HK1069019A - A magnetic filter - Google Patents

Description

Magnetic filter

Technical Field

The present invention relates generally to filtering electrical signals. More particularly, the present invention relates to an apparatus, system, and method for filtering a plurality of signals at different frequencies.

Background

Communication systems typically transmit a plurality of carrier signals, each of which is transmitted at a different transmit frequency. Each transmission signal is typically modulated by an information signal. Each transmission signal may be received separately and the information signal may be detected.

Fig. 1 shows the frequency spectrum of a plurality of transmission signals. Each transmission signal includes carrier frequencies FC1, FC2, FC3, FC 4. The spectrum allocated to each carrier frequency is commonly referred to as a transmission channel. The amount of spectrum allocated to each transmission channel generally determines the amount of information that can be transmitted over the transmission channel. It is desirable to utilize as much of the allocated spectrum as possible.

The frequency spectrum of fig. 1 shows the transmission signals 110, 120, 130, 140 at the carrier frequencies FC1, FC2, FC3, FC 4. The frequency spectrum adjacent to each transmission signal 110, 120, 130, 140 is typically occupied by information modulated onto the transmission signal 110, 120, 130, 140. Generally, the greater the modulation rate of the information (which is generally proportional to the amount of information), the greater the amount of spectrum occupied by each transmitted signal and associated modulated information. The modulation rate of each transmission signal should not be too great so that the modulation information of one transmission signal interferes with the modulation information of an adjacent transmission signal.

Fig. 2 shows a frequency spectrum of multiple transmission signals 210, 220, 230, 240, where information from adjacent transmission channels overlap each other. That is, information that should be transmitted in one transmission channel is not intentionally transmitted in another transmission channel. For example, the modulation information of the first transmission signal 210 overlaps the modulation information of the second transmission signal 220, as shown at 215. The modulation information of the second transmission signal 220 overlaps the modulation information of the third transmission signal 230 as indicated at 225. The modulation information of the third transmission signal 230 overlaps the modulation information of the fourth transmission signal 240 as shown at 235.

The overlap may be due to distortion of the transmitted signal due to non-idealities of components in the transmission system. The distortion may include noise, spurious signals, and harmonics of the transmission signal that overlap with adjacent transmission signals.

Channel frequency overlap of information signals from one transmission channel to another introduces transmission errors. Transmission errors reduce the effectiveness of the transmission system. In addition, transmission errors also reduce the transmission bandwidth of the communication.

There is therefore a need for a method and apparatus that provides selective filtering of multiple frequency signals. It is also desirable to provide high frequency filtering of the communication signals to reduce the amount of spectral overlap between the transmission signals of the communication signals.

Disclosure of Invention

The present invention includes apparatus and methods for providing selective filtering of a multi-frequency signal. The apparatus and method may filter the communication signals to reduce spectral overlap between transmission signals of the communication signals.

Embodiments of the invention include a magnetic filter. The magnetic filter includes a magnetic transducer for generating a magnetic field in response to a plurality of information-transfer signals. The magnetic filter further includes a magnetic tunnel junction. The magnetic tunnel junction can be tuned to switch states in response to a selected frequency of a magnetic field. The magnetic filter may further include a magnetic tunnel junction sensor for detecting a state of the magnetic tunnel junction.

Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

Drawings

Fig. 1 shows the frequency spectrum of several modulated carrier signals.

Fig. 2 shows the frequency spectrum of several modulated carrier signals with spectral overlap between adjacent channels.

FIG. 3 illustrates a magnetic tunnel junction sensor.

FIG. 4 shows waveforms depicting magnetic tunnel junction sensor transitions when magnetic transition signals of different pulse widths are applied to the magnetic tunnel junction sensor.

Fig. 5A shows the magnetic dipole and the applied magnetic field.

FIG. 5B illustrates the sense layer of the magnetic tunnel junction.

FIG. 5C illustrates precession of the magnetization vector of the magnetic tunnel junction.

FIG. 6 shows a graph depicting the probability of a magnetic tunnel junction changing state when magnetic switching signals of different pulse widths are applied to a magnetic tunnel junction sensor.

Fig. 7 shows a magnetic comb filter according to an embodiment of the invention.

Fig. 8 illustrates a transmit comb filter according to an embodiment of the present invention.

Fig. 9 illustrates a receive comb filter according to an embodiment of the present invention.

Fig. 10 shows the frequency response of a magnetic comb filter according to an embodiment of the invention.

FIG. 11 is a flowchart including operations according to embodiments of the present invention.

Detailed Description

As shown in the drawings for purposes of illustration, the present invention is embodied in an apparatus and method for high frequency filtering of communication signals to reduce the amount of spectral overlap between the transmission signals of the communication signals.

FIG. 3 illustrates an embodiment of a magnetic tunnel junction sensor 300 that includes a reference layer 310, a sense layer 320, and an insulating layer 330.

The magnetic tunnel junction sensor 300 may be used to detect the presence of a magnetic field. Magnetic tunnel junction sensors based on tunneling magneto-resistive devices may include spin-dependent tunneling junctions. The orientation of the magnetization of the reference layer 310 is fixed and therefore does not rotate in the presence of an applied magnetic field of interest. The magnetization of the sense layer 320 can be oriented in either of two directions. If the magnetization of the reference layer 310 and the sense layer 320 are in the same direction, the orientation of the spin-dependent tunneling junction is said to be parallel. If the magnetization of the reference layer 310 and the sense layer 320 are in opposite directions, the orientation of the spin-dependent tunneling junction is said to be antiparallel. These two stable orientations, parallel and antiparallel, may correspond to logical values "0" and "1".

The magnetic orientation of the sense layer 320 generally coincides with a direction corresponding to the most recent external magnetic field in the vicinity of the sense layer 320. The external magnetic field must have sufficient magnetic strength to change the orientation of the sensing layer 320 in order to detect the magnetic field.

The magnitude of the resistance across the magnetic tunnel junction sensor 300 varies with the magnetic orientation of the sense layer 320 relative to the magnetic orientation of the reference layer 310. In general, if the magnetic orientation of the sense layer 320 is in the opposite direction of the magnetic orientation of the reference layer 310, the resistance across the magnetic tunnel junction sensor 300 is large. If the magnetic orientation of the sense layer 320 is in the same direction as the magnetic orientation of the reference layer 310, the resistance across the magnetic tunnel junction sensor 300 is smaller. Therefore, the resistance across the magnetic tunnel junction sensor 300 can be used to detect the direction of the magnetic field, which determines the resistance of the magnetic sensor 300 because the direction of the magnetic field determines the magnetic orientation of the sensing layer 320 relative to the reference layer 310.

The reference layer 310 and the sensing layer 320 may be made of ferromagnetic materials. The reference layer 310 may be implemented with a soft magnetic reference layer or a magnetic pinned layer.

The orientation of the magnetic tunnel junction sensor 300 may be said to be "parallel" if the magnetizations of the sense layer 320 and the reference layer 310 of the magnetic tunnel junction sensor 300 are in the same direction. The orientation of the magnetic tunnel junction sensor 300 may be said to be "antiparallel" if the magnetizations of the sense layer 320 and the reference layer 310 are in opposite directions. These two orientations, parallel and anti-parallel, may correspond to a low resistance or high resistance magnetic sensor state.

The insulating tunnel barrier layer 330 may enable quantum mechanical tunneling between the reference layer 310 and the sensing layer 320. The tunneling effect is spin dependent on the electrons, resulting in a resistance of the magnetic tunnel junction sensor that varies with the relative orientation of the magnetization directions of the reference layer 310 and the sense layer 320. The presence of a magnetic field may be detected by determining the orientation of the magnetization of the reference layer 310 and the sense layer 320.

If the magnetization orientations of the magnetic tunnel junction sensor 300 are parallel, the resistance across the magnetic tunnel junction sensor 300 is a first value (R); if the magnetization orientations of the magnetic tunnel junction sensor 300 are antiparallel, the resistance across the magnetic tunnel junction sensor 300 is a second value (R + Δ). The invention is not limited to the orientation of the magnetization of said two layers, i.e. to only two layers.

The insulating tunnel barrier layer 330 may be made of aluminum oxide, silicon dioxide, tantalum oxide, silicon nitride, aluminum nitride, or manganese oxide. Other dielectrics and certain semiconductor materials may also be used as the insulating tunnel barrier layer 330. The thickness of the insulating tunnel barrier layer 330 may be from about 0.5nm to about 3 nm. The present invention is not limited in this context.

The sensing layer 320 may be made of a ferromagnetic material. Both the sense layer 320 and the reference layer 310 may be implemented with a Synthetic Ferromagnet (SF), also known as an artificial antiferromagnet.

The sense layer 320 of the magnetic tunnel junction sensor 300 is generally aligned in a direction corresponding to the direction of the applied magnetic field.

FIG. 4 illustrates transition waveforms of the magnetic tunnel junction sensor when magnetic transition signals of different pulse widths are applied to the magnetic tunnel junction sensor. A first waveform 410 represents the state of the magnetic tunnel junction sensor when a magnetic field having a pulse width of 125ps is applied. A second waveform 420 represents the state of the magnetic tunnel junction sensor when a magnetic field having a pulse width of 250ps is applied. A third waveform 430 represents the state of the magnetic tunnel junction sensor when a magnetic field having a pulse width of 350ps is applied. The magnitude of the applied magnetic field is 200 oersted (Oe) for each waveform.

These waveforms indicate that the magnetic tunnel junction sensor is more likely to switch states for a particular pulse width than for other pulse widths. For example, the waveforms of FIG. 4 show the switching states of the magnetic tunnel junction sensor at a pulse width of 250 ps.

Due to the switching characteristics of the magnetic tunnel junction (commonly referred to as precessional switching), the magnetic tunnel junction will switch states for pulses of a particular pulse width, but will not switch states for pulses of other widths. There are typically many ranges of pulse widths that can be selected that result in switching of the magnetic tunnel junction. The pulse width may be equivalent to the period of the sinusoidal waveform. A sinusoidal waveform containing a period corresponding to the selected pulse width "on" causes the magnetic tunnel junction to change state. This time/frequency selective characteristic of the magnetic tunnel junction enables the magnetic tunnel junction to be used as a selection switch or filter.

Such selective time/frequency switching characteristics of the magnetic tunnel junction can be determined experimentally or computationally. Therefore, the magnetic tunnel junction can be tuned to selectively pass signals containing a particular frequency.

There are many applications that can utilize the selective signal frequency passband of the present invention. The description provided herein of a signal transmitter and signal receiver utilizing the frequency comb filter of the present invention is but a few examples of the useful applications of the present invention. The invention can also be used in many different applications where high frequency selective filtering is required.

Precessional conversion

Precessional switching is a phenomenon that can be used to illustrate the transition region of the transfer curve of fig. 4. The precessional switching when applied to a single magnetic dipole will first be described, followed by a description of the application to the magnetic tunnel junction of the present invention.

FIG. 5A shows the magnetic moment m of a single magnetic dipole. If a magnetic field Heff is applied to the magnetic dipole, the magnetic dipole precesses around the axis of the applied magnetic field Heff in an effort to align with the applied magnetic field Heff. The precession is represented by a circular rotation 510 about the axis of the applied magnetic field Heff.

As shown in FIG. 5A, the axis of the applied magnetic field Heff makes an angle A with the illustrated z-axis, and the magnetic moments of the dipoles make an angle B with the illustrated z-axis.

Precession can be calculated using the following equation of motion:

(1/γ) (dm/dt) ═ m × Heff, where m is the magnetic moment of the dipole and γ is the well-known gyromagnetic ratio. The standard value of gamma is 1.76X 10⁷Oe^-1s^-1。

The estimated precession of the magnetic tunnel junction of the present invention also includes damping and exchange interactions between the large number of dipoles used to mimic the magnetic tunnel junction sensing layer. If these factors are included and the calculations made are summed over all dipoles of the small ferromagnet of the magnetic tunnel junction, the precession of the magnetic tunnel junction can be determined using a final equation of motion, commonly referred to as the Landau-Lifshitz-Gilbert equation. The equation of motion can be expressed as:

(dM/dt) — γ (Mx δ W/δ M) - (α/M) (Mx (dM/dt)); where M is the magnetization vector, γ is the gyromagnetic ratio, α is the damping ratio, and (δ W/δ M) is the total derivative of the energy density of the magnetization Heff.

FIG. 5B illustrates the sense layer and corresponding x, y, and z axes of a magnetic tunnel junction.

FIG. 5C shows an example of the precession of the sense layer magnetization M (also referred to as the net magnetic moment) calculated using the aforementioned Landau-Lifshitz-Gilbert equation.

As shown in FIG. 5C, initially, the magnetization M of the magnetic tunnel junction is oriented along the x-axis. Upon application of the magnetic field Heff, the magnetization vector M starts to rotate and changes direction as a result of the magnetization vector M striving to align with the magnetic field Heff, according to line 520. The speed at which the magnetization vector M changes direction depends on the damping factor of the motion as well as the geometry and material of the magnetic tunnel junction. Using a micromagnetic model of the dynamic domain (i.e., scalar less than 1ns), the Landau-Lifshitz-Gilbert equation can be used to model this motion.

For the purposes of the present invention, precession is modeled to provide a prediction of the precise magnitude and duration of the magnetic pulse that causes the magnetization vector of the magnetic tunnel junction to switch. The duration and amplitude of the applied magnetic pulse can be varied to identify the particular frequency at which the transition of the magnetic tunnel junction occurs. This selective switching frequency is used to provide the filter effect of the present invention.

FIG. 6 shows a graph depicting the probability that a magnetic tunnel junction will switch state when magnetic switching pulses of different pulse widths are applied to the magnetic tunnel junction sensor. The first peak 610 occurs at a pulse width of 150 ps. The second peak 620 occurs at a pulse width of 290 ps. The third peak 630 occurs at a pulse width of 430 ps.

All of these pulses include an amplitude of 200 Oe. The pulse width may be different for different pulse amplitudes.

According to the curves, pulse widths of 150ps, 290ps, and 430ps more easily cause the magnetic tunnel junction to change states. The curve can be used to determine the passband frequency of a comb filter formed by the magnetic tunnel junction. In general, the following signals can be passed, namely: the signal frequency contains a time period corresponding to the pulse width of the pulse that causes the magnetic tunnel junction to switch.

For example, if a signal includes a magnitude greater than 200Oe for a duration of 150ps, this signal will cause the magnetic tunnel junction to switch states and the signal will not be filtered out by the magnetic tunnel junction (i.e., the signal will pass through the magnetic tunnel junction).

The passband of the magnetic comb filter of the present invention can be tuned. I.e. the passband of the comb filter can be tuned. The tuning may be done by controlling the materials in the MTJ or controlling the physical properties of the magnetic tunnel junction. The actual tuning frequency can be simulated and determined by experiment.

Furthermore, the passband of the comb filter of the present invention can also be tuned quickly. That is, a magnetic field is applied in a direction orthogonal to (or different from) the magnetization directions of the reference layer and the sense layer. This applied magnetic field changes the desired pulse width that enables switching of the magnetic tunnel junction. Therefore, the passband frequency of the comb filter changes accordingly. The effect of applying a magnetic field in orthogonal directions can be simulated and determined experimentally.

The passband frequency of the magnetic tunnel junction can be determined either experimentally, by simulation, or by a combination of the two.

Fig. 7 shows a magnetic comb filter 700 according to the invention. The magnetic comb filter 700 includes a magnetic transducer 710 for generating a magnetic field 720 in response to a plurality of information-conveying signals. The magnetic comb filter 700 includes a magnetic tunnel junction 730. The magnetic tunnel junction 730 may be tuned to switch states in response to a selected frequency of the magnetic field 720. The magnetic comb filter 700 may also include a magnetic tunnel junction sensor 740 for detecting the state of the magnetic tunnel junction 730.

The magnetic transducer 710 must be sensitive enough to generate a magnetic field at the frequency of the information transfer signal. Magnetic transducer 710 may be implemented with, for example, an inductive coil that induces a magnetic field proportional to the current conducted by the inductive coil.

The magnetic tunnel junction 730 is tuned to contain a frequency pass band that coincides with the carrier frequency of the information transfer signal.

The magnetic tunnel junction sensor 740 must be sensitive enough to detect a change in the state of the magnetic tunnel junction 730 at the desired filter passband frequency.

Fig. 8 shows an emitter comb filter 800 according to an embodiment of the invention. The transmitter comb filter 800 includes an adder 810 for adding the plurality of carrier signal sources S1, S2, S3 to generate a plurality of transmission signals. Each transmission signal has a unique carrier frequency and includes transmission information. Magnetic transducer 820 generates a magnetic field 825 in response to a plurality of transmission signals. The transmitter comb filter 800 includes a magnetic tunnel junction 830. The magnetic tunnel junction 830 can be tuned to switch states in response to a selected frequency of the magnetic field 825. The transmitter comb filter 800 includes a magnetic tunnel junction sensor 840 for detecting the state of the magnetic tunnel junction 830.

Fig. 9 illustrates a receiver comb filter 900 according to an embodiment of the invention. The receiver comb filter 900 includes a receiving unit 910 for receiving a plurality of transmission signals. Each transmission signal has a unique frequency and includes transmission information. The receiver may include an optical transducer for converting an optical signal that has passed through the optical medium. The receiver comb filter 900 includes a magnetic transducer 920 for generating a magnetic field 925 in response to a plurality of transmit signals. The receiver comb filter 900 includes a magnetic tunnel junction 930. The magnetic tunnel junction 930 may be tuned to switch states in response to a selected frequency of the magnetic field 925. The receiver comb filter 900 includes a magnetic tunnel junction sensor 940 for detecting a magnetic tunnel junction state.

Fig. 10 shows an ideal frequency response of a magnetic comb filter according to an embodiment of the invention. The frequency response may pass certain frequency components through a comb filter while filtering or attenuating other frequency components. More precisely, the frequency response comprises frequency passbands 1010, 1020, 1030, 1040. The present invention provides selective filtering of electromagnetic signals. As previously mentioned, the filter passband provided by the present invention may be adjusted. In addition, the passband can be actively tuned in real time by applying an additional magnetic field to the magnetic tunnel junction of the present invention. Typically, the additional magnetic field is applied in a direction orthogonal to the magnetic field generated by the applied signal to be filtered.

As previously mentioned, the selective time/frequency switching characteristics of the magnetic tunnel junction may be determined experimentally or computationally. The magnetic tunnel junction can be tuned to selectively pass signals containing a particular frequency.

There are many applications that can utilize the selective signal frequency passband of the present invention. The description of signal transmitters and signal receivers provided herein that may utilize the frequency comb filter of the present invention is but a few examples of the useful applications of the present invention. The invention can also be used in many different applications where high frequency selective filtering is required.

FIG. 11 illustrates a flowchart including operations according to embodiments of the present invention. These operations provide a method of filtering a plurality of transmission signals having respective frequencies.

A first operation 1110 includes accumulating the plurality of information delivery signals.

A second operation 1120 includes generating a magnetic field in response to the plurality of information conveying signals received by the accumulator.

A third operation 1130 includes filtering the plurality of information transfer signals with a magnetic tunnel junction tuned to switch states in response to a selected frequency of a magnetic field.

A fourth operation 1140 includes detecting a state of the magnetic tunnel junction.

Although specific embodiments of the invention have been illustrated and described, the invention is not to be limited to the specific forms or arrangements of parts so illustrated and described. The invention is limited only by the appended claims.

Claims (10)

1. A magnetic filter (700) comprising

A magnetic transducer (710) for generating a magnetic field (720) in response to a plurality of information-conveying signals;

a magnetic tunnel junction (730), said magnetic tunnel junction (730) tuned to switch states in response to a selected frequency of said magnetic field (720); and

a magnetic tunnel junction sensor (740) for a state of the magnetic tunnel junction (730).

2. The magnetic filter of claim 1, wherein: the plurality of information signals are received by a multi-frequency signal accumulator (810) that drives the magnetic transducer (710).

3. The magnetic filter of claim 1, wherein: the plurality of information signals are received by a receiver (910) that receives the plurality of information signals after the signals pass through a transmission medium.

4. The magnetic filter of claim 1, wherein: the tuning of the magnetic tunnel junction (730) may be adjusted.

5. The magnetic filter of claim 4, wherein: tuning of the magnetic tunnel junction (730) may be adjusted by appropriate selection of the material of the magnetic tunnel junction (730).

6. The magnetic filter of claim 4, wherein: tuning of the magnetic tunnel junction (730) may be adjusted by appropriate selection of the physical dimensions of the magnetic tunnel junction (730).

7. The magnetic filter of claim 4, wherein: the tuning of the magnetic tunnel junction (730) may be additionally adjusted by applying different magnetic field biases to the magnetic tunnel junction (730).

8. The magnetic filter of claim 1, wherein: the magnetic tunnel junction sensor (740) is sufficiently sensitive to detect a state of the magnetic tunnel junction (730) at a rate at which the magnetic tunnel junction (730) changes state.

9. The magnetic filter of claim 1, wherein: the magnetic transducer (710) is sufficiently sensitive to generate a magnetic field at a rate at which the magnetic tunnel junction (730) changes state to detect the state of the magnetic tunnel junction (730).

10. The magnetic filter of claim 1, wherein: the magnetic transducer includes an inductive coil.