate controls for the various tuning circuits, by reason of failure of the tuning circuits exactly to match or fulfill the desired tuning relation.

For these reasons, when a radio receiver is desired having single tuning control operation, which is the full equal from the standpoint of sensitivity, selectivity, and reliability of a receiver embodying the same number of stages but having independent tuning controls, further re-- .35 nnements beyond matching of coils and condensers are desirable. In general, the apparatus employed should be arranged in particular ways which are hereinafter pointed out. In order to It will be understood that bythe use of all the features herein disclosed, in their preferred form, a single control receiver may be produced which is the full equal in every respect of a receiver having separate tuning controls; however, the cost of such a receiver will naturally be somewhat increased thereby, and in general, various of such features may be omitted for the sake of decreasing the cost, it being understood, however, that the benefits of the invention will in general, be sacrificed thereby to some extent.

It is an object of this invention to provide a uni-control receiver which will eliminate the disadvantages referred to above and, in general to produce'a receiver which shall operate by only a single control, without vernier tuning elements, and which shall be fully as sensitive and selective as if the same were used. It is a further object of this invention to provide a method and means for modifying or adjusting the main tuning elements of a uni-control receiver to compensate for unavoidable residual differences in the tuning circuits, so as to .provide exactly the desired relation between the tuning characteristics of the various tuning circuits over the entire range of the receiver, which method I term "pretuning the receiver.

It is a further object of this invention to produce a receiver of the class described, which may be quickly, easily and economically readJusted, in case it shall be thrown out of adjustment,

as for instance by rough handling in transportation. i

It is still a further object of this invention to provide a receiver of the class described which shall be as economical as possible to manufacture and in which the pretuning processes shall interfere as little as possible with the quantity production of such receivers. Still further obiects will be apparent from the specification.

. The features of novelty which I believe .to be characteristic of my invention are pointed out with particularity in the appended claims. My invention itself however, both as to its underlying principles and as to its practical application, will best be understood by reference to the specification and to the accompanying drawings in which:.

Fig. 1 represents a radio receiver of the type commonly known as a tuned radio frequency amplifier constructed according to my invention;

Fig. 2 represents a different type of receiver most commonly known as a superheterodyne, as constructed in accordance with my invention.

Fig. 3 shows a form of multiple condenser constructed in accordance with my invention.

Fig. 4 is a sectional view of lines 44 of Fig. 3;

Figs. 5, 6, '7 and 8 are detail sectional views showing specific forms of condenser plate deforming means according to my invention:

Fig. 9 illustrates a particular type of wavemeter which may be used to advantage in pretuning a receiver according to my invention; and

Fig. 10 is a chart showing the operation of my invention.

According to my invention I utilize a multiple condenser, or a plurality of condensers controlled and driven together, as for example by being mounted upon a single shaft, connected together by gears or the like, in such manner that they all operate by the same control for varying the tuning in a plurality of tunable circuits. In the preferred form of my invention, one or more of these condensers is provided with one or more plates which may be deformed, bent, warped, or otherwise modified so as to vary the capacity of that particular condenser as desired. More specifically in the case of two condensers arranged for uni-control I provide a flexible plate on one of them and also provide means for deforming, bending or warping, or otherwise modifying the flexible plate at various points, to change the spacing normally existing between said plate and the adjacent plate of opposite electrical polarity, thus changing the capacity of the condenser. It will be clear that each condenser may be provided with such means if desired but in general I prefer to leave one condenser without any such means and to provide all the others with it. This simplifies the adjustment or pretuning of the receiver, since it is necessary to adjust only a part and not all of the condensers.

In carrying out my invention I prefer to make use of electrical condensers which are normally as closely matched as it is possible to make them without incurring excessive expense and I prefer also to use coils which are similarly matched as closely as possible as this decreases the amount of adjustment necessary to cause each of these circuits to tune as desired, as will be readily understood.

In the preferred form of my invention I provide a rotor member adapted to interleave with a stator member, each of the members consisting of a plurality of spaced plates as is well known in the art and the rotor being provided with a plate physically outside the stator plates when the condenser is set to maximum capacity. The outside rotor plate will preferably be made of a flexible metal and at a little distance therefrom will be provided a relatively inflexible plate which I term a carrier plate. This carrier plate may be of sufficient thickness to be inflexible or of difierent material, or may be arranged in any suitable way so as to be sufficiently strong to stand the stress which it will be called upon to bear, without being appreciably deformed. This carrier plate is provided with means such as push-pull screws extending over and attached to the outer rotor plate, and the arrangement of these push-pull screws and their attachment to the flexible plate and to the carrier plate is made such that the flexible plate may be pushed or pulled toward or away from the adjacent stator plate at various points or zones by operation of the push-pull screws.

Referring now more particularly to Fig. 1, I have shown my invention as applied to a radio receiver of the type known as a tuned radio frequency amplifier and comprising a first stage of radio frequency amplification having a vacuum tube amplifier l, a second stage of radio frequency amplification having vacuum tube amplifier 2 and a detector circuit having vacuum tube detector 3. The operation of such a system being well known in the art, I do not consider it necessary to go into detail in describing it except as regards the application of my invention therein.

As will be readily understood, incoming signals are received by means of a suitable antenna 4 and a signal E. M. F. is thereby developed in coil l6 which under the proper conditions is amplifled by tube I, passed on to tube 2, there amplified again, and passed-on to tube 3 where it is detected. The output of said detector may operate a suitable indicator diagrammatically shown as telephones 5, or may be amplified to the extent desired and used to operate a loud speaker. Suitable batteries are provided to energize the various tube circuits, such for instance as A battery having adjustable resistance 2| in series therewith for controlling the filament lighting circuit, B batteries of 22% and volts for example for energizing the plate circuits of the detector and amplifier tubes respectively, and C battery of the proper value for biasing the grids of the amplifier tubes to the extent desired. In addition suitable by-pass condensers such as II, II and 20 are provided having the proper values to perform their functions as is well known in the art.

For the purpose of tuning coils I8, I and i to the frequency of the incoming signals simultaneously I provide condensers II, II and I! simultaneously operable by common driving means I! provided with a suitable indicator or knob ll. The negative side of the filament lighting circuit will preferably be grounded as at 22 and taps I! will be provided connected to coils I5 and I. which in turn are connected to the rotor of condenser II. The voltage developed in the input circuit of tube l controls the grid-filament potential of said tube, thereby controlling the space current in said tube, and through coil I developing a voltage within coil I. Coil I is tuned by condenser II and its voltage is impressed upon the'grld-filament circuit of tube 2 thereby controlling the flow of current through coil I and developing voltage upon coil 9. Condenser l2 tunes coil 9 and the voltage developed therein is impressed upon detector tube 3 through grid condenser 23. The detecting action of the tube may be brought about by means of grid leak 24. Suitable means may be utilized to prevent oscillation, such as resistances, tube capacity neutralization, or the like, as will be understood. In Hg. 1 there are shown, as a conventional example of such oscillation prevention means, stabilizing resistors R, each resistor being disposed in the grid circuit of each stage, as is well known to those skilled in the art.

When all of the grid circuits in a tuned radio frequency receiver designed to cover the present broad cast range, are tuned by means of a condenser shunted across the secondary winding of the step-up transformer used for coupling adiacent tubes, the instability of the system will be such that it will generally be desirable to use stabilizing resistances or tube capacity neutralization, in order to eliminate the necessity for the user to prevent such oscillation by other expedients, such as decreasing the tube plate potential or filament heating current, which decreases the sensitivity and may distort the outputof the receiver. In generaL'the lower the range of frequencies desired to be covered, the less will be the necessity for the use of such stabilizers.

In practice I prefer to mount the rotors of condenser 10, H and I2 upon conducting shaft l8 and ground said shaft through by-pass condenser 28 which has practically zero impedance to currents of radio frequency. If the condensers are not so mounted they will preferably have their rotors connected together and grounded for a purpose to be described later. I5, I I and H, are preferably constructed as shown in Fig. 3. In this figure I have shown condensers Ill and II as provided with suitable means for warping one of the plates on the rotor of each of said condensers, to effect a minor variation in the capacity of each of said condensers at various points in the setting thereof. Condenser I2 is not provided with such means as the same is not necessary. However, it will be understood that if for any reason it is desired to provide each of the condensers with such means it may be done.

In assembling the multiple condenser, I provide a plurality of plates 58 held in spaced parallel relation and at fixed distances from each other by suitable sections of rod 5| threaded and tapped and secured at their ends by nuts 52. On certain of the plates 58 I secure blocks of suitable insulating material 55 as for instance by screws 54 projecting thereinto. The stators of the various condensers are assembled from plates 55 held in spaced relation by suitable spacing washers or the equivalent and the entire stator assembly is held in position by means of bolts 58 passing through the plates 55 and spacing washers into the insulating blocks 53.

A shaft 88 is provided which may be a single piece or a plurality of pieces jointed together passing through all of the plates 55 and properly mounted and secured for rotation therein. The rotor members of the condenser II, II and I! are mounted upon and carried by the shaft 55. Each of the rotors consist, for example, of a plurality of plates 51 supported by suitable spacing washers and held in place upon the shaft 85 by suitable collars, lock nuts or the like. The stator and rotor plates may both be of any desired shape although I prefer to form them as described in the joint application of Harold M. Lewis and Jackson H. Pressley filed May 19, 1925, Serial No. 31,272, entitled Variable electrical condensers and tuned circuits, so that equal mechanical motion of the condenser shaft produces equal increases in the resonant frequency of the circuit. In other words, I providefor what is commonly called straight-line frequency tuning.

As will be seen by reference to Fig. 8. condenser if has a rotor consisting of four plates 51 of similar construction. Condensers II and II however, have plates 51 of this construction and in addition have a plate 58 of flexible material adapted to be deformed. bent, warped, or otherwise modified to vary the capacity of the condenser within a particular region or zone by a small amount. As an example of a suitable way in which this may be done, there may be pro- Condensers I vided for condensers l and II a relatively stiff plate 59 which I term a carrier plate. This plate may be made much thicker than the other plates in order to be stiff, or it may be made of material having greater rigidity. Mounted upon the plate 59 at a plurality of points around its periphery I have provided screws 60 as will be seen more clearly by reference to Fig. 4. The plates 59 are provided with holes screw-threaded to engage the threads of screws 68 whereby these screws may be longitudinally positioned in plate 59 as desired. Referring to Fig. 5 showing a detail of plates 58 and 59 and screw 60, it will be noted that each of the screws 60 is provided with a projecting portion 53 of smaller diameter than the remainder of the screw, thereby defining a shoulder at the end of the threaded portion. Flexible plates 58 are provided with holes of sufiicient diameter to permit projection 63' to pass through and be rotatable therein after which a head 84 may be formed on projection 83 as for example by hammering thereby rotatably attaching screw 60 to plate 58. It will now be clear that when screw 60 is rotated, it is caused to move longitudinally which in turn will cause flexible plate 58 to be pushed or pulled in the region adjacent thereto.

It will be understood that the amplitude of the movement of the flexible plate 58 will be greatest in the region which is the closest to the particular screw 80 in question. The amplitude of movement will progressively diminish in a radial direction toward the shaft 80 and will also diminish in a peripheral direction toward adjacent screws 68 at which point no appreciable movement can take place because the periphery of plate 58 is fixed in position at these points by the adjacent screws 60. It will thus be seen that by means of any one of the adjusting screws 60 the position of plate 58 may be modified as desired to effect small increases or decreases in the capacity of the condenser at any particular setting whereby the tuning characteristic of the circuit controlled thereby is appropriately modified.

After each of the screws has been positioned so as to give the desired capacity to condensers ill and I i in the particular zones, the screws are fixed in position by tightening down lock nuts 62 against carrier plates 59 after which the flexible plate 58 will be securely held in the position to which it has been adjusted.

While I have shown and described a particular means for bending or warping the plate 58 I do not wish to be limited thereto, as clearly other means may be used to perform the same function. For example, Fig. 6 shows an alternative form. In this instance screw 68 instead of being provided with a head 5| is provided with aknurled portion 65 somewhat less in-diameter than the screwthreaded portion and in this instance plate 59 is not screw-threaded but is simply provided with a hole through which screw 60 may pass freely. An additional lock nut 86 is provided on the opposite side of plate 58 from lock nut 52. The method of fastening this modified screw to plate 58 may be the same as previously described. In order to adjust plate 58 in this instance to bring it closer to plate 59 lock nut 86 will be loosened and lock nut 52 tightened while screw 88 is held by portion 85 until plate 58 has been positioned as desired. If, on the other hand, it is desired to move plate 58 away from lock nut 82 it will be loosened and lock nut 85 will be tightened.

Still another possible modification is illustrated in Figure 7. In this instance, a pin 61 may be provided which may be welded to plate 58 so as to project therefrom and pass through a hole in carrier plate 59 and project on the opposite side therefrom. In order to adjust the position of plate 58 the projecting portion of pin 61 may be grasped by the fingers, or by means of a suitable tool and pushed in or pulled out to the desired point. A small drop of solder may be then placed upon the pin to secure it in position with reference to plate 59. Still another possible modification is shown in Fig. 8 in which pin 61 is provided with screw-threaded portion 68 adapted to engage internal screw-threads in a suitable sleeve 69 positioned within a hole in carrier plate 59 and secured therein by bushing I0. A portion of the sleeve 69 may be spun or turned over bushing 10 to secure the sleeve in position with sufllcient clearance to allow for rotation. It will be apparent that rotation of this sleeve will force plate 68 either toward or away from plate 09.

Referring again to Fig. 1 it will be evident from what has been said that it is desired that the in-put circuits of the tubes I, 2 and 3 should all tune to the same identical frequency, i. e.,. the desired signal frequency, for any given setting of the indicator I4. It will also be understood that although the circuits will tune approximately to the same frequency for any particular setting the frequencies will not be identical. Forinstance, and merely by way of example, at a given setting of indicator I4 the input circuit of tube 3 may be tuned to 500 kilocycles; the input of tube 2 to 501 kilocycles and the input of tube I to 499 kilocycles. By the adjustment of the plate warping means, according to my invention it is possible to modify the capacity of condensers H and III so that the input circuits of tubes 2 and I will tune to 500 kilocycles as desired.

It is to be borne in mind however, that the adjustment made to these condensers so that the circuits will tune to 500 kilocycles should not prevent or interfere with the adjustment of the condensers at some different frequency, say 1000 kilocycles, to bring the circuits all in tune at that frequency. This is possible only when the adjustment of each of the condensers is made in a certain way, or in other words when a predetermined procedure is carried out in making the adjustment which I term pretuning.

In order to make the adjustment, 9. suitable source of external oscillations is desirable. This may be awave-meter such as is shown diagrammatically at II in Fig. 9 and may comprise a radio frequency oscillator, for example, a vacuum tube provided with a feed back circuit, whose output may be modulated in any suitable way as for instance by an audiofrequency oscillator which again may be a vacuum tube provided with a feed back circuit, to provide an audible signal. In the output circuit of the wavemeter I have provided a pair of condenser plates I2 and 13 and an additional plate I4 normally balanced as to electrostatic exposure between said plates and carrying practically no current, but which may be moved to destroy the balance, at which time currents will flow through conductor I5 connected to said plate I4 and to conducting spring clip I6. Assuming that the wavemeter 'II produces signals of' any frequency desired, within the range of the receiver to be pretuned, the procedure may best be carried out as follows: Indicator or control knob I will be set at such a point that condensers III, II, and I2 are set at substantially minimum capacity, or near such minimum capacity, or so that the first adjusting screw is about to pass or has just passed the edge of the adjacent adjusted so that the frequency of oscillations which it generates, is the frequency to which the input circuit of detector tube 3 is tuned. The clip I6 is now removed from the plate lead of tube 2 and attached to the plate lead :from tube I,

thereby using coil 0 to impress the wavemeter signal on coil 1, thereby bringing amplifying tube 2 into operation to amplify the wavemeter sig- 7' nal. The input circuit of tube 2 is approximately in tune with the detector input circuit, but may now be brought into exact tune by adjusting the first screw 60, thereby warping the plate 58 until the minimum capacity 'of condenser is correct asevidenced by a maximum signal re-' sponse onthe indicator I. This indicates that -condensers II and. II have now exactly equal capacities, or at any rate .thattheir capacities are such that the input circuits of tubes 2 and 3 are I now exactly. tuned to the same frequency. a

At this point the adjustment is set by means of the lock nut 02 and spring clip" may be dcto the input terminal'of coil I. An actual antenna or a -dummy antennais likewise connected at one ofthe points II. Condenser I0 will now be adjusted by means of the. screw 60 nearest the edge of the adjacent stator plate, asv

above described for condenser II, until the input circuit of tube I in operative condition, withthe antenna connected, is tuned to the wavemeter frequency as indicated by a maximum response in indicator 5. From what has been said it will be understood that the receiver has now been adto this new setting of the input circuit of detached from its former position and connected tector tube 3. Clip I8 is then advanced to the plate lead from tube I and condenser II adjusted at this frequency by the adjusting screw nearest the edge of the stator plate. Similarly condenser III is adjusted at this new setting of the condensers by advancing clip I6 to the input terminal of coil I5. It will be understood that a suitable volume control is provided by movable plate H of the wavemeter, which may be adjusted to maintain a comfortable level of signal from indicator 5 as the number of stages in use is varied in the operation just described. The

same procedure will be carried out until all of the I adjusting screws on each of the condensershave been adjusted to give maximumresponse in the indicator, after which it will be found that .the receiver is entirely pretuned. That is to say, at

each position of the indicator I4, each of the circults will be tuned exactly to the same frequency or so close thereto that no difference canbe detected, and since the rotors of the condensers are all grounded the body capacity of the operator will not interfere with either the adjustment of the receiver or its subsequent operation. I

It will be understood that adjusting screws 60 may be spaced closer together and a greater'number may be provided than the number-which I have shown. I have found however, that 4 adjusting screws to each condenser positioned subtheretofore made.

stantially as shown in Fig. 4 are suflicient to pretune a broadcast receiver such as that shown in Fig. 1 as closely as is necessary to give it the desired characteristics, for the reason that the departure from exact synchronism of the several circuits before pretuning is naturally distributed and gradual in the nature of a smooth curve, over the range of frequencies to be received, and is not local and abrupt. The limited number of adjusting screws shown therefore sufllce to correct, over the range of frequencies to be tuned to, the tuning of the several circuits although appliedat a limited number of points.

It should be noted at this time that the adjustment must be made in such a way that the setting of an adjusting screw other than the first screw on the condenser does not interfere with the settings already made. For this reason I have found it necessary to begin adjusting with the condenser at the minimum capacity setting and to progress toward the maximum capacity setting. The reason for this will be understood when it is pointed out that if it were attempted to start the maximum capacity setting and progress to minimum capacity setting each progressive adjustment would upset the adjustment For instance, if at maximum capacity setting it were found necessary to move the end adjusting screw, the position to which it would be moved would of course depend upon the position of all other portions of the plate. In other words, the minimum capacity areas of the condenser plates are of course effective in determining the capacity of the condenser at the maximum setting. If, then, the condenser were moved to some smaller capacity setting and a different portion of the plate adjusted, this would upset the adjustment already made at the maximum capacity setting. The effect of this would be that each subsequent adjustment would interfere with the prior adjustment whereas by progressing from minimum capacity setting to maximum capacity setting this does not take place.

It is clear that my invention may be applied to any type of receiver in which a plurality of circuits are employed which tune to resonance in a desired and predetermined manner. As an example of the application of my invention to a different type of receiver, I have shown in Fig. 2 its application to a receiver of the type known as a superheterodyne.

Referring now to Figure 2, I have shown a radio frequency amplifier 30 deriving signal energy from a. loop antenna 42 which is tuned by a suitable condenser 43. The output of amplifier tube 30 is passed through coil 33 coupled to coil 34 which in turn is included in the grid circuit of detector tube 3|. The output of detector tube 3| is fed to coil 35 forming the primary of an intermediate frequency transformer of which the coil 33 forms the secondary. The voltage produced in coil 36 is amplified by intermediate frequency amplifier 31 to the extent desired as is well understood in the art. The output of the intermediate frequency amplifier 31 may be detected and used to operate any suitable indicator either before or after audiofrequency amplification.

As will be understood in the art, a suitable local oscillator will be provided consisting of a vacuum tube 32 provided with a tuned input circuit consisting of coils 33 and 33 and condenser 45 and a feed back arrangement which may for example comprise output coil 43 and input coil 39. As this arrangement is well understood in the art it wi l not be further described. A coil 4| will preferably also be provided in the input circuit of detector tube 3| which coil 4| is coupled to coil 40 of the local oscillator for the purpose of combining the local oscillations with the incoming signal oscillations to produce oscillations of intermediate frequency after detection. A suitable grid leak 5| may be provided in shunt with grid condenser 50 and the necessary batteries and bystood, these circuits do not tune to the same frequency but they do tune in a predetermined relation. The tuned circuits associated with tubes 30 and 3| should tune to the same frequency. The local oscillator, however, should tune to a frequency different from that of the circuits just described by the value or amount of the intermediate frequency. When this is the case, beat frequency currents are produced which are amplified by the intermediate frequency amplifier as is well understood.

The application of my invention to a circuit of this type it is believed will be readilyunderstood from what has already been said and from a. further description. Condensers 43, 44 and 45 are all controlled by a single indicator or common dial. They may, for example, be mounted upon the same shaft or as shown, on different shafts geared together, and should be of the type already described for producing straight line frequency tuning. The constants of the various circuits are adjusted so that for any position of the single control means 49, circuits 30 and 3| will tune approximately to the same frequency and the local oscillator will be tuned to a frequency differing therefrom by the intermediate frequency as described in the application referred to above. This relation will hold approximately true for any setting of the common indicator 49 but it will not be exactly true for the reasons heretofore pointed out. In other words, variations in well matched apparatus will occur so that there may be a slight departure from the desired relation for any setting.

The tuning characteristics of the local oscillator may be as represented byline A of Fig. 10, the abscissa: of which represent dial setting and the ordinates of which represent the resonant frequency of the circuit. Curve B may be considered to represent the tuning characteristics of the detector circuit and curve C the tuning characteristics of the amplifier circuit. It will be seen that the relation desired is expressed by the curve A and the dotted curve, where curve A represents the tuning characteristics of the local oscillator as already described, and the dotted curve the tuning' characteristics of tubes 30 and 3|. In order to bring about the desired relation, curve B must be swung to coincide with the dottedcurve by the removal of capacity from condenser 44 at the minimum capacity setting and the addition of capacity thereto at the maximum capacity setting as shown by the arrows of Fig. 9 whereas curve C must be varied in the opposite sense. The procedure for adjusting a receiver of this type will be the same in principle as that already described, modified as may be desired by the exigencies of the particular case as will be well understood by those skilled in the art.

While I have shown and described in Fig. 2 the adjustment of condensers 43 and 44 to the frequency of the local oscillator it will be clear that any other circuit may be used as a standard if desired or that all of the condensers may be provided with adjusting means.

While I have referred to the use of a wavemeter it will be understood that the receiver may be pretuned to actual incoming 8 -1 For instance, if during transportation of the receiver to a purchaser from the factory, rough treatment has distorted the condensers and spoiled the adjustment, the user may readily readjust the receiver himself, while connected to the-antenna with which it is to be used.

Ordinarily, it might be supposed that the use of different antenna with my receiver would upset the tuning thereof. I have found, however,

that the average antenna has a definite set of characteristics, and if coils l5 and I. are loosely coupled, the variation of constants between average antennae does not affect the operation of my receiver. If, however, it is to be used with a very long or very short antenna, it is well to reset the adjusting screws of the first radio frequency amplifier tuning condenser.

In general slight variations in antenna need not be compensated for if an average antenna is used in pretuning, as several terminals II can be provided for loading the antenna actually used so that it conforms with the average antenna used in pretuning.

While similarly shaped plates may be used condensers Ill, Ii and I2 and the effect of the average antenna taken care of by the adjusting screws of condenser l0, timewill be saved in pretuning the receiver if the plates of condenser II are shaped so that while its maximum capacity is thesame as before, its capacity near the minimum setting is less. The plates may conveniently have theform shown in dotted lines in Fig. 4, thereby largely decreasing the time n to pretune the first amplifier circuit.

Also, it should be noted that I find it advantageous to provide condensers having plates of relatively large area and few in number so that warping of a single plate produces a relatively large percentage change of condenser capacity.

While I have shown and described certain specific forms of my invention and methods of carrying the some out, it will be apparent that modifications and changes may be made without departing from the spirit and scope of my invention.

I claim:

1. In a radio receiver, a plurality of thermionic tubes in cascade, each tube having an anode, cathode and at least one grid, a tunable oscillatory circuit connected between the cathode and grid of each tube, each of said oscillatory circuits including a variable tuning condenser, a conductive, actuating shaft having the rotor units of the said condensers directly amxed thereto in spaced relation along the length of the shaft thereby electrically connecting the respective rotor units of the condensers at a common potential, the shaft being arranged to operate said nits in unison, a common means for maintaining the grids of said tubes, said shaft and rotor units biased at a common potential which is more negative than that of the cathodes, and a capacitive path of low impedance for radio frequencies directly connected between said conductive actuating shaft and the cathodes.

2. In a radio receiver, a plurality of thermionic tubes in cascade, each tube having an .anode, cathode and at least one grid, a tunable oscillatory circuit connected between the cathode and grid of each tube, each of said oscillatory cir- 'means and said cathodes to maintain said actuating means and units at said different potential.

3. In a radio receiver, a plurality of thermionic tubes in cascade, each tube having an anode, cathode and at least one grid, a tunable oscillatory circuit connected between the cathode and grid of each tube, each of said oscillatory circuits including a variable tuning condenser, a conductive, actuating shaft having the rotor units of the condenser directly afiixed thereto thereby electrically connecting the respective rotor units of the condensers at a common potential, the shaft being arranged to operate said units in unison, a common means for maintaining the grids of said tubes, said shaft, and rotor units biased at a common potential more negative than that of any portion of the cathodes, and a capacitive path of low impedance for radio frequencies directly connected between said conductive actuating shaft and the cathodes.

4. In a radio receiver, a plurality of thermionic tubes, each tube having an anode, cathode and at least one grid, a tunable oscillatory circuit connected between the cathode and grid of each tube, each of said oscillatory circuits including a variable tuning condenser, a conductive, actuating means electrically connecting the respective rotor units of the condensers at a common potential and being arranged to operate said units in unison, a common means for maintaining the grids of said tubes biased at a potential more negative than that of any portion of the cathodes, and a condenser of low impedance for radio frequencies directly connected between said conductive actuating means and the cathodes, said common biasing means being connected between the said conductive actuating means and said cathodes, and a connection between one side of said condenser and ground tomaintain said actuating means and rotor units at ground potential wi respect to radio frequency potentials.

5. In a radio receiver, a plurality of thermioni tubes in cascade, each tube having an anode, cathode and at least one grid, a tunable oscillatory circuit connected between the cathode and grid of each tube, each of said oscillatory circuits including a variable tuning condenser, a conductive,-actuating shaft electrically connecting the respective rotor units of the condensers at a common potential and being arranged to operate said units in unison, and a condenser of low impedance for radio frequencies directly connected between said conductive actuating means and the cathodes.

6. In a radio receiver, a plurality of electron dischargetubesincascadaeochtubehavingan anode, cathode, and at least one grid, an oscillatory circuit connected between the cathode and grid of each tube, each oscillatory circuit including a variable tuning condenser, a metallic shaft electrically connecting the respective rotor units of the condensers, at a common potential, the shaft being arranged to have said units physically affixed thereto thereby to operate said units in unison, a path of substantial impedance for radio frequencies between the shaft and the cathodes of said tubes, and a condenser of low impedance to radio frequencies connected across said first path and directly between the shaft and the said cathodes.

'7. In combination in a radio receiver, a plurality of multi-electrode tubes connected in cascade and each having an anode, a cathode, and at least one grid, tuned oscillatory circuits operatively connected with said tubes and each including a variable tuning condenser, a conducting shaft supporting the respective rotor units of said condensers in electrically conductive relation, means for connecting the conducting shaft with the grid of each of said tubes whereby the grids have the same normal direct current potential as the shaft, means interposed between the cathodes of the tubes and the conducting shaft for maintaining the shaft and the grids negatively biased with respect to the cathodes, and a condenser for bypassing radio frequency energy connected directly between the shaft and said cathodes.

8. In combination in a radio receiver, a plurality of multi-electrode tubes connected in cascade and each having an anode, a cathode, and at least one grid, tuned oscillatory circuits operatively connected with said tubes and each including a variable tuning condenser, a conducting shaft supporting the respective rotor units of said condensers in electrically conductive relation, means for connecting the conducting shaft with the grid of each of said tubes whereby the grids have the same normal direct current potential as the shaft and rotor units, means interposed between the cathodes of the tubes andthe conducting shaft for maintaining the shaft, rotor units, and the grids negatively biased at a common potential with respect to the cathodes, a condenser for by-passing radio frequency energy connected directly between the shaft and said cathodes, and additional means for preventing self-oscillation when said circuits are tuned alike.

9. In combination in a radio receiver, a plurality of multi-electrode tubes connected in cas-- cade and each having an anode, cathode, and at least one grid, tuned oscillatory circuits operatively connected with said tubes and each including a variable tuning condenser, a conducting shaft supporting the respective rotor units of said condensers in electrically conductive relation, means for connecting the conducting shaft with the grid of each of said tubes whereby the grids have the same normal direct current potentialas the shaft, means interposed between the cathodes of the tubes and. the conducting shaft for maintaining the shaft and the grids negatively biased with respect to the cathodes, a condenser for by-passing radio frequency energy connected directly between the shaft and said cathodes, and additional means for shielding said condensers from each other to prevent reaction between said oscillatory circuits.

10. In a radio receiving system, in combination, a plurality of thermionic vacuum tubes for amplifying incoming signals, and a tube for detecting the amplified incoming signals, a potential source for biasing the grids of said amplifier tubes more negative than the cathodes thereof, step-up transformers interposed between adjacent tubes for coupling said tubes in cascade, a multiple variable condenser comprising a frame and a -plurality of continuously variable condenser sections carried by the said frame, each section including a stator and a cooperating rotor, connections for operatively associating the secondary winding of each transformer with one of said condenser sections respectively, to provide a plurality of continuously variable tuning circuits, interposed between said tubes, means for preventing the generation of self-excited oscillations in said system when said tuning circuits are tuned substantially alike, and a single shaft electrically connecting the rotors of said sections and the aforesaid grids for operating said condenser sections in unison to permit the selection of desired signals with only a single tuning adjustment, said biasing source having its negative terminal conductively connected to all of said rotors through said single means and its positive terminal connected to the cathodes of said amplifier tubes.

HAROLD M. LEWIS.

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