Microstrip Ferrite Phase Shifters Having Time Segments Varying In Length In Accordance With Preselected Phase Shift Characteristic

Abstract

An electronically controllable RF (radio frequency) phase shifter that can be designed to provide either true time delay characteristics or constant phase versus frequency characteristics. The non-reciprocal microstrip phase shifters of the invention utilize a ferrimagnetic substrate with a metallized strip conductor on one face and a metallized ground plane on the other face. The strip conductor has a plurality of adjacent line sections selected to provide a circularly polarized RF magnetic field at a point between each two adjacent conductors with the plane of the circularly polarized magnetic field being orthogonal to the plane of a DC magnetic bias field, the latter plane being provided parallel to the face of the substrate. The circularly polarized RF magnetic field is developed by selecting the line sections with a length so that the signals in adjacent lines are 90.degree. out of phase from each other. When either the level or direction of the DC bias is changed, the permeability of the ferrimagnetic material changes causing a change in the propagation constant and a resultant phase shift change. To provide a relatively wide bandwidth to the phase shifter and a desired delay characteristic, the adjacent conductor sections have their line length varied so that different portions of the line effects the phase shift for different frequencies.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to RF phase shifters and particularly to an electronically controlled RF phase shifter having a wide bandwidth and having desired phase shift characteristics such as true time delay or constant phase versus frequency.

2. Description of the Prior Art

Two basic families of wide bandwidth RF phase shifter devices would be desirable for different applications, with the first family having the characteristic of true time delay and the second family having the characteristic of constant phase versus frequency. The true time delay phase shifters could be employed in antenna array applications, and the constant phase versus frequency phase shifters could be employed as the active phasing device in Butler matrix applications, in Blass matrices and in switchable four-port differential phase shift circulators. Suitable phase shifters with true time delay characteristics are presently not known, and prior art phase shifters with a constant phase versus frequency characteristic have bandwidth limitations. In the past, systems and devices have been limited to the bandwidth of the phase shifters, approximately 12 percent, thus providing a substantial limitation to the system operation. Another substantial requirement for phase shifters in certain operations is that the amount of phase shift be electronically controllable.

SUMMARY OF THE INVENTION

Briefly, the phase shifters in accordance with the invention are non-reciprocal, microstrip ferrite phase shifters fabricated by using a ferrimagnetic substrate with a metallized conductor line on one face or surface, positioned to form a plurality of adjacent line segments and with a metallized ground plane on the other face. The devices develop a circularly polarized RF magnetic field between adjacent line segments in a plane orthogonal to the plane of a DC magnetic bias field, the latter plane being perpendicular to the plane of the substrate. The circularly polarized RF magnetic field is generated by selecting the length of the line segments to provide the RF signal 90.degree. out of phase in the adjacent line segments. In response to a DC magnetic bias applied to the substrate, either the level or direction of the DC magnetic field or magnetic bais field is changed so that the permeability of the substrate material is controlled with a resultant desired phase shift. Due to the closed magnetic path in the substrate, the DC magnetic field remains constant without a continuously applied external driving source. Any desired number of phase shift conditions may be selected by controlling the DC magnetic bias. In order to provide a phase shift versus frequency characteristic that is either true time delay or constant phase, the length of the line segments are varied with a distribution along the plurality of line segments such that phase shift only occurs in the group of lines whose length is approximately equal to a quarter wavelength of the operating frequency. Also, the variation of line length provides a desired wide bandwidth to the phase shifter.

It is therefore an object of this invention to provide an improved microstrip RF phase shifter having desired time delay characteristics.

It is another object of this invention to provide an improved RF phase shifter device having a substantially wide bandwidth.

It is another object of this invention to provide an improved electronically controllable phase shifter for reliably selecting desired phase shift characteristics.

It is another object of this invention to provide RF phase shifters that can be developed to provide a characteristic of true time delay or to provide a characteristic of constant phase versus frequency.

It is still another object of this invention o provide an electronically controlled microstrip RF phase shifter that has a wide bandwidth and that has a desired time delay characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the invention itself will become apparent to those skilled in the art in the light of the following detailed description taken in consideration with the accompanying drawings wherein like reference numerals indicate like or corresponding parts throughout the several parts:

FIG. 1 is a schematic diagram of a simplified microstrip phase shifter utilizing a meander line for explaining the operation thereof in accordance with the principles of the invention;

FIG. 2 is a schematic sectional drawing taken at line 2--2 of FIG. 1 for explaining the theory of operation thereof;

FIG. 3 is a schematic diagram of vectors useful in explaining the operation of the phase shifters by reference to FIG. 2;

FIG. 4 is a schematic diagram of a microstrip phase shifter utilizing a meander line with a log-periodic line length distribution and indicating other possible line length distributions in accordance with the invention;

FIG. 5 is a schematic diagram of hysteresis curve of magnetic flux density versus current for explaining the DC magnetic bias utilized to provide the electronically controlled phase shifts in accordance with the invention;

FIG. 6 is a schematic plan view of a single conductor microstrip phase shifter in a circular or spiral format in accordance with the invention;

FIG. 7 is a schematic diagram of phase shift versus frequency for explaining the characteristics and the selection of characteristics of the phase shifters in accordance with the invention; and

FIG. 8 is a curve of phase shift versus frequency showing experimental results from the performance of a phase shifter in accordance with FIG. 4 having a log-periodic line length variation.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGS. 1 and 2, a simplified ferrimagnetic phase shifter 10 in accordance with the invention may be fabricated by using a ferrimagnetic substrate 12 which may be of any suitable material such as ferrite or garnet with a metallized strip conductor 14 on one face and a metallized ground plane 18 on the other face. The conductors utilized in the phase shifters of the invention may be of any suitable conductive material such as aluminum, copper, silver, gold, for example, and may be deposited or mounted on the substrate by any suitable technique. For example, the conductor may be a thin film deposited by vapor deposition or by sputtering, or may be a film placed on the substrate by a thick film process in which a mixture including silver and glass particles are painted on the substrate and then fused in an oven. A source of RF input signals 20 applies RF (radio frequency) signals to the line 14 to excite line segments 15, 16 and 17. Each line segment such as 16 has a line length or line segment length between points such as 23 and 25 or between similar points. Coupled to the lead 14 at the output of the phase shifter 10 is a utilization unit 28. For electronically controlling the DC (direct current) magnetic field in the substrate 12, a pulse control source 30 is provided for appplying current pulses of a waveform 31 through a lead 32 which is wound through an opening or hole 36 in the substrate, to establish the DC magnetic field or magnetic state therein indicated by a dashed line 33. The pulse control source 30 is a suitable voltage driving source controlled to selectively provide current pulses of the waveform 31 with a predetermined amplitude and of a predetermined duration. The control source 30 in some arrangements in accordance with the invention for a digital source, may include a current source, a switch such as a single pole double throw switch selectively passing positive or negative current pulses from the source through a gate to the drive line 32, with a peak current sensor controlling the gate in response to the drive line current reaching a predetermined level, as is well known in the art. The control source 30 for some arrangements in accordance with the invention for an analog source, may include a voltage source, switch such as a single pole double throw switch selectively passing either positive or negative current pulses to a gate and a controllable counter closing the gate for selected periods of time to pass pulses to the drive line 32. It is to be understood that the control source 30 is not limited to any particular arrangement but may include any suitable mechanization in accordance with the invention.

A first requirement for the non-reciprocal phase shifter in accordance with the invention is that the RF magnetic field be circularly polarized at a point A in the material. The line length of line segments such as 15 and 16 is equal to a quarter wavelength of the RF signal frequency so that adjacent signals are 90.degree. out of phase from each other to develop the circularly polarized region. The line length for the effected signal is .lambda./4 or .eta..lambda. + .lambda./4 where .eta. is any integer and .lambda. is the wavelength of the RF signal effected or phase shifted at that line area. The second requirement for the phase shifter of the invention is that a DC magnetic field be produced orthogonal to the plane of the circularly polarized field developed at point A and a third requirement is that the RF signal must pass through that RF circular polarized region A. The DC magnetic field H.sub.DC is shown passing into the paper in the region of the line segments in FIG. 2 by arrowheads 48 and 50 and coming out of the paper in a region away from the meander line by an arrow point 47, as developed by the conductor 32.

Referring now also to FIG. 3, the RF signal passing through line segments 15 and 16 shown as I.sub.rf for each line segment at a time t = 0 are respectively maximum in the line segment 15 with current conducting into the plane of the drawing and has a value of zero in the line segment 16. The magnetic field h .sub.rf at point A has the direction and magnitude of a vector 36 resulting from magnetic fields 34 and 35 (when current flows through the conductors). At a second time t = T/4 where T is the period of a cycle of the RF signal, the RF current is at a zero level through the line segment 15 and is passing with a maximum amplitude through the line segment 16 in a direction into the paper. As a result, the magnetic field has a valve and amplitude of a vector 38. At a time t = T/2, RF current passes through the line segment 15 with a maximum amplitude in a direction out of the paper and zero current passes through the line segment 16 to provide a magnetic vector 40 at point A. At a fourth time t = 3T/4 zero current passes through the line segment 15 and current of maximum amplitude passes through the line segment 16 in a direction out of the paper, resulting in a magnetic vector 41 at point A. Thus, a circular polarized condition is developed at the point A and an elliptically polarized condition is developed in adjacent regions such as 44 and 46. It is to be noted that the interaction which provides the phase shift in accordance with the invention is developed in the elliptically polarized regions as well as at the circularly polarized region or point A. Less interaction is provided in the elliptical polarized regions than in the circularly polarized regions. It is to be noted that a similar circularly polarized point A is developed between each pair of conductors for acting on the RF phase such as between conductors 16 and 17 as well as between conductors 15 and 16.

The operation of the DC magnetic bias field H.sub.DC orthogonal to the plane of the circular polarization will now be further explained. The ferrite material 12 may be considered as being divided into magnetic domains, each domain having its atoms with a certain moment and a certain direction and angle of precession. The electrons spin in each domain with a certain moment as determined by that group of atoms. When a DC magnetic field is provided, all of the domains align so that the atoms have the same direction of moment and the same precession angle. The RF magnetic field changes the precession angle of the atom due to the interaction of the magnetic forces. A change of the precession angles of the atoms changes the effective permeability of the ferrite material and in turn the amount of time delay provided therethrough, resulting in a different phase shift. For example, an increase of the precession angles of the spinning atom, increases the effective permeability of the ferrite, slows down the transfer of the RF signal therethrough and provides a larger phase shift. The DC field which is orthogonal to the plane of the RF field controls the direction of the magnetic moment and therefore the direction of the precessional spin relative to the direction of the circularly polarized magnetic field, thus changing the effective permeability of the ferrite. For example, if the DC field changes to its opposite direction of polarity, the precession is in the opposite direction and the precession angle will be smaller which results in a reduction of the RF energy density. Since the group velocity of RF energy passing through the ferrite type material is equal to the power flow density divided by the energy density in the ferrite, a reduction of energy density will cause an increase in group velocity. Thus, the phase shifter of the invention provides an effective change in the material so that the energy passes through the ferrite at selected speeds, the change in material being a change in the RF permeability of the ferrimagnetic material.

Referring now to FIG. 4 which shows a phase shifter 60 having a single conductor 64 as a meander line and which is formed of a ferrimagnetic substrate 68 on which the conductor 64 is deposited or positioned to establish the meander line generally indicated as 70. A ground plane (not shown) is provided as shown in FIG. 2. In the illustrated arrangement, the line length defined as the distance between points 72 and 74, for example, is varied along the meander line in accordance with selected characteristics such as log-periodically so that the longer lines on the left-hand side of the device provides a phase shift at the lower operating frequencies and the shorter lines at the right-hand side of the device provides a phase shift at the higher operating frequencies. The meander line 70 is shown with a log-periodic distribution of line length to provide a true time delay phase shift characteristic. Other variations are linear having a configuration of dotted lines 78 and 79 and a variation of lines 80 and 81 which may be selected to provide a constant phase characteristic. The variation of the line length is such that a phase shift only occurs in the group of lines whose length is approximately equal to a quarter wavelength of the operating frequency because this is the only frequency at which a circular polarized condition is developed at the point A and other similar points as explained relative to FIG. 2. For the log-periodic line length distribution, every frequency is effected by an equal number of lines, but not by the same ones, and a wide bandwidth operation is achieved as well as the desired phase shift versus frequency characteristic. For line length distributions other than log-periodic for providing other desired phase shift versus frequency characteristics, different frequencies are effected by different numbers of lines but a substantially wide bandwidth is achieved. When the line length is varied log-periodically, as shown in FIG. 4, for example, and the line spacing such as 75 is held constant, a true time dealy characteristic is achieved. A log-periodic distribution means that for adjacent line segments equally spaced along the meander line, each adjacent line segment length varies as a constant, K, times the length of the previous adjacent line segment. The DC magnetic field is provided by passing conductor 86 through openings 88 and 90 in the substrate 68 so that the DC magnetic fields are all in the same direction, and applying a DC control current therethrough from a DC control source 94. The line 64 has an input port 98 and an output port 100 thus requiring only a single strip conductor being placed upon the substrate 68.

When the line segments of the meander line of FIGS. 2 and 4 have a length .eta..lambda. + .lambda./4 and .eta. is an integer greater than zero, some additional loss may be developed. In this condition with additional wavelengths, there are additional circular polarized points along the center between adjacent conductor segments. The RF rotation changes direction between adjacent circular polarized points by a change from a circular polarization point to elliptical and then to linear and then back to elliptical and a circular polarized point. For a line length of .lambda. + .lambda./4 there are five points (like point A) between adjacent segments with three providing a positive phase shift and two providing a negative phase shift to develop a net positive phase shift, for example. For a line length of 2.lambda. + .lambda./4 there are nine points with five providing a positive phase shift and four providing a net positive phase shift, for example.

Referring now also to FIG. 5 which shows the hysteresis curve of the ferrimagnetic material 68, the control source 94 may provide digital control so that the material is in either state 103 or 105 resulting in a reversal of the polarity of the magnetic moment. Also within the scope of the invention, the control source 94 may select any analog magnetic state such as 107 to 110 for respective phase shifts of 45.degree., 90.degree., 180.degree. or 270.degree., for example. The DC source 94 may be a suitable circuit for digital or analog operation as discussed relative to FIG. 1. For analog operation the source 94 may be any suitable voltage source circuit as well known in the art for providing a predetermined voltage for a predetermined time for each controlled phase shift state.

Referring now to FIG. 6 which shows a single conductor microstrip phase shifter 120 employing a circular or spiral format and which, for example, may have a log-periodic variation of circular line segment length of a plurality of adjacent line segments. The spiral phase shifter includes a ferrimagnetic substrate 122 and a spirally arranged strip conductor 124 having a first port 126 and a second port 128 either of which may be the input or output ports. A ground plane (not shown) is provided as shown in FIG. 2. The spiral is arranged so that the distance around any circular portion or line segment of the conductor such as portions 130 and 132 from a position of a line 134 is 90.degree. less for the signal in the inner portion 132 than in the outer portion 130. The line segments are spaced from each other as a function of the selected line length distribution. The length of each portion further out from the next inner portion is .lambda./4 more than the adjacent inner segment. The k.sup.th spiral segment has a length .eta..lambda..sub.k + .lambda..sub.k /4 where .eta. is 0 or any integer and .lambda..sub.k is the wavelength which receives a maximum phase shift in this segment. Thus, for each line segment, l.sub.k, a different frequency as defined by the expression .eta..lambda.+ .lambda./4 = l.sub.k receives the maximum phase shift. The length of adjacent segments is varied as a selected function (such as log-periodic) so that the 90.degree. phase difference is provided between adjacent segments and so that frequencies are effected in each line as defined by the .lambda. of that line. Each of the circular line segments may control only the RF signal at a single frequency (as defined by the .lambda. used in the line length) or narrow band of frequencies in the phase shifter bandwidth. In the spiral configuration a continuous line of the point A circularly polarized condition is provided between the conductors. A DC conductor 136 passes through the opening 141 to provide a DC magnetic field of dotted lines 137 and may be controlled by a suitable digital or analog pulse control source such as source 30 of FIG. 1 or source 94 of FIG. 4.

Referring now also to FIG. 7, the selection of the line length distribution along the length of the conductor will be further explained. Synthesis has shown that the line lengths can be varied to provide a desired phase shift versus frequency characteristic that is consistent over an extremely wide band of operation. If the line length distribution of FIGS. 4 or 6 varies with a log-periodic line variation, a true time delay of a curve 69 is provided over a relatively wide frequency band. If the distribution of the length of the line segments is selected to provide a constant phase shift versus frequency characteristic, a wide frequency band of operation is provided as shown by a curve 71. One arrangement for determining the line length distribution of the conductor segments is to draw from calculated or experimental phase shift amplitudes, curves such as 70, 72 and 74, each for a portion of the conductor segments between the input and output terminals. Longer line length at the input provides a phase shift at lower frequencies as shown by the curve 70 and the shorter line lengths at the output provides a phase shift at higher frequencies as shown by a curve 74. The curve 72 may represent the phase shift at the central portion along the length of the conductor. Any number of curves may be utilized to provide this graphical distribution of line lengths. The combined result, when the line length distribution is properly selected to provide proper curves such as 70, 72 and 74, is the resultant phase shift curve 69, for example, when the line length between the two adjacent conductor segments is log-periodic, that is, the line length for each segment is a constant times the line length of the previous segment (for equal line spacing). Thus, any desired phase shift versus frequency curve such as 69 or 71 may be provided in accordance with the invention. It has been determined in accordance with the invention, that for a true time delay the variation in line length is typically log-periodic. For a log-periodic variation, the amount of line segments utilized for the different frequencies is less for the lower frequencies and substantially more for the higher frequencies. For a constant phase shift versus frequency characteristic the distribution of the line lengths is more line segments for the low frequencies and less line segments for the high frequencies. The distribution may be determined, for example, either by computer simulation, by graphical simulation as explained relative to FIG. 7, or by calculation or experimentally.

Referring now to FIG. 8 which shows an experimental curve 170 of phase shift versus frequency developed from the phase shifter of FIG. 4 having a log-periodic line distribution. The constructed phase shifter provided approximately one wavelength or 360.degree. or true time delay phase shift over the frequency band of 5.0 to 6.0 GHz (gigahertz). The length of the lines were varied log-periodically and the spacing was held constant to provide a true time delay. The device performance was in accordance with the solid curve 170 and the frequency band of operation represents a bandwidth of 18 percent. It is to be noted that this achieved bandwidth is not an inherent limitation to the device but rather resulted from a particular design. Broken lines 171 and 173 show a theoretical perfect true time delay characteristic. The solid experimental line 170 varies a relatively small amount from the theoretically perfect characteristic. In addition to this wide bandwidth performance, the device could be switched from one phase state to a second within five microseconds. The energy required to perform the switching operation was 20 microjoules. The size of the constructed device was 1.0 .times. 2.0 .times. 0.025 inches with a weight of 4.6 grams (0.010 lbs.).

Thus there has been provided electronically controllable RF phase shifters which may develop over a 360.degree. phase shift, a characteristic of true time delay or which may be designed to provide a constant phase versus frequency characteristic. The phase shifters in accordance with the invention provide a bandwidth substantially greater than any similar type devices known in the art today. The phase shifters of the invention operate with either terminal or port being the input so that the input RF signal can be applied initially to either the short or the long line segments. The principles of the invention include controlling the phase shift electronically and varying the line length in the configuration of a meander line or other arrangements having adjacent line segments. The phase shifter devices which employ log-periodic line spacing develop a true time delay in accordance with the principles of the invention. However, it is to be understood that phase shifters having a line spacing other than log-periodic such as to develop a constant phase versus frequency characteristic are within the scope of the invention.

Claims

What is claimed is:

1. A wide bandwidth RF phase shifter responsive to applied control signals, for phase shifting applied RF signals and wherein the relative phase shift across the bandwidth is in accordance with a preselected function of the frequency of the RF signals, said device comprising:

a ferrimagnetic substrate having first and second surfaces;

a ground plane conductor on the first surface of said substrate;

a continuous conductor on the second surface of said substrate said conductor being a meander line having a plurality of equally spaced substantially parallel adjacent line segments with the length of the adjacent line segments varying log-periodically from one end to the other end of said meander line; whereby the phase delay applied to said RF signals is approximately a linear function of the frequency of said RF signals.

means for applying said Rf input signals to a first end of said conductor;

output means for receiving the RF signals from a second end of said conductor; and

means for providing a DC magnetic bias field in said substrate in response to the applied control signals;

whereby the value of phase delay applied to said RF signals is controllable in response to said control signals, and the relative phase shift across the bandwidth is in accordance with a predetermined function of frequency established by said preselected segment length distribution pattern.

2. A wide bandwidth RF phase shifter responsive to applied control signals, for phase shifting applied RF signals and wherein the relative phase shift across the bandwidth is in accordance with a preselected function of the frequency of the RF signals, said device comprising:

a ferrimagnetic substrate having first and second surfaces;

a ground plane conductor on the first surface of said substrate;

a continuous conductor on the second surface of said substrate said conductor being a meander line having a plurality of equally spaced substantially parallel adjacent line segments with the length of the adjacent line segments varying approximately linearly from one end to the other end of said meander line,

means for applying said RF input signals to a first end of said conductor;

output means for receiving the RF signals from a second end of said conductor; and

means for providing a DC magnetic bias field in said substrate in response to the applied control signals;

whereby the value of phase delay applied to said RF signals is controllable in response to said control signals, and the relative phase shift across the bandwidth is in accordance with a predetermined function of frequency established by said preselected segment length distribution pattern.

3. A wide bandwidth RF phase shifter responsive to applied control signals, for phase shifting applied RF signals and wherein the relative phase shift across the bandwidth is in accordance with a preselected function of the frequency of the RF signals, said device comprising:

a ferrimagnetic substrate having first and second surfaces;

a ground plane conductor on the first surface of said substrate;

a continuous conductor on the second surface of said substrate said conductor having a generally spiral-shaped configuration from one end to the other end thereof with each turn of said spiral forming a conductor section and with adjacent conductor sections varying in length from a first end to a second end of said spiral in accordance with a log-periodic distribution pattern; whereby the phase delay applied to said RF signals is approximately a linear function of the frequency of said signals;

means for applying said RF input signals to a first end of said conductor;

output means for receiving the RF signals from a second end of said conductor; and

means for providing a DC magnetic bias field in said substrate in response to the applied control signals;

whereby the value of phase delay applied to said RF signals is controllable in response to said control signals, and the relative phase shift across the bandwidth is in accordance with a predetermined function of frequency established by said preselected segment length distribution pattern.