US3082383A - Ferromagnetic limiter - Google Patents

Description

March 19, 1963 E. STERN FERROMAGNETIC LIMITER Filed Nov. 22. 1960 2 Sheets-Sheet l ou"r RES SUB

FIG.5.

" INVENTORI ERNEST STERN,

BY m1 HIS AGENT.

March 19, 1963 E. STERN 3,

FERROMAGNETIC LIMITER Filed Nov. 22. 1960 2 sheets sheet 2 3 L0 (AT SUBSIDIARY RESONANCE) P LOG PIN )l asA W-u ase- INVENTORI ERNEST STERN BY J. I

HIS AGENT.

United States Patent 3,082,383 FERROMAGNETIC LIMITER Ernest Stern, Liverpool, N.Y., assignor to General Electric Company, a corporation of New York Filed Nov. 22, 1960, Ser. No. 70,943 '7 Claims. (Ci. 333-1.]l)

This invention relates to microwave solid state limiters employing a ferromagnetic limiter element having a low limiting threshold and a short response time.

It is well known that ferromagnetic materials exhibit a precession phenomenon. This phenomenon, as other magnetic properties of ferromagnetic materials, is usually described in terms of unpaired electron spins occurring in some atoms such as iron and nickel which produce a net magnetic moment. The unpaired electron spin model leads to the precession phenomenon which may be considered an analog of the mechanical gyroscope. That is, in the presence of a constant magnetic field, the axis of rotation of the electron precesses around the direction of the magnetic field. In a crystal lattice, the precessional phenomenon is coupled between neighboring atoms causing spin Waves, which are propagated throughout the lattice. These spin waves may be of long or short wavelength, varying from two lattice lengths to the full dimensions of the member and randomly distributed in the member.

The precession phenomenon and the accompanying spin waves lead to complex efiects. One such effect is that ferromagnetic materials exhibit a nonlinear dissipative response to electromagnetic waves which excite the spin waves. This nonlinear effect is such that for electromagnetic waves below a given amplitude, there Will be little signal attenuation, but for signals exceeding this given amplitude, there will be a substantially constant amplitude output. An example of a ferromagnetic limiter operating upon this principle is disclosed in the IRE Transactions on Microwave Theory and Techniques, January 1959 (Characteristics of Ferrite Microwave limiters, by G. S. Uebele) in which a slab of ferrite is piaced in the microwave magnetic field of a waveguide.

The prior ferromagnetic limiters have had inherent limitations which have made practical applications difficult. One of the most serious problems has been the spike leakage which the ferromagnetic limiters pass when an applied pulsed signal has a rapid rise time and a magnitude which exceeds the limiting threshold. The leakage spike approaches the input signal amplitude and has a time duration typically on the order of to 10- seconds. Qualitatively, this leakage arises from the finite time required to develop excitation of the ferromagnetic material, a time which increases with the Q of the material. This is inherent in the nature of the gyromagnetic effect. The energy passed during this transient effect can be reduced by increasing the linewidth, i.e. lowering the Q of the ferromagnetic material since the spike duration time is inversely proportional to the linewidth. Unfortunately, ferrites exhibiting increased linewidth also exhibit increased limiting thresholds. For a signal substantially coinciding with the subsidiary resonance frequency, the limiting threshold varies directly with the linewidth. This limitation is in addition to that imposed by the direct proportionality relationship between the microwave magnetic field in the ferromagnetic material and the square root of the incident signal power which limits the portion of the incident energy operated upon.

It is an object of this invention to provide an improved microwave limiter employing a ferromagnetic medium in a configuration which intensifies the microwave signal field.

It is a further object of this invention to provide a microwave limiter employing a ferromagnetic medium with a low limiting threshold and a leakage spike of shortened duration.

3,682,383 Patented Mar. 19, 1%53 Briefly stated, in accordance with one aspect of the invention, a ferromagnetic limiter element operating in a DC. magnetic field is arranged to limit microwave signals in a section of waveguide. The limiter element is com prised of a conductive filament terminating in enlarged end portions and surrounded with a body of ferromagnetic material. The end portions of the limiter element are shaped to prevent breakdown and maximize the filament current. The ferromagnetic body of the limiter element is dimensioned to give a demagnetization geometry such as to provide a subsidiary resonance at the signal frequency near to but separated from the main resonance. The limiter element is supported at a point of maximum electric signal field and in alignment with the direction of the signal field. The magnetic field is oriented parallel to the axis of the ferromagnetic limiter element and is adjusted to bring the subsidiary resonance frequency in coincidence with the signal frequency.

The invention will be better understood from the following description taken in connection with the accompanying drawings and its scope will be pointed out in the appended claims. 7

FIGURE 1 is a block diagram of a suitable limiter combination incorporating a limiter element constructed in accordance with the applicants invention.

FIGURE 2 is a cross section of a limiter element constructed in accordance with the applicants invention.

FIGURE 3 is a diagram of the equivalent circuit of the FIGURE 2 limiter element.

FIGURE 4 is a graph of the power output to power input ratio as a function of the constant magnetic field, H applied to the limiter.

FIGURE 5 is a graph of output power as a function of time for a limiter incorporating the invention.

FIGURE 6 is a graph of the log of power output as a function of the log of power input for a novel limiter incorporating one, two and three limiter elements.

FIGURE 7 is an example of another limiter configuration.

A typical circuit environment for a limiter constructed in accordance with the disclosed invention is illustrated in FIGURE 1 in which the limiter serves as a detector protector in an X-band radar system in addition to the usual duplexer. A conventional circulator 1 providing counterclockwise connection between the ports is shown. It provides a connection between a duplexer 2 and a radar receiver 3 through a path incorporating the limiter circuit in a radar system requiring the limiting function. The circulator may be constructed in accordance with the disclosure found in the Journal of Applied Physics, supplement to volume 30, No. 4, April 1959 Y circulator, by H. N. Chait and T. R. Curry). The input signal, before it is transmitted to the output port, is connected through the limiter and through a quarter wavelength delay line 4 to a short 6 where it is reflected back through the circulator 1. The limiter serves to attenuate the input signals above a given threshold power amplitude. Suitable dimensions for the waveguide interconnections are one inch by one half inch in which the signals are propagated in a TE mode.

The circuit configuration of FIGURE 1 produces a standing wave as seen by the limiter 4. This feature is not essential to the operation of the limiter 4 per se. However, it is a convenience which presents the signal a second time for limiting action and with the reflecting short arrangement, it is easier to design the limiter. This is because the requirement that the limiter impedance match the waveguide impedance is relaxed.

FIGURE 2 illustrates a longitudinal cross section in elevation of a preferred embodiment of the limiter 4 of FIGURE 1. A waveguide section 21 is arranged to support a limiter element 22 at a point of maximum electric field intensity and in alignment therewith. The limiter element includes a conductive filament 23 terminating at each end in enlarged, disk-shaped conductive portions 24 which reduce the density of charge accumulation resulting from currents induced by a signal in the waveguide. The filament is surrounded by a-ferromagnetic sheath 25 extending the length of the filament and preferably contiguous therewith. The fabrication and properties of the ferromagnetic sheath will be described below. The limiter element is supported by quartz studs 27 cemented to the waveguide walls and to the disks 24. The limiter element is subjected to a magnetic bias field by pole pieces 28, 29 which are orieinted to create a field aligned with the limiter element. The bias field strength is adjusted to produce correspondence of the ferrite subsidiary resonance frequency with that of the signal frequency.

The limiter element 22 serves to concentrate the energy of the waveguide signal into the ferromagnetic material 25 which produces an improved coupling between the signal and the ferromagnetic material. The coupling results from the alternating current induced in the limiter filament by the electric field of the signal, which in turn creates a magnetic field acting upon the ferromagnetic material. A filament to concentrate a microwave signal has been disclosed by D. Rodbell in the Journal of Applied Physics, volume 30, No. 4, November 1959 (Microwave Magnetic Field Near a Conducting Perturbation) and has been described and claimed in an application for US. Letters Patent entitled Magnetic Microwave Device, Serial No. 65,085, filed October 26, 1960, by Donald S. Rodbell and assigned to the assignee of the present invention. When the limiter element configuration consists of a good conductor surrounded by ferromagnetic material, the result can be a concentration of the effective microwave field by several orders of magnitude. This concentration makes it possible to reduce the threshold signal power required to excite the subsidiary resonance and permits the use of a broader linewidth material.

The positioning of the limiter element in the waveguide is not overly critical, but variations from the optimum position will increase the threshold level for limiting bydecreasing the current induced in the conductive filament 23. For example, misalignment of the limiter filament with the direction of the electric field by an angle will reduce the threshold level by a cos 2 0 factor. Displacement from the position of maximum electric field will result in a similar reduction of the threshold level.

As shown in FIGURE 2, one type of limiter element utilizes a ferrite as the ferromagnetic material 25 formed on the filament conductor 23. One method of applying the ferrite is to imbed a fine wire in the unfired ferrite material. The techniques of either dry pressing the ferrite in a mold in which a Wire is centered or extruding the ferrite around a wire inserted in an extrusion die are suitable. After the combination is thus formed, it is fired at sufliciently high temperatures to sinter the ferrite. It is necessary to use metals such as platinum for the filament wire to avoid melting and reaction of the wire with the ferrite at the high temperatures in firing.

The ferrite type of limiter element must be formed with the proper shape to produce the desired limiting efiect. That is, the subsidiary gyromagnetic resonance of the ferrite body must be tuned to the signal frequency. FIG- URE 4 is a graph of the ratio of power output to power input against bias field for an input signal of a given frequency and power amplitude above the limiting threshold. This curve is characterized by two resonance lines, the main resonance line at H and the lower, subsidiary resonance line at H The main resonance line is essentially determined by the properties of the material and the external dimensions thereof. Within the range of signal power where the subsidiary resonance effects occur, the main resonance represents a loss factor which is not of utility in a limiter application. The subsidiary resonance line, however, only appears for input signals above the limiting threshold and provides the limiting mechanism relied on in the present invention. The position of the subsidiary resonance line relative to the main resonance line is a variable factor. The variation obtainable extends from coincidence between the resonances to a position of substantial displacement of the subsidiary resonance line below the mainresonance line as shown in FIGURE 4. This separation factor is determined by the demagnetization effects in the ferromagnetic material and for a given material is a function of the geometry of the body as explained hereinafter.

The optimum design of a limiter element involves consideration of several interrelated factors. These factors include maintaining proportions of the limiter element dimensions to a satisfactory impedance match of the conductive structure with the waveguide; the selection of a material with the desired magnetic properties; shaping the body of ferromagnetic material to produce the desired small displacement of the subsidiary resonance from the main gyromagnetic resonance and obtaining the desired relation between the amplitude of the signal in the waveguide and the amplitude of the magnetic field produced by the conductive filament so that limiting action is introduced at the desired signal level.

The first consideration in designing a limiter element is the selection of proper physical dimensions to produce satisfactory coupling of the signal from the waveguide to the limiter element. It has been found that the limiter element, considered as a lumped circuit element must present a good impedance match to the waveguide to avoid substantial reflection of the signal.

The equivalent circuitof the limiter element and Waveguide is illustrated in FIGURE 3. Capacitance appears between each of the metalized ends of the limiter element 24A and 24'13, and between each of the metalized ends and the walls of the waveguide 21'. The current bearing filament presents an inductance 23' and the ferromagnetic absorption in the ferrite approximates a nonlinear resistance 25'. Accordingly, the limiter appears as a parallel resonant circuit in series with a pair of capacitances. It is essential that this circuit resonate in the region of the ,signal frequency to achieve the desired coupling of the signal to the limiter element.

The underlying physical phenomena giving rise to the lossy element 25 of this parallel resonance circuit will now be discussed.

For optimum sensitivity, the subsidiary resonance frequency should be positioned as close to the main resonance as possible. The threshold level for the subsidiary resonance decreases as the subsidiary resonance is removed from the main resonance. However, if the signal frequency should fall within the main resonance line width, the signal will see an additional fixed attenuation factor which would appear as an undesired limiter insertion loss. Accordingly, the limiter element is designed to have a small but clearly delineated displacement between the two resonance peaks. In terms of the applied magnetic field intensity, the displacement is typically twice the main resonance linewidth 2AH.

For a cylinder, the main resonance and subsidiary resonance are respectively determined as follows:

where H is the main resonance, H is the subsidiary resonance, N, is the demagnetization factor, to is the signal frequency, 41rM is the saturation magnetization and 'y is the gyromagnetic constant. To provide the desired ZAH displacement, Equations 1 and 2 are combined to produce the requirement:

(3) Ni fall 1g,

From this expression a solution for N can be obtained. From the article by Osborn in the Physical Review, volume 67, June 1945 (Demagnetizing Effects of the General Ellipsoid), the required ratio of diameter to length can be obtained.

The absolute dimensions may vary over a substantial range, having, of course, a maximum dimension limited by the size of the transmission line.

Expressions 1 and 2, which apply only to the solution for a cylinder, also are of the same general nature as those of the simple geometrical shapes.

The objective of a limiter is to remove all signal power above a given threshold signal power level. In the limiter element configuration of FIGURE 2, the magnetic field operating on the ferromagnetic material is not primarily the waveguide field at the signal frequency, but rather the field produced by the current in the conductive structure of the limiter. Since the conductive structure con-- figuration, which determines the filament current is subject to design, the relation between waveguide field amplitude and the amplitude of the magnetic field acting upon the ferromagnetic material can be controlled by this design.

For example, in the FIGURE 2 limiter element, the current is primarily determined by th conductive filament diameter, the impedance of the element, and the area of the disk shaped metallized end portions 24. The charge induced in the ends is if the field terminating on the limiter element end portions is equal to the electric field intensity E of the empty waveguide at the signal frequency, where A is the area of the end portions and s is the permittivity constant.

Accordingly, from the Biot-Savart law, the magnetic field produced at the filament surface is aproximately where w is the signal frequency, r is the filament radius.

The particular application will specify the threshold power, the spike leakage and signal frequency. A material is selected that will provide the minimum h consistent with the spike leakage requirements. The dimensions of the ferrite element, the wire diameter and disk diameter are then chosen to produce the specified threshold power.

The response of the novel limiter element is illustrated in FIGURE 6. For a step input signal pulse shown in dashed lines the output power is shown by the solid line. Initially, the output amplitude essentially equals the high signal pulse but it rapidly falls to a plateau substantially at the threshold power amplitude, P This initial spike response as pointed out earlier is an inherent feature of the resonance mechanism of the ferromagnetic material. Although it cannot be eliminated, it can be alleviated by reducing the spike width to minimize the energy content of the spike.

The spike duration is equal to where 'y is the gyromagnetic constant of the material and AH is the spin wave linewidth. By selecting a material having an appropriate linewidth, the specified spike duration can be achieved.

The required threshold of microwave field intensity is determined theoretically by the following relation:

An appropriate material and shape can be selected to provide a low threshold field.

FIGURE 5 is a graph of the log of output power response of the limiter against the log of input signal power at the subsidiary resonance frequency. Curve 41 is the response of a single limiter element and curves 42 and 43 are for two and three limiter element configurations, respectively. As all of these curves show, the response of the limiter is comprised of three regions. The first region is characterized by a linear relation between input and output power with a small differential due to insertion loss. This region is followed by a second region in which there is no further increase in output power as the input power increases. This is the limiting region which typically extends over a thirty decibel range. Above this range, is a region of no limiting where the lossy spin wave mechanism is saturated. Here, for each increment of input power there is an equal increment of output power. The efiect of a plurality of limiter elements is the multiplication of the limiting range. The onset of limiting action remains fixed, but the region 'of limiting is extended to twice the single element limiting range for a two element configuration as shown by curve 42, and the limiting range is tripled for three limiter elements as shown by curve 43. This multiplication effect holds when the ele ments are connected in series.

The conductive disk-shaped end portions 24 of the limiter element illustrated in FIGURE 2 are easily formed elements for charge accumulation. These portions are produced by the application of silver paint over the end portions of the ferrite resulting in the conductive disks. One of the primary criteria in the design of the end portions is the prevention of break down which is determined by factors such as the dielectric in the waveguide and the signal strength. The enlarged conductive end portions also produce the important result that limiter element current is increased by the enhanced charge storage. The desired signal field intensity in the material is proportional to this current so that this factor is usually critical. Clearly, shapes other than the disk-shaped configuration such as tear-shaped end portions can reduce the charge density. Another approach is to ground the ends of the limiter element to the waveguide walls, but the enlarged end portion arrangement can produce larger currents than the grounded limiter element arrangement.

Another type of limiter element is comprised of a film of Permalloy as the ferromagnetic material 26 formed on the filament conductor 23. The method of film formation is conveniently electrodeposition in accordance with the process disclosed in the 44th Proceedings of the American Electroplaters Society, 1957 (Further Studies on Nickel-Iron Alloy Electrodeposits, by I. W. Wolf) and the 43rd Proceedings of the American Electroplaters Society, 1956 (Nickel-Iron Alloy Electrodeposits for Magnetic Shielding, by I. W. Wolf and V. P. McConnell). By this method, films on the order of one thousand angstroms in thickness are formed using a one mil diameter gold wire as the fihn substrate and filament conductor.

A second exemplary limiter configuration is illustrated in FIGURE 7. In this configuration, a pair of limiter elements are arranged oposite one another on the walls of a waveguide 31. A pair of conductive filaments 33A and 33B are positioned in alignment and in contact with the waveguide walls. Surrounding the filaments 33A and 33B are coaxial cylindrical bodies of ferromagnetic ceramic 35A and 35B, respectively. The choice of materials and dimensions of the limiter elements are determined by the same considerations as the FIGURE 2 embodiment. The limiter elements are separated by an insulator 36 selected to provide mechanical and electrical stability to this configuration. Between each cylinder of ferromagnetic material 35A and 35B and the insulator 36 is placed a conductor 34in electrical contact with corresponding conductive filaments 33A and 33B. These 7 conductors 34 serve the same function as the conductive portions 24 in the FIGURE 3 limiter element and are conveniently produced by the application of silver paint on the cylinders 35A and 35B. The FIGURE 7 embodiment also operates in the same manner as that described for the FIGURE 2. embodiment. The electric field of a signal in the waveguide sets up oscillating currents in the filaments 33A and 33B which are dissipated by magnetic coupling to the lossy spin waves in the ferrite 35A and 353 for signals exceeding the threshold limiting amplitude.

The ferrite materials which have particular advantage in the present invention are those having an inreased subsidiary resonance line width since they are characterized by a shorter response time. An adverse property concurrently shared by these favored materials and offset in accordance with the present invention is that the required field intensities must be considerably higher before this broadened linewidth capability comes into play. Suitable material for use at X-band are those having. linewidths of large AH containing in their lattice structure fast relaxers such as the cobalt ion.

The ferrite limiter designed in accordance with the disclosed invention has as a principal advantage the reduction of spike duration from between and 10* to 10- seconds. The selected configuration, employing a conductive filament, concentrates the signal energy producing a localized magnetic field in the ferrite substantially exceeding the magnetic field strength normally applied to the ferrite. This makes possible the use of'those ferromagnetic materials having broader spin absorption line propertes which require higher field concentrations. As explained earlier, the ability to use broader linewidth materials shortens the response time, and thus reduces the duration of the spike leakage.

While particular embodiments of the invention have been shown and described, it should be understood that the invention is not limited thereto and it is intended in the appended claims to claim all such variations as fall within the true spirit of the present invention.

What is claimed is:

l. A solid sate limiter comprising: a signal transmitting wave guide; a conductor positioned in said waveguide subject to the oscillatory electric field of the waveguide to produce current oscillations therein giving rise to an oscillatory magnetic field having an amplitude exceeding that of the waveguide magnetic field strength, a body of ferromagnetic material positioned in said waveguide proximate said conductor for substantial flux linkage with said oscillatory magnetic field, said body exhibiting a subsidiary resonance separated from the main gyromagnetic resonance frequency; and magnetic means arranged to produce a constant magnetic bias field in said body of ferromagnetic material of a magnitude such as to make the subsidiary resonance coincident with the frequency of the waveguide signal.

2. The limiter of claim 1 wherein: at least one end of said conductor is conductively connected to said waveguide.

3. A solid state limiter comprising: a signal transmitting waveguide; a conductive filament positioned in said waveguide subject to the oscillatory electric field of the waveguide to produce current oscillations therein giving rise to an oscillatory magnetic field having an amplitude exceeding that of the waveguide magnetic field strength; a substantially cylindrical body of ferromagnetic material surnounding said filament and proximate thereto and dimensioned to exhibit a subsidiary resonance separated from the main gyromagnetic resonance frequency; and magnetic means arranged to produce a constant magnetic bias field in said body of ferromagnetic material of a magnitude such as to make the subsidiary resonance coincident with the frequency of the waveguide signal.

4. The solid state limiter of claim 3 further including: a pair of conductive charge storage elements, each element being connected to one end of said conductive filament.

5. The solid state limiter of claim 3 further including: a pair of conductive elements formed on the end surfaces of said cylinder in electrical contact with said filament, said elements being shaped to prevent electrical breakdown between said filament and said waveguide and to produce increased current oscillations in said filament.

6. A solid state limiter comprising: a signal transmitting waveguide; a conductive filament positioned in said waveguide subject to variations in the electric field of the waveguide signal to produce current oscillations therein giving rise to an oscillatory magnetic field having an amplitude exceeding that of the waveguide signal magnetic field strength; a substantially cylindrical body of ferromagnetic material having a broad spin absorption line property surrounding said filament and proximate thereto and dimensioned to exhibit a subsidiary resonance removed from the main gyromagnetic resonance frequency; and magnetic means arranged to produce a constant magnetic bias field in said body of ferromagnetic material of a magnitude such as to make the subsidiary resonance coincident with the frequency of the waveguide signal.

7. A limiter comprising: an isolating microwave element having first, second and third ports, said element having the property that an input signal received at one of said ports is transmitted only to said port of succeeding number; a waveguide element connected to one of said ports; reflecting means connected to said waveguide element arranged to produce a standing wave pattern therein; and a limiter element positioned in said waveguide element, said limiter element being comprised of a fila mentary conductor producing current oscillations and a body of ferromagnetic material exhibiting a subsidiary resonance separated from the main gyromagnetic resonance frequency, said body being proximate said conductor for substantial flux linkage with the magnetic field accompanying said current oscillation.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. A SOLID STATE LIMITER COMPRISING: A SIGNAL TRANSMITTING WAVE GUIDE; A CONDUCTOR POSITIONED IN SAID WAVEGUIDE SUBJECT TO THE OSCILLATORY ELECTRIC FIELD OF THE WAVEGUIDE TO PRODUCE CURRENT OSCILLATIONS THEREIN GIVING RISE TO AN OSCILLATORY MAGNETIC FIELD HAVING AN AMPLITUDE EXCEEDING THAT OF THE WAVEGUIDE MAGNETIC FIELD STRENGTH, A BODY OF FERROMAGNETIC MATERIAL POSITIONED IN SAID WAVEGUIDE PROXIMATE SAID CONDUCTOR FOR SUBSTANTIAL FLUX LINKAGE WITH SAID OSCILLATORY MAGNETIC FIELD, SAID BODY EXHIBITING A SUBSIDIARY RESONANCE SEPARATED FROM THE MAIN GYROMAGNETIC RESONANCE FREQUENCY; AND MAGNETIC MEANS ARRANGED TO PRODUCE A CONSTANT MAGNETIC BIAS FIELD IN SAID BODY OF FERROMAGNETIC MATERIAL OF A MAGNITUDE SUCH AS TO MAKE THE SUBSIDIARY RESONANCE COINCIDENT WITH THE FREQUENCY OF THE WAVEGUIDE SIGNAL.

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