US2398694A - Carrier-wave generating system - Google Patents

Description

April 16, 1946. CASE 2,398,694

CARRIER-WAVE GENERATING SYSTEM Filed March 20 1942 3 Sheets-Sheet 5 NELSON P. CASE ATTORNEY Patented Apr. 16, 1946 2,888,894 CARRIER-WAVE GENERATING SYSTEM Nelson P. Case, Great Neck. N.

Hueltlne Y assignmto rporatlon, a corporation 0! Delaware Application Mai-en so. 1942, semi! No. 435.441 19 Claims. (Oletc-so) The present invention relates to carrier-wave generating systems and, particularly, to such systems adapted to generate a carrier wave having any selectable one of a number of predetermined constant frequencies in a predetermined frequency band, each such frequency corresponding to a desired transmission channel.

It is frequently desirable to provide a highirequency carrier-wave generating system capable of generating a carrier wave having a constant or stabilized frequency on any selectable one of a predetermined number of transmission channels located in a predetermined high-frequency band. Such systems have many useful applications. For example, in the field of marine communication, a predetermined number of transmission channels are allocated for use in ship-to-shore communication in the vicinity of a given port and, since the number of ships in that locality desiring to use such channels may exceed the number of channels assigned to such service, it would be undesirable to assign a particular channel to a given ship. A person having available the carrier-wave generating system of the type described can listen on the air to find a channel which is not used at that moment and may then quickly and readily set his transmitting apparatus for operation on a non-used channel.

There are only a limited number of radio transmission channels available in the portion oi the frequency spectrum normally used at the present day. The demands upon the available channels necessitate that interchannel interference he reduced to a minimum by the use or carrier-wave generating systems having a high degree of frequency stability. Piezoelectric crystals are con ventionally used for this purpose. Where the carrier-wave generating system must operate on any selectable one of a number of pro-determined transmission channels, the problem arises as how best to stabilize the frequency of the system on each of these channels. Individual crystals may, of course, be provided for each such channel. This has the disadvantage, however, that it requires a large number of relatively expensive crystals and requires a complicated switching arrangement by which the crystals are selectively included in circuit with the oscillator of the gen. erating system.

In one arrangement of the prior art it has been proposed that multihannel transmission be pro-- vided by the use of a relatively high-frequency carrier-wave generator, a tunable low-frequency oscillator which is relatively stable in absolute frequency by virtue of the fact that it generates only low frequencies, a modulator by which the high-frequency carrier wave is modulated with the low-frequency generated oscillations to provide a modulated-carrier wave including a carrler-wave component and modulation sideband components, and a selector for selecting one sideband of the modulated-carrier wave to derive a modulation component which is used as the carrier wave for radiation. This prior art arrangement has several disadvantages. No provision is made for precisely tuning the generating system to definite predetermined transmission channels, this being done only by the method of manually tuning the low-frequency oscillator as closely as manual manipulation will permit to any desired channel. There is the further disadvantage that the system is capable of operating only over a very limited operating-frequency band. Additionally, there is the disadvantage that the selector which selects the desired sideband component of the modulated-carrier wave generally cannot, in practice, he designed to select only the desired component to the exclusion of the carrierwave component and the other modulation sideband component, with the result that the latter components are apt to be radiated as undesired spurious carrier waves which may create undesired interference on other transmission channels.

In accordance with another prior art arrangement, it has been proposed that precise multiplechannel operation of a carrier-wave generator on definite selected transmission channels be provided by the use of a low-frequency crystal-controlled osclllator having a frequency equal to the frequency separation of desired transmission channels. There is derived from this oscillator a modulation signal having frequencies which are harmonically related to the frequency of the lowfrequency oscillator. A high-frequency crystalcontrolled oscillator generates a carrier wave which is modulated with the modulation signal to derive a carrier wave having modulation sideband components which define desired equally-spaced transmission channels. A selected component of the modulated-carrier wave is used as the carrier wave which is radiated. While this prior art arrangement is adapted to operate over a wide band of operating frequencies, it is subject, as in the arrangement previously described, to the radiation of undesired modulation components as undesired spurious carrier waves. The radiation of such spurious carrier waves causes, of course, undesired interference with communications on channels other than that which, at any given time, is used by the carrier-wave generating system described.

It is an object of the present invention, therefore, to provide a new and improved carrier-wave generating system which avoids one or more of the disadvantages and limitations of the prior art systems.

It is a further object of the invention to provide an improved carrier-wave generating system in which a single frequency standard is effective to maintain the frequency of a relatively highfrequency carrier wave substantially constant on any selectable transmission channel in a wide band of such channels.

It is an additional object of the invention to provide a carrier-wave generating system adapted to generate a relatively high-frequency carrier wave and in which the frequency of the wave generated is manually adjusted app y 130 that suitable for operation of the system on any selected transmission channel in a wide band of such channels and the generator-is thereafter automatically controlled to adjust it more accurately to and maintain it on such selected channel.

It is a further object of the invention to provide a carrier-wave generating system in which a plurality of frequency standards are effective to establish and maintain the frequency of a relatively high-frequency carrier wave substantially constant on any selectable transmission channel in one of a plurality of wide bands of such channels after the frequency of the carrier wave has been manually adjusted approximately to the frequency corresponding to such selected channel, each of such bands of channels being defined by a particular one of the frequency standards and the transmission channels of one band intervening between the channels of at least a portion of one or more of the others of the bands.

In accordance with the invention, a carrierwave generating system comprises means for generating a. carrier wave including a carrier-wave component of relatively high and substantially constant frequency and sideband components of substantially constant frequency and extending over a predetermined frequency band. The system includes means for generating a second wave having any selectable frequency in a predetermined band equal to but displaced by a predetermined value in the frequency spectrum from the first-mentioned frequency band. There is also included in the system means responsive Jointly to the frequency of the second wave and the frequency of that one of the aforesaid components of the first wave which is displaced in the frequency spectrum by a predetermined value from the desired operating frequency of the second wave for maintaining the frequency of the second wave at a substantially constant desired value.

For a better understanding of the present 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.

Referring now to the drawings, Fig. 1 is a schematic circuit diagram of a complete carrier-wave generating system embodying the present invention; Fig. 2 is a frequency scale used as an aid in explaining the operation of the Fig. 1 arrangement; Fig. 3 is a schematic circuit diagram of a complete carrier-wave generator embodying a modified form of the invention; and Figs. 4 and 5 are frequency scales used in explaining alternative modes of mention of the Fig. 3 arrangement.

Referring now more particularly to Fig. 1 of the drawings, there is represented schematically a complete carrier-wave generating system embodying the present invention in a particular form. The system includes means for generating a carrier wave of high and substantially constant frequency comprising an oscillator Hi, this means including means for maintaining the frequency thereof substantially constant comprising, preferably, a piezo-electric crystal l I. The system also includes means for generating a modulation signal having frequency components of substantially constant frequencies and predetermined frequency spacing relative to each other. preferably a frequency spacing equal to that between equally-spaced desired transmission channels, comprising an oscillator I2 and a harmonic generator is having an input circuit coupled to the output circuit of the oscillator i2. Since the frequency components of the modulation signal derived by the harmonic generator l3 are harmonically related, it will be seen that they have equal frequency spacings relative to each other and, as will become apparent hereinafter, define equally-spaced predetermined desired transmission channels. The system additionally includes means for modulating the carrier wave generated by oscillator lfl with the modulation signal generated by units l2 and I3 to derive a carrier wave including a carrier-wave component of relatively high and constant frequency and sideband components of substantially constant frequencies and extending over a predetermined frequency band. This last-named means comprises a modulator if having one input circuit coupled to the output circuit of harmonic generator 13 and another in put circuit coupled to the output circuit of oscillator Ill and havin an output circuit in which is developed the aforementioned carrier wave including the carrier-wave component and sideband components.

The carrier-wave generating system of the invention also includes means for generating a second wave having any selectable frequency in a predetermined frequency band equal to but displaced in the frequency spectrum from the fre quency band of the above-mentioned modulated carrier wave, hereinafter referred to as the control carrier wave for simplicity, developed by units I044, inclusive. This means comprises an oscillator l5 having an adjustable tuning element comprising a condenser l6 for tuning the oscillator l5 approximately to a desired operating frequency. Since in practice it is desirable for numerous reasons, such as lower cost, the increased ease of making, and increased ruggedness, that the piezoelectric crystal ii have a frequency as low as the circumstances permit, the frequency band of the oscillator l5 preferably lies above the frequency band of the control carrier wave. I

There is also included in the carrier-wave generating system means responsive jointly to the frequency of the second wave, which is generated by the oscillator l5, and the frequency of that one of the frequency components of the control carrier wave which is displaced in the frequency spectrum by a predetermined selectable value from the carrier-wave component of the latter wave for maintaining the frequency of the second wave at a substantially constant desired value. This last-named means comprises a modulator if having a first input circuit coupled aaaacoc to the output circuit of modulator It and a second input circuit coupled to the output circuit of oscillator II, the modulator il having an output circuit coupled to an intermediate-frequency amplifier ll of one or more stages. There is coupled to the output circuit of amplifier it, in the order named, a frequency detector l9 and a frequency shifter 20, which may comprise a reactor tube, coupled across a frequency-control circuit of oscillator II. The output circuit of oscillator I! is coupled to the input circuit of a unit II which may include an amplitude-modulator stage and a tunable radio-frequency amplifier of one or more stages. The modulation signal to be transmitted is applied to the modulator of unit 21 in conventional manner. The output of the radio-frequency amplifier of unit 2! is coupled to an antenna system 22, 23. The radio-frequency amplifier of unit 2i includes an adjustable tuning element N which is mechanically connected for unicontrol operation with the adjustable tuning element ii of oscillator l5, as indicated by the broken line 28.

Considering now the operation of the arrange ment just described, and referring to the frequency scale of Fig. 2, assume that the carrierwave generating system is to operate on any selectable one of a predetermined number of transmission channels having equal frequency spacing h. The oscillator i2 generates oscillations having the frequency 11 and, since this frequency is normally relatively low, for example of the order of 100 or 200 kilocycles, this oscillator has good absolute frequency stability without requiring that it be of the piezoelectric crystal-controlled type. If desired, however, the oscillator I! may be of the crystal-controlled type to insure even better frequency stability.

The signal generated by the oscillator i2 thus has a frequency equal to the frequency spacing between the equally-spaced desired transmission channels and this signal is applied to the input circuit of the harmonic generator l3. There is derived in the output circuit of generator it a modulation signal having frequency components which may, for example, include the fundamental-frequency component ii of the signal of oscillator I2 and nine harmonic components fz-fm, inclusive, which are harmonically related to, and preferably, are a continuous band of harmonics of the signal generated by the oscillator I2, whereby the frequency components of the modulation signal have relative spacings corresponding to that of desired transmission channels. This modulation signal is applied to an input circult of the modulator it and there is applied to another input circuit of this modulator a carrier wave having a relatively high and substantially constant frequency in generated by the oscillator II.

There is consequently derived in the output circuit of the modulator it the "ontrol carrier wave including a carrier-wave component of relatively high and substantially constant frequency and sideband components of substantially constant frequency and extending over a predetermined frequency band, such components of the control carrier wave having, in the preferred form of the invention, equal spacing in the frequency spectrum with relation to each other and the frequency of the components individually defining predetermined desired transmission channels. The term "define" is here used to denote that these components individually so control the oscillator it that the frequency of the carrier wave generated by oscillator It at any given time corresponds to that of a selected one of a plurality of transmission channels equal in number to the components but lying in the frequency spectrum in predetermined relations with respect to the frequency band of the components, preferably lying above the latter band. The frequency components of this control carrier wave are indicated in Fig. 2 as a spectrum of frequencies shown over the frequency In and extending over a frequency band Ill-fut, the carrierwave component being designated by the frequency {'11, the lower-sideband components by the frequencies I'm to 1'1, inclusive, and the upper-sideband components by the frequencies )'i to {"m, inclusive.

The control carrier wave is applied from modulator ll to one input circuit of the modulator I! while the carrier wav developed by the oscillator I5 is applied to another input circuit of the modulator H. The oscillator I6 is tunable by manual adjustment of the tuning element it over a predetermined frequency band Ila-I13 which is equal to but displaced in the frequency spectrum from the frequency band fiirflfi of the control carrier wave, preferably displaced above the latter band, the mean frequency of the band fie-11a being designated as In. The carrier wave generated by the oscillator It may thus have any selectable frequency in this predetermined operatlng frequency band Ila-11a. The frequency of the oscillator I0 is so chosen that the carrier wave component In of the. control carrier wave fit-I15 is below and at a predetermined frequency spacing {m from the mean operating frequency in of the oscillator ii. The frequency spacing 11s is preferably chosen greater than one-half of the frequency range fit-{1s of the control carrier wave for reasons presently to be explained.

There is thus developed in the output circuit of modulator H in a manner well known, an intermediate-frequency control signal having the difference value of frequency in: whenever the carrier wave of oscillator l5 has a frequency spaced by the frequency difference fit from any of the sideband components fl-fm or f'i-f'uc or the carrier-wave component )11 of the control carrier wave fld-flS- This intermediate-frequency control signal is amplified in the amplifier I 8 and applied to the frequency detector id to develop in the output circuit thereof a unidirectional control potential the magnitude and polarity of which vary, as in conventional automatic frequency-control systems, with the deviation and sense of deviation of the intermediatefrequency signal from the mean resonant frequency 11a to which the frequency detector is tuned. This derived unidirectional control potential is applied to the input circuit of the frequency shifter 20 so to control the action thereof, as by varying th simulated reactance of its reactor tube, that the frequency of the oscillator i5 is changed in such a direction and to such an extent that the frequency of the intermediate-frequency control signal is very nearly the value m. This action occurs, of course, each time that the oscillator i5 is so tuned that its frequency is approximately spaced by the frequency spacing fit from any selected one of th components of the control carrier wave. Each time that the frequency of the oscillator I5 is so controlled, the frequency of the carrier wave generated thereby is maintained substantially constant on that one of the transmission channels which is defined by the fmquency component of the control carrier wave that produced the intermediate-frequency control signal. Thus, as the tuning control It or the oscillator II is varied continuously in one direction over its operating frequency range fir-ha, the carrier wave generated thereby is stabilized for intervals, depending upon the rate of such tuning, on predetermined transmission channels defined by each of the frequency components oi the control carrier wave. In using the arrangement of Fig. i, the oscillator i5 is manually tuned by adjustment of its tuning element It approximately to an operating frequency corresponding to a desired transmission channel and the units l8, l9 and 2B thereupon operate to establish and maintain the frequency of the oscillator l5 more accurately at the desired operating frequency and upon the desired transmission channel which, as pointed out, is defined by one of the frequency components of the control carrier wave I14 to 115.

It will be understood, of course, that the bandpass characteristics of the intermediate-frequency amplifier l8 and frequency detector I! are such that the automatic-frequency control system comprising units l8, l8 and 20 relaxes at least during some portion of the time required manually to tune the oscillator i5 by adjustment of its tuning element It from one transmitting channel to an adjacent channel. If this were not so. the oscillator is could not be tuned from any given channel to every other channel.

The carrier wave generated by the oscillator I5 is applied to the modulator included in unit H where, after amplitude modulation by the signal which it is desired to transmit, the modulated-carrier signal is amplified by the radiofrequency amplifier of this unit and applied to the antenna system 22, 23 for radiation therefrom.

In lieu of the use of an amplitude-modulator stage in unit 2| as described, th signal which it is desired to transmit may alternatively be applied to the frequency shifter 20 in conventional manner to frequency-modulate in accordance with such signal the carrier wave generated by the oscillator 15.

It has previously been stated that the frequency in of the oscilator III is spaced by the frequency difference flfi from the mean operating frequenc in of the operating frequency band jlZ-flli of the oscillator l5 and that the frequency spacing fit is preferably greater than half the frequency band fir-fie over which the components of the control carrier wave extend. This is desirabie from the standpoint that there are then no components of the control carrier wave which can beat with the carrier wave of oscillator i5 to produce image-frequency control signals in the output circuit of the modulator ll. While such frequency spacing of the oscillator i is desirable, it is not essential and the frequency spacing may be less than one-half of the frequenc range fii)15. Where this is the case, however, the value of the frequency spacing fld should be an odd muliiple of one-quarter of the channel spacing defined by the frequency components of the control carrier wave. This insures that any image intermediate-frequenc control signals which are produced by intermodulation between the carrier wave of oscillator l and nonselected components of the control carrier wave have frequencies sufficiently widely spaced from the desired control signal that they are not translated with appreciable amplitude by the amplifier I! to the detector trolled oscillator, for generating a plurality of i ll. Whatever small-intensity image signals that are applied to the detector l'l, however, have frequencies such that the unidirectional control potential produced therefrom in the output circuit of the detector i9 so controls the frequency of the oscillator Ii that the latter is tuned away from the one of such nonselected components which produces th largest-intensity image signal and, in being so tuned, is tuned toward the selected component.

The arrangement of Fig. 1 ha the important advantage that the input circuit of the unit 2| is substantially completely decoupled from any of the units Ill to I 4, inclusive, and, hence, the control carrier wave is substantially completely isolated from the input circuit of unit 2|. Consequently, none of the frequency components 01 the control carrier wave can be translated through the unit II to the antenna system 22, 23 for radiation therefrom as spurious undesired carrier waves which would cause interference with communications carried on with other apparatus.

The band of transmission channels over which a carrier-wave generating system embodying the arrangement of Fig. 1 can be designed to operate depends, of course, upon the number of harmonic frequencies of reasonably large amplitude that can be derived b the harmonic generator it from the oscillations generated by the 0s illator l2. Where this limited number of transmi sion channels is inadequate for a particular purpose, the modified form of the invention represented by the schematic circuit diagram of Fig. 3 is useful. The arrangement of Fig. 3 is essentially similar to that of Fig. 1, similar circuit elements being designated by similar reference numerals and analogous circuit elements by similar reference numerals primed. The arrangement of Fig, 3 differs from that of Fig. l in that it comprises means including at least one oscillator having means for maintaining the frequency thereof substantially constant, preferably at least one piezoelectric crystal-concarrier waves each including a carrier-wave component of relatively high and substantially constant frequencies and sideband components of substantiall constant frequencies and extending over a predetermined frequency band. The frequencies of the components of each of such carrier waves individually define predetermined desired transmission channels. This means comprises an oscillator Ill having a plurality of piezoelectric crystal devices H, H, H", il', any selected one of which may be included in circuit with the oscillator ill, by manual operation of the switch 26. The instant arrangement also includes selector means for selecting a desired one of the components of the control carrier waves, this means comprising a tunable selector 21 having an input circuit coupled to the output circuit of the modulator i4 and having an output circuit cou-- pied to the input circuit of the modulator H. The selector 2'! include an adjustable tuning element 28 which is connected for unicontrol operation with the adjustable tunin elements It and 24 of the respective units I5 and 21, as indicated by the broken line 25'. The oscillator ii in this arrangement is tunable over a predetermined operating-frequency band equal to, but displaced in the frequency spectrum from, the combined frequency bands of the control carrier waves developed in the output circuit of the modulator ll.

Considering now the operation of this modified form of the invention, and referring to the frequency scales of Figs. 4 and 5, it may be stated that, in general, its operation is essentially similar to that of Fig. 1 and will not be repeated in detail. There are two alternative manners in which the present arrangement may be used to extend the available number of transmission channels on .which the system may operate. The first of these is illustrated in Fig. 4 wherein, for purposes of simplicity, only two control carrier-wave frequency spectra are shown. The carrier-wave component and sideband components of one control carrier wave. for example, that produced by the oscillator ll under control of the piezoelectric crystal H, extends over the range fie-fit. while the frequency of the carrier wave generated by the oscillator Ill under control of another of the crystals, for example the crystal- II, is so chosen that the components of the control carrier wave corresponding thereto extend over a higher frequency Ila-fin, the adjacent end frequencies lit and he of these two frequency ranges being spaced by the width of one transmission channel. The operating frequency band fro-1:1 of the oscillator I5 is now equal to the combined frequency band )ll'jlfl of these two control carrier waves, and, actually to the combined frequency bands of all of the control carrier waves used in the system. The frequencies of the crystals II to H', inclusive, are additionally so chosen that the mid-range of the combined frequency band of the control carrier waves produced thereby is spaced by a frequency In from the mean operating frequency in of the operating frequency band her-I21 of the oscillator IS. The operation of the Fig. 3 arrangement when operated in this manner is otherwise essentiall similar to that of the Fig. l arrangement except that the desired component of a given control carrier wave is selected by the selector 21 and applied to the modulator l'l substantially to the exclusive of undesired image components thereof. The intermediatefrequency control signal developed in the output circuit of the modulator H has a frequency in in this instance and the units l8, I9 and 20 operate to establish and maintain substantially constant the frequency of the oscillator IS on a selected one of the transmission channels defined by the control carrier-wave component which is utilized.

An alternative method of operating the Fig. 3 arrangement is illustrated by the frequency scale of Fig. 5. Here the harmonic components of the modulation signal generated by the harmonic generator 13 are more widely sp ced than is the case in the arrangement of Fig. l and define every fourth desired transmission channel where there are four crystals used in the oscillator ill. The frequencies of'the crystals II to II'", inclusive. are so chosen with relation to each other that the frequencies of the components of the control carrier wave produced by each individually define predetermined desired transmission channels but the channels defined by a given one of the control carrier waves are different from, and intervene between, at least a portion of the transmission channels deflnedby at least one other of the control carrier waves. The oscillator I! in this case has a frequency corresponding to the spacing of four desired transmission channels, whereby the frequency spacing of the frequency components of the control carrier wave derived by unit H correspondingly define every fourth desire transmission channel. The frequencies of the crystals I l-l l'" are so chosen with relation to each other that any crystal has a frequency displaced from the next preceding crystal by an amount corresponding to one desired transmission channel. The control carrier wave developed in the output circuit of modulator H from the carrier wave of the oscillator it under control of the crystal II. for example, is thus as indicated in Fig. 5 by the legend A; that corresponding to the crystal II by the legend B; that corresponding to the crystal II" by the legend C: and that corresponding to the crystal II'" by the legend D. It will be seen that the components of the control carrier wave 3 are each displaced by one desired transmission-channel spacing from the corresponding component of the control carrier wave A. Similarly, the components of the carrier wave C are displaced from the corresponding components of the carrier wave 18 by one desired transmission-channel spacin and the components of the control carrier wave D are similarly displaced from the corresponding components of the control carrier wave 0.

In using the arrangement of Fig. 3 and the lastdescribed mode of operation, one component of one of the control carrier waves A, B, C or D will define the transmission channel upon which it is desired to operate. Assuming that this is a component of the control carrier wave C, the switch 26 is adjusted to include the piezoelectric crystal device II" in circuit with the oscillator lli' to develop in the output circuit of modulator II the control carrier wave C. The adjustable tuning element it of the oscillator I5 is then adjusted approximately to the desired operating frequency and the mechanical connection between the adjustable tuning element It and the adjustable tuning element 28 of the selector 21 causes the latter tuning element simultaneously to be adjusted to condition the selector 21 to select the desired component of the control carrier wave C which defines the transmission channel upon which it is desired to operate. The selected component of the control carrier wave C is applied to one input circuit of modulator I'l to develop in the output circuit thereof an intermediatefrequency control signal having a frequency approximating frequency for. Units i8, i9 and 2|) thereupon operate to control the frequency of the oscillator 15, as in the arrangement of Fig. 1, to establish and maintain the frequency of the carrler wave generated thereby more exactly on the desired transmission channel corresponding to the selected component of the control carrier wave.

It will thus be evident that, by selection of one of the piezoelectric devices il-I l', inclusive, and suitable adjustment of the tuning elements 24, I6 and 28, the carrier-wave generating system of Fig. 3 may be caused to operate on any desired transmission channel included in the frequency band over which the system is designed to operate. In this respect, it may be noted that the carrier wave generated by the oscillator l5 may have any selected frequency in a predetermined frequency band fro-{'21, this band being equal to but displaced in the frequency spectrum from the combined frequency bands of the control carrier waves A, B, C and D. The operation of this modifled form of the invention is otherwise essentially similar to Fig. 1 and will not be repeated.

While a tunable selector 21 has been included in the Fig. 3 arrangement, the use of this unit is not essential and the unit may be dispensed with, if desired, in which case the output circuit of modulator I4 is directly connected to one input circuit of the modulator l1. Furthermore a tunable selector similar to unit TI may be included in the arrangement of Fig. 1 between the modulator II and the modulator if. if desired, and when so included will operate for the same purpose and in the same manner as the tunable selector 21 of the Fig. 3 arrangement. A selector of this nature is primarily useful where the mean frequency of the operating-frequency band over which the oscillator I! is tunable'is spaced from the frequency of the crystal II in the Fig. 1 arrangement or any of the crystals lli l" in the Fig. 3 arrangement by less than one-half of the frequency band over which the components of the corn "pending control carrier wave extend. In this event, the tunable selector prevents the application to the modulator I I of such non-selected components of a control carrier wave as would produce image intermediate-frequency control signals in the output circuit of the modulator II.

While it has previously been stated that the sideband components of the control carrier wave preferably have equal spacings in the frequency spectrum with relation to each other and to the carrier-wave component, it will be evident that only selected ones of the harmonic frequencies of the modulation signal generated by the harmonic generator I: may be used. In this event, the sideband components of the control carrier wave may have unequal spacings with relation to each other, but nevertheless define desired unequallyspaced transmission channels. An arrangement of this nature, is useful, for example, where it is desired to transmit on, say, only the second, fifth, ninth and tenth transmission channels in a band of ten equally-spaced such channels.

From the above description of the invention, it will be apparent that a carrier-wave generating system embodying the invention is adapted to generate a carrier wave having ahlgh degree of frequency stability and a frequency accurately corresponding to that of any selected one of a plurality of predetermined desired transmission channels. There is the further advantage in a system of this nature that there are produced no undesired spurious carrier waves which might cause interference with communications carried on by other persons on other channels. A carrier-wave generating system embodying the invention has the additional advantage that the frequency of the generated carrier wave may be and preferably is higher than that of a frequency standard used in the system. This is particularly important where the frequency standard comprises a piezoelectric crystal in which case it is desirable for numerous reasons, for example the lower cost of the crystal, its ruggedness, and the greater ease with which it may be manufactured. that the resonant frequency of the crystal be as low as the circumstances will permit.

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 it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A carrier-wave generating system comprising, means for generating a carrier wave including a carrier-wave component of relatively high and substantially constant frequency and sideband components of substantially constant frequencies and extending over a predetermined frequency band, means for generating a second wave havin any selectable frequency in a predetermined frequency band equal to but displaced by a predetermined value in the frequency spectrum from said first-named frequency band, and means responsive Jointly to the frequency of said secand wave and the frequency of that one of said components of said carrier wave which is displaced in the frequency spectrum by a predetermined value from the desired operating frequency of said second wave for maintaining the frequency of said second wave at a substantially constant desired value.

2. A carrier-wave generating system comprising, means for generating a carrier wave including a carrier-wave component of relatively high and substantially constant frequency and sideband components of substantially constant frequencies and. extending over a predetermined frequency band, the frequency of said carrier-wave component and the frequencies of said sideband components being individually related to the carrier frequencie 01' predetermined desired transmission channels, means for generating a second wave having any selectable frequency in a predetermined frequency band equal to said firstnamed frequency band, and means responsive jointly to the frequency of said second wave and the frequency of one of said components of said carrier wave for maintaining the frequency of said second wave substantially constant on the one of said transmission channels related to said one component.

3. A carrier-wave generating system comprising, means for generating a carrier wave including a carrier-wave component of relatively high and substantially constant frequency and sideband components of substantially constant -frequencies and extending over a predetermined frequency band, the frequency spacings of said sideband components with relation to each other and to said carrier-wave component being equal to predetermined corresponding frequency spacings of desired transmission channels whose operating frequencies are related to said components, means for generating a second wave having any selectable frequency in a predetermined frequency band equal to said first-named frequency band, and means responsive jointly to the frequency of said second wave and the frequency of one of said components for maintaining the frequency of said second wave substantially constant on the one of said transmission channels related to said one component.

4. A carrier-wave enerating system comprising, means for generating a carrier wave including a carrier-wave component of relatively high and substantially constant frequency and sideband components of substantially constant frequencies and extending over a predetermined frequency band, said components of said carrier wave having equal spacings in. the frequency spectrum with relation to each other and the frequencies of said components being individually related to the carrier frequencies of predetermined desired transmission channels, means for generating a second wave having any selectable frequency in a predetermined frequency band equal to said first-named frequency band, and means responsive Jointly to the frequency of said second wave and the frequency of one of said components for maintaining the frequency of said second wave substantially constant on the one of said transmission channels related to said one component.

5. A carrier-wave generating system comprising, means for generating a carrier wave including a carrier wave component of relatively high and substantially constant frequency and sideband components of substantially constant frequencies and extending over a predetermined frequency band, means for generating a second wave having any selectable frequency in a predetermined frequency band equal to but displaced by a predetermined value higher in the frequency spectrum from said first-named frequency band. and means responsive jointly to the frequency of said second wave and the frequency of that one of said components which is displaced in the frequency spectrum by a predetermined value from the desired operating frequency of said second wave for maintaining the frequency of said second wave at a substantially constant desired value.

6. A carrier-wave generating system comprising, an oscillator including means for maintaining the frequency thereof substantially constant for generating a carrier wav including a carrierwave component of relatively high and substantially constant frequency and sideband components of substantially constant frequencies and extending over a predetermined frequency band, means for generating a second wave having any selectable frequency in a predetermined frequency band equal to said first-named frequency band, and means responsive jointly to the frequency of said second wave and the frequency of that one of said components which is related in a predetermined manner to the desired operating frequency of said second wave for maintaining the frequency of said second wave at a substantially constant desired value.

'7. A carrier-wave generating system comprising, a piezoelectric crystal-controlled oscillator for generating a carrier wave including a carrier-wave component of relatively high and substantially-constant frequency and sideband components of substantially constant frequencies and extending over a predetermined frequency band, means for generating a second wave having any selectable frequency in a predetermined frequency band equal to said first-named frequency band, and means responsive jointly to the frequency of said second wave and the frequency of that one of said components which is related in a predetermined manner to the desired operating frequency of said second wave for maintaining the frequency of said second wave at a substantially constant desired value.

8. A carrier-wave generating system comprising, means for generating a carrier wave including a carrier-wave component of relatively high and substantially constant frequency and sideband components of substantially constant frequencies and extending over a predetermined frequency band, means for generating a second wave having any selectable frequency in a predetermined frequency band equal to but displaced by a predetermined value in the frequency spectrum from said first-named frequency band, means for tuning said second-named generating means approximately to a desired operating frequency and for selecting that one of said carrier-wave components which is displaced in the frequency spectrum by the said predetermined value from said desired operating frequency, and means responsive jointly to the frequency of said second wave and the frequency of said selected component for establishing and maintaining the frequency of said second wave more accurately at said desired operating frequency.

9. A carrier-wave generating system comprising, means for generating a carrier wave including a carrier-wave component of relatively high and substantially constant frequency and sideband components of substantially constant frequencies and extending over a predetermined frequency band, means including an adjustabl tuning element for generating a second wave having any selectable frequency in a predetermined frequency band equal to but displaced by a predetermined value in the frequency spectrum from said first-named frequency band, selector means for selecting a desired one of said carrier-wave components, means for operating said adjustable tuning element and said selector means in unison for tuning said second-named generating means approximately to a desired operating frequency and for selecting that one of said carrier-wave components which is displaced in the frequency spectrum by the said predetermined value from said desired operating frequency, and means responsive jointly to the frequency of said second wave and the frequency of said selected component for establishing and maintaining the frequency of said second wave more accurately at said desired operating frequency.

10. A carrier-wave generating system comprising, means for generating a carrier wave having a relatively high and substantially constant frequency, means for generating a modulation signal having frequency components of substantially constant frequencies and predetermined spacin relative to each other, means for modulating said carrier wave with said modulation signal to derive a modulated carrier wave including a carrier-wave component and sideband components extending over a predetermined frequency band. means for generating a second wave having any selectable frequency in a predetermined frequency band equal to said first-named frequency band. and means responsive jointly to the frequency of said second wave and the frequency of that one of said components of said modulated carrier wave which is related in a predetermined manner to the desired operating frequency of said second wave for maintaining the frequency of said second wave at a substantially constant desired value.

11. A carrier-wave generating system comprising, means for generating a carrier wave havin a relatively high and substantially constant frequency, means for generating a modulation signal having frequency components of substantially constant frequencies and having frequency spacings relative to each other corresponding to the frequency spacings between predetermined desired transmission channels, means for modulating said carrier wave with said modulation signal to derive a modulated carrier wave includin a carrier-wave component and sideband components extending over a predetermined frequency band and being individually related to the carrier frequencies of said predetermined desired transmission channels, means for generating a second wave having any selectable frequency in a predetermined frequency band equal to said firstnamed frequency band, and means responsive Jointly to the frequency of said second wave and the frequency of one of said components of said modulated carrier wave for maintaining the frequency of said second wave substantially constant on the one of said transmission channels related to said one component.

12. A carrier-wave generating system comprising, means for generating a carrier wave having a relatively high and substantially constant frequency, means for generating a modulation signal having frequency components of substantially constant frequencies and having equal frequency spacings relative to each other and corresponding to equally-spaced predetermined desired transmission channels. means for modulating said carrier wave with said modulation signal to derive a modulated carrier wave including a carrier-wave component and sideband components extending over a predetermined frequency band and being individually related to the carrier frequencies of said predetermined desired transmission channels, means for generating a second wave having any selectable frequency in a predetermined frequency band equal to said firstnamed frequency band, and means responsive jointly to the frequency of said second wave and the frequency of one of said components of said modulated carrier wave which is displaced in the frequency spectrum by a predetermined selectable value from said first wave for maintaining the frequency of said second wave substantially constant on theone of said transmission channels related to said one component.

13. A carrier-wave generating system comprising, means for generating a carrier wave having a relatively high and substantially constant frequency, means for generating a signal having a frequency equal to the frequency spacing between equally-spaced desired transmission channels, means for deriving from said signal a modulation signal having frequency components which are harmonically related to said signal, means for modulatin said carrier wave with said modulation signal to derive a modulated carrier wave including a carrier-wave component and sideband components extending over a predetermined frequency band and being individually related to the carrier frequencies of said desired transmission channels, means for generating a second wave having any selectable frequency in a predetermined frequency band equal to said first-named frequency band, and means responsive Jointly to the frequency=of said second wave and the fre quency of one of said components of said modulated carrier wave for maintaining the frequency of said second wave substantially constant on the one of said transmission channels related to said one component.

14. A carrier-wave generating system comprising, means for generating a plurality of carrier waves each including a carrier-wave component of relatively high and substantially constant frequency and sideband components of substantially constant frequencies and extending over a predetermined frequency band, the frequencies of the components of each of said carrier waves being individually related to the carrier frequencies of predetermined desired transmission channels and the channels related to the frequencies of the components of a given one of said carrier waves being different from but intervening between the transmission channels related to the others of said carrier waves, means for generating a second wave having any selectable frequency in a predetermined frequency band equal to the combined frequency bands of said firstnamed carrier waves, and means responsive jointly to the frequency of said second wave and the frequency of one of said components for maintaining the frequency of said second wave at a substantially constant desired value, thereby asoacos to maintain said second wave on the one of said transmission channels related to said one component.

15. A carrier-wave generating system comprising, means for generating any selectable one of a plurality of carrier waves each including a carrier-wave component of relatively high and substantially constant frequency and sideband components of substantially constant frequencies and extending over a predetermined frequency band. the frequencies of the components of each of said carrier waves being individually related to the carrier frequencies of predetermined desired transmission channels and the channels related to a given one of said carrier waves being different from but intervening between the transmission channels related to the others of said carrier waves, means for generating a second wave having any selectable frequency in a predetermined frequency band equal to the combined frequency bands of said first-named carrier waves, and means responsive jointly to the frequency of said second wave and the frequency of one of said components for maintaining the frequency of said second wave at a substantially constant desired value, thereby to maintain said second wave on the one of said transmission channels related to said one component.

16. A carrier-wave generating system comprising, means including at least one oscillator having means for maintaining the frequency thereof substantially constant for generating a plurality of carrier waves each including a carrier-wave component of relatively high and substantially constant frequency and sideband components of substantially constant frequencies and extending over a predetermined frequency band, the frequencies of the components of each of said carrier waves being individually related to the carrier frequencies of predetermined desired transmission channels and the channels related to a given one of said carrier waves being different from but intervening between the transmission channels related to the others of said carrier waves, means for generating a second wave having any selectable frequency in a predetermined frequency band equal to the combined frequency bands of said first-named carrier waves, and means responsive jointly to the frequency of said second wave and the frequency of one of said components for maintaining the frequency of said second wave at a substantially constant desired value, thereby to maintain said second wave on the one of said transmission channels related to said one component.

17. A carrier-wave generating system comprising, piezoelectric crystal-controlled oscillator means for generating a plurality of carrier waves each including a carrier-wave component of relatively high and substantially constant frequency and sideband components of substantially constant frequencies and extending over a predetermined frequency band, the frequencies of the components of each of said carrier waves being individually related to the carrier frequencies of predetermined desired transmission channels and the channels related to a given one of said carrier waves being different from but, intervening between the transmission channels related to the others of said carrier waves, means for generatin a second wave having any selectable frequency in a predetermined frequency band equal to the combined frequency bands of said first-named carrier waves, and means responsive jointly to the frequency of said second wave and the frequency of one or said components tor maintaining the frequency or substantially maintain said second wave on the transmission channels defined by said one component.

18. A carrier-wave generating system comprisins, means for generating waves each of relatively high and quency and sideband components of substantially constant frequencies determined frequency the components or each or said carrier waves being individually related or said carrier waves tervening between at least a portion or the transmission channels said carrier and wave having any selectable frequency in a aseaecs quency of said second wave and the frequency one 01' said components quency of said second constant desired value, second wave on the one of nels related to said one component.

19. A carrier-wave genera ins, means tor generating a c in; a carrier-wave componen and substantially constant band components 0 quency and extending over a quency band, means for generating a second we having any selectable frequency said second wave at a constant desired value, thereby to one oi said a plurality oi carrier arrier wave inclu including a carrier-wave component substantially constant 1112- 1 and extending over a preband. the frequencies or to the carrier fredesired transmission and means responsive jointly being diflerent from but inenemy or that one related to at least one other or means for generating a sec- 20 waves, second wave for main the frequency sired value.

NELSON P. CASE.

Certificate of Correction NELSON P. CASE at errors a pear in the rinted correction as follows: age 5, t column, l ne 39, for 8, first column, lines 22 to 24, claim l2 strike out the predetermined selectable e fs lpect 1' 5 1 11 i defined by" e 9 t co umn, 1 Q mm or value from Bald first wave pa'g '0 should be read with these corrections (1 elated t and that the said Letters Paton tli ere'sln that the same may conform to the record of the case in the Patent Office.

of October, A. D. 1946.

Signed and sealed this 15th day Patent No. 2,398,694. April is, 1946.

It is hereby certified th numbered patent requ ring exclusive read emcluston; page words which is displaced m the sirgcification of the above LESLIE FRAZER for maintaining the frewave at a substantially thereby to maintain said said transmission chanting system comprist of relatively high frequency and sidesubstantially constant frepredetermined frein a predeterband equal to said first-named the frequency of said second wave and the fre- 01 said components of said carrier wave which is related in a predetermined manner to the desired operating frequency or said said second wave at a substantially constant dequency of one or said components tor maintaining the frequency or substantially maintain said second wave on the transmission channels defined by said one component.

18. A carrier-wave generating system comprisins, means for generating waves each of relatively high and quency and sideband components of substantially constant frequencies determined frequency the components or each or said carrier waves being individually related or said carrier waves tervening between at least a portion or the transmission channels said carrier and wave having any selectable frequency in a aseaecs quency of said second wave and the frequency one 01' said components quency of said second constant desired value, second wave on the one of nels related to said one component.

19. A carrier-wave genera ins, means tor generating a c in; a carrier-wave componen and substantially constant band components 0 quency and extending over a quency band, means for generating a second we having any selectable frequency said second wave at a constant desired value, thereby to one oi said a plurality oi carrier arrier wave inclu including a carrier-wave component substantially constant 1112- 1 and extending over a preband. the frequencies or to the carrier fredesired transmission and means responsive jointly being diflerent from but inenemy or that one related to at least one other or means for generating a sec- 20 waves, second wave for main the frequency sired value.

NELSON P. CASE.

Certificate of Correction NELSON P. CASE at errors a pear in the rinted correction as follows: age 5, t column, l ne 39, for 8, first column, lines 22 to 24, claim l2 strike out the predetermined selectable e fs lpect 1' 5 1 11 i defined by" e 9 t co umn, 1 Q mm or value from Bald first wave pa'g '0 should be read with these corrections (1 elated t and that the said Letters Paton tli ere'sln that the same may conform to the record of the case in the Patent Office.

of October, A. D. 1946.

Signed and sealed this 15th day Patent No. 2,398,694. April is, 1946.

It is hereby certified th numbered patent requ ring exclusive read emcluston; page words which is displaced m the sirgcification of the above LESLIE FRAZER for maintaining the frewave at a substantially thereby to maintain said said transmission chanting system comprist of relatively high frequency and sidesubstantially constant frepredetermined frein a predeterband equal to said first-named the frequency of said second wave and the fre- 01 said components of said carrier wave which is related in a predetermined manner to the desired operating frequency or said said second wave at a substantially constant de-

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