US2112686A - Radio receiver - Google Patents

Description

Malmhv 29, 1938.

| E. BARTON RADlo RECEIVER Filed Deo. l, 1933 2 Sheets-Sheet l March 29, 1938. E. BARTON RADIO RECEIVER Filed Deo. 1, 1955 2 Sheets-Sheet` 2 L. W w 1 1 w n Mau 4 MM flow/2 9 1 9. 1 1 1 -ill z ww m Wad u Niv. fr 1 ATTORNEY Patented Mar. 29, 1938 YUNITED STATES PATENT CFFICE RADIO RECEIVER Delaware Application December 1, 1933, Serial No. 700,471

16 Claims.

'Ihe present invention relates to radio receivers and particularly to tuning control devices and circuits therefor.

An objectionable feature of present high quality radio receivers is that they are so selective that they are somewhat difficult to tune. Furthermore, it is a recognized fact that it is desirable to make the tuning of radio receivers still more exact, preferably by means of a very sharp noise suppressor circuit, the noise suppressor circuit serving both for eliminating background noise when tuning between stations and for insuring the proper tuning of the receiver. The selectivity provided by a noise suppressor may be referred to as the apparent selectivity since a noise suppressor increases only the sharpness of tuning and does not increase the selectivity of the signal channel.

The advantage which is obtained by increasing the apparent selectivity of the receiver is usually oifset, however, by the fact that the tuning becomes very critical, and it becomes difficult for the average person to stop the tuning control at the proper tuning point, the proper tuning point being both the correct position for tuning in a signal without distortion and the correct position for releasing the noise suppressor so that the signal is heard.

It is, accordingly, an object of my invention to provide a tuning device for radio receivers which will make them less difficult to tune than when they are equipped with a conventional tuning control device.

It is a further object of my invention to provide improved tuning means for preventing a radio receiver from being tuned inaccurately during the reception of a signal.

More specifically it is an object of my invention l to provide automatic tuning control means which permits a sharply tuned radio receiver to be tuned easily to the exact station position.

It is a still further object of my invention to provide a radio receiver which may be tuned exactly to an incoming signal without the aid of either sight or hearing, and without preselecting the stations.

A still further object of my invention is to provide a radio receiver having an improved indieating device which will indicate when the receiver is tuned exactly to the carrier frequency of an incoming radio signal whereby distortion of the signal due to inaccurate tuning is avoided.

In practicing my invention, in one embodiment thereof I provide a superheterodyne receiver with a very sharp noise suppressor circuit and with an (Cl. Z50-20) automatic volume control circuit so connected that it prevents the signal input to the noise suppressor from exceeding a predetermined value. 'Ihe tuning control device of the receiver is provided with a magnetic brake which is controlled ,5 by the noise suppressor circuit. When the` receiver is tuned exactly to an incoming signal, the noise suppressor circuit functions to permit the signal to pass through the receiver and to energize the magnetic brake, whereby the tuning control device is instantly locked in the correct tuning position.

Before the receiver is tuned to another station position, the magnetic brake must be released. This may be accomplished by means of a release circuit which may be closed by some predetermined movement of the tuning knob, such as an axial movement toward the panel of the receiver, or the brake may be released by means of a circuit which automatically de-energizes the brake 20 after it has been energized a predetermined length of time.

Other objects, features and advantages of'my invention will appear from the following description taken in connection with the accompanying :2-5 drawings in which Figure 1 is a circuit and schematic diagram of one embodiment of my invention;

Fig. 2 is a front view of the tuning dial and magnetic brake shown in Fig. 1; f3@

Fig. 3 is a side view of the structure shown in Fig. 2; and

Fig. 4 is a view illustrating a modified form of my invention.

Referring to Fig. 1, an embodiment of my in- L35 vention is shown applied to a superheterodyne receiver which comprises a radio frequency amplii'ler I having an input circuit coupled to an antenna 3 through a radio frequency transformer 5. The output circuit of the amplifier I is coupled E4.0 to a rst detector 'I through a radio frequency transformer 9.

The secondary windings of the transformers 5 and 9 have variable tuning condensers II and I3, respectively, connected thereacross in the usual ,45 manner, for tuning the input circuitsy of the amplier I and first detector 1 to the desired incoming signal.

An oscillator I5 is coupled to the first detector 1 for the purpose of heterodyning an incoming 50 signal to a lower intermediate frequency signal. The frequency of the oscillator output may be varied by means of a variable tuning condenser l1.

The tuning condensers II, I3, and I1 are pref.- ,55

erably units of a gang condenser, and may be varied simultaneously through a common tuning control indicated by a dotted line |9.

'I'he intermediate frequency output of the first detector is impressed upon the input circuit of an intermediate frequency amplifier 2| through an intermediate frequency transformer 23, the primary and secondary 0f Which are tuned to make the transformer function as a band-pass filter having a pass range of sufficient width to pass the intermediate frequency carrier and. at least one signal side band.

It will be understood, of course, that the tuned' radio frequency circuits of the amplifier I and detector 1 are also tuned broadly enough to pass the carrier of the incoming radio signal and' at least one of its side bands. Y

The output of the intermediate frequency amplifier 2| is fed to the second detector 25 throughk an intermediate frequency transformer 21 tuned in the'same mannerfas the preceding transformer 23.

In the' particular embodiment. illustrated', the second detector 25 is a diode rectifier which is one unit of a double diode-triode vacuum tube 29'.

One terminal ofthe secondary 3| of transformer 21 is connected' tothe detector diode plate 33, While the other terminal ofy the' secondary 3| isl connected to the cathode through a. resistor 3T, the resistor being shunted by means of an intermediate' frequency bypass condenser 39.

The triode section 4| of the Vacuum tube 29 functions as the first audio frequency amplifier. Its control grid 43 is connected to the plate end of the resistor 31, whereby any audio signal appearingacross the resistor 31 is impressed across the input of the triode section and an amplified audio signal appears in the plate circuit thereof. A resistor 38" reduces' the intermediate frequency voltage' that may otherwise' get to the grid 43'.

The amplified audio sigr'ial: is' impressed upon the input circuit of another audio frequency amplifier stage comprising an electric discharge devic'e such as a vacuum tube 45,- The signal? is impressed upon said' input circuit through a cou- `pling system which, in the embodiment shown, includes an audio frequency choke coil 41 inthe plate circuit of the trlode section 4|. The upper end of the chokecoil 41 is coupled to' a volume control resistor 49 through akcoupling condenser 5|, the lower end of the resistor being connected to ground. The potentiometer 49 maybe tapped to obtain the usual compensation of audio frequency signals at various volume levels.

The control grid 53 of the amplifier 45 is connected to the volume control resistor 49 through a variable volume control tap 55.

The output of the amplifier may be further ainplifed by any suitable audio frequency amplifier, generally indicated at 51,- and supplied to a loudspeaker 59.

Y It will be noted thatl the input circuit of the audio frequency amplifier 45 includes an electric discharge device 6| which may be a vacuum tube of the pentode type. The cathode-anode circuit of the tube 6I is connected between the cathode 63 of tube 45 and ground to function as a selfbiasing resistor which may be given a very high resistance to block the amplifier tube 45 when tuning between stations. In other Words, the tube 6| applies noise suppressor control to the audio frequency amplifier tube 45 by functioning as a self-biasing resistor. This feature of my invention is described and claimed my Patent l a pentode section 12 consisting of the cathode 1|,

a control grid 13, a screen grid 15, a suppressor gri'd11?, and an anode 19.

The input circuit of. the pentode section 12 is coupled't'o the resistor 31 in any suitable manner for impressingthe low intermediate frequency voltage drop of the resistor across the pentode amplifier input circuit. In the drawings, this coupling device is shown as a small coupling condenser 8|, having a comparatively low impedance tosignals at the intermediate frequency, but having an impedance` to audio frequency signals sufficient to prevent appreciable audiosignal being applied to the control grid 13.

A- coupling resistor 8-3 is provided which is connectedat one end to the control gridv 13v through a conductor 851, and connectedv at the other end tothe cathode 1|- through an audio-frequency bypass condenser 81. The coupling resistor 83 is shuntedby a variable condenser 89 which may be adjusted for the purpose of controlling the input voltage to the pentode amplifier 12.

The plate circuit `of the pentode section 12 includes the primary winding 6.6 of the transformer 61- anda resistor 9|, the primary winding and the resistor being connected in series, and the lower end of the resistor being connected to a positive point on the voltage divider 93 of the power supply 95.

The secondary winding 68 of the transformer 61 is loosely coupled to the primary winding 66, and both the primary and the secondary are tuned so that the transformer is tuned very sharply to the intermediate frequency.

The amplified intermediate frequency output of the pentode section 12 is transferred through the sharply tuned transformer 61 and impressed uponl the elements 69 and 1| of the diode rectifier through a conductor 91 which connects the diode plate 69 to one terminal of the secondary winding 68, and through a connection from the cathode 1| to the other terminal of the secondary 68, which may be traced from the cathode 1| through a self biasing resistor 99 to ground, through ground to the lower terminal of a resistor IBI, (which may have a` resistance of the order of one megohm), and through the resistor |0| to the said other terminal of the secondary 68.

The sole function of the diode rectifier 10 is to increase the negative bias on the control grid 13 of the pentode Sectio-n upon reception of a radio signal. Since a flow of current through the diode rectifier 10 causes the upper end of the resistor ||l| to become negative, the control grid 13 also becomes more negative because of its connection to the negative end of the resistor |0| through a high impedance resistor |03.

Obviously an audio frequency signal will appear in the circuit of rectifier 10 which should not be impressed upon the control grid 13 of the pentode section. This is prevented by the audio frequency bypass condenser 81.

It will be noted that the self biasing resistor 99, the resistor |0|, and the plate resistor 9| are shunted by the usual intermediate frequency bypass condensers |01, |05, and |09, respectively.

It will be apparent from the foregoing description that as soon as an intermediate frequency signal is impressed upon the input circuit o-f the pentode amplifier 12, the control grid 13 of that amplifier will become more negative because of the current flow through thel circuit of the diode 10.

It may be noted that the change in bias on the control grid 13 depends, to a certain extent, upon the voltage drop produced in the self-biasing resistor 99, since this Voltage drop is in series with the voltage drop along the resistor of the diode circuit. That these voltage drops are in series will be made evident by tracing the direct current path of the pentode section input circuit. It may be traced from the control grid 13 through the resistor 83, the high impedance resistor |03, through the diode circuit resistor 0| to ground, through ground to the self-biasing resistor 99, and through the self-biasing resistor 99 to cathode 1|.

When there is no incoming signal, the plate current of the pentode amplifier portion 12 is large, since the only negative bias on the control grid 13 is that due to the self-biasing resistor 99, and this negative bias is comparatively small. Consequently there is a large voltage drop in the plate resistor 9|, whereby the upper end of the resistor 9| is at a fairly high negative potential with respect to the upper end of the Voltage divider 93. Also, the gain of amplifier 12 is low because of the low voltage on its anode 19.

As soon as the receiver is tuned to an incoming carrier wave, amplifier 12 supplies signal energy to the diode 10, the voltage drop in the diode circuit resistor |0| increases the negative bias on the control grid 13, whereby the plate current of amplifier 12 decreases and the upper end of the plate resistor 9| becomes less negative and the voltage applied to anode 19 is increased. It is the potential at this end of the plate resistor 9| which is utilized for actuating both the noise suppressor and the automatic tuning control, this point in the circuit being identified as point A on the drawings.

It should be noted that although the decrease in plate current tends to cause a reduction in the biasing voltage in the self-biasing resistor 99, the increased negative bias provided by the current flow through the diode circuit is much greater than any decrease in bias due to reduction in plate current.

The above described action of the diode-pentode circuit whereby the upper end of plate resistor 9| becomes less negative in response to reception of a radio signal is preferably made a trigger -action by giving plate resistor 9| a proper resistance value. This trigger action depends upon the fact that the increase in gain caused by the increase in the plate voltage is greater than the decrease in gain caused by the increase in negative bias on the grid until a more or less definite bias is reached. In practice, a tube of the type known as RCA 2B7 has been found satisfactory for use as the tube 65.

A trigger amplifier employing a resistor in the plate circuit of a pentode tube is described and claimed in my co-pending -application Serial No. 704,510, filed December 29, 1933, and assigned to the same assignee as this application.

In order to provide the proper control voltages for the noise suppressor biasing tube 6| and for the magnetic brake control circuit, which is described hereinafter, a potentiometer is connected between the upper end of the plate resistor 9| and the negative end of the voltage divider 93. This potentiometer consists of three resistor sections, R5, R6, and R1, the sections R5 and R1 having a high resistance value which may be of the order of one megohm, while the resistor section R6 preferably has a lower value of the order of 100,000 ohms.

Referring now more specically to the biasing tube 6|, it has a cathode |l3, a control grid I5, a screen grid ||1, a suppressor grid ||9, and an anode 2| The anode |2| is connected to the cathode 63 of the audio frequency amplifier 45, while the cathode ||3 is connected to ground, and through ground to the lower end of the volume control resistor 49. An audio frequency bypass condenser |23 (preferably having a capacity of from 4 to 8 microfarads) is connected between the cathode ||3 and anode |2| for bypassing the audio frequency signal around the plate impedance of the tube 6|. Thus it will be seen that the plate impedance of the tube 9| is connected in the cathode circuit of the audio frequency `amplifier 45 to act as a self-biasing resistor, while the audio frequency signal appearing across the volume control resistor 49 is impressed upon the input circuit of the audio frequency amplifier 45 through the conductor |25 and through ground and the bypass condenser |23.

The anode |2| of the tube 6| preferably is connected to a positive point on the voltage divider 93 through a resistor |21 which may have a value of from one-fourth to one megohm. This resistor serves to maintain a high anode to cathode voltage when the tube 45 is blocked because of any leakage current taken by tube 6|. The effect of the use of resistor |21 in the circuit is to make the noise suppressor more rapid and positive in operation.

When there is no incoming signal, the plate impedance of the biasing tube 6| is maintained at a very high value by means of the control grid ||5 which has a high negative potential sufficient to block the tube. 'I'his high negative grid potential is obtained by connecting the control grid ||5 to the lower end of the resistor R1, preferably through a resistor |29, this end of resistor R1 being identified as point B. Resistor |29 may be given a value of approximately one-half megohm.

The negative voltage applied to the control grid l5 from the point B is not critical in value, approximately 20 volts on the grid, with no incoming signal, having `been found satisfactory when employing an RCA 57 tube as the tube BI.

Point B is given the desired negative voltage with respect to ground, the above-mentioned 20 volts, for example, by the proper selection of the resistor values of the potentiometer In order to understand the operation of the circuit more clearly, specific voltage values for the voltage divider 93, the plate resistor 9|, and the potentiometer will be given.

In one specific embodiment, the voltage divider 93 is connected to ground at such a point intermediate its ends that the lower end of the voltage divider is 150 volts negative with respect to ground, while the upper end of the voltage divider is 250 volts positive with respect to ground. With no incoming signal, and a plate resistor 9| having a value of from 50,000 to 100,000 ohms, the plate current of the pentode produces a voltage drop in the plate resistor 9| Ofsuicient magnitude to make the upper end of the resistor 9| and potentiometer III (identified as point A) 25 to 50 volts positive with respect to ground. The point B is then approximately 20 Volts negative with respect to ground, and the upper end of kresistor R5 (identified as point C) is still more negative Withrespect to ground.

Thus, with no incoming signal, a constant biasing voltage is applied from the point B to the control grid I I5 of the biasing tube 6|, a bypass condenser I3I being connected between the control grid II5 and the cathode ||3 for preventing audio frequency voltages or other ripple voltages from being applied to the grid.

The biasing voltage applied to they control grid II5 when there is no signal input is suflicient to block the tube 6|, thereby making its plate resistance so high that the audio frequency amplifier tube 45 is likewise blocked. The blocking potential applied to tube 45 may become as high as 80 volts, this being much higher than necessary to block a. tube such as an RCA56 which has been found satisfactory for use with the RCA57.

When the receiver is tuned exactly to an incoming signal, an intermediate frequency voltage is impressed upon the input circuit of the pentode section 12, whereby the voltage drop in the plate resistor 9| is decreased in value, causing the point B to assume a value sufficiently less negative to unblockrthe biasing tube 6| and greatly lower its plate impedance. This causes a normal biasingvoltage to be applied to the grid 53 and the audio frequency amplifier 45 is immediately changed to its normal condition for an amplifier.

It should be noted that in case the point B becomes positive with respect to ground, it will cause vgrid current to flow through the resistor |29 so that the grid I I5 will be maintained at substantially zero bias.

As fully explained in my above-mentioned patent, the characteristics of the pentode 6I are such that the plate impedance of the tube remains substantially constant within the' range of voltage applied to thecontrol grid during the time the amplifier 45 is effective. 'I'his range of voltages is determined by the adjustment of. the trigger amplifier 65. In a preferred adjustnient the potential of point B is changed instantly from approximately 20 volts negative to about 3 Volts negative upon reception of a signal.

Further slight voltage changes may be caused by variations in the strength of an incoming signal but they will not cause changes in the impedance of tube 6|. For example, if the tube 6I is an RCA57, its plate impedance will remain substantially constant for control grid voltages ranging from about 4 volts negative to zero and, since the .variation inplate voltage of tube is much greater than is necessary for the control of grid II5, considerable variation in signal may take place without affecting the bias on tube 45.

The above-described circuit may be made so selective that it is an impractical circuit to employ if the receiver is to be tuned in the ordinary way. To overcome this diiculty and also to increase the ease of tuning, even when the receiver is not unusually selective, I provide a tuning control circuit whichincludes a control tube |33 and a magnetic brake |35.

The control tube |33 includes a cathode |31 which is connected tonground, a control grid 39 which is connected to the upper end of. the resistor R5 through a grid leak resistor |4I (which may have a resistance of the order of one megohm), and an anodey |43 which is connected through a conductor |45, the magnet coil |41 of the brake |35, and a conductor |49 to a positive point on the voltage divider 93. The control grid |39 is also connected to the point A 'through a condenser I5| which may have a value of approximately 0.1 microfarad. f

So long as no signal is being received, as when the receiver is tuned between stations, the point C applies aynegative potential to the control grid |39 which is sufficient to block the tube |33 so that there is no plate current flow through the brake winding |41, and the brake |35 is deenergized.

There is, however, a small iiow of current through the magnet winding |41, which flows in a direction opposite to the flow of plate current, for the purpose of removing residual magnetism from the brake after it is de-energized. This iiow of current may be traced from the positive end of the voltage divider 93, through a resister |53 (which may have a resistance of. the order of 100,000 ohms) to the conductor |45, and through the magnet winding |41 and the conductor |49 to the positive point on the voltage divider 93.

The instant the receiver is tuned to the car.-

-rier wave of an incoming signal, the voltage applied to the control grid |39 of the brake control tube I 33v is changed. If it is changed sufciently, plate current flows through the magnet coil |41 of the brake |35 and the brake is instantly energized, thereby clamping the rotors of the tuning condenser II, I3, and I1, and the tuning knob |55, in the exact position at which the signal is properly tuned in.

In the apparatus illustrated in Fig. 1 the control grid of the brake control tube |33 is so connected to the potentiometer II I that the brake |35 is automatically released `a predetermined length of time after it has been energized. When employing this automatic release circuit the receiver is tuned by rotating the tuning knob |55 rather slowly in case it is desired to tune in th next incoming signal.

As soon as the receiver is tuned in this manner to the carrier of the next incoming signal, the potential of the point A becomes more positive, and this more positive potential is transferred through the condenser I5I tothe control grid |39 of the tube |33. 'I'he comparatively slow rotation of the tuning knob |55 gives suflicient time for the control grid |39 to assume a potential such that plate current flows through the magnet coil |41 and the brake is energized. This locks the tuning condensers II, I3, and I1 securely in position, and the operators fingers may slip on ther tuning knob if further rotation is attempted. After an instant (one half to two seconds), the grid |39 again assumes the potential of the point C which is sufficiently negative to block the tube |33 even when there is an incoming signal. This releases the brake |35, and the receiver may be tuned to the next station in the same manner.

If it is desired to tune through one or more stations without having the brake operate, the tuning knob is rotated more rapidly so that the potential at A cannot increase appreciably because of the condenser |09 and the control grid I 39 does not acquire the necessary more positive potential for operating the brake each time the receiver is tuned to. a carrier wave.

It will be understood that, in general, the band pass filters 23 and 21 will be designed to pass the intermediate frequency carrier and the upper and lower side bands, the pass range of each filter ,being made so narrow for selectivity purposes that the intermediate frequency carrier must fall at the mid-point of the pass range in order to avoid the cutting off of a side band. It is essential, therefore, that the receiver be tuned accurately to an incoming signal so that the intermediate frequency carrier falls at the proper point in the pass range of the filters, as otherwise the quality of the received signal will be impaired. If the transformer 61 in the output circuit of tube 65 is tuned sharply enough to the intermediate frequency, the magnetic brake |35 will be operated only when the receiver is tuned in accordance with the above requirements for high quality reception.

In practice, it is found both difcult and eX- pensive to provide a control circuit for the magnetic brake which is so sharp that the selectivity of the circuit is itself sufficient to prevent the magnetic brake from being operated too soon if strong signals are impressed upon the control circuit. This will readily be understood since the brake is operated when the plate current of tube 65 reaches a predetermined value. If signals of varying strength are impressed upon the input circuit of tube 65, a very strong signal at a frequency on one side of the resonant curve for the transformer 61 might produce the same plate current flow as a weaker signal at the intermediate frequency which is located at the middle or peak of the resonant curve.

It has been found that the magnetic brake |35 may be made to operate at the proper time when employing a control circuit having reasonably sharp selectivity if the strength of all signals applied to the control circuit is held substantially uniform. A convenient way of accomplishing this is to provide the receiver with an automatic volume control circuit or AVC circuit which is applied to the portion of the receiver preceding the point supplying voltage to the brake control circuit.

While various forms of automatic volume control may be utilized, the circuit illustrated in the drawings is simple and effective in operation. The AVC circuit includes one of the diode rectifiers |6| of the vacuum tube 29, this diode comprising the cathode 35 and a plate |63. The plate |63 is connected through a filter resistor |65 and la volume control resistor |61 to ground. The junction point of the resistors |65 and |61 is connected to the control grids of the tubes 1 and 2| through resistors |69, |1|, and |13, respectively. A lter condenser |15 is connected between the grid end of the iilter resistor |65 and.l ground.

The cathode 35 is connected to the negative end of the voltage divider 93 through a cathode resistor |11. 'I'his resistor is bypassed by means of an audio frequency bypass condenser |19.

The operation of the AVC circuit is as follows: Assuming that a strong signal is being received, there is then maximum flow of current through the second detector resistor 31 and the control grid 43 is at a rather high negative potential, whereby the plate current of tube 29 is small and there is a small voltage drop in the cathode resistor |11; Therefore, a large part of the voltage across the lower section I 8| of the voltage divider 93 is impressed upon the diode rectifier I6 This will be seen by noting that the grounded point of the voltage divider 93 is connected to the plate |63 of the diode rectier |6| through ground and through the volume control resistor |61, this grounded point being positive with respect to the cathode 35. The voltage across the voltage divider section |8| is large compared with the voltage drop in the volume control resistor |11 under the conditions assumed, so that although the two voltages are in opposition, the greater part of the voltage across the voltage divider section IBI is applied to the rectifier |6|.

This results in a comparatively large flow of current through the resistor |61, with the result that the voltage drop in resistor |61 maintains the control grids of the tubes 1, and 2| highly negative with respect to their cathodes, whereby their gain is held at a low value.

If the strength of the received signal becomes less, the control grid 43 becomes less negative, the plate current and the voltage drop in the resistor |11 increase, and a lower voltage is impressed across the rectifier |6|. As a result, the current ow through the volume control resistorv |61 decreases and the control grids of the tubes 1, and 2| become less negative, whereby the gain of the receiver is increased to bring up the signal strength.

From the above description it will be apparent that the output of the transformer 21 remains substantially constant in amplitude so that signals of uniform amplitude are applied to the brake control circuit whereby it will function uniformly regardless of the strength of incoming signals. Thus, the tuning condensers will never be stopped by the brake before they have been rotated to the proper position.

The above described automatic volume control circuit is described and claimed in my application. Serial No. 640,946, filed November 3, 1932, and assigned to the same assignee as this application.

While various forms of magnetically operated brakes may be employed in my automatic tuning circuit, the brake |35 shown in detail in Figs.,2 and 3, has been found to be especially effective.

t comprises a U-shaped core |83 of magnetic material having a pair of magnet windings |41 on the legs of the core. The ends. of the core legs are flattened on one side to permit their use as a braking surface. f

A flat bar |81 of magnetic material is. mounted opposite the at surfaces of the core legs by means of hinge supporting members |89 which permit the bar or armature |81 to rest against a disc segment |9|. 'I'he supporting members |89 may be supported from a bar |90 of non-magnetic material secured tothe core |83. l The disc segment |9| is fastened to the shaft of the tuning condensers with the edge thereof positioned between the flat surface of the magnet core |83 and the armature |81. In a preferred embodiment, the disc segment |9| is mounted upon the tuning dial |93 at the outer edge of the dial. This disc segment |9| is preferably made of magnetic material in order tov increase the effectiveness of the brake.

An inspection of Fig. 3 willshow that the air gaps in the brakes magnetic circuit arey very short so that the reluctance of the circuit is low, and the brake Very efficient. Referring to Fig. 4, there is shown a manually operable release for the magnetic brake |35, this release being actuated by pushing in on a tuning knob |95. In Figs. 1 and 4, like parts are indicated bythe same reference numerals. In the circuit shown in Fig. 4, the control grid |39 of the tube |33 is connected to a point D on the poten` tiometer which is sufficiently less negative than the point C to permit the control grid |39 becoming positive, or nearly so, upon the reception of an incoming signal whereby tube |33 is unblocked and the brake is energized.

It will generally be desirable to have the tube |33l and the biasing tube 6| start drawing plate current at the same time. Therefore, the point on the potentiometer to which the control grid 39 should be connected depends upon the relative characteristics of the two vacuum tubes. If the tube |33 Were the same type as the tube 6|, the control grid |39 preferably would be connected to the same point as the control grid of tube 6|, that is, to the point B.

Although the condenser |5| has been shown in connection' with the manual brake release, it may be omitted if desired. Its use, however, will cause the magnetic brake to respond somewhat more rapidly to the reception of an incoming signal than it would otherwise since the increasing potential of point A will be applied momentarily to the grid |39.

The tuning knob |95 of the receiver is slidably mounted upon a shaft |91 in non-rotatable relation therewith, and provided with a flange |99 for actuating a switch when the tuning knob is pushed in. 'I'he tuning knob is normally held pushed out away from the panel of the receiver, indicated in dotted lines at 203, by means of a spring 205 as indicated in the drawings.

The tuning knob shaft |91 may be connected directly to the tuning dial and tuning condenser shaft, or it may be connected thereto through a friction drive. In the drawings, the two shafts are shown mechanically connected through a friction clutch 201, comprising two friction plates which are held against each other in any suitable manner. Intuning the receiver, the tuning knob |95 is rotated While it is in the out position indicated in Fig. 4. As soon as the receiver is tuned to an incoming signal, the bias on the control grid |39 is so reduced in value that plate current flows through the magnetic brake |35 and the tuning condensers are locked in position. Any further rotation of the tuning knob |95 will cause the friction clutch 201 to slip and the tuning of the receiver will not be changed.

In order to tune the receiver to the next incoming signal, the tuning knob is pushed in to close the switch 20|. This places a high negative potential on the control grid |39, thereby blocking the tube |33 and releasing the brake |35. 'Ihe tuning knob |95 is then rotated far enough to tune the receiver away from the station that was being received, which action will cause the point D to become sufficiently negative to block the tube |33; 'Ihe tuning knob is then permitted to move back to its original position away from the panel 203, thereby opening the switch 20| and bringing the receiver to its former condition. By further rotation of the tuning knob, the next incoming signal may be tuned in, thus causing the brake to be energized again.

If desired, a lamp may be substituted for the magnetic brake as a device for indicating when the receiver is tuned exactly to the desired radio station. For example, a neon lamp 209 may be supported in the rear of the tuning dial |93, as shown in Fig. 4', and so connected that it may be included in the' plate circuit of tube |33 by moving a switch arm 2|| to its middle position.

It will be noted that when the switch arm 2| is in the middle position, the magnetic brake |35 is disconnected so that it cannot operate. Therefore, the release switch 20| has no function where the lamp alone is used. In tuning the receiver, the receiver is tuned until the lamp 209 lights up, thus showing that the receiver is accurately tuned to an incoming signal.

In one form of my invention, the lamp 209 may be used as described above with the noise suppressor omitted. In this form, the signal may be heard before the receiver is tuned Ato the point where the signal is undistorted but the operator Will know that the receiver is not properly tuned unless the lamp: has become lighted. The main feature of a receiver designed in this manner is that the circuit which controls the lamp 209 is sharply tuned to the intermediate frequency and, therefore, is much more sharply tuned than the signal channel.

If preferred, a signal lamp may be employed in combination With the magnetic brake. Thus a lamp 2|3 is positioned behind the tuning dial |93 or in -any other suitable location and provided with any suitable source of current, such as a battery 2|5. A relay 2|'1 may be connected in the plate circuit of tube |33 by closing a switch 2|9. When the receiver is tuned exactly to an incoming signal, the relay 2 1 is actuated and the lamp 2|3 is lighted.

In some cases, and particularly where the release mechanism of Fig. 4 is employed, it may be preferred to have the radio receiver respond to strong incoming signals only, such as those of local stations, whereby the brake will be actuated by a comparatively small number of radio stations. This result can be obtained by providing any suitable form of sensitivity control for the receiver. In the circuit shown in Fig. 1, such a control is provided by making the self-biasing resistors of tubes I, 1, and 2| variable. The resistance values of these resistors may be varied simultaneously by means of a common control device indicated by the dotted line 22|.

The use of the magnetic brake in combination with the noise suppressor has a further advantage, in addition to those mentioned in the foregoing description. Because of an appreciable time lag in the action of the noise suppressor, an operator might easilytune through a desired station before the suppressor released if the brake were omitted. It will be noted that a considerable part of this time lag is introduced by the resistor |29 and condenser |3|, the circuit of condenser |3| h-aving a fairly large time constant since condenser |3| must be large enough to bypass audio frequencies, preferably having a capacity of the order of 0.25 microfarad.

'Ihe control circuit for the brake may be, and is in the circuit above described, given a time constant less than that of the noise suppressor control circuit. Thus the brake responds to an incoming signal and stops the tuning condensers before the noise suppressor releases.

Various other modifications may be made in my invention without departing from the spirit and scope thereof and I desire, therefore, that Cil ually operable tuning knob, means 'for making a movement of said tuning knob effective normally to produce a corresponding .movement of said tuning element, and means responsive to the reception of a signal for making said tuning knob ineective to produce a movement of said tuning element.

2. A radio receiver for the reception of modulated carrier waves, said receiver comprising a demodulating device and converting means for converting the carrier Wave of an incoming signal to a predetermined frequency, said means including a tunable oscillator, lter means for transferring said converted carrier wave to said demo-dulating device, said lter means having a pass range sufciently wide to pass the converted carrier having said predetermined frequency and at least one side band thereof, a control circuit coupled to receive energy from the output of said converting means and sharply tuned to said converted carrier frequency, means responsive to the energy passed by said control circuit upon the reception of a si-gnal for controlling the tuning of said oscillator, a manually operable tuning knob, means for making a movement of said tuning knob effective normally to produce a corresponding change in the tuning of said oscillator, and said means which is responsive to the energy passed by the contr-ol circuit upon the reception of a signal being operative to render the tuning knob movement ineffective to produce a change in the tuning of the oscillator.

3. In -a radio receiver tunable over a predetermined frequency range, a variable tuning device, sensitivity control means for adjusting the response of said receiver to signals of predetermined strength, means for rendering a portion of sai-d receiver inoperative until the receiver is tuned to an incoming signal of said predetermined strength, and means responsive to the reception of said incoming signal for automatically preventing undesired movement of said tuning device, said second and last-named means having differing time constants of operation whereby the tuning movement is stopped in advance of the release of the inoperative condition of the receiver.

4. In a radio receiver, a signal selecting circuit, a variable tuning element in said selecting circuit, a manually operable tuning knob coupled to said tuning element, means including a magnetically operated brake for locking said tuning element in position in response to the reception of a signal, and means for releasing said brake by a predetermined movement of said tuning knob.

5. A magnetic brake for a radio tuning device, said brake comprising a disc segment of magnetic material coupled to said tuning device, a U-shape core positioned with its ends opposite one side of said disc segment, a magnet winding on said core, and an armature bar of magnetic material movably supported on the opposite side of said disc segment and opposite the ends of said core.

6. In a radio receiver tunable over a predetermined frequency range, a variable tuning device, a noise suppressor circuit comprising means for rendering a portion of said receiver inoperative when the receiver is not tuned to an incoming signal and for rendering said portion operative in resp-onse to tuning said receiver to an incoming signal, and a control circuit comprising means responsive to said receiver being tuned to an incoming signal for preventing undesired movement of said tuning device, said control circuit having a time constant which is shorter than the time constant of said noise suppressor circuit.

7. In a radio receiver having a continuously variable tuning device, a manually operable tuning knob, means for making a movement of said tuning knob effective normally to produce a corresponding movement of said tuning device, means responsive to the reception of a signal for making said'tuning knob ineffective to pro-duce a movement of said tuning device, and adjustable means for making said last means effective only when the strength of said incoming signal is above a predetermined amplitude.

8. In a radio receiver having a tuning device, a magnetic brake for said device, said brake comprising a magnetic core having a winding thereon, means for producing a flow of current through said winding in one direction in response to said receiver being tuned to an incoming signal, and means for producing a ow of current through said winding in the opposite direction in response to the receiver being tuned away from said signal.

9. In a radio receiver, a signal selecting circuit, a variable tuning element in said selecting circuit, a tuning knob for said tuning element, a magnetic control device for said tuning element, said control device including a winding, an electric discharge tube having a control grid and having an output circuit which is coupled to said winding, a resistor section, a condenser and a grid resistor connected in series across said section, said grid being conductively connected to the junction point of said condenser and said grid resistor, means normally for maintaining a voltage across said resistor section such that said grid is made sumciently negative to substantially block said tube, and means for so changing the voltage across said section in response to tuning said receiver to an incoming signal that said tube is unblocked for a period determined by the time constant of the circuit including said condenser and said grid resistor.

10. In a radio receiving system, the combination of a plurality of tuning condensers, a shaft directly connected with and movable to operate said condensers simultaneously thereby to tune said receiver continuously through a predetermined frequency range, a plate directly carried by said shaft, a magnetic brake for locking said plate, shaft and condensers when said brake is energized, means for energizing said brake including a circuit sharply tuned to a predetermined frequency whereby said tuning condensers are locked in position when said receiver is tuned exactly to a predetermined frequency, and means for causing said brake to operate in response to a signal of a predetermined strength only in response to movement of said tuning shaft at a rate below a predetermined value.

1l. A radio receiver for the reception of a signal modulated carrier Wave, said receiver having a signal channel response of sufficient width to pass said carrier wave and at least one and not more than both side bands, a signal selecting circuit having a movable tuning element therein, and means including a sharply selective signal circuit for abruptly locking said tuning element in response to the reception of a signal, said last circuit being responsive to said carrier wave and more selective than said signal channel, and means for adjusting said response to signals of a predetermined strength.

12. A radio receiver comprising a signal selecting circuit tunable over a predetermined fre- 'que'nc'y range, an intermediate frequency amplier coupled to said selecting circuit, automatic volume control means for said amplifier, a tuning control circuit coupled to said amplifier to receive signals therefrom at substantially constant amplitude, said control circuit being more selective than said signal selecting circuit to the same signal, means including a brake responsive to the energy passed by said control circuit for controlling the tuning of said signal selecting circuit, and means for automatically deenergizing said brake after it has been energized for a predetermined length of time.

13. A radio receiver comprising a signal selecting circuit tunable over a predetermined frequency range, an intermediate frequency amplifier coupled to said selecting circuit, automatic volume control means for said amplier, a tuning control circuit coupled to said amplifier to receive signals therefrom at substantially constant amplitude, said control circuit being more selective than said signal selecting circuit to the same signal, means responsive to the energy passed by said control' circuit for controlling the tuning of said signal selecting circuit, said last-named means including a tuning device for said firstnamed circuit having a movable tuning element, a magnetic brake for said element and a control tube through which said brake is energized, said control tube having acontrol grid connected with said tuning control circuit, and means in said last-named connection for causing said brake to operate only when said tuning element is moved relatively slowly.

14. In a radio receiving system, the combination of a plurality of tuning condensers, a shaft directly connected with and movable to operate said condensers simultaneously thereby to tune said receiving system continuously through a predetermined frequency range, a plate carried by said shaft, a magnetic brake for locking said plate, shaft and condensers when said brake is energized, means for energizing said brake including a circuit sharply tunedto a predetermined frequency, Whereby said tuning condensers are locked in position when said receiver is tuned exactly with said predetermined frequency, said circuit including an amplifier tube, automatic volume control means for applying signals of substantially constant amplitude thereto, means for deriving .a signal-responsive control vpotential therefrom, a control tube for the brake operative in responsev to said potential, Aand means providing a predetermined time constant for the' operationl of said control tube' and brake.

15. In a radio receiving system, the combination of a plurality of tuning condensers, a shaft` directly connected with and movable to operate said condensers simultaneously thereby to tune said receiving system continuously through a predetermined frequency range, a plate carried by said shaft, a magnetic brake for locking said plate, shaft and condensers when said brake is energized,- and means for energizing said brake' including a circuit sharply` tuned to a predetermined frequency whereby said tuning condensers are locked in position when said receiver is tuned exactly with said predetermined frequency, said circuit including an amplifier tube, automatic volume control means for deriving a signal-responsive control potential therefrom, and means for causing said brake to operate when said tuning shaft is moved relatively slowly, said last-named means including a signal rectifier, a control tube for the brake connected with said rectifier to receive a controlling potential therefrom, and a filter in said connection for regulating the time constant of the operation of the control tube in response to change in the controlling potential.

16. In a radio signal receiving system, the cornbination with a variable tuning device therefor having a movable element, of a tuning control circuit including a control tube and an electromagnetic brake, one element of said brake being rigidly connected with said movable tuning element to move therewith, means for energizing said brake to lock said movable tuning element against tuning movement, including an electromagnet Winding for the brakeA in circuit with said control tube to receive anode current therefrom, a grid circuit for said control tube having a time delay filter means therein, means for applying a signal-responsive control potential to said grid circuit through said filter, and automatic volume control means for controlling the amplitude of said potential, whereby said last-named means may be more broadly responsive to a received signal of relatively high amplitude.

LOY E. BARTON.

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