US2080646A - Visual resonance indicator - Google Patents

Description

May 18, 1937.

H. A. WHEELERv VISUAL masolmw INDICATOR Filed July '7, 1927 2 Sheets-Sheet Sve INVENTOR Haro/d A. Wqe/@l' BY 15a/1, Wma fnd?.

ATTORNEYS May 18, 1937. H. A. WHEELER 2,080,646

VISUAL RESQNANCE INDICATOR Filed July '7, 1927 2 Sheets-Sheet 2 3y farmi!) Patented May 18, 1937 UNITED STATE VISUAL RESONANCE INDICATOR Harold A. Wheeler, Baltimore, Md., assigner to Hazeltine Corporation, Jersey City, N. J., a corporation of Delaware Application July 7, 1927, Serial No. 203,879

14 Claims.

'I'his invention relates to visual resonance indicators and more particularly to said indicators utiliaed in modulated carrier-current signaling systems wherein the limit of amplification-is au- 5 tomatically maintained substantially at a predetermined level.

When amplifiers are employed for amplifying a signal voltage it becomes desirable for various reasons to control automatically the amplitude l of this amplified signal voltage. Certain embodiments of an automatic amplication control arrangement in a modulated carrier-current receiver are dcribed and claimed in United States Letters Patent Re. 19,744, granted October 29, l 1935, upon my application Serial No. 745,651, being an application for reissue of Patent No. 1,879,863, granted September 27, 1932, upon my original application Serial No. 495,386,1iled November 13, 1930, which is a division of the present application. Automatic amplification control such as describedin that patent is particularly advantageous in radio receivers such as are employed for receiving broadcast signals, because it prevents the overloading of the last amplifier stage of the receiver, which overloading Awould result in distortion of the reproduced signal, as well as loud and harsh reproduction.

Another advantage of such an arrangement resides in uniform reproduction of the amplied carrier-current signal irrespective of whether the signal is received from a nearby station or from a distant station, or a high-power station, Y or a, low-power station, since it has been found in former radio receivers that when the receiver was reproducing strong signals as from a nearby, or a high-power station, the audibly reproduced signal was very loud, whereas when the signal was received from a distant, or a. low-power station, it was relatively weak, with the result that if signals were to be reproduced uniformly from both near and distant stations, and from highpower and low-power stations, it became necessary to readjust some volume controlling means in the receiver to compensate for these unequal 45 signals.

It has also been a common experience in the use of former radio receivers that the reproduced signal was not uniform due to the phenomenon of fading, whereby the received signal occasion- 50 ally, or periodically, became much weaker, or faded almost to the point of inaudibility. Since an arrangement as disclosed in the above-mentioned patent provides an amplifier which automatically compensates for inequalities in the re- 55 ceived carrier-current signal strength, when (Cl. Z50-20) fading takes place the degree of amplificationV is correspondingly increased and the reproduced signal maintained at its former volume, so that a listener is unaware that variation of the received carrier-current signal is occurring. This 5 automatic compensation for signal fading is especially advantageous in commercial radio telephony and likesystems.

In present-day receivers in which operating current is derived from the municipal power l0 system, it has been found that when there is considerable variation in the line voltage supply, the volume of the reproduced signal is not uniform. An additional advantage of Vthe arrangement described above is that of automatically l5 compensating for such line voltage variations with the result that the reproduced signal is uniformin volume.

A still further advantage is the saving in plate current which is automatically effected during the receptionof powerful signals, for the reason that the arrangement described above incidentally provides means for reducing the plate current of one or more amplifying tubes as the signal strength increases.

There is, however, one disadvantage attendant upon applying automatic amplification control to a radio broadcast receiver and that is the dimculty o f determining when the receiver is precisely tuned to the desired signal. In the ar- 30 rangements of the prior art, the condition of precise tuning corresponds critically to the maxlmum sound output of the receiver so that accurate tuning of the receiver can readily be accomplished by observing the sound output. With 35 automatic amplification control applied to the receiver, however, the sound output varies only within a relatively narrow range so that it is impracticable, or at least difiicult, to tune the receiver by its audible response.

It is an object of the present invention, therefore, to provide a novel and improved means for obtaining an indication of the condition of tuning of a modulated-carrier signal receiver embodying means for maintaining its signal output amplitude within a narrow range for a wide range of received signal amplitudes.

It is another object of the invention to provide in a receiver of the type described, means for giving a visual indication of resonance with a desired signal-carrier frequency.

In accordance with the preferred form of the invention, a modulated-carrier signal receiver comprises a signal-translating channel including at least one carrier-frequency vacuum-tube ampliner having a tunable signal-selecting circuit coupled to its input circuit and a. sound-reproducing means coupled to its output circuit. The receiver includes also means responsive to variations of received signal intensity for controlling the amplification, and thus the space current, of such tube or tubes inversely in accordance with amplitude of the received signal carrier to maintain the sound output of the receiver within a relatively narrow range for a wide range of received signal intensities, whereby audible indication of ce with the desired signal is difiicult or impracticable. In order to obtain an indication of the accuracy of tuning of the receiver to a desired signal, there is provided a visual rce indicating means connected to be responsive to the output of the amplifier or signal-translating channel. It will be apparent that,allsignalstobepreciselytuned in, the response of the visual resonance indicator will be a measure of the amplitude of the received signal carrier, that is, the resonanceindicator acts also as an indicator of relative signal strength. y

Fig. 1 is a circuit diagram of a complete radio receiver which includes the present invention, and consists of a three-stage radio-frequency amplifier followed by a rectifier, a two-stage audio-frequency amplifier, and a loud speaker, or other suitable indicating device.

Fig. 2 shows curves disclosing the relation be- I tween the radio-frequency antenna voltage and the radio-frequency amplified voltage, with and without the application of automatic amplification control. Y

Fig. 3 shows a" circuit diagram of a second embodiment of the invention in which there is disclosed a three-stage tuned radio-frequency ampliiler, a, rectifier, and a three-stage audio-frequency amplifier.

Flg. 4 shows graphically a comparison between the perfomance of the two-electrode valve or rectifier, and ofthe three-electrode detector.

Referring in detail to Fig. 1, there is shown an antenna 5 connected to ground I thru the primary winding I of a radio-frequency transformer, the secondary winding I of which, tuned by a variable condenser 8, is connected at one point to the filament 21 of the vacuum tube 9 in the rst radio-frequency amplifying stage and at another point to the grid II of this vacuum tube. 'I'he output circuit of this vacuum tube extends from the filament system, thru a high-voltage battery B, a milliammeter Il, primary winding I3 of a. second radio-frequency transformer to the anode or plate Il of this vacuum tube. In order to neutralize the inherent capacity between the grid II and the plate Il, and thereby to prevent oscillations, a neutralizing winding I9, electromagnetically coupled to winding Il, and a neutralizing condenser 3 are employed in' the manner described in the U. S. patents to `Hazeltine Nos. 1,489,228 and 1,533,858.

A second stage of radio-frequency amplication including the vacuum tube I5 neutralized by cooperation of coil 26 and condenser I, like the first stage, comprises the secondary winding I6 of the last-mentioned radio-frequency transformer tuned by a variable condenser I1 connected between the filament system of the vacuum tube I5 and the grid I8 thereof. The output circuit of this vacuum tube also includes the high-voltage battery B and a primary winding 20 of a second `radio-frequency transformer, while the secondary winding 2l of this tl'lllml' lllned by a,oso,s"4e

a vanbie condenser n u included m me input circuit of a third stage oi radio-frequency smplification which includes vacuum tube 23. The

inherent capacity effective -between the electrodes 2l and Il is neutraliaed by a network -in alent such as a three-electrode vacuum tube. as.

shown, having its grid I2 and its plate or anode I6 directly connected together to comprise in effect a single anode.

In the absence of the control circuit Si, to be described, the signal-translating channel comprising the three-stage tuned radio-frequency ampliner, including the vacuum tubes I, Il, and 23 and rectifier Il, functions in a manner well known in the art to amplify selectively the incoming modulated-carrier signals of any carrier amplitude within a wide range which are intercepted on the antenna l. The output circuit of the rectier 3l includes what may be termed a reiector" circuit for stopping radio-frequency currents which have passed thru the rectier, and consists of a network including a resistance ll and a by-pass condenser 81 connected between the anode l5 and the filament 3l of the rectiner. The output circuit of the rectifier -iscoupled to the input circuit of an vaudio-frequency amplifying vacuum tube ll thru an audio-frequency-pass nlter including a ilxedcondenser II and an impedance resistance 4I connected between the filament 42 and the grid 4l of this vacuum tube. As appears from the constants hereinafter given, the characteristics of the. illter are such that it passes the audio-frequency component to the input circuit of the audioor modulation-frequency amplifier, while preventing the uni-directional component from being impressed upon the input circuit thereof. The output circuit of this amplifier is connected between the lament 42 and plate 44 thru the highvoltage battery B and the primary winding l5 of an audio-frequency transformer the secondput circuit of a second audio-frequency tube 41,

while a resistance 4I connected across the winding 46 serves to give the audio amplifier substantially uniform amplification over the desired frequency range. Instead of employing resistance III, a closed copper band of suitable size may be placed around the transformer winding so as to be electromagnetlcally-coupled thereto. A loud speaker or other reproducing device Il, or if required, a coupling device for a telephone system, isconnected in the output circuit of the last audio-frequency amplifying tube Yl'l. It is presumed that adequate precautions against undesired electromagnetic coupling between the various radio-frequency coupling transformers are included in all of the arrangements herein dis closed.

'I he degree of amplification effected in the radio-frequency amplifying stages is automatically controlled by a biasing potential` obtained by rectifying the modulated signal carrier in a twoelectrode rectifier 33, having a resistance 5I, connected between the filament 38 and the anode or output electrode 35 of the rectifier, thru which the pulsating rectified or converted current fiows, thereby developing .a negati\'e voltage at terminal 52. This negative voltage is applied over Conductor 36 thru the resistance 53 and the secondary winding I of the first radio-frequency transformer to grid IIVof the first radio-frequency amplifier stage. Resistance 53, together with blocking condenser 54, is effective in producing a time constant predetermined to filter out and rject voltage fluctuations at frequencies of modulation of the selected and amplified signal, that is, any audio-frequency voltages which otherwise might be applied from conductor 36 to the grid I I.

To complete the description of the system illustrated in Fig. l certain design data or constants are given herewith., It should be understood. howeyer, that these, as well as other constants appearing in the present specification. are mentioned merely by way of example in describing certain specific embodiments which in practice have proved satisfactory, and are not intended to suggest any specific limitations as to the scope of this invention. Accordingly, fixed condenser 2 may be of 0.0005 microfarads; 31 of 0.0001 microfarads;` 54 of 0.01 microfarads; 40 of 0.005 microfarads; resistance 5I of 1 megohm; 34 of 1 megohm; and 4I and 53 of 2 megohms each. The tubes may be assumed to be all of the well-known 201A type.

In the operation of the receiver shown in Fig. 1, a modulated-carrier signal intercepted on the antenna 5 is selected and successively amplified thru the neutralized tuned radio-frequency amplifier stages indicated by the vacuum tubes 9, I5 and 23 connected in cascade. The selected and amplified signal is then rectified by the rectifier 33, and the rectified pulsating current is successively amplified by the audio amplifying stages,

including vacuum tubes 39 and 41, after which it may be reproducedas sound by the loud speaker 50. The high resistance 5I connected between the filament 38 and the anode 35 of the rectifler, through which a small space current flows in the absence of signal output from the radiofrequency amplifier, maintains the anode normally negative relative to at least part of the filament of the rectifier. Since all the filaments are connected in parallel, the rectifier filament is maintained at substantially the same potential as the filament 21 of the rst radio-frequency amplifier tube 9. Therefore, the resistance 5I is connected effectively between the rectifier anode 35 and the amplifier filament 2,1, and thereby determines the normal negative potential of the rectifier anode relative to at least part of the amplifier filament or cathode.' When the rectified or converted signal voltage across resistance 5I increases with signal output beyond a predetermined value, there is developed at the anode terminal 52 sufficient increase of negative biasing voltage which in turn is impressed, thru the conductor 36, upon the grid or control electrode II of the vacuum tube 9, to reduce the amplification of this tube. Conversely, it will be apparent that as the magnitude of the rectified current fiowing thru resistance 5I decreases with decreasing signal strength, the direct voltage at terminal 52 becomes less negative, and the negative biasing voltage impressed upon the grid II also diminishes so that the vacuum tube 9 effects an increased degree of amplification.v In this manner, the radio-frequency voltage applied to the input of the rectifier is maintained within a relatively narrow range, and the volume of the reproduced signal is substantially uniform for a wide range of received signal intensities. The degree of volume of the reproduced signal is then determined by adjustment of rheostat 49 which controls the heating current in the filament 42 of the first audio-frequency amplifying tube `39. In the above operation it is noted that the twoelectrode or diode rectifier 33 functions as the detector and also effects rectification of the radio-frequency carrier current to control amplification in the first radio-frequency stage of the receiver. The audio-frequency component of the detector output is transferred to the input circuit of the audio-frequency amplifier for further amplification. The neutralization of the gridplate capacity of the radio-frequency amplifying tubes is, in the system disclosed, particularly valuable in that it allows an increase in the effectiveness of the amplification control, because such neutralization prevents radio-frequency energy from passing thru the grid-plate capacity of the tubes. Thus the relay action of the tubes is almost entirely subject to the control by grid bias voltage.

The time required for operation of the control system would ordinarily be determined by the lowest audio-frequency modulation which must be reproduced. Fading, for example, might be considered a form of modulation; the frequency of the rise and fall of signals due to fading being the frequency of modulation. If this frequencyof modulation be increased sufiiciently, the effect will be audio-frequency modulation. It will thus be seen that if the automatic control Vbe allowed to respond too quickly, it will tend to smooth out the desired modulation of the signals at the lower audio frequencies. Hence, a time constant of operationgis chosen which will be greater than the period of the audio frequencies which lthe system is intended to amplify. 'Ihis time constant of the control circuit is equal to the product of the series resistance and the shunt capacitance of the grid bias circuit, represented in Fig. 1 by resistance 53 between conductor 36 and the anode terminal 52, in the direct-current connection back to the grid II, and condenser 54 connected between the amplifier filament 21 and a point on conductor 36. Since the time constant can always be reduced to a value equal to the period of the lowest modulation frequency, it may readily be set to meet the requirements of nearly any special case which may arise. For example, a value of two million ohms resistance and of 0.1 microfarad capacitance gives a time constant of one-fifth of a second, which does not appreciabiy affect the modulation of frequencies above five cycles. While this constant is greater than required from the point of view of satisfactory audio-frequency quality in the reproduction of music, there appears to be no need for more rapid control under the conditions usually encountered. Even with a time constant of the order of one-fifth of a second, however, the time lag of the automatic amplification control is so small as to allow it to follow closely variations in the signal output intensity of the receiver as it is tuned in the vicinity of resonance, that is, the control is substantially inertialess, so that audible indication of resonance is lmpracticable. The use in this connection of conlil densers of large capacitance, such as one-tenth microfarad, likewise introduces another convenience in that the condensers may also serve to by-pass radio frequencies in order to prevent undesired coupling between the detector circuit and the rst radio-frequency amplifying tube because of some ,impedance common to those two portions of the apparatus.

For a better understanding of the system described above, reference is made to Fig, 2 from which it will be appreciated that, in a system similar to that illustrated in Fig. l but in which n' means for automatically limiting the degree of amplification is includedthe amplified radiofrequency voltage is proportional to the radiofrequency antenna voltage, as indicated by curve |02. When, however, automatic amplification control is applied to such an amplifier, the relation between the radio-frequency antenna voltage and the amplified radio-frequency' voltage is indicated by curve |03 from which it will be seen that when at least a certain predetermined `radio-frequency antenna voltage is present,

(herein referred to as the threshold antenna voltage") the amplified radio-frequency voltage approaches-but is always less thananother certain predetermined voltage value (herein re-l ferred to as the cut-off vo1tage).

As stated above, in the case of a modulatedcarrier signal receiver having automatic amplification control as just described, it is impracticable to attempt to tune the receiver accurately by the audible response, since the sound output varies only within relatively narrow limits which are not readily detectable by the ear. In order to provide a visual indication of resonance, there is provided a visual indicating device connected to be responsive to the output of the signal-translatingv channel. As shown,4 this device comprises a milliammeter I0 connected in the anode circuit of the amplifying vacuum tube 9. The effect of the control 'circuit is to vary the amplification in the tube 9, and incidentally to vary the plate current through the milliammeter I0 inversely in accordance with variations of received signal intensity. When the receiver is precisely tuned to the signal frequency, the amplitude' of the signal voltage developed across tuned circuit l, 8 and impressed upon the control grid of the tube 9 is a maximum. Under these conditions, a minimum amplification in this tube is required, theplate current ofrtube 3 reaches a minimum value, and the milliammeter I0 so indicates. Thus the milliammeter I0 visually indicates the degree of amplification and, thus, the condition of resonance with the desired signal-carrier frequency. Because of the fact thatthe automatic amplification control maintains the variations/in the input to the detector 33, and thus the variations of the space current of the tube 9, within a relatively narrow range, the range of the milliammeter I0 may be 'correspondingly small.

' When the receiver has been precisely tuned, as

just described, it is in condition to provide a maximum range of control'of amplification to compensate for variations in the signal input intensity due to fading or other causes. Since, under any given operating conditions, the milliammeter III indicates the amplitude of the signal voltage impressed uponv the control grid of the tube 9, it is also effective to indicate relative signal strength as between different signals, assuming that in each case the receiver is prequency cascade amplifier including the vacuun tubes ll, l0 and l2 coupled by transformers l.

and 0I. The last stage of the amplifier is connected by a transformer I8 to a two-electrode rectifier I4 of the type already described, the output circuit of which, including the resistance Il, isI

connected' between the anode 6l and nlament l of the rectifier, as previously explained. Resistance 12 and condenser 08 associated with this output circuit, constitute a reiector network which flltersout the radio-frequency current component in the output circuit of the rectifier N, while the network including condenser 69 and resistance l0 constitutes an audio-frequency-pass filter for coupling the output circuit of the rectifier to the input circuit of the audio-frequency amplifier which includes vacuum tube 1|. Rheostat 13 controls the heating current supplied to the filament 14 of this vacuum tube, and thereby permits a manual adjustment of the volume of the reproduced signal desired by the listener. Audiofrequency transformer 16, which is preferably of a low ratio of transformation, couples theoutput circuit of vacuum tube 1| to a second audio-frequency amplifying tube 1l. This last vacuum tube in turn is coupled by a second audio-frequency transformer 18 to a third audio-frequency amplifyingtube 18 in the output circuit of which there is includeda loud speaker 00.

In this arrangement automatic amplification control is effected in the same manner as in Fig. 1, except that in this instance the radio-frequency voltage of the signals intercepted by the antenna 56 is successively amplified by three neutralized tuned radio-frequency amplifying stages including the vacuum tubes 68, 60 and B2, the amplification of two of which is controlled in accordance with received' signal intensities. The amplified radio -frequency current is rectified by the rectifying valve 64 and successively amplified at audiofrequency by the vacuum tubes 1|, 11 and 19. 'I'he rectified current in the output circuit of the rectifier flows thru the resistance il, and thereby develops a negative voltage at the terminal 8|. which voltage is applied, thru the resistances l2, 02, 83 and 85 to the grids I4 and i6 of the radio-frequency amplifying tubes 6l and 60. By thus simultaneously controlling the degreefof amplification of two successive radio-frequency amplifylng stages, a greatly increased uniformity of regulation is attained. Resistance l2 and the condenser Bl constitutel an` audio-frequency-stop filter, so that substantially only direct-voltage is impressed upon the grids ll and O6. It will be understood that the voltage developed at terminal 8| is a function of the amplified radio-frequency voltage delivered to the input circuit of the rectifier by the radio-frequency amplifying tubes 58, 60 and 02, and therefore, as the negative voltabe at terminal 8| tends to increase with the increasing signal, the resulting increase of biasing voltage impressed upon the grids of the tubes Il and 60 limits the degree of amplification eil'ected in the 'very small range of voltages.

ample, be the same as those for the corresponding elements in Fig. 1. .In addition the grid resistances 83 and 85 may have a value of 2 megohms each; 4and the grid condensers connected at the junctions of these resistances and the grid electrodes 84 and 86 may each be of 0.001 microfarad capacity.

There are advantages attending the use of the two-electrode rectifier circuit typified by Figs. 1 and 3, which may not be apparent from the foregoing discussion. It is impossible to overload this type of rectier, and the rectified output voltage is directly proportional to the applied alternating signal voltage when-this voltage is large, say over two volts. The control system in the circuits of the gures referred to requires a large operating voltage, say ten volts, so that the latter condition of large signal voltage is realized. No such simple relationship is possible in a three-electrode detector, whose rectified output never exceeds a limiting upper value, and is never pro-` portional to the applied voltage, except over a This distinction will be seen from Fig. 4 where the abscissae A. CL represent the alternating signal voltages, whereas the ordinates D. C. represent the rectified output voltages. It is well known that the linear curve is much more desirable when minimum distortion of a modulated signal is desired, and it will be observed from Fig. 4 that the preferred type of curve is obtained from the twoelectrode rectifier. A further advantage of the linear type of detector with automatic amplication control connection and a visual resonance indicator in the anode circuit of the amplier whose grid bias is being automatically confi-olled, lies in the fact that the visual resonance indicator will give an indication which is proportionate to the received signal intensity. This follows from the fact that the negative grid bias on the amplifier` is directly proportional to the strength of the signal, and hence the anode current bears a similar relation to the signal.

If a three-electrode detector were used in an automatic amplification control system, and the rectified output voltage would be approximately proportional to the square of the applied voltage, i. e., to the power associated with the applied voltage. For this reason the rectified voltage would increase with the carrier wave modulation. Therefore, if such a detector were so used, the total power from the radio-frequency amplifier would be maintained at a substantially constant level, the amplitude of the carrier Wave being decreased in the presence of modulation. It is desirable to maintain the carrier wave at a constant amplitude at the output of the amplifier, and this is accomplished by the two-electrode rectifier as shown in Figs. 1 and 3. The control system maintains constant the average signal amplitude which is equal to the carrier wave amplitude and independent of the degree of modulation.

It will be observed that in a system employing a two-electrode rectifier'such as represented by tube 33 of Fig. 1, and 64 of Fig. 3, the control bias voltage is independent of the B or anode battery voltage. Since the rectier is not an amplier, is not critical, and requires neither anode nor biasing battery, no adjusting devices are required. 'Ihis is not the case in three-electrode detector circuits, in which an adjustment must be made as by a potentiometer to accommodate thecontrol bias to any particular combination of tubes and B voltage.

In the foregoing description, tuned radio-frequency receivers ofthe neutralized type have been ceivers of the super-heterodyne type. For these 1 reasons the present disclosure of typical embodiments of the invention should not be construed as a limitation, but merely as illustrative of the principles of the invention, the 4scope of which is defined in the appended claims.

What is claimed is:

l. A carrier-frequency signal receiver normally responsive to signals having a wide range of input intensities, comprising means for tuning the receiver to resonance with a desired signal, substantially inertialess electrical control means responsive to received signal intensity for automatically maintaining the signal output intensity oi the receiver within a relatively narrow range, and visual resonance indicating means responsive to variations of said output intensity.

2. A carrier-frequency signal receiver normally responsive to signals having a wide range of input intensities, comprising a signal translating channel including a vacuum tube having an an- 0de circuit, means for automatically maintaining the signal output of said channel within a relatively narrow range comprising means responsive to variations of the received signal intensity for varying the space current of said tube, and means responsive to said space current for visually indicating variations of received signal intensity. 1

3. A carrier-frequency signal receiver normally responsive to signals having a wide range of input intensities, comprising an automatic controlv electrically linking portions of said receiver for maintaining the signal output intensities of said system within a relatively narrow range, and visual indicating means of small range responsive to l said output intensity for automatically indicating Ireceiver for maintaining said audible response within a relatively narrow range for a wide range of signal input intensity variation, and means for visually indicating when the tuning "means is in proper adjustment for automatic control of said audible response over thev maximum range of variation of said desired signal input intensity.

5. In a modulated-carrier signal receiver including selective means tunable to a desired signal, the method of tuning the receiver to resonance with said desired signal which comprises amplifying said signal at a carrier frequency, producing an audible response to the amplified signal, automatically controlling the amplification to maintain the audible response within a relatively narrow range for a wide range of signal input intensities with a time lag of automatic control so small that audible indication of resonance is impracticable, and visually indicating the relative degree of amplification and thereby the tuning adjustment at which said desired signal is received with optimum selectivity.

6. A carrier-frequency signal receiver including a vacuum-tube amplifier tunable to resonance with a signal frequency, means for automatically intensities, whereby audible indication of reso-- nance is impracticable, and means for visual indicaticn of resonance comprising a device responsive to variation inthe unidirectional space current ci one or more of said ampliner tubes. y

7. Y A modulated-carrier signal receiver comprising tuning means for tuning the `receiver to a desired signal, means for producing, in lresponse to modulationsin the received signal, an audible output varying within a narrow range for a wide rangev oi received signal intensities, and visual means for-'indicating when the tuning means is in proper adjustment for control of said audible output by said second means over the maximum range of said received signal intensities.

8. A modulated-carrier signal receiver comprising a vacuum tube having an anode circuit, 'a cathode and a grid, Aa tunable circuit connected between the cathode and'grid, whereby the receiver may be adjusted vto respond to a desired signal, sound reproducing ymeans, means i'or translating the signal from said tube to said sound reproducing means, means for lcontrolling the bias potential on said grid in response tothe intensity o1' the received signal, thereby to maintain the sound output from said sound reproducingmeans within a narrow range for a wide range of received signal intensities, and means responsive to the magnitude of the unidirectional space current owing in said anode circuit for visually indicating the condition of adjustment of said tunable circuit with reference to the carrier frequency oi the received signal.

9. A carrier-frequency signal receiver including a vacuum-tube ampliiier, means for adjustably tuning the receiver to resonance with a desired signal, means for automatically maintaining the amplified signal intensity within a narrow range for a wide range of received signal intensities by control of the space current o! one or more ofI the tubes oi.' said amplifier, whereby the usual audible indication of resonance is impracticable, and a meter connected in circuit with the space current circuit of one or more of the automatically controlled tubes of said amplifier for visual indication oi' resonance.

10. A modulated-carrier signal receiver including a vacuum-tube amplifier, means for tuning the receiver to resonance with the carrier irequency of a desired signal, a rectier coupled to the output of said ampliiier for detecting the modulations, means for automatically maintaining the modulation signal intensity within a narrow range for a wide range of received signal intensities by automatic control oi thespace current of said amplifier, and resonance indicatingmeansdirectlyresponsivetochangesinthe space current oi saiil vacuum-tube ampliiier.

l1. A carrier-frequency signal. receiver normallyresponsivetosignalshavingawiderange oi signal input intensities. including a signaltranslating channehmeansior timing the receiver to resonance with a desired signal, means responsive to received signal intensity for maintaining the signal output intensity oi said chan'- nel within a relatively narrow range, and visual resonance indicating means connected to respond to the output-of said channel,

' 12. A carrier-frequency isnsl receiver normally responsive to signals having a wide range oi signal input intensities, comprising a vacuumtube ampliiier, means for tuning said receiver `to resonance with a dired signal, means responsive-to received signal intensity for maintaining the signal output intensity of said arn- Y pliner within a relatively narrow range, and visual in signal intensity a response suillcientiy rapid to follow closely variations in the signal output in- A tensity of said stage in the vicinity of resonance with said desired signal, whereby audible indication oi resonance is impracticable, and visual resonance indicating means connected to respon to the output of said channel.`

14. A modulated-carrier signal receiver ,normally responsive to signals having a wide range oi-signal input intensities, including a vacuumtube ampliiler, means for tuning the receiver to resonance with a desired signal, means for producing an audible response to received signalcarrier modulations, means responsive to received signal intensity for maintaining said audible response within a relatively narrow range, said last-named means having a time lag so small that audible indication of resonance is impracticable, and visual resonance indicating means connected to respond to the output of said amplifier.

y HAROLD A. HLM vi' CERTIFICATE OF CORRECTION.

.Patent No. 2,080,646. May 18, 19575 HAROLD A. WHEELER.

It is hereby Certified that error appears in the printed' specification of the above numbered patent 'requiring Correction as follows: Page l first column, line 2, for the word "said" read such; line 30, strike ont "carrier-Current" and insert the same before the word "signal" inline 31, page 2, second column, line 42, strike out "impedance"; page 4, second column, line 64, for "voltabe" read Voltage; page 45, firsti'oolumn, line 45,I strike out "and".; same page, second column, line 39, Claim 3, for "intensities" read intensity; line 40, Same claim, for "System" read receiver; line 48, Claim 4, strike out "to" after "signal-carrier" and inf sert the same after "response" in same line and claim; and that the said Letters Patent should belread with these'corrections therein thatthe same may conform to the record of the Case in the Patent Office.

4 Signed and sealed this 27th day of July, A. D. 1937.

Henry Van Arsciale (Seal) ActingI Commissioner of Patents.y

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