US3036277A - Ferrite modulators - Google Patents

Description

3,fl36,277 Patented May 22, 1962 3,036,277 FERRITE MODULATORS Joseph Gindsherg, Newton Center, Mass, assignor to Raytheon (Iompany, a corporation of Delaware Filed Oct. 29, 1958, Ser. No. 770,391 11 Claims. (Cl. 33216) This invention relates to waveguide modulating devices and, more particularly, to waveguide modulating devices of the ferrite rotator type which provide frequency and amplitude modulation of microwave energy in an electromagnetic wave transmission system.

Ferrite rotators utilizing the Faraday rotation effect have been used in an effort to produce phase or frequency modulation by varying the phase shift in the electromagnetic energy passing through the ferrite. However, phase shift produced by a ferrite rotator of this type generally varies in a non-linear manner with the axial magnetic field applied to the ferrite. Moreover, a constant biasing magnetic field is generally required to maintain the ferrite operating point within the relatively linear portion of the phase shift-magnetization curve of the ferrite rotator and to obtain a modulation output of substantial magnitude and relatively low distortion. Also, as is known, frequency modulation of magnetrons or similar microwave sources by power-supply modulation, sometimes referred to as pushing, often results in nonlinearity of the modulated output and is generally accompanied by amplitude modulation.

In numerous applications, therefore, it would be desirable to utilize the Faraday rotation effect to provide an accurately controlled degree of pure frequency modulation of the microwave energy passing through the ferrite rotating device without attendant amplitude modulation. It is further desirable to utilize this rotation effect to produce a controlled percentage of amplitude modulation or a mixture of frequency and amplitude modulation. It is also desirable to produce the aforesaid pure frequency modulation without requiring application of a direct-current magnetic bias field to the ferrite rotating element and without the necessity of varying any parameter in the microwave source or its power supply.

In accordance 'with the invention, pure frequency modulation of microwave energy can be achieved by providing a ferrite element axially mounted in a circular or square waveguide and by providing a coil in the region of the ferrite element to which is applied an alternating current for producing an alternating axial magnetic field. This field produces a time-varying rotation of the plane of polarization of microwave energy passing through the ferrite. The output of the ferrite element or rotator is connected to a two-mode transducer or T-section of waveguide having a pair of cross-polarized output arms. Thus, when a sinusoidal modulating current is appliedv to the alternating-current winding of the ferrite rotator, and

if there is no steady magnetic field applied to the ferrite, the output energy in the output arm of the T-section,. which is polarized in a plane parallel to the polarization of energy in the input portion of the T-section, will contain a microwave carrier at the unmodulated-input frequency and even-order sidebands. The output energy in the T-section which is cross-polarized with respect to the energy in the input T-section will contain odd-order sidebands.

These sideband frequencies differ from the carrier frequency by even or odd multiples of the modulating frequency, which is the frequency of the current applied to the rotator coil. These two T-section outputs are then connected by means of rectangular waveguide sections to the input arms of a magic-T hybrid junction and are combined in the hybrid junction to provide modulated output energy at each of its output arms; The arms' are preferably terminated in well-known loads of low standing-wave ratio, not shown. The output energy from the two output arms differs by degrees in modulation phase. In addition, when a phase shifter or line stretcher, such as a dielectric card, is inserted into either rectangular waveguide section feeding the hybrid junction, the effective path length from the ferrite rotator to the hybrid junction can be made to differ by an odd number of quarter wavelengths. At such settings of the phase shifter card, the outputs from the output arms of the hybrid junction will contain the carrier and all sidebands having such relative magnitudes and phasing as to constitute pure frequency modulated signals. On the other hand, when the phase shifter card is adjusted such that the effective lengths of the paths from the rotator to the hybrid junction differ by an even number of quarter wavelengths, that is an integral number of half wavelengths, the outputs at the hybrid junction will be amplitude modulated.

In some applications, it is desirable to provide a steady direct-current magnetic field to the ferrite. When a steady bias is applied, the aforementioned carrier and even-order sidebands become mixed with the odd-order sidcoands in both paths leading to the hybrid junction. If such bias is applied and if the phase shifter card is set to operate the device as a frequency modulator, the amount of bias will control the total phase shift or electrical length of the modulator. However, the purity of the frequency-modulated output will be independent of the amount of the applied bias.-

If, on the other hand, the aforementioned steady bias is applied while the phase shifter card is set to operate the device. as an amplitude modulator, the amount of bias will control the division of electromagnetic energy among. the two output arms of the hybrid junction. In this instance the purity of the amplitude modulated output will be independent of the applied bias.

In this manner, pure frequency modulation combined with electrical line stretching can be achieved.

Other objects and advantages will be more readily perceived upon analysis of the drawing, in which:

FIG. 1 is an isometric view, partly in section, of the ferrite modulator according to the invention; and

FIG. 2 is an isometric View of a section of waveguide that may be substituted for the waveguide section 17 in the embodiment shown in FIG. 1.

Referring now to FIG. 1, the ferrite modulator is indicated generally by the reference numeral 10 and includesa ferrite rotator 12, a T-section of waveguide or atwomode transducer 14, a line stretcher or phase shifter 16, and a. magic-T hybrid section 18 and connecting waveguide sections 20, 1'7 and 66. More particularly, FIG. 1 shows the rectangular input section of waveguide 20 which. is connected to a source of unmodulated microwave energy. A flange 22 is shown which couples therectangular waveguide section 2il= to the circular portionv ofthe ferrite rotator 12 by means of rotator flange 23. The ferrite rotator 12 is connected to the-two-mode input portion of the T-section of waveguide 14 by means of waveguide flange 28 connected to rotator flange 3t). The input portion 26'of the T-section 14' functions such that a vertically polarized wave entering from the rotator will In the present instance, rotator 12 includes a cylindrical ferrite element 36, positioned within a section of circular Waveguide 37 by means of low-loss sections of dielectric material 38 and 39, such as Teflon, which act as a solid supporting medium for the ferrite element 36. The Teflon dielectric material may be cut to the inner diameter of the circular waveguide section 37 and divided into two sections 38 and 39 in the region of the ferrite element. A hole is then bottom-drilled into each section, and the ferrite element inserted into the Teflon, Many other methods for mounting the ferrite within the Teflon or other materials will suggest themselves to those skilled in the art.

The ferrite device 12 further includes means for obtaining both static and dynamic polarization rotation of microwave energy passing through the ferrite. To produce dynamic polarization rotation, a means for producing an alternating magnetic field, such as field coil 40, surrounds the circular waveguide section 37 in the region of the ferrite element 36. The field coil 40 is connected to an alternating-current source 44 by means of terminals 45 and 46. The alternating-current source 44 provides an alternating modulating voltage at the frequency at which it is desirable to modulate microwave energy entering the rectangular waveguide section 20. To produce static polarization rotation in the ferrite rotator 12, a second field coil 48 surrounds field coil 40 and is connected to a source of direct current 50 by way of terminals 51 and 52. These terminals are connected to a well-known polarity reversing network.

- As shown in FIG. 1, the network for reversing the polarity of direct-current source 50 comprises voltage divider resistors 52 and 54 connected to the direct-current source 50. Terminal 51 is then connected to the junction of the resistors 52 and 54, A potentiometer 55 is also connected across direct-current source 50 and is provided with a slidable arm 56 for adjusting the magnitude and polarity of direct current applied to field coil 48. As is known, at the mid position of the potentiometer substantially no direct current will be applied to the field coil 40 when resistors 52 and 54 are alike. The alternating-current field coil may, for example, consist of one thousand turns of Wire while the direct-current field coil may consist of ten thousand turns. The alternatingcurrent source 44 may provide a modulatingcurrent of ten milliamperes to the field coil 40 at five volts. The direct-current source 48, typically, may provide a current of approximately five milliamperes at five volts. In this manner, alternating-current and direct-current fields may be simultaneously applied to the respective field coils.

Extending from the vertically polarized arm 15 of T-section 14 is a rectangular waveguide section 17 which is secured to the T-section by means of waveguide flange 59 and T-section flange 60. The rectangular waveguide section 17 is connected to the H-plane arm 62 of the hybrid junction 18 by means of flanges 63 and 64. Also, extending from the rectangular output arm 32 of T-section 14 is a rectangular waveguide section 66 into which is inserted a dielectric card 68 which is positioned along the longitudinal axis of the rectangular waveguide sec tion and may be moved to either side of the waveguide by means of a slidable rod 69. The adjustable card operates as a simple line stretcher or phase shifter and is used to adjust the elfective length of the horizontally-polarized path from the ferrite rotator to the hybrid junction. While the phase shifter 16 is shown integral with waveguide section 66, a removable phase shifting unit may be used. Waveguide section 66 is connected to arm 32 of T-section 26 by means of flange 71 and 73, The output of waveguide section 66, into which the phase shifting card 68 has been inserted, is connected to the E-plane arm 74 of the hybrid junction 18 by means of Waveguide flanges 76 and 77. The modulated output energy may be taken from either one or both of collinear arms 80 and 82 of the hybrid junction 18. It should be understood, however, that while the H and E-plane arms, asshown, serve as input branches and the collinear arms as output branches of the hybrid junction, the collinear arms may be connected as input branches, with the E and H-plane arms serving as output members. This alternative arrangement requires a ninety-degree Waveguide twisting section in one of the input branch arms of the hybrid junction.

In operation, therefore, plane-polarized unmodulated microwave energy entering the rectangular waveguide 20 will enter the ferrite rotator 12. When a sinusoidal modulating current from source 44 is of such amplitude as to produce a peak rotation of K radians in the rotator 12, and if there is no steady magnetic field applied to the ferrite, the output 15 of T-section 14, which is parallelpolarized relative to the input, will contain the carrier and even-Order sidebands. The cross-polarized output 32 of the T-section 14 will contain odd-order sidebands. In addition, the amplitude of each even or odd-order sideband, which here includes the microwave carrier as a zero-order sideband, is a fraction 1,,(K) of the input amplitude; 1,,(K) being a Bessel function of the first kind, of order n and argument K where K is the amplitude of rotation in radians, A detailed analysis of the sidebands produced by Faraday rotation is contained in A Note On Sidebands Produced By Ferrite Modulators" by P. A. Rizzi and D. J. Rich, in the Proceedings of the Institute of Radio Engineers, vol. 44, page 556; April, 1956. When a steady direct-current magnetic field is applied to the ferrite rotator 12 to produce a steady angular rotation, K both sets of odd and even-order sideband signals will mix in the two rectangular waveguide outputs of the T-section 14, in proportion to the sine and cosine of K When the phase shifter 16 in Waveguide section 66 is adjusted such that the elfective lengths of the paths from the ferrite rotator 12 to the hybrid junction 18 differ by an odd number of quarter wavelengths, the outputs in the collinear arms 86 and 82 of the hybrid junction will nevertheless contain all sidebands and carrier in such phase as to constitute pure frequency modulated microwave energy. Each output from arm and 82 contains half the input power to waveguide section 20 when ferrite losses are considered negligible. It should be understood that phase shifter 16 can alternately be placed in Waveguide section 17 and is not restricted to the dielectric-card type shown in FIG. 1. The index of modulation of the resultant frequencymodulated output is equal to the amplitude of rotation K. The modulation index may be defined as the ratio of peak frequency deviation divided by the modulation frequency. Moreover, if there is a steady rotation, K in the ferrite 36 resulting from direct current applied to winding 48, the two outputs at arms 80 and 82, in addition, will differ by 2K radians in microwave phase.

By adjusting the card 68 in phase shifter 16 to provide a path difference of an integral number of half-wave lengths, the microwave energy output at arms 8!! and 82 will be amplitude modulated at the rotator drive fre quency and its harmonics. If the direct current rotation K is set to zero, by adjusting the potentiometer arm 56 and if the rotation amplitude K is small, the resultant output in arms 80 and 82 will be essentially amplitude modulated microwave energy, modulated only at the frequency of the modulating source, and having an amplitude modulation factor equal to the peak rotation K. The amplitude modulation factor is defined as the ratio of the peak change in amplitude to the average amplitude of the modulated signal.

In this manner, the adjustable phase shifter 16 can be set for either frequency or amplitude modulation, respec tively, by either nulling or maximizing the amplitude modulated output which may be detected by a wellknown means such as an output monitor crystal, not shown. For intermediate positions of the phase shifter, the outputs are modulated both in phase and amplitude.

equal when an amplitude modulated output is desired,

when pure frequency modulation is desired, the difference in path length is selected to be precisely one-quarter wavelength. Small adjustments are made by waveguide shims 89 and 90 positioned between flanges 9192 and 9394. In this instance, the phase shifter 16 can be removed from waveguide section 66. By maintaining the aforementioned path difference at the minimum required for the desired type of modulation, broadband operation of the device is achieved.

This completes a description of the embodiments of the invention illustrated herein. However, many modifications thereof will be apparent to persons skilled in the art without departing from the spirit and scope of this invention. For example, it should be understood that the rotation of the plane of polarization of the electromagnetic energy passing through the rotator can be achieved by means other than field coils, such as permanent' magnets and the like. Also, the functions of the alternating and direct-current magnetic fields can be combined in a single coil. Accordingly, it is desired that this invention not be limited to the particular details of the embodiments disclosed herein, except as defined in the appended claims.

What is claimed is:

1. A system for providing modulation of electromagnetic energy comprising a rotating element adapted to receive and rotate linearly polarized electromagnetic energy, a T-section of waveguide having an input arm and first and second cross-polarized output arms, said inputarm connected to the output of said ferrite rotating element, a magic-T hybrid junction having a pair of collinear output arms and first and second input arms, a first waveguide section connecting the first output arm of said T-section to the first input armof said magic-T hybrid junction, a second waveguide section connecting the second output arm of said T-section to the second input arm of said magic-T hybrid junction, an alternating magnetic field applied to said rotating element, and a line stretcher connected in one of said waveguide sections to provide a predetermined phase relationship of electromagnetic energy in said input arms'of said hybrid junction.

2. A waveguide modulating device for providing modulation of electromagnetic energy comprising a polarization rotating element adapted to receive and rotate linearly polarized electromagnetic energy, a two-mode transducer having an input arm and a pair of cross-polarized output arms, said input arm connected to the output of. said polarization rotating element, a hybrid junction having two output arms and first and second input arms, each of said output and input arms adapted to support electromagnetic energy, said first and second input arms being coupled to said output arms in a manner such that electromagnetic energy entering said hybrid junction from either input arm is coupled to both output arms but not to the other input arm, a first waveguide section serially connecting one output arm of said two-mode transducer to the first input arm of said hybrid junction, at second waveguide section of predetermined electrical length such that electromagnetic energy in said first and second waveguide sections is simultaneously combined in said hybrid junction serially connecting the other output arm of said two-mode transducer to the second input branch arm of said hybrid junction, and an alternating magnetic field applied to said polarization rotating element to provide alternating rotation of the plane of-polarization of said linearly polarized electromagnetic energy.

3. In combination, a waveguide rotating device for providing modulation of electromagnetic energy comprising a polarization rotating element adapted to receive linearly polarized. electromagnetic energy, a T-section of waveguide having an input arm connected to said polarization rotating element, a magic-T hybrid junction having two collinear output arms and first and second input branch arms adapted to receive electromagnetic energy, said first and second input branch arms being connected respectively to the electric and magnetic planes of said collinear output arms ata common junction, a first waveguide section serially connecting the first output arm of said T-section to the first input branch arm of said magic- T hybrid junction, a second waveguide section serially connecting the second output arm of said T-section to the second input branch arm of said magic-T hybrid junction, an alternating magnetic field applied tosaid rotating element to provide apredetermined rotation ofsaid electromagnetic energy, and a line stretcher connected in circuit with one of said waveguide sections to control the phase of electromagnetic energy passing therethrough. 4. In combination, a waveguide rotating device forproviding modulation of electromagnetic energy comprisinga polarization rotating element adapted to receive linearly polarized electromagnetic energy, a T-section of waveguide having an input arm and first and second crosspolarized output arms, said input arm connected to said polarization rotating element, a magic-T hybrid junction having two collinear output arms and first and second input arms adapted to receive electromagnetic energy, said first and second input arms being connected respectively to the electric and magnetic planes of said collinear output arms at a common junction, a first wave-- guidev sectionseri'ally connecting the first output arm of' to control the amount of phase and'amplitude modulation of electromagnetic energy in the collinear output. arms of said magic-T hybrid junction.

5. A waveguide rotating device for-providing modula-- tion of electromagnetic energy comprising a ferrite rotating element adapted to rotate, the. plane. of polariza: tion of linearly polarized electromagnetic energy, a T- section of waveguide having an input arm and first and second cross-polarized rectangular output arms, saidinput arm being connected to the output of saidferrit'e rotating element, a magic-T hybrid junction having two collinear rectangular output arms and first and second rectangular input arms adapted to receive electromagnetic energy, said first and secondrectangular input arms being connected respectively to the electric and magnetic planes of said collinear output armsrat a common junction, a first rectangular waveguide section. connecting thefirst output? arm: of said T-sectionto the first rectangular-input arm. of said magic-T hybrid junction, at. second rectangular waveguide section connecting the second output arm of said T-section tothe second rectangular input arm of said magic-T'hybrid section, said. first and secondrectang ular waveguide sections differing in. length by substantially an odd. number of quarter wavelengths, a direct current magnetic field'applied tosaidferrite rotating, element to produce a quiescent rotation of the plane of polarization of said electromagnetic energy through a predetermined angular distance, thereby to produce an adjustable'phase shift at the output arms of said magic- T hybrid junction, and an alternating magnetic field applied to said rotating element to'produce' a predetermined alternating rotation of the planeof polarization of said? electromagnetic energy, thereby to produce frequency 7. modulation of electromagnetic energy at the output arms of said magic-T hybrid junction.

6. A waveguide rotating device for providing modulation of electromagnetic energy comprising a ferrite rotating element adapted to rotate the plane of polarization of linearly polarized electromagnetic energy, a T- section of waveguide having an input arm and a pair of cross-polarized rectangular output arms, said input arm connected to the output of said polarization rotating element, a magic-T hybrid junction having two collinear rectangular output arms and first and second rectangular input arms, said first and second rectangular input arms being coupled respectively to the electric and magnetic planes of said collinear output arms at a common junction, at first rectangular waveguide section serially connecting one output arm of said T-section to the first input arm of said magic-T hybrid junction, a second rectangular waveguide section serially connecting the other output arm of said T-section to the second input arm of said magic-T hybrid section, said first and second rectangular waveguide sections differing in length by an integral number of half wavelengths, and an alternating magnetic field applied to said rotating element to rotate the plane of polarization of said linearly polarized electromagnetic energy passing therethrough.

7. A waveguide rotating device for providing modulation of electromagnetic energy comprising a ferrite rotating element adapted to rotate the plane of polarization of linearly polarized electromagnetic energy, a T- section of waveguide having an input arm and a pair of cross-polarized rectangular output arms, said input arm connected to the output of said polarization rotating element, a magic-T hybrid junction having two collinear rectangular output arms and first and second rectangular input arms, said first and second rectangular input arms being coupled respectively to the electric and magnetic planes of said collinear output arms at a common junction, a first rectangular waveguide section serially connecting one output arm of said T-section to the first input arm of said magic-T hybrid junction, a second rectangular waveguide section serially connecting the other output arm of said T-section to the second input arm of said magic-T hybrid section, said first and second rectangular waveguide sections difiering in length by substantially an integral number of half wavelengths, an alternating mag netic field applied to said rotating element to rotate the plane of polarization of said linearly polarized electromagnetic energy passing therethrough, and a direct current magnetic field applied to said ferrite rotating element to produce a quiescent rotation of the plane of polarization of said electromagnetic energy through a predetermined angle, thereby to control the division of electromagnetic energy among the collinear output arms of said magic-T hybrid junction.

8. A waveguide rotating device for providing modulation of electromagnetic energy comprising a rotating element adapted to rotate the plane of polarization of linearly polarized electromagnetic energy, a T'section of waveguide having an input arm and a pair of cross-polarized rectangular output arms, said input arm connected to the output of said rotating element, a magic-T hybrid junction having two collinear rectangular output arms and first and second rectangular input arms, said first and second rectangular input arms being coupled respectively to the electric and magnetic planes of said collinear output arms at a common junction, a first rectangular waveguide section serially connecting one output arm of said T- section to the first input arm of said magic-T hybrid junction, a second rectangular waveguide section serially connecting the other output arm of said T-sectionto the second input arm of said magic-T hybrid section, said first and second rectangular waveguide sections differing in length by substantially an integral number of half wavelengths, and a direct current magnetic field applied to said ferrite rotating element to produce a quiescent rotation of the plane of polarization of said electromagnetic energy through a predetermined angle thereby to control the division of electromagnetic energy among the collinear output arms of said magic-T hybrid junction.

9. A waveguide rotating device for providing modulation of electromagnetic energy comprising a ferrite rotating element adapted to rotate the plane of polarization of linearly polarized electromagnetic energy, a T-section of waveguide having an input arm and first and second cross-polarized rectangular output arms, said input arm being connected to the output of said ferrite rotating element, a magic-T hybrid junction having two collinear rectangular output arms and first and second rectangular input arms adapted to receive electromagnetic energy, said first and second rectangular input arms being connected respectively to the electric and magnetic planes of said collinear output arms at a common junction, a first rectangular waveguide section connecting the first output arm of said T-section to the first rectangular input arm of said magic-T hybrid junction, a second rectangular waveguide section connecting the second output arm of said T-section to the second rectangular input arm of said magic-T hybrid section, said first and second rectangular waveguide sections difiering in length by substantially an odd number of quarter wavelengths, an alternating magnetic field applied to said rotating element to produce a predetermined alternating rotation of the plane of polarization of said electromagnetic energy, thereby to produce frequency modulation of electromagnetic energy at the output arms of said magic-T hybrid junction.

10. A device for producing a predetermined modulation of electromagnetic energy from a source of electromagnetic energy whose plane of polarization has a predetermined quiescent and dynamic rotation comprising a T- section of waveguide having an input arm and first and second cross-polarized output arms, said input arm adapted to receive electromagnetic energy polarized in either of two mutually perpendicular planes, a hybrid junction having two output arms and first and second input arms adapted to receive electromagnetic energy, said first and second input arms being connected respectively to the electric and magnetic planes of said output arms at a common junction, a first rectangular waveguide section connecting the first output arm of said T-section to the first rectangular input arm of said hybrid junction, a second rectangular waveguide section connecting the second output arm of said T-section to the second rectangular input arm of said hybrid junction, such first and second rectangular waveguide sections difiering in effective electrical length by a predetermined amount to control the character of modulated electromagnetic energy in the output arms of said hybrid junction.

11. A waveguide rotating device for providing modulation of electromagnetic energy comprising a ferrite rotating element adapted to rotate the plane of polarization of linearly polarized electromagnetic energy, a T-section of waveguide having an input arm and first and second cross-polarized rectangular output arms, said input arm being connected to the output of said ferrite rotating element, a magic-T hybrid junction having two collinear rectangular output arms and first and second rectangular input arms adapted to receive electromagnetic energy, said first and second rectangular input arms being connected respectively to the electric and magnetic planes of said collinear output arms at a common junction, a first rectangular waveguide section connecting the first output arm of said T-section to the first rectangular input arm of said magic-T hybrid junction, a second rectangular waveguide section connecting the second output arm of said T-section to the second rectangular input arm of said magic-T hybrid section, said first and second rectangular waveguide sections differing in length by substantially an odd number of quarter Wavelengths, a direct current magnetic field applied to said ferrite rotating element to produce a quiescent rotation of the plane of polarization of said electromagnetic energy through a predetermined angular distance, thereby to produce an adjustable phase shift at the output arms of said magic-T hybrid junction.

References Cited in the file of this patent UNITED STATES PATENTS 2,408,684 Roberts Oct. 1, 1946 2,748,353 Hogan May 29, 1956 2,333,992 McKay et al May 6, 1958 2,885,677 Zaleski May 5, 1959

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