US2088203A - Radio receiving system - Google Patents

Description

July 27, 1937, c. w. HANsELl.

RADIO RECEIVING SYSTEM Original Filed May 13, 1952 5 Sheets-Sheet l lNvx-:NToR rCLARENC W. HANSELL BY 6 A-rr'oRNEY July 27, 1937. c. w. HANsELL RADIO RECEIVING SYSTEM 5 Sheets-Sheet 2 Original Filed May 13, 1932 AALAAA VVVVVV INVENTOR LLARENCE w. HANSELL ATTORNEY July 27, 1937. c. w. HANsELL RADIO RECEIVING SYSTEM Original Filed May 13, 1932 5 Sheets-Sheet 3 ANSELL MVL-W Patented July 27, 1937 UNITED STATES PATENT OFFICE 2,088,203 K RAnIo RECEIVING SYSTEM Clarence W. Hansell, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Original application May 13., 1932, Serial No. 611,050. Divided and this application May 13,

1933, Serial No. 670,846

7 Claims. (Cl. Z50-20) I have-observed, in connection with fading phenomena that distortions in the detected signals occur when the strength of lthe received car- 10 lrier current fades down to a value less than the peak strength of the side bands produced by modulation. To eliminate this diiculty is a prime object of my present invention and in order to fulfill it, I provide a receiving system 15 utilizing one or more detector tubes each of which is reached by only one side band of the received signalrand the carrier. Consequently, there are no discontinuous distortions such as previously referred to when the carrier and both side bands 20 reach a single detector tube and the maximum value of the side bands is greater than the carrier. 'I'hat is to say, by allowingonly the carrier and one side band to reach any one detector tube of my improved receiving system, it is possible to receive signals which have either been sent with, or which have been given by fading, more than 100% modulation, without producing the distortion inthe receiver output which would otherwise be the case.

30 While this object may be carried out by the `use of one detector tube, as I have already indicated, I prefer to use two detector tubes and allow only one side band to reach one tube ,and the other side band to reach the other tube,

35 while at the same time the carrier is made to act upon both tubes. In ,this manner there is more efficient utilization of the side bands, in

fact lboth side bands lmay -be used in producing useful signal output as will be described more 40 fully in connection with the receiving systems illustrated on the attached drawings. f

From what I have stated before, and considering the known characteristics of detectors, it should `be apparent that, in order to obtain the 45 best quality reproduction of the modulation or transmitted signal, it is desirable to have the carrier reach the detectors with its strength much higher than the maximum resultant of the side bands. In other words the harmonic `dis- 50 tortion in detectors is reduced by keeping the effective percentage modulation 10W. To do so forms a further object of my present invention. To accomplish this purpose, I place ahead of my detectors, band pass filters having such a char- 55 acteristic, that for very narrow frequency bands around the carrier frequency, or preferably, in the event that heterodyning is used,\around the intermediate frequency carrier, they pass energy -of the carrier frequency with much lower attenuation than in the main portions of the passed bands. p

These narrow bands of Very low attenuationv in the two filters may be made identical but preferably they are made slightly different but `overlapping in the same manner as the normal pass band in order to allow making the automatic control of the frequency of the first beating oscillator more positive, a feature which will be described more fully hereinafter. l

To further exalt, or intensify,.the intermediate or carrier frequency relative to the other frequencies of the passed bands, I may increase the attenuation of the said frequencies by the use of impedances or resistors, and increase the intermediate carrier frequency by means of a by-passing crystal filter which also will be more fully described later,

Moreover, in the ordinary superheterodyne type of receiver itis common practice to takev the incoming signal picked 'up by a receiving an- 25 tenna and amplify it through one or more stages o-f a high frequency amplifier. The output of the amplifier is then applied to a heterodyne dei tector, and commonly supplied with beat frequency energy from av separate oscillator. The Outputof the heterodyne detector is usually an intermediate frequency which is still further ampliiied. The intermediate frequency amplifier which has a relatively narrow frequency pass band, permits the desired signal to pass with, dueto its narrow pass band, a minimum of accompanying undesired signals and noise. When i the signal being received is an amplitude medulated phone signal, the output of T/the intermediate frequency amplifier is applied to a recti- 40 .fyingtype of detector and produces variations in the detector plate current which are substantially a true copy of the useful modulation.

When a receiver of this type is used for receiving very high frequencies, towards which my present invention is particularly directed, diiliculty is encountered due to the fact that it is impossible to maintain absolutely accurate frequencies in the transmitted carrier and in the beat frequency oscillator for the rst detector of `the receiver. The variations in frequency o-f the transmitter and receiver oscillators increase about in proportion to the carrier frequency used,

Y and cause variations in the intermediate fre- For this reason quency energy of the receiver.

it is found necessary to allow a sufficiently wide frequency band inA the intermediate frequency amplifier so that the desired signal does not move outside the received frequency band, if receiving system is to function satisfactorily in commercial service. The breadth of the band necessary, however, adversely affects to an unwanted degree, the design of commercial receiving equipment. For example, in an inter-island telephone circuit set up in the Hawaiian Islands partly under my supervision, in order to make it possible for the signal to be always within the receiver frequency band, it was necessary to widen the frequency band of the intermediate frequency receiver circuits to about 60,000 cy- `cles whereas the maximum band required for the useful signal was only about 6,000 cycles. As a result, this receiver allows through it an amount of noise energy substantially 10 times as great as would be the case were there no necessity to allow for variations in transmitter and receiver oscillators in setting the frequency selectivity of the receiver.

Another example of the deleterious, effect of frequency variations is to be found in commercial short wave telegraph receivers in long distance short wave circuits wherein the receivers require a frequency band of only about 250 cycles to accommodate the side bands produced by telegrapl. keying at speeds of 20D-words per minute. However, due to the necessity for allowing for frequency variations in transmitter and receiver, the intermediate frequency lter circuits must be designed with a band width approximately 2,500 cycles wide. Here again, approximately 10 times as much` interfering energy is admitted as would be necessary if the effect of frequency variations in the output of the receiver could be eliminated.

Accordingly, a further prime object of my present invention is to eliminate the foregoing defects arising from variations in frequency of the transmitted carrier and variations in frequency in the locally generated heterodyning wave. Briefly to do so, I eliminate variations in the intermediate frequency by utilizing signal energy applied to two detector or demodulator tubes, or electron discharge devices to cause differential variations in potential which are utiliredA to correct the frequency of, say, the first beating oscillator. In this manner, the first beating oscillator is always made to follow the frequency of the transmitter with a substantially constant frequency difference and therefore in the intermediate frequency amplifier circuits, there is no necessity to pass a frequency band wider than that necessary to pass the useful modulation.

With my automatic tuning arrangement in simplified form, some difficulty may be encountered in actual operation during times of extremely bad fading when the incoming signal may practically cease altogether for appreciable periods. Thus, if the amount of correction being applied to the first beating oscillator were quite great, in the event that the incoming signal dropped out the oscillator would change greatly in frequency in returning to its natural frequency, so that when the signal came in again, the local oscillator might cause an intermediate frequency after the first detector beyond the pass bands of either of the filters for the last detector.

To avoid this possibility which would require atv tention from an operator to manually tune in the signal again, since under the circumstances outlined, the signal cannot control the first beating oscillator, I further provide means, which will be described more fully later on, to keep the receiver circuits-always closely adjusted to the correct value so that in the case of absence of signal the first beating oscillator does not change greatly in frequency.

It is undesirable to have the automatic frequency controlling feature' just referred to in operation while initially tuning in a station, since once tuned to a station, the receiver would automatically hold its own condition to receive that station and render the operator powerless to tune to some other desired station. I provide as a further feature of my present invention, means for rendering the automatic frequency controlling apparatus inoperative during tuning periods.

There are additional objects in my invention of a more specific nature which will be self apparent in connection with more detailed descriptions of my invention which will be given hereinafter.

In the accompanying drawings wherein I have illustrated preferred embodiments for carrying into effect the various features of my present invention, Figure l illustrates a diversity receiving system particularly adapted for reception of phase modulated waves and having the feature of automatic frequency control for the heterodyning oscillator;

Figure 1a is a graph illustrating the frequency characteristics of band pass filters to be used ahead of the second or last detectors of the receiving system shown in Figure l;

Figure 2 illustrates a preferred filtering arrangement to be used in connection with my improved receiving system wherein the carrier fre-- quency may be exalted above the side frequencies in order to reduce the effective percentage modulation at the detectors and so reduce their distortion;

Figure 3 illustrates a band pass lter which may be utilized in connection withthe arrangement shown in Figure 2;

Figure 4 illustrates graphically, the characteristic of a pair of band pass filters such as illustrated in Figure 2 for my improved receiving system; and,

Figure 5 illustrates .an improved modification of the receiving arrangement shown in Figure 1 having in addition to the arrangement shown in Figure 1, means for preventing ill effects due to dropping out of a signal entirely.

Referring to Figure 1, which illustrates a preferred form of receiving system having many features of my present invention, the receiving antenna 2 feeds energy to a high frequency amplifier 4, preferably of the cascaded vacuum tube type having tuned coupling circuits. The output of the high frequency amplifier 4 is fed to a first detector B preferably of the power type. The first detector 6 is also supplied with locally generated heterodyning waves through an inductive link 8. 'I'his link 8 is not essential for, inthe event that the beat frequencyoscillator I0 is not shielded, coupling to the detector is obtained unavoidably.

In fact no special coupling circuit under Vthose value of this intermediate frequency fed to the intermediate frequency amplifier I2 should be sufuciently low to permit the desired Vdegree of selectivity in the intermediate frequency amplier. However, the value of the intermediate frequency in cycles vper second should always be greater than half the width of the pass band of the high frequency amplifier and preferably it should be greater than the Width of the pass band of the high frequency amplifier in order that the noise energy in the intermediate frequency circuits may be made a minimum. In receiving very high frequency, or short wave signals, it may sometimes be necessary to pass through two or more stages of heterodyne detector, each preceded by amplifiers and frequency selective circuits, in order to permit carrying out this requirement for obtaining minimum noise. It should be understood that,where necessary this would be done.

In receiving phase modulated signals the intermediate frequency amplifier is operated preferably as a limiter, that is to say, it ampliiies the minimum amount of energy fed to it so as to bring its output at all values of input to a constant value. In other words, the intermediate frequency amplier is designed'and adjusted in such a way that the minimum excitation applied to the first tube of the amplifier is more than that required to produce saturation of the last tube. To assist in this action, or in place of it, a portion of the intermediate frequency amplier output may be fed to a volume control or further detector Ill of any well known type, whose unidirectional output is utilized to control the grid bias `through conductor I6 of the tubes forming the high frequency amplifier so that the high frequency amplifier is made to furnish substantially s uiicient constant input to the intermediate frequency amplifier through the intermediary of the rst detector. Although, in Figure 1 I have shown volume control detector It as being supplied with energy from the nal output of the intermediate frequency amplifier it should be understood that the energy might be taken from any of the intermediate frequency amplifier stages.

The intermediate frequency energy is passed through the intermediate frequency amplifier and the constant'amplitude intermediate frequency energy (due of course to the limiting action of the intermediate frequency Aamplifier together with the volume control circuit I4,` IB) is fed to two bandpass filters A B having overlapping characteristics as illustrated in Figure 1a. The wiring diagram of each of the filters A and B may be as indicated in Figure 3 which, of course, is only illustrative of many of the forms which the band pass filters A and B may take. The overlapping characteristic of the two filters should be such as illustrated in Figure laso that the intermediate4 frequency carrier may be allowed to reach both of the second or last detector tubes I8, 20. By adjusting the. local oscillator I the intermediate frequency carrier `is made to fall within the band common to `both detector filters A B. It should be understood,

however, that the amount of overlapping frequency band in the two filters' should be small so that substantially only the carrier and one side band can reach each of the nal detector tubes I8,'2Il.

However, I prefer to make each of the band pass filters in the form illustrated vin Figure 2 such that the combined characteristic of both filters will be that illustrated'in Figure 4. Thus,

,referring to Figure 2 by shimting in parallel with any type of electrical circuit band pass filter 22 which may be of the type illustrated in Figure 3, a neutralized piezo-electric crystal circuit 2li which passes only a very narrow band of frequencies, this narrow band is increased in value relative to the other side frequencies. This predominant passing action for the very narrow band may be further increased by the use of attenuating impedances or resistors R1 Rz as illustrated in Figure 2. These attenuating resistors serve to further attenuate the side frequencies passed by the band filters 22 orvfllters A and B of Figure 1. 'I'his particular arrangement of ltering circuit giving a filter a desired characteristic is more fully described in my copending application, Serial Number 564,770, filed September 24, 1931, patented May 14, 1935 No. 2,001,387, any of the arrangements of which may, of course, be used here. 'I'he neutralizing feature is more fully described in my copending application, Serial Number 203,901, filed July 7, 1927, patented June 18, 1935 No. 2,005,083.

'I'he carrier and one side band which reaches 4 each detector of the last detectors I0, 20 produces beats in the outputs of the detectors corresponding to the modulation introduced at the transmitter. Since these beats from the two detectors will be 180 degrees out of phase with one another if phase modulation is used, the plate output coils 26, 28 should be coupled magnetically to the common audio output coil 30 in pushpull relationship so that. their outputs add rather than cancel. In the event that the transmitter has its carrier wave amplitude modulated, the outputs of the two detectors should be coupled with like polarities or effectively in parallel which may be done by reversing the connections from one of the coils relative to the other so that the output in the output coil 30 is cumulative. For frequency modulated waves, the connections should be made similar to those for phase modulated Waves, that is to say, the coils 26, 28 should be coupled effectively in push-- pull relationship or vsubstantially in push-pull relation to the two detector tubes 20, I 8. Insofar as the operation of the receiver shownin Figure 1 is concerned, the only change lnecessary to modify reception from phase lor frequency modulation to amplitude modulation is merely the reversal of the connections to one primary winding of the output transformer comprising primary windings 26, 28 and secondary winding 30.

In the case of diversity reception, the output conductors of, say, receiver number 2, are connected as illustrated at 26a., 28a' and 30a, corresponding 'to the connections for coils 26,28 and 30. For simplicity only the final detector connections of the other receivers are shown. The conductors I9, 2I lead, of course, to the plates of the second detectors for receivir f: set number 2, which detectors would correspond to detectors I 8, 20. The final output of each antenna from each receivermay then be fed as shown to a common transformer 32, the primaries of which are Aconnected to the respective coils of each radio receiver such as coils 30 and 30a. In the sec- -ondary'of transformer 32, there will appear, of

course, the combined output which may be amplied by a suitableampliiier 34 and fed to a suitable translating device 36.

receiver, the impedance of the output circuit is such as to take approximately maximum power from the one receiver. Then, when all receivers are working," their mutual reaction upon one another changes their output impedances, lowering the output from each receiver and so tending to give the same output as if only one receiver were working. Thus the combined power output of the group of receivers tends to remain constant.

In addition to the output transformer 26, 28, 30 of each receiver, or more simply in the receiving set Number 1 completely illustrated in Figure 1, two resistances R1, R2 may be connected as shown. The amount of direct current through these two resistances varies differentially as the intermediate frequency carrier varies in frequency towards one or the other of the pass bands of the two band pass filters A, B. This differential variation in current is used to cause differential variations in the adjustments of the oscillator IU for the first detector and these variations in adjustments move the frequency of the oscillator in such a direction as to tend to keep the currents through R1 R2 equal or in other words, to maintain the constant frequency difference between the output of the local oscillator SI) and the received high frequency carrier.

To accomplish this I may use any oscillator capable of having its frequency controlled. In the circuits shown in Figure 1 I make use of a form of oscillator quite similar to my improved variable oscillator described in my copending applications Serial Number 36,772, filed February l, 1929, patented July 19, 1932, No. 1,867,567, and Serial Number 463,614, filed June 25, 1930, patented January 14, 1936, No. 2,027,975. As illustrated here my variable oscillator I0 consists of two screen grid oscillator tubes 38, 40 having separate tuned grid circuits 42, 44 with a common tuned plate circuit 46. The two grid circuits are tuned to different frequencies, one being tuned to a frequency higher than the desired heterodyning frequency, while the other is tuned to a fre-A quency lower than the desired frequency of operation of the oscillator I0. The plate`circuit 46, however, is tuned substantially to the desired beating frequency which is intermediate'to the frequencies for which the grid circuits or control electrode circuits are tuned.

As explained in my copending applications, in

an oscillator of this type the frequencies of the.

oscillations produce a compromise between the frequencies which each of tubes 38, 40 would produce if oscillating alone.

By differentially varying the outputs of the tubes 38, 40 by applying the voltage variations across resistances R1 and R2 to the screen grids thereof as shown, one tube or the other predominates in producing oscillations in the common output circuit 46. As a result, the output frequency ofoscillator III may be varied back and forth between the limits set by the frequencies of oscillation of each tube when used alone.v

Consequently, the output frequency of the oscillator I0 is put under control of the plate currents of the detector tubes since the voltage variations across resistances R1 and Rz are.. proportlonal to the said detector plate currents and the frequency of the local oscillator I0 is automaticallyadjusted to such a value that the detector plate currents are always substantially balanced across resistors R1 and R2 and the intermediate frequency of the receiver is substantially in the center of the receiver pass band.

If for any reason the transmitter frequency varies or the circuits of the local oscillator vary their tuning due to changes in temperature, humidity and so forth, in sucha wayv as to produce a variation in the intermediate frequency of the receiver, the variations are automatically compensated for by the method shown. For example, if desired, the transmitter frequency might be varied over a considerable range and the receiver adjustments would automatically follow the transmitter so that the desired signal might be received always with maximum efficiency and a minimum of undesired noise.

In addition, this arrangement also provides for the retention of a receiver in tune with a transmitter whose frequency is varied for secret signaling. That is to say, my present receiver may be applied to a secret signaling system without the need of any complicated synchronizing arrangements at the receiving end as now required in such systems. The transmitter frequency may be varied through wide limits and the receiver with my receiving apparatus would always follow the transmitter frequency so that there is a constant intermediate frequency which may be detected as shown.

In the circuits shown in Figures 1 and 5 I have assumed that each receiver has its own separate beating oscillator for its first heterodyne detector. However, ifv desired, a common beating oscillator may be used for all receivers in which case it is preferable that all the detectors contribute to the control of the common beating oscillator. Thus I may utilize the drop in resistors R7, Re, as well as R1, R1, through resistors R10, R11, and R4, R5, to control the frequency of the beating oscillator, as shown in Figure l. In this case, so long as carrier energy reaches either receiver the oscillatorA frequency remains under control. Of course, any number of receivers may be connected up in the same way to contribute to the oscillator control and the probability of controlbeing lost, due to fading of the carrier, is greatly reduced. Also, for the other receivers similar coupling units 8 may be used for supplying the local frequency thereto, or, if desired, the output of the local oscillator I0 may be radiated or carried by Wire line to each receiver. Or, receiving apparatus may be placed together and exposed to the fields of the oscillator I0 which is left unshielded.

It is not necessary, of course, to use the form of oscillators 40, 38 illustrated', but any other type of oscillators such as referred to in my copending applications may be used. Also detector I4 and control circuit I6 may be omitted for receiving phase or frequency modulation or a common detector I4 may be used and the conductor I6 be used to control the bias on all of the high frequency amplifiers or detector I4 may be duplicated at each receiver and the detector outputs paralleled to obtain a common volume control which will tend to make the output from the receiver with the strongest received carrier predominate in giving output. For receiving amplitude modulation with my system I would not use limiting in the intermediate frequency amplifier system, because that would remove the modulation. Instead I would rely chiey on the automatic volume control through I4 and I6 and upon the characteristic of the detectors I8, 20 inconjunction with voltage drop in resistor R3 to obtain relatively constant output from each receiver. The biasing means for the detectors illusl any other negative polarizing source. 'Ihe common resistor R3 maybe inserted in the platersupply lead 52, as shown, and the voltage drop across this resistance in the event of a good signal on receiver I will be sufficient to more than proportionally reduce the outputs of receivers " numbers 2 and 3 so that the noise level may be reduced. In other words, with Rs in circuit, usually only one antenna supplies a useful energy output while the other antennae are effectively disconnected from circuit. However, resistor Rs is not essential and may be omitted. In that case all of the antennae are coupled into circuit. y In other words, with resistor R3 in circuit, as the strength of. the incoming carrier picked up by the various receiving antennae and delivered to the various receivers varies, there would be produced a potential drop across Iresistor R3 to cause the receiver with the strongest carrier to predominate over the other receivers furnishing audio output. 'Ihis action is more fully described in the copending application of Harold O. Peterson Serial Number 319,938, led November 16, 1928. The system which I have shown here has marked advantages over others in that there need beV no time delay in the action of the voltage drop in resistor R3 so that a more perfect final output results.

I have stated that in receiving phase and frequency modulation limiting may be used in the l intermediate frequency amplifier I2, but it is to be clearly understood that such action vmay also be used in addition in th high frequency ampliiier II to reduce the effect of fading upon the strength of output from each receiver.

Also," the potential variations `across the resistors R1 and R2 may be used as will be apparent to those skilled in the art to vary the plate'potentials on the local oscillator tubes 38, 40 or, to vary their control grid potentials in order to vary their outputs differentially. In fact any system in which frequency variationsmay beproduced by variations of potential might be used. Y

' Obviously, it would be almost impossible to tune the receiving system from one station to another with thev frequency controlling arrangement in circuit,ffor, under such circumstances, or if the receiver happened to be tuned into an undesired station inadvertently, the receiver would automaticallyl hold its own condition to receive the undesired transmitter and no other. Consequently, to allow change in tuning from one station to another, the' local oscillator I0 is provided with a switch S which, when closed, short circuits voltage variations across resistors R1,R2 away from the screen grids of tubes 38, 40. The resistors R4, R 'are in circuit in order to reduce the D. C. voltage applied to the screen grids of tubes.38, 4I) to a suitable value. With the switch S closed, the receiver can be tuned to any desired station, and, once tuned, opening of the switch S will cause automatic frequency controlling action of the receiving system.

In addition to the switch S I have provided a zero center voltmeter .5I which may be used to indicate the condition of tuning adjustment of the receiver at, all times. With this voltmeter, the operator may adjust the circuits of the beat frequency oscillator to put a minimum demand upon the automatic frequency correction equipment, which is another feature of my present' invention. To facilitate tuning and to allow the outputs from the receivers to be initially equalized I have added ammeters A1 and A2 which normally arrangement of Figure 5. As the grid bias con-` nections of the last detectors I8 and 20 shown in Figure 5 are illustrated in the form of grid leak and condenser arrangements 68, for volume lcontrol a resistor 62 is inserted in series with the ground and filament connection of the detector tubes II`8, 20. Consequently, with strong signals, the output plate currents of the last detectors I8, 20 are vrelatively small allowing a high negative bias from source 66 to be impressed upon the control grids of the high frequency amplier 4- and intermediate frequency amplifier I2. However, with Weak signals, the detector output currents are increased and the voltage arising across resistor 62 will be in such a direction as to compensatethesource 66 and make the bias upon the high frequency amplifier and intermediate frequency amplifier tubes less negative thereby increasing the output of the intermediate frequency ampliler I2. In view of the reversed current action in the detector tubes care must then be taken so that the differential voltage variations across resistors R1, Rz cause the correct variation in output frequency of the local oscillation generator IIJ. Y

'I'his local oscillation generator I 0 of Figure 5 isa little different from the one described in connection with Figure 1 and has the refinement that the local oscillator will not greatly vary in' frequency when the signal drops out at the antenna 2. 'Ihat is to say, if the amount of correction being applied to the first beating oscillator I8 were quite great and should the incoming signal drop out at antenna 2, the oscillator would change greatly in frequency in returning to its natural frequency so that when the filters A, B, for the last detectors I8, 20.

In rthis case, the signal cannot regain control of the rst beating'oscillator Where one antenna is used and the attention of an operator is re'- quired totune in the signal again..

To avoid such a difficulty, a polarized relay III is added to the local oscillator I0 of Figure 5, which in turn is operated by' the unbalanced currents through the detector tubes of the re ceiver. 'I'he relay is used to control a small motor 'I2 in one direction or the other to keep the receiver always near the exactly correct adjustment. In eifect, therefore, the voltage variations applied to the screen grids as illustrated in Figure 5, compensate the relatively rapid frequency fluctuations while the motor operated control follows the slow variations.

By tracing the circuits shown in Figure 5, it will be found that the polarized relay is operated by diierential variations of plate currents of the oscillator tubes. This relay, of course', could also be operated by potential dierences across' resistors R1, Rz.

To simplify adjustment of the local oscillator I0, unicontrol is provided forall of the tuning condensers as in the case of the beating oscillator I0 of Figure 1. l

As an alternative arrangement for the motor 0f Figure 5, which as shown hasseries field wind-I ings, only one of which is used at a time and which is so connected as to reverse the field and direction of rotation depending upon which is energized, a shunt motor maybe used and the relay arranged to reverse the direction of the armature current. Still another alternative method, rather than the one illustrated, may be to allow the motor to run always in onel direction and engage electrically operated reversing clutches operated by the relay.

The relay 10 should be so designed magnetical- 1y, or preferably should be so equipped with springs as to hold the contacts open unless the tube current unbalance exceeds a chosen value. Adjustable springs are preferred so the relay can be biased to compensate for tube and circuit characteristics.

As illustrated in Figure 5, the final output coils of the detectors are coupled for giving an audio output from either phase or frequency modulated waves. For amplitude modulated waves one of the primary coils 26, 28 may be reversed in polarity relative to the other. The secondary audio output appearing in coil 30 may be combined with audio outputs from similar receiving systems utilizing energy from the local oscillator I0. The other receiving systems, preferably, have independent antennae separated apart a considerable fraction of the operating wave length. .v

Having thus described my invention, what I claim is:

1. The method of reception which includes collecting transmitted signal energy, generating locally yheterodyning Waves with a local oscillator, heterodyning the locally generated waves with the incoming energy to produce intermediate frequency energy, dividing the intermediate frequency energy into two portions, one of which includes intermediate frequency carrier energy and one side band, and the other portion of which includes intermediate frequency carrier energy and the other side band of the intermediate frequency energy, separately detecting the energy portions so separated, and utilizing resultant voltages derived from the detection to control the frequency of the local oscillations generated by said local oscillator in such a direction as to maintain a desired difference between the frequency of the incoming carrier and the locally generated Waves.

2. A receiving system comprising an antenna for collecting propagated phase modulated carrier energy, a local oscillator comprising a pair of generators coupled together to operate at a mean frequency, for beating the collected modulated carrier energy to characteristic intermediate frequency energy, a filter for separating from the resulting intermediate frequency energy one side band and intermediate frequency carrier energy, another filter for separating from lthe resulting intermediate frequency energy the other side band and intermediate frequency carrier energy, means for producing a potential dif ference from the separated energies, and means for utilizing that potential difference to Vary the relative outputs of said generators and thereby vary the mean frequency of the local oscillator to maintain a constant frequency difference between it and the received carrier wave.

3. A heterodyne receiving system comprising means for collecting modulated carrier energy, a local oscillator for beating the collected energy t9 a desired intermediate frequency, Controlling means responsive to said intermediate frequency for maintaining constant frequency difference between the local oscillator and the received carrier despite relative frequency variations of said carrier and local oscillator, and means for rendering inoperative said last mentioned constant frequency difference controlling means in order to enable adjustment of said receiving system' independently of said constant frequency difference controlling means.

4. In a radio receiver system of the superheterodyne type, a first detector network, a local oscillator network, means for tuning the detector and oscillator networks to different station settings, an intermediate frequency transmission network coupled to the rst detector output circuit, means for rectifying signal energy appearing in said intermediate frequency network and deriving therefrom a uni-directional voltage whose magnitude is dependent upon the difference in frequency between the operating intermediate frequency and the said signal energy frequency, additional means electrically connected between said rectifying means and said oscillator network, and responsive to said unidirectional voltage, for automatically adjusting the frequency of the oscillator supplementally of said tuning means, and means for rendering said additional means ineffective prior to the adjustment of said tuning means to each new station setting.

5. In a radio receiver system of the superheterodyne type, a first detector network, a local oscillator network, means for tuning the detector and oscillator networks to different station settings, an intermediate frequency transmission network coupled to the first detector output circuit, means for rectifying signal energy appearing in said intermediate frequency network and deriving therefrom a uni-directional voltage whose magnitude is dependent upon the difference in frequency between the operating intermediate frequency and the said signal energy frequency, additional means electrically connected between said rectifying means and said oscillator network, and responsive to said uni-directional voltage, for automatically adjusting the frequency of the oscillator supplementally of said tuning means, said rectifying means comprising a pair of detectors, one detector having a signal input circuit tuned to the operating intermediate carrier frequency and its lower side bands, the other detector having a signal input circuit tuned to said carrier frequency and its upper side bands, and a resistive path arranged to be traversed by the space currents of the detectors to produce said voltage.

6. In a radio receiver system of the superheterodyne type, a first detector network, a local oscillator network, means for tuning the detector and oscillator networks to different station settings, an intermediate frequency transmission network coupled to the rst detector output circuit, means for rectifying signal energy appearing in said intermediate frequency network and deriving therefrom a uni-directional voltage whose magnitude is dependent upon the difference in frequency between the operating intermediate frequency and the said signal energy frequency, additional means electrically connected between said rectifying means and said oscillator network', and responsive to said uni-directional voltage, for automatically adjusting the frequency of the oscillator supplementally of said tuning means, an audio network coupled to the said rectifying means and adapted to derive therefrom the audio frequency component of the rectied signal energy, control means connected to said rectifying means for deriving a direct current voltage from the rectified signal energy which is dependent in magnitude on the signal carrier amplitude, and an automatic gain control circuit connected to the control means for utilizing said direct current Voltage.

7. In a radio receiver of the superheterodyne type, a rst detector network, a local oscillator network, means for tuning the detector and oscillator networks to different station settings, an intermediate frequency transmission network coupled tothe first detector network, means for 7 rectifying signal energy appearing in said intermediate frequency network and deriving therefrom a direct current voltage whose value is dependent upon the frequency difference between the operating intermediate frequency and the frequency of said signal energy, frequency control means electrically connected between said rectifying means and said oscillator network, and

responsive to said direct current voltage, for automatically adjusting the frequency of the oscillator supplementally of said tuning means, and means for optionally rendering ineffective the action of the frequency control means at desired settings of the tuning means. r

CLARENCE W. HANSELL.

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