US2115676A

US2115676A - Selectivity control

Info

Publication number: US2115676A
Application number: US86290A
Authority: US
Inventors: Harold A Wheeler
Original assignee: Hazeltine Corp
Current assignee: BAE Systems Aerospace Inc
Priority date: 1936-06-20
Filing date: 1936-06-20
Publication date: 1938-04-26
Anticipated expiration: 1955-04-26

Description

H. A. WHEELER SELECTIVITY CONTROL Filed June 20, 1936 2 Sheets-Sheet 1 INVENTOR.

EELER HAROLD A.

ATTORNEY.

H. A. W HE ELER SELECTIVITY CONTROL "April 26, 1938.

2 sheets-sheet 2 Filed June 20, 1936 L fly. -11. F/ M 0 0% p00 mm mum 3 I INVENTOR. HAROLD A.W HE ELER Patented Apr. 26, 1938 PATENT OFFICE ssmc'rrvrr'r comnor.

Harold A. Wheeler, Great Neck, N. Y., minor to Hazeltine Corporation, a corporation of Delaware Application June 20, 1936, Serial No. cam

21 Ulaims.

This invention relates to modulated-carrier signal-translating systems such as are employed in radio receivers, and particularly to methods of, and means for, controlling the selectivity and fidelity of reproduction of such receivers.

In accordance with present radio broadcasting practice, each broadcasting station is allotteda carrier frequency on which a signal is transmitted having two sidebands of signal modulation which usually extend 5 kilocycles or more on either side of the carrier frequency. The allotted carrier frequencies are uniformly spaced throughout the broadcast frequency range, the spacing generally being 10 kilocycles. In many instances, therefore, the sideband frequencies of one carrier overlap those of adjacent carriers received at the same location, or closely encroach thereon. This condition frequently makes it dlflicult to tune a receiver to a desired signal without interference from undesired signals on carrier frequencies near the desired signal carrier, particuiii lariy when the strength of such an undesired signal is comparable to, or exceeds that of, the desired signal. So-called background noises present at the outer frequencies of the sidebands may also interfere with quiet reception.

in order to avoid interference from undesired signals on carriers adjacent the desired signal carrier and from background noise present at the higher frequencies of modulation, so that the dill most faithful reception and reproduction of the desired signal consistent with the prevailing re- 'ceived signal conditions may be obtained, it is necessary to utilize a selecting'system which is effective to pass a band of the desired modulation frequencies which is sufficiently narrow greatly to attenuate the undesired signals and noise. Since the outer frequencies of the sidebands which are suppressed by such narrowing of the selected band correspond, in radio broadcasting, to the higher audio frequencies of modulation, the fidelity of reception is thus impaired in proportion to the extent of suchnarrowing. It is, therefore, highly desirable that the width of the band of frequencies be contracted only when such undesired signals or noise are present with suf ficient amplitudes to cause appreciable interference and that, in the absence of such interference conditions and with a sufficiently strong desired signal, the selected band be maintained in expanded condition so as to admit and pass all of the useful sideband frequencies of the desired signal. v

Various systems have heretofore been devised 55 for selecting and controlling the expansion and ii-d contraction of a desired band of frequencies in accordance with received signal conditions to secure the desirableoperating characteristics described above. Means have been employed to effect the control of such selecting systems both manually and automatically. For ideal performance, such selector systems should be characterized by stability, simplicity and efficiency with regard to their construction and operation.

It is a primary object of the present invention to provide an improved method of, and means for, controlling the selectivity of a modulatedcarrier signal-translating system to obtain maximum fidelity of reception consistent with the relative intensities of the received desired signal and undesired signals and noise on frequencies adjacent the desired signal-carrier frequency.

It is a further object of the invention to provide an improved method and means of the character described, whereby the selectivity is con- 20 trolled automatically in accordance with the received signal conditions. I

It is a still further object of the inventiont provide a method and means of the character described utilizing electronically controlled circuits for eflecting the adjustments of the selectivity of the system.

It is a still further object of the invention to provide a method and means of the character described utilizing ed frequency selector circuits.

This invention in its present preferred form is described and explained as embodied in a modulated-carrier signal receiver including a tunable frequencychanger for converting any desired received modulatedecarrier signalwithin a wide range of frequencies into a first intermediate modulated-carrier frequency in the manner of conventional superheterodyne systems. A first fixed selector may be coupled to this frequency changer and preferably is broadly tuned tov the intermediate-frequency output thereof.

In accordance with the present invention, a second frequency changer, a second fixed selector, a third frequency changer, and a third fixed selector circuit are coupled in cascade to the output of the first selector in the order named. The second frequency changer serves to convert the first intermediate-frequency signal to a second intermediate-frequency signal, and the third frequency changer serves to convert the second intermediate-frequency signal to a third intermediate-frequency signal, the second and third selectors being preferably equally sharply tuned respectively to the normal carrier frequencies of the second and third intermediate frequency sigsystem.

In the preferred embodiment of the invention,

.a single oscillator is employed for both the second and third frequency changers, and preferably the fundamental oscillation frequency is utilized for one of these frequency changers and the second harmonic of this fundamental frequency is utilized for the other. The fundamental oscillation frequency is preferably substantially less than the first intermediate-carrier frequency and is combined therewith in the second frequency changer to produce the second intermediate-irequency signal which differs in one sense from the first intermediate-carrier frequency by a difference equal to the fundamental. oscillation. frequency, while the second intermediatefrequency signal is combined with the second harmonic of the oscillation frequency in the third frequency changerto produce the third intermediate-ire quency signal differing from the first intermediate-frequency signal by an equal frequency diiference, but in the opposite sense. The fundamental frequency of the oscillator and hence its second harmonic frequency are adjusted to shift the second and third intermediate-carrier irequencies in opposite senses, changing the extent to which one of the selectors favors one sideband and the other selector favors the othersideband, as mentioned above. This adjustment is preferably, accomplished by automatically controlling the oscillation frequency in accordance with prevailing conditions of reception, or more particularly by adjusting the oscillation frequency, thereby to adjust the width of the band .passed by the selector directly in accordance with the amplitude of the received desired signal carrier and inversely in accordance with the amplitude of undesired signals on carrier frequencies adjacent the desired signal carrier frequency.

For a better understanding of this invention,

together with other and further objects thereof,

reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the accompanying drawings, Fig. l. is a circult diagram, partly schematic, of a complete superheterodyne radio receiver employing the present invention; Fig.2 is a circuit diagram of modified forms of the oscillation generator and the control circuits of the embodiment shown in Fig. 1, while Fig. 3 is a graphical representation of certain relationships obtaining in the receiver shown in Fig. 1.

Referring now more particularly to Fig. i, there is shown schematically a complete superheterodyne radio receiver embodying the present invention in a preferred form. In general, the receiver includes a tunable radlo-frequency ampliiier l0 having its input circuit connected with an antenna II and ground I2 and its output circuit connected to a first frequency changer or oscillator-modulator I3 of conventional type. The tuning elements of the amplifier l0 and frequency changer I! are connected for unicontrol angers in a conventional manner, so that the intermediate frequency developed by the oscillator-moduwhich is broadly tuned to pass the first intermediate-frequency carrier and its sidebars modulation frequencies with reasonable unite iity.

In accordance with the present invention, as hereinafter described in more detail, there is connected to the output circuit of the selector M, in cascade and in the order named, a second frequency changer l5, 2. second intermediatefrequency selector it, a third frequency changer il and a third intermediate-frequency selector it. While the blocks 35 and i? are given the general designation of frequency changer these parts or the system, in fact, are modulators and together with oscillator it constitute frequency changers. The output circuits of the adjustable frequency oscillator it are connected to the modulators of frequency changers l5 and. as shown.

The selector 18 may be connected by way of a non-selective or broadly responsive intermediatcfrequency amplifier 2b to a detector 2i, hereinafter described in detail. The detector output circuit is connected to pass audio fre 'TlClQS 512- rived thereby to a conventional 1-iircquency amplifier and loud-speaker 22, and is also counected to apply negatively the unidirectional voltages developed thereby to the control grid or grids of one or more of the tubes of the amplh fier 2G for automatically controlling the amplification therein in conventional manner, and to apply suchunidirectional voltages also to a control circuit arrangement 23, hereinafter further described, for controlling the frequency of the oscillator iii.-

. In order to control automatically the amplification of signals in the initial stages of the receiver, anauxiliary intermediate-frequency am.- plifier 23 is connected to the output circuit of the frequency changer i3 and its output circuit is, in turn, connected with a diode rectifier quency portion of the system when fully ex" panded, this relation being highly desirable, if the full benefits of expansion are to be obtained. The intermediate-frequency amplifier 2 however, is designed to pass a band of frequencies which is substantially wider than that of the radio frequency amplifier ill, that is, it passes and amplifies not only the desired signal but also all undesired signals which are passed by the radio-frequency amplifier with sufficle'ct amplitude to be capable of overloading the first frequency changer it or causing interference. The rectifier 25 is designed and operates in the conventional manner to develop an amplification control or first AVC bias voltage proportional to the amplitude of the signal supplied thereto. The bias voltage thus developed is applied negatively to the control grids of one or more of the tubes in the radio-frequency amplifier it, the first frequency changer It, to decrease the ampliflcation therein proportionately to the input to the rectifier 25. The unidirectional voltage developed by the rectifier 25 may also be applied progressively to one or more of the tubes of the succeeding stages 01 the system as, for instance, to the second frequency changer l5, as shown, to procure substantially constant signal output eration of the apparatus for controlling the selectivity of the system, which constitutes the principal feature of the present invention, the system described above comprises the elements'of a conventional superheterodyne receiver, the additional circuits'provided in accordance with the present invention effecting conversion of the signal into two additional intermediate frequencies. The operation of a superheterodyne receiver being well understood in the art, a detailed explanation thereof is, therefore, unnecessary herein. In brief, however, signals intercepted by the antenna are selected and amplified in the tunable radio-frequency amplifier I0 and delivered to the first frequency changer it wherein they are converted into a first intermediate-frequency signal which is translated by the broad band selector it to the second frequency changer it. In the second frequency changer the signal is modulated by oscillations developed by the oscillationsystem It to produce a second intermediate-frequency signal which is translated by the selector it to the third frequency changer iii. In the frequency changer i! the signal is again modulated with oscillations developed by the oscillator id to produce a third intermediate-frequency signal which is translated by the selector it to the nonselective intermediate-frequency amplifier it. The signals are thereupon amplified and delivered to the rectifier 2i, wherein the audio frequencies of modulation are derived and delivered to the audio-frequency amplifier and loudspeaker 22 for amplification and reproduction in the conventional manner.

The regressive and progressive automatic amplification control, provided by the amplifier 2t and rectifier 25, maintains signal amplitude at the output of the frequency changer it within a relatively narrow range for wide variations of received signal intensities. Furthermore, in the presence of received interfering signals, the amplitude of the desired modulated carrier at the output of the second frequency changer it is reduced to produce the same effect on the band width control as a decrease in the amplitude of the received desired signal, per se, as will be hereinafter further described. The amplification ,in the amplifier 2t is controlled in conventional manner by means of the unidirectional voltage supplied by the detector til, so as to maintain the amplitude of the signal input to the rectifier more nearly constant.

In accordance with the present invention, the selectivity of the system is controlled by means of the frequency changers it and Ill, the intermediate-frequency selectors it and it, and the oscillator l9, together with the control means therefor. While a separate oscillator may be employed for each of the modulators 15, ii, in the present preferred embodiment of the invention illustrated in Fig. 1, the single oscillator i9 is employed for this purpose. The oscillator it comprises a pentode vacuum tube 26 having a winding 2! connected between its anode and cathode by way of a source of operating potential, as, for example,

the battery 28. The winding 21' constitutes thetors hi, hi and 53, and condensers 5t and 55 reare preferably grounded, as shown.

cillation circuit is connected to the input circuit of the tube 26 through a coupling condenser 32, the input circuit including a. biasing resistor 33.

There is included in series in the cathode circuit of the tube 26 a parallel resonant circuit 35 comprising a condenser 36 and inductance 31, and a resistor 38. The circuit 35 is broadly tuned to pass the fundamental frequency of the oscillator as well as its second harmonic frequency. The lower terminal of the circuit 35 is coupled to the input modulator l5 for delivering the oscillations of fundamental frequency thereto to effect the desired frequency conversion therein. The inductance 31 also serves as the primary winding ,of a transformer, the secondary winding 39 of which constitutes the inductance arm of a parallel resonant circuit Ml, which is tuned to the second harmonic of the oscillator frequency by a condenser M. The circuit W is suitably coupled to the frequency changer '91 for delivering the second harmonic of the oscillator frequency thereto to effect the desired frequency conversion therein. The ground connection. of this coupling circuit may include a suitable biasing resistor 42 and condenser t3.

The control circuit it comprises a pentode vacuum tube 44 having its input circuit connected across the resistor 3! of the oscillation circuit 29, 30 by way of coupling condensers t5 .and shunt resistor 46. The anode circuit of the tube M is connected across the entire oscillation circuit, operating voltage for the tube M being supplied by the battery M. A suitable potential is supplied to the screen grid by means of a battery M, the

- suppressor grid is connected to ground, as shown,

and a proper control grid-bias voltage is provided by a battery 43 in the cathode circuit.

The detector 2i comprises a double diode 5!] having as a load circuit series-connected resisspectively connected between the opposite ends of the resistor 5i and the cathodes. The cathodes Audio-frequency voltages developed across the resistor 53 are applied to the audio-frequency amplifier 22 in conventional manner. The unidirectional component of the voltage developed across the resistor 53 is applied negatively to the control electrodes of one or more of the tubes of the intermediate-frequency amplifier it, by way of a filter including'a series resistor ht and shunt condenser bl, to provide additional automatic amplification control as described above. For the purpose of controlling the selectivity of the system, the unidirectional component of the voltage developed across resistors 52 and 53 is applied negatively to the control grid of the tube M by way of a filter including series resistors 58 and 59 and shunt condenser 60.

The operation of the receiver embodying the .present invention, as just described, will be explained with reference to the diagram of Fig. 3 wherein the abscissae represent frequencies in kilocycles, and characteristic selectivity curves for the'selectors I4, l6 and it and amplifier 20 are indicated at Ma, Ilia, Mia and 20a respectively. The ordinates in Fig. 3 are not of importance, the vertical spacing of the curves in the figure being merely for the purpose. of clarity. The selectors I6 and is, as mentioned above, may be of any conventional type and are designed to pass rela-- tively narrow bands of frequencies, that is, to favorequally sharply the respective intermediatefrequency signal components to which they are tuned, as indicated by the curves 16a and l8a.

Ti l

iii

The selector i6 is broadly responsive, however, as shown by the curve Ma, and the amplifier 20 is non-selective; that is, designed to pass substantially uniformly the band including all of the frequencies delivered thereto.

The first intermediate-carrier frequency developed by the frequency changer i3 is substantially constant for all tuning adjustments of the receiver and may, for example, be 485 kilocycies, indicated in Fig. 3 as ii. The circuit constants of the oscillation system it) are so proportioned that, in the absence of signal conditions efiecting expansion of the band width of the system, a predetermined fundamental frequency and second harmonic frequency are produced. These frequencies may, for example, be 140 kilocycles and 280 kilocycles, respectively, and are indicated in Fig. 3 as I and Zfo. In the preferred embodiment of the invention illustrated, the selector i8 is tuned to the normal second intermediate-carrier frequency indicated as f2, developed by the frequency changer l5, which is equal to the sum of the first intermediate-carrier frequency plus the fundamental oscillation frequency, or f1 plus 10, in this instance 605 kilocycles. On the other hand, the selector I8 is tuned to the normal third intermediate-carrier frequency, indicated as 7'3, developed by the frequency changer it, which is equal to the difference between the second intermediate-carrier frequency and the second harmonic frequency, or f2 minus 2fo, in this instance 325 kilocycles. The band passed by the system under these conditions is, therefore, relatively narrow as indicated by the curve 2%, theonly effect of the amplifier 26 being uniformly to amplify all of the frequencies passed by the selector system.

Referring now particularly to the operation of the control circuit 23, since the input of the tube Ml is connected across the resistor M, which is in the capacitance arm of the oscillation circuit, the input voltage of the tube M leads the voltage across the oscillation circuit 29, 30 by 90 degrees. The outputcurrent' of this tube, which is supplied to the oscillation circuit, also leads the voltage across the oscillation circuit by 90 degrees and the tube simulates a condenser of low power factor. The bias voltage applied negatively to the grid of the tube 44 from the detector 2i varies in accordance with the amplitude of the desired signal input to the receiver and thus controls the amplitude of the current supplied to the oscillation circuit by the tube M, and hence shifts the effective resonant frequency of the oscillation circuit directly in accordance with the amplitude of the desired signal.

An increase in the oscillation frequency in response to an increase in the signal input, which may be referred to as a frequency increment A), serves to increase the frequency f: of the second intermediate-frequency signal developed by the frequency changer i5 by a corresponding increment, as indicated in Fig. -3, so that it now has a value of 605+Aj lzilocycles. At the same time the second harmonic of the oscillation frequency is increased by twice the frequency increment, 2M, which serves to decrease the third intermediate frequency fa developed by the modulator II, also by a net amount Af, so that it now has a value of 325A,f kilocycles.

Since, as mentioned above, the selectors l8 and I! are permanently tuned sharply to the normal second and third intermediate-carrier frequencies f: and f3, respectively, the shift of the second intermediate-carrierfrequency causes the selector it to favor the lower sideband of modulation frequencies while the shift of the third intermediatecarrier frequency causes the selector 8 to favor the upper sideband of modulation frequencies. This condition is shown by the positions of the vertical broken lines in Fig. 3, representing the adjusted values of the second and third intermediate-carrier frequencies. In this manner, the band of frequencies passed by the system as a whole is effectively symmetrically expanded with increase of the oscillation frequency, so that the resultant band passed by the entire intermediateirequency amplifier has a characteristic as indicated by the curve 2%.

An increase of the amplitude of the received desired signal, in the absence of interfering signals, thus results in a symmetrical expansion of the band of frequencies passed by the system to increase the fidelity of reception in accordance with the input amplitude of the desired signal, depending on prevailing conditions of reception. When, however, interfering signals are received on carrier frequencies adjacent the desired signal-carrier frequency, since the amplifier 2G is responsive to these interfering signals, the rectifier 25 develops an increased bias voltage and this condition effects a reduction of the gain of the ampliiler iii and frequency changers i3 and i5, causing a decrease in the amplitude of the desired signal input to the detector 2i. The same effect on. the band width control means is thus effected by an increase in the amplitude of the interfering signals as by a decrease in the amplitude of the desired signal, that is, the width of the band passed by the system is contracted and the effects of the interfering signals are thus minimized.

In Fig. 2 there are illustrated modified forms of the oscillation generator and control circuit shown in Fig. 1, which may be similarly embodied in a complete receiver, and which operate in substantially the same manner as the corresponding circuits of Fig. 1. Here the oscillator iila is substantially the same as the oscillator 59 of Fig. 1, excepting that in the oscillation circuit 29a, 3% the resistor am is included in the inductance arm of the circuit. The control circuit 230. is identical to the control circuit 23 of Fig. 1, the input circuit of tube a, which corresponds to tube M, being connected across the resistance 3 la. It will be apparent, with the resistor 31a thus included in the inductance arm of the oscillation circuit, that the input voltage to the tube Ma, will lag the voltage across the oscillation circuit by 90 degrees, so that the output current of the tube Ma supplied to the oscillation circuit likewise lags the voltage across the oscillation circuit by 90 degrees and the tube Ma will simulate a variable inductance of low power factor across the oscillation circuit. Increase of the output current of the tube, caused by increase of the incremental bias voltage applied to the tube Ma, causes a decreas of the apparent inductance of the oscillator circuit to increase its resonant frequency.

The control circuit Zia is generally similar to the circuit 2! of Fig. 1, and includes a double diode 50a which may be similarly connected to the output circuit of a preceding amplifier 20. I

The load circuit of the rectifier Zia comprises a pair of resistors BI and 62 which are grounded at their junction and are shunted by

condensers

63 and 64, respectively. The audio-frequency voltage developed across the resistors 6i and 62 is applied negatively by way of filter resistors 65, 66, and

condensers

61, 68 and lead 69 to the audio-frequency amplifier. The unidirectional component of the voltage developed across resistors 6i and 62 may also be applied negatively, by way of the filtering elements just mentioned, over the lead 10, as well as by way of further filtering elements (not shown) for removing the audio-frequency component, as automatic amplification control bias voltage to the preceding intermediate-frequency amplifier. In this instance, the unidirectional voltage developed across the resistor 62 is applied positively by way of filtering elements including series resistors 1|, 12 and i3 and shunt condensers M and to the control grid of the tube 440.. The unidirectional control voltage being applied to the tube tively, increases in the bias voltage developed by the rectifier 5011 will effect increases in the lagging current supplied by the tube a effectively to decrease the apparent inductance of the oscillation circuit, as mentioned above. Hence, the resonant frequency of the oscillation circuit is increased with increases in the desired signal input to the detector 28, and the operation of this modified form of the invention is substantially the same as that of the embodiment shown in Fig. 1.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein Without departing from the invention, and, therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is:

1. In a modulated-carrier signal-translating system, frequency-changing means including an oscillator for deriving from a desired modulatedcarrier signal of a. given frequency a second modulated-carrier signal normally of a predetermined different frequency, selecting means niost responsive at said predetermined frequency coupled to said frequency-changing means, and means for automatically adjusting the ed'ective width of a modulation sideband of said second signal passed by said system comprising means responsive to the amplitude of received signals for so changing the frequency of said oscillator as to shift the frequency of said second modulated-carrier signal relative to the response frequency oi said'selecting means. s v

2. In a modulated-carrier signal-translating system, frequency-changing means, including oscillation generating means generating a plurality of frequencies, for deriving from a first desired modulated-carrier signal a second modulated-- carrier signal normally of a first predetermined frequency and for deriving from said second modulated-carrier signal a third modulated-carrier signal normally of a second predetermined frequency, selecting means for individually translaiing said second and third modulated carrier signals and most responsive at said first and second predetermined frequencies, respectively, and l. loans for adjusting the width of the band of frequencies passed by said system comprising means for adjusting the frequencies of the generated oscillations, said generated frequencies being so related to the frequencies of said first and second signal carriers that said adjustment is effective to shift the carrier frequencies of said second and third modulated-carrier signals in opposite senses relative to the response frequencies of their respective selecting means.

3.111 a modulated-carrier signalstranslating a posi- 5 system, frequency-changing means, including oscillation generating means generating a plurality of frequencies, for deriving from a first desired modulated-carrier signal a second modulatedcarrier signal normally at a first predetermined frequency and for. deriving from said second modulated-carrier signal a third modulated-carrier signal normally of a second predetermined frequency, selecting means for individually translating said second and third modulated-carrier signals and most responsive at said first and second predetermined frequencies, respectively, and means for adjusting the width of the band of frequencies passed by said system symmetrically with respect to the mean frequency of said vband, comprising means for adjusting the frequencies of the generated oscillations, said generated frequencies being so related to the frequencies of said first and second signal carriers that said adjustment is effective to shift the frequencies of said second and third carriers in opposite senses relative to the response frequencies of their respective selecting means and to equal extents.

4. In a modulated-carrier signal-translating system, frequency-changing means, including oscillation generating means generating a plurality of frequencies, for deriving from a first desired modulated-carrier signal, including a carrier at a first frequency and its modulation sidebands, a second modulated-carrier signal normally at a predetermined second frequency and for deriving from said second signal a third modulatedcarrier signal normally at a predetermined third frequency, selecting means for individually translating said second and third signals most responsive to said second and third frequencies, respectively, and means for automatically adjusting the width of the band of frequencies passed by said system symmetrically with respect to the mean frequency of said band comprising means responsive to the received signal conditions for adjusting the frequencies of the generated oscillations, said generated frequencies being so related to the frequencies of said first and second signal carriers that said adjustment is effective to shift the carrier frequencies of said second and third modulated-carrier signals relative to the response frequencies of their respective selecting meansin opposite senses and to equal extents.

. 5. In a modulated-carrier signal-translating system, frequency-changing means, including oscillation generating means generating a plurality of frequencies, for deriving from a first desired modulated-carrier signal, including a carrier at a first frequency and its modulation sidebands, a second modulated-carrier signal normally at a predetermined second frequency and for deriving from said second signal a third modulatedcarrier signal normally at a predetermined third frequency, a first selective circuit sharply responsive to said second frequency for translating said second signal, a second selective circuit sharply responsive to said third frequency for translating said third signal, said circuits being proportioned to have substantially equal degrees of selectivity, and means for adjusting the width of the band of frequencies passed by said system comprising means for adjusting the frequencies of the generated oscillations, said generated frequenciesbeing sorelated to the frequencies of said first and second signal carriers that said adjustment is effective to shift the carrier frequencies of said second and third modulated-carrier signals relative to the response frequencies of their respective selective circuits in opposite senses and to equal. extents.

6, In a modulated-carrier signal-translating system, frequency-changing means, including an oscillator having an oscillation circuit and generating a plurality of frequencies, for deriving from a first desired modulated-carrier signal a second modulated-carrier signal normally of a first predetermined frequency and for deriving from said second modulated-carrier signal a third modulatedwarrier signal normally of a second predetermined frequency, individually translating said second and third modulated-carrier signals most responsive to said first and second predetermined frequencies, respectively, and means for adjusting the width of the band of frequencies passed by said system comprising a vacuum tube connected in circuit with said oscillation circuit for adjusting the apparent reactance of said oscillation circuit to shift the frequencies generated thereby upon ad-= justment of the operating potentials thereof, said generated frequencies being so related to the frequencies of said first and second signal carriers that said adjustment is effective to shift the carrier frequencies of said second and third modulated-carrier signals in opposite senses relative to the response frequencies of their respective selecting means, and means for adjusting said operating potentials.

'7. In a modulated-carrier signal-translating system, frequency-changing means, including an oscillator having an oscillation circuit and generating a plurality of frequencies, for deriving from a first desired modulatedwarrier signal a second modulated-carrier signal normally of a first predetermined frequency and. for deriving from said second modulatedcarrier signal a third modu lated carrier signal normally of a second predetermined frequency, selecting means for individually translating said second and third modulated-carrier signals most responsive to said first and second predetermined frequencies, respectively, and means for adjusting the width of the band of frequencies passed by said system symmetrically with respect to the mean frequency of said band, comprising a vacuum tube having acontrol electrode and connected in circuit with said oscillation circuit for adjusting the apparent reactance in said oscillation circuit to shift the frequencies generated thereby with adjustment of biasing voltage applied to said control electrode, said generated frequencies being so related to the frequencies of said first and secondv signal carriers that said adjustment is effective to shift the carrier frequencies of said second and third modulated-carrler signals in opposite senses relative to the response frequencies of their respective selecting means and to equal extents, and means for adjusting the biasing voltage applied to said control electrode.

8. In a modulated-carrier signal-translating system, frequency-changing means, including an oscillator having an oscillation circuit and generating a plurality of frequencies, for deriving from a first desired modulated-carrier signal a second modulated-carrier signal normally of a first predetermined frequency and for deriving from said second modulatcdwarrier signal a third modulated carrier signal normally of a second predetermined frequency, selecting means for individually translating said second and third modulated-carrier signals most responsive to said first and second predetermined frequencies, respectively, and means for adjusting the width of selecting means for mated-carrier afsqore the band of frequencies passed by said system symmetrically with respect to the mean frequency of said band, comprising a vacuum tube having a control electrode and connected in circuit with said oscillation circuit for adjusting the apparent reactance in-said oscillation circuit to shift the frequencies generated thereby with adjustment of biasing voltage applied to said control electrode, said generated frequencies being so related to the frequencies of said first and second signal. A

carriers that said adjustment is effective to shift the carrier frequencies of said second and third modulated-carrier signals in opposite senses relative to the response frequencies of their respective selecting means and to equal extents, a rectifier for developing a unidirectional biasing voltage in response to the received signal conditions, and means for applying said unidirectional voltage as a control biasing voltage to said control electrode.

9. In a modulatedwarrier signal-translating system, a first frequency changer for deriving from. the desired modulated-carrier signal, including a carrier of a first frequency and its sidebands of modulation frequencies, a second modsignal normally of a predetermined second frequency, a first selector coupled to said'flrst frequency changer and most responsive at said second frequency, a second frequency changer coupled to said first selector for deriving from said. second signal a third modulatedcarrier signal normally of a predetermined third frequency, a second selector coupled to said second frequency changer and most responsive at said third frequency, and means for automatically adjusting the width of the band of frequencies passed by said system. comprising means for simultaneously adjusting said second and third frequencies relative to the response frequencies of their respective selectors to cause selectors to favor one of the signal sidebands and the other of said selectors to favor the other of the signal sidebands.

ill. in. amodulated-carrier signal-translating system, a first frequency changer including an 05- x clllator effective to generate a plurality of hermonically related frequencies for deriving from a desired modulated-carrier signal, including a carrier of a first frequency and its sidebands of modulation frequencies, a second modulated-carrier signal normally of a predetermined second frequency differing" from said first frequency in one sense by an amount equal to one of said oscillator frequencies substantially less than said first frequency, a first selector coupled to said first frequency changer and most responsive at said second frequency, a second frequency changer utilising the harmonic of double said one of said oscillator frequencies for deriving from said second signal a third modulated-carrier signal. normally of a predetermined third frequency differing from said first frequency in the opposite sense to that of said second frequency and by an equal amount, a second selector coupled to said second frequency changer and most responsive at said third frequency, and means for adjusting the width of the band of frequencies passed by said system comprising means for adjusting said oscillator frequencies to adjust the extent to which one of said selectors favors one of the signal sidebands and to adjust equally the extent to which the other of said selectors favors the other of said signal sidebands.

11. In a modulated-carrier signal-translating system, a tunable first frequency changer for deone of said .1

riving from a desired modulated-carrier signal including a carrier at any given frequency in a wide range and its sidebands of modulation frequencies, a second modulated-carrier signal including a carrier at a predetermined second frequency, a first selector coupled to said first frequency changer and broadly tuned to said second frequency, a second frequency changer coupled to said first selector and including an oscillator effective to generate a plurality of harmonically related frequencies for deriving from said second signal a third modulated-carrier signal normally at a third predetermined frequency differing from said second frequency in one sense by an amount equal to one of said oscillator frequencies substantially less than said second frequency, ,a second selector coupled to said second frequency changer and most responsive to said third frequency, a third frequency changer utilizing the harmonic of double said one of said 0scillator frequencies for changing said third signal to a fourth modulated-carrier signal normally at a fourth frequency differing from said second frequency in the opposite sense to that of said third frequency and by an equal amount, a

third selector coupled to said third frequency w changer and most responsive to said fourth frequency, and means for adjusting the width of the band of frequencies passed by said system comprising means for adjusting said oscillation frequencies to adjust the extent to which one of said selectors is caused to favor one of the signal sidebands and to adjust equally the extent to which the other of said selectors favors the other of the signal sidebands.

12. In a modulated-carrier signal-translating system, frequency-changing means including an oscillator having an oscillation circuitand effective to generate a plurality of harmonically related frequencies for utilizing one of said oscillator frequencies to derive from a desired modulated-carrier signal including a carrier of a first frequency and its sidebands of modulation frequencies, a second modulated-carrier signal normally of a predetermined second frequency, said frequency of said oscillations being substantially less than the frequency of said desired modulatecl-carrier and for utilizing the harmonic of double said first one of said oscillator frequencies to derive from the second signal a third modulated-carrier signal normally at a predetermined third frequency, selectingmeans for translating said second and third modulated-carrier signals most responsive at said second and third frequencies respectively, means for adjusting the width of the band of frequencies passed by said system comprising means for adjusting the apparent reactance of said oscillation circuit to adjust the frequencies of the generated oscillations, thereby to shift the carrier frequencies of said second and third modulated-carrier signals in opposite senses relative to the response frequencies of their respective selecting means, and means for controlling said adjusting means automatically in accordance with the received signal conditions.

13. In a modulated-carrier signal-translating system, a first frequency changer including an oscillator having an oscillation circuit and effective' to generate a plurality of harmonically related frequencies for deriving from a desired modulated-carrier signal including a carrier of a first mally of a predetermined second frequency differing from said first frequency in one sense by cies substantially less than said. first frequency;

a selector coupled to said first frequency changer and most responsive at said second frequency, a second frequency changer coupled to said second selector utilizing the harmonic of double said one of said oscillator frequencies for deriving from said second signal a third modulated-carrier signal normallyat a predetermined third frequency differing from saidflrstfrequency-in the opposite sense to that of said second frequency andby an equal amount, a second selector coupled to'said second frequency changer and most responsive at said third frequency, means for adjusting the width of the band of frequencies passed by said system comprising a vacuum tube connected in circuit with said oscillation circuit for adjusting the apparent reactance of said oscillation circuit to adjust the fundamental frequency thereof and thereby its harmonic frequencies in accordance with adjustments of bias voltage on said vacuum tube thereby to vary in opposite senses the effective responsiveness of said selectors, and means for adjusting said bias voltage in accordance with the amplitude of the desired signal carrier.

14. An electric circuit arrangement for controlling the selectivlty of a modulated-carrier signal-translating system automatically in accordance with the received signal conditions comprising frequency-changing means including an oscillator for deriving from a desired modulated-carrler signal of a given frequency a second modulated-carrier signal normally of a predetermined different frequency, selecting means most responsive at said predetermined frequency coupled to said frequency-changing means, and means for adjusting, the effective width of a modulated sideband of said second modulatedcarrier signal passed by said system comprising means responsive to the amplitude of received signals for so changing the frequency of said oscillator as to shift the frequency of said second modulated-carrier signal relative to the response frequency-of said selecting means.

15. An electric circuit arrangement for controlling the selectivity of a modulated-carrier signal-translating system comprising frequencychanging means, including oscillation generating means generating a plurality of frequencies, for deriving from a first desired modulated-carrier signal a second modulated-carrier signal normally of a first predetermined frequency and for deriving from said second modulated-carrier signal a third modulated-carrier signal normaly of a second predetermined frequency, selecting means for translating said second and third modulated-carrier signals and most responsive at said first and second predetermined frequencies respectively, and means for adjusting the frequencies of the generated oscillations, said generated frequencies being so related to said first and second signal carriers that said adjustment changer and most responsive at said second frequency, a second frequency changer coupled to said first selector for deriving from said second signal a third. modulated-carrier signal normally of a predetermined third frequency, a second selector coupled to said second frequency changer and most responsive at said third frequency, and means for automatically adjusting the width of the band of frequencies passed by said system comprising means for simultaneously adjusting said second and third frequencies relative to the response frequencies of their respective selectors to cause one of said selectors to favor one of the signal sidebands and the other of said selectors to favor the other of the signal sidebands.

l'l'. The method of controlling the selectivity of a modulated-carrier signal-translating systcrn which comprises generating oscillations, modulating a desired modulated-carrier signal of a given frequencywith said oscillations to derive a second modulated-carrier signal normally of a predetermined difierent frequency, translating saici'second modulated-carrier signal with frequency discrimination sharply in favor of said predetermined frequency, and so adjusting the frequencies of said oscillations in re sponse to and simultaneously with changes in the amplitude of received signals as to shift the ire quency of said second modulated-carrier signal relative to said predetermined frequency, thereby to adjust the effective width of" a modulation sideband of said second carrier passed by said,

system.

18. The method of controlling the selectivity of a modulated-carrier signal-translating system which comprises generating oscillations, modw lating a desired modulated-carrier signal. of a given frequency with certain of said oscillations to derive a second modulated-carrier signal normally of a first predetermined frequency and modulating said second modulated-carrier signal Wl other of said oscillations to derive a third modulated-carrier signal normally at a second predetermined frequency, translating said second and third modulated-carrier signals with discrimination sharply in favor of said first and second predetermined frequencies, and adjusting the frequency of said oscillations to shift the carrier frequencies of said second and third signals in opposite senses relative to said desired carrier frequency, thereby to adjust the width of the hand of frequencies passed by said system.

19. The method of controlling the selectivity of a modulated-carrier signal-translating system which comprises generating oscillations, modulating a desired modulated-carrier signal with certain of said oscillations to derive a sec- 0nd modulated-carrier signal normally of a first predetermined frequency and modulating said second modulated-carrier signal with certain other of said oscillations to derive a third modulated-carrier signal normally at a second predetermined frequency, translating said second and third modulated-carrier signals with frequency lated-carrier signal normally discrimination sharply in favor of said first and second predetermined frequencies, and adjusting the frequency of said oscillations in response to and simultaneously with ceived signal conditions to shift the carrier frequencies or" said second and third modulatedcarrier signals in opposite senses relative to said desired carrier frequency and to equal extents, thereby to adjust the width of the band of frequencies by said system symmetrically with respect to the mean frequency of said hand in accordance with said received signal conditions. I

20. The method of controlling the selectivity of a modulatedwarrier signal-translating system which comprises changing the frequency of a desired modulated-carrier signal including a carrier at a first frequency and its sidebands of modulation frequencies to derive a second modulated-carrier signal normally at a predetermined second frequency, selecting said second signal with discrimination-sharply in favor of said second frequency, changing said second car- Her-frequency signal to derive a third moduat a third predetermined frequency, selecting said third signal with sharp discrimination frequency, adjusting said second. and third frequencies vii-stile selecting said second and third signals with said. discrimination sharply in favor q of said second and third predetermined frequencies, thereby to favor one of the signal sidebands during one of said selections and to favor the other or the signal sidehands during the other of said selections.

21. The method of controlling the selectivity of a modulated-carrier signal-translating system which comprises generating a plurality of harmonically related frequencies, modulating a desired modulated-carrier signal with a first of said oscillation frequencies to derive a second modulated-carrier signal normally at a first predetermined frequency differing from the carrier frequency or" said desired signal by the amount of said first oscillation frequency, modulating said second modulated-carrier signal with a second harmonic of said first oscillation frequency to derive a third modulated-carrier signal normally at a second predetermined frequency diffaring from the carrier frequency of said desired signal in an opposite sense to said first predetcrmined frequency and in the same amount, translating the second and third modulated-carrier signals with discrimination sharply in favor of said first and second predetermined frequencies respectively, and adjusting the frequency of said oscillations to shift said second and third carriers changes in the re-' in. favor of said third in opposite senses relative to their respective HAROLD A. i l/ HEELER.