All references, including publications, patent applications and patents cited herein are hereby incorporated by reference in their entireties to the extent allowable to the same extent as if each reference was individually and specifically indicate to be incorporated by reference and was set forth in its entirety.
Modem ultra wideband communication systems and radars, and metrology systems all need configurable subsystems such as tunable bandpass filters that are compact, lightweight, and power efficient. At the same time, isolators with a large bandwidth are widely used in communication systems for enhancing the isolation between the sensitive receiver and power transmitter. If a new class of non-reciprocal RF devices that combines the performance of a tunable bandpass filter and an ultra-wideband isolator is made available, new RF system designs can be enabled which lead to compact and low-cost reconfigurable RF communication systems with significantly enhanced isolation between the transmitter and receiver.
Another issue for magnetostatic surface wave (MSSW)-based YIG resonator devices is the unwanted reflected waves from the straight edges of the YIG slab, which will induce spurious resonance due to the standing wave modes, formed from the forward and backward wave. Several kinds of techniques have been reported to suppress the unwanted reflection by depositing a resistive absorbing film or attaching an additional ferrite material on to the edges of the YIG films to absorb the MSW, or using tapered YIG slab edges at an angle (≠90°), or local low bias field at the edge of the film. These approaches, however, need extra effort to implement.
Conventionally, YIG MSW filters based on single resonance modes have a relatively low power handling, typically below 0 dBm, due to the narrow spin wave linewidth of a single resonance mode. Increasing the power handling capability has been an open challenge for such YIG devices.
A new type of non-reciprocal C-band magnetic tunable bandpass filter with ultra-wideband isolation is presented. The bandpass filter was designed with a 45°-rotated yttrium iron garnet (YIG) slab loaded on an inverted-L shaped microstrip transducer pair. This filter shows an insertion loss of 1.6-2.3 dB and an ultra-wideband isolation of more than 20 dB, which was attributed to the magnetostatic surface wave. The non-reciprocal C-band magnetic tunable bandpass filter with ultra-wideband isolation with dual functionality of a tunable bandpass filter and an ultra-wideband isolator will have many applications in RF frontend and other microwave circuits.
In one aspect, a nonreciprocal tunable bandpass filter includes a transducer comprising parallel coupled conductive lines; and a ferrite body having at least two opposing parallel edges, the ferrite body disposed over the microstrip transducer such that the parallel edges of the ferrite layer are tilted at a non-zero angle θ with respect to the parallel coupled microstrip lines of the microstrip transducer.
In one or more embodiments, the transducer comprises microstrip lines.
In any of the preceding embodiments, the microstrip transducer comprises an inverted-L shaped microstrip transducer pair.
In any of the preceding embodiments, the angle θ is in the range of 15°-75°, or. the angle θ is in the range of 30°-60°, or. the angle θ is in the range of 40°-50°.
In any of the preceding embodiments, the ferrite material comprises a ferrite material with ferromagnetic resonance linewidth of <200˜300 Oe at X-band.
In any of the preceding embodiments, the ferrite material is selected from yttrium iron garnet (YIG), spinel ferrites such as Ni-ferrite, NiZn-ferrites, MnZn-ferrites, Li-ferrite, hexaferrites.
In any of the preceding embodiments, the ferrite body comprises yttrium iron garnet (YIG).
In any of the preceding embodiments, the ferrite body has shape selected from the group consisting of square, rectangular, hexagonal, octagonal, trapezoidal and parallelapedal.
In any of the preceding embodiments, the bandpass filter has an isolation of greater than 10 dB.
In any of the preceding embodiments, the bandpass filter has an isolation of greater than 15 dB.
In any of the preceding embodiments, the bandpass filter acts as a ultra-wideband isolator with more than 20-dB isolation at the passband with insertion loss of 1.6-3 dB.
In any of the preceding embodiments, wherein the bandpass filter further includes an electric current source disposed proximate to the ferrite body.
In another aspect a microwave circuit include the nonreciprocal tunable bandpass filter of any of the preceding embodiments.
In another aspect, a method of filtering a signal includes providing a bandpass filter according to of the preceding embodiments; applying a signal as an input signal to the tunable bandpass filter; and controlling a bandwidth of the signal as a function of an applied magnetic field.
In another aspect, a method of producing a signal includes providing a bandpass filter according to any preceding embodiment; applying a signal as an input signal to the tunable bandpass filter; and subjecting the bandpass filter to an external electric field to permit the signal to propagate in only one direction.
The non-reciprocal propagation performance of magnetostatic surface waves in microwave ferrites such as yttrium iron garnet (YIG) provides the possibility of realizing such a non-reciprocal device. Planar ferrite structures with straight edges have been applied in filters utilizing magnetostatic wave theory (MSW). A bandpass filter using two microstrip line antennas was prepared by exciting the magnetostatic surface waves (MSSW) which can be tuned by electric field.
A new method of suppressing the spurious resonance is proposed. The YIG slab is rotated by a proper angle to diminish standing wave modes in order to get a much smoother pass band, and achieve a tunable nonreciprocal bandpass behavior. The designed C-band tunable bandpass filters show a central frequency shift from 5.2 GHz to 7.5 GHz under in-plane magnetic fields from 1.1 kOe to 1.9 kOe with an insertion loss<3 dB. The oblique angle between the DC bias field and the propagation direction leads to non-reciprocal transmission characteristics of the forward and backward MSSW, which provide more than 20 dB isolation across all measured frequency ranges.
Advantages of the system according to one or more embodiments include:
The invention is described with reference to the following figures, which are presented for the purpose of illustration only and are not intended to be limiting. In the Drawings:
A nonreciprocal tunable bandpass filter having wideband isolation is described. The bandpass filter is a frequency selective filter circuit used in electronic systems to separate a signal at one particular frequency, or a range of signals that lie within a certain “band” of frequencies from signals at all other frequencies. This band or range of frequencies is set between two cut-off or corner frequency points labeled the “lower frequency” (fL) and the “higher frequency” (fH) while attenuating any signals outside of these two points. A nonreciprocal bandpass filter is one that only allows electromagnetic waves (signals) to flow in one direction. In one or more embodiments, the device includes a microstrip transducer and a ferrite body having at least two opposing parallel edges. The ferrite body is disposed on the microstrip transducer such that the parallel edges of the ferrite body are tilted out of alignment with respect to the parallel coupled microstrip lines of the microstrip transducer.
In conventional bandpass filters, the ferrite body 140 is arranged so that the edges of the ferrite body are parallel to the microstrips, as shown in
Conventional bandpass filters such as illustrated in
According to one or more embodiments, a nonreciprocal tunable bandpass filter is achieved by positioning the ferrite body at an angle with respect to the longitudinal direction defined by the parallel coupled microstrip lines. In one or more embodiments, θ can range from 15° to 75°, or θ can range from 30° to 60°, or θ can range from 35° to 55°, or θ can be about 45°.
Microstrip lines are known in the art and the materials and circuitry used for their manufacture and use will be readily apparent to one of skill in the art. Exemplary materials for the microstrip lines includes copper; dielectric substrates commonly used in microelectronics can also be employed in the preparation of the microstriplines. The ferrite body or slab can be any of a number of ferrite materials used in the preparation of magnetically tunable bandpass filters. In certain embodiments, the ferrite material can be a low-loss RF/microwave ferrite material with a relatively low ferromagnetic resonance linewidth of <200˜300 Oe at X-band. Suitable ferrite materials include yttrium iron garnet (YIG), spinel ferrites such as Ni-ferrite, NiZn-ferrites, MnZn-ferrites, Li-ferrite, hexaferrites, etc. In one or more embodiments, the ferrite body has a thickness of greater than 10 μm, or a thickness in the range of 75 μm to several millimeters. In one or more embodiments, the ferrite body is about 100 μm in thickness. The ferrite body can be of any dimension (length, width) or aspect ratio. Thus, the ferrite body can be square, rectangular, hexagonal, octagonal, trapezoidal or a parallelapedal, etc.
To diminish the splitting modes and achieve the nonreciprocity characteristics, the ferrite slab can be rotated, e.g., by 45°, as is shown in
In one or more embodiments, θ can range from 15° to 75°, or θ can range from 30° to 60°, or θ can range from 35° to 55°, or θ can be about 45°. A range of angles can be acceptable, in particular, because small variations in ferrite slab properties will occur along its length. Thus, tilt angles that are bracketed around an ideal 45 degree tilt are suitable and can provide a population of Magneto-Static Backward Volume Waves (MSBVW) that will propagate in the direction of the bias field.
In operation, a d/c current can be applied, e.g., using a current carrying wire near the ferrite body or using a winding which encircles the parallel coupled microstrip lines and the ferrite body, to the bandpass filter to produce a magnetic biasing field H (indicated by arrow H) in
The dissipation of the MSBVW energy provides bandpass filters with exceptional wideband isolation capabilities. The reflected waves are essentially dissipated, meaning that there is no reflected energy in the system. Antennae are typically capable of both transmitting and receiving signals. However, a bandpass filter according to one or more embodiments can possess ultra-wide band isolation that permits only transmission or receiving. The antenna operates in essentially a single direction, e.g., either as a transmitter or a receiver. Ultra-wide band isolation of more than 20 dB can be achieved.
In one embodiment, a bandpass filter was designed with a 45° rotated yttrium iron garnet (YIG) slab loaded on an inverted-L-shaped microstrip transducer pair. With external in-plane magnetic fields from 1.1 to 1.9 kOe, the central frequency of the filter was tuned from 5.2 to 7.5 GHz, with an insertion loss of 1.6-3 dB and an ultra-wideband isolation of more than 20 dB, which was attributed to the nonreciprocity characteristics of the magnetostatic surface wave. In addition, the measured result demonstrated power-handling capabilities of over 30 dBm under room temperature.
The design parameters and performance of a two port nonreciprocal MSSW filter are provided. Relevant parameters include geometrical parameters (slab length L and width W, thickness d, rotation angle θ, and overlap length of the transducer L′), magnetic parameters (external bias magnetic field (H0), ferrite-film saturation magnetization (4πMs), FMR linewidth (ΔH0), and resonator spin-wave linewidth (ΔHk)) and filter performance parameters (nonreciprocity, group delay τg, and 3-dB bandwidth f3 dB).
1) Ferrite Slab width W: For the parallel aligned YIG case, the width W will determine the resonance frequency of the standing-wave modes (n=1, 2, 3 . . . ). Moreover, because the separation between the main resonance and the finite length modes (m=1, 2, 3 . . . ) is inversely proportional to W, the interference of width modes with the main resonance can be minimized by choosing the parameter W to be as small as possible. After rotating the YIG film, the standing-wave modes are eliminated, the slab width W will affect the propagation loss rather than the resonant frequency.
2) Slab length L: the parameter L determines the wavelength of the finite-length mode resonances. From the finite-length mode dispersion relation calculations (m=1, 2, 3 . . . ), reducing the value of L will increase the frequency separation between the resonances and result in better rejection of the (1, 2) resonance with respect to the main resonance (1, 1). However, as L decreases, the power-handling capability of this filter will decrease, due to the decrease of volume of the device.
3) Overlap length of the transducer L′: this parameter determines the coupling between transducers and the YIG slab, which affect the input and output reflection coefficients. According to (9) and
5) Thickness d: a thicker YIG slab leads to wider 3-dB bandwidth. At the same time, since the power compression level of the resonator is proportional to its volume, for a given dimension of L and W, the thicker the YIG is, the better the power-handling ability of the filter will be.
1) External bias magnetic field (H0): the orientation of the bias magnetic field determines the FMR frequency of MSSW filters, as well as the operating frequency.
2) YIG-film saturation magnetization (4πMs): the orientation of the bias magnetic field determines the FMR frequency through the permeability tensor.
3) Resonator spin-wave linewidth (Hk): this parameter is defined as (Hk=f3 dB/γ): f3 dB is the half-power bandwidth of the resonator. The power-handling capability of MSSW filters is proportional to bandwidth.
1) Nonreciprocity of the bandpass filters is determined by rotation angle θ. A 45 degree rotation angle leads to minimum insertion loss and maximum isolation.
2) Group delay τg: group delay is the rate of change of phase response with frequency. It can be estimated by in by τg=ΔΦ/Δω in both HFSS simulation and VNA measurement. Also, analytically, group delay can also be derived via the dispersion relation, as τg=W/vg=W/(dk/dw), where W is the length along the propagation path. The group delay of the nonreciprocal filter was analyzed with rotated aligned YIG thicknesses d=108 μm, under bias magnetic field 1.6 kOe. From
3) Bandwidth: the 3-dB bandwidth of the bandpass filter can attribute to both propagation losses (PL) and transduction losses (TL).
Aspects of the invention is illustrated in the analysis that follows, which is presented for the purpose of analysis only and is not intended to be limiting of the invention. From these parameter analyses, one can select the desired specification for filter designs.
MSW can be excited in a YIG slab loaded on an inverted-L shaped microstrip transducer pair as shown in
where ωm=−γ4πMs, ω0=−γH0 is the dc bias field, and w is the angular frequency.
With the magnetostatic approximation, the wave propagation in an infinite YIG slab follows the Walker's equation
(1+χ)(kx2+ky2)+kz2=0, (2)
Supposing that the YIG slab was infinite size and ignoring in-plane boundary conditions, the dispersion relation of MSW was calculated and plotted as shown in
where k is the wave number along the x-axis and d is the thickness of the slab.
Under the bias condition {right arrow over (k)}∥{right arrow over (Hdc)}, magnetostatic back volume wave (MSBVW) will be excited inside the YIG slab. The magnetic potential has a sinusoidal distribution. The back volume wave consists of multimodes with the same cutoff frequencies given by
For practical filter designs, the MSBVW will suffer from ripples due to the multiresonance modes, while MSSW usually has a better resolution due to its single resonance.
E. Nonreciprocity in Ferrite Slab with Finite Size
When the YIG slab is placed parallel to the transducers and the bias magnetic field, {right arrow over (Hdc)}, is as shown in
(kx++kx−)W=2πn, n=1,2,3 . . . (5)
where k+ and k− are the wave numbers for forward and backward propagation in the YIG slab, respectively, and is the distance between the two edges of the YIG slab. In addition, the finite length of the films generates additional modes
The dispersion relation of MSSW propagating in a finite YIG slab can be expressed as
where t is the thickness of the substrate and k=kx indicates the wave number of MSSW. The dispersion relation was plotted in
Experimentally, it is easy to excite the MSSW by placing a current carrying wire near a YIG slab. Most commonly, microstrip structures with short pins to the ground plane at the end of the strip line are utilized to achieve the excitation. Parallel microstrip have been used as the transducers. A T-shaped microstrip coupling structure and YIG films can also be used to achieve a low-loss C-band tunable bandpass filter. An L-shaped microstrip transducer was observed to enhance the coupling to a minimum insertion loss of 5 dB. In order to improve the insertion loss and isolation and achieve the nonreciprocal behavior at the same time, an inverted-L-shaped transducer can be used, as shown in
Usually, the coupling between the current flowing on the microstrip transducer and the MSSW propagating in the ferrite slab can be modeled as an equivalent lossy transmission line. As the incident wave propagates along the transducer, energy is lost to the MSSW excitation. The radiation resistance per unit length for surface waves traveling in the v (±1) direction (±{circumflex over (x)}) can be written as
where F indicates array factor for the current flowing on microstrip transducer with F=Ie−ksJ0(kw/2), k is the MSSW wave number, w is the width of the transducer, J0 is the Bessel function of zeroth-order, and s is the vertical spacing between the transducer and YIG/air interface. Here, it is 40 μm for bottom surface of YIG and 148 μm for the top surface.
With an open end, the current distributes nonuniformly across the inverted-L-shaped transducer. The total radiation resistance can then be estimated as
where I=I0 sin(βy) indicates the current on the transducer, β=ω√∈μ0 considers ∈r=9.8 of the substrate, 0<y<L is the distance from the open end, and L is the overlap length of YIG slab and transducer.
At 6.7 GHz, Rvx+=57.5Ω and Rvx−=0.5Ω. Therefore, the coupling to the top surface can be neglected, while the bottom coupling dominates the MSSW propagation in the YIG slab. If we suppose the feeding transducer is ideally 50Ω at 6.7 GHz, the transduction loss due to impedance mismatch on the bottom surface (forward transmission) can be approximated as 2·TL=0.04 dB, including the transmit and receive transducer. Here,
TL=−10*log(1−((Rvx−50)/(Rvx+50))2).
If the YIG is aligned parallel to the transducer, the reflection from the edges generate surface wave on the top surface, which leads to reciprocal performance and splitting resonance modes. On the other hand, when rotated YIG is applied, the surface wave is limited on the bottom surface due to the nonreflection edges. Nonreciprocity and nonsplitting characteristics can be achieved.
At a given frequency, the propagation loss of MSSW can be approximated as
where ΔH is the FMR linewidth of YIG in Oe and τg is the group delay in the YIG slab, defined as dω/dk. The propagation loss under a bias field of 1.6 kOe was calculated as plotted in
The proposed transducers were simulated with Ansoft High Frequency Structure Simulator (HFSS) 12.1 and then fabricated and measured via a vector network analyzer (Agilent PNA E8364A). The input power for the measurement is −12 dBm.
When the YIG resonator was rotated 45° around its center, S12 and S21 showed nonreciprocal transmission behavior. Also, the passband becomes much smoother due to the suppression of the reflections from the edges. The insertion loss of forward transmission is about 1.65 dB at 6.7 GHz, with a bandwidth of 220 MHz (3.2%), while the reverse transmission S12 has isolation greater than 22 dB, as was shown in
The bandpass filter with 45° rotated YIG resonator was also measured from 5.3 to 7.5 GHz under a dc magnetic field of 1.1-1.9 kOe, as shown in
S11 and S22 are also plotted in
Another possible reason for higher insertion loss at lower frequencies is the impedance mismatch. At 5.2 GHz (1.1-kOe bias), the return loss is 8.8 dB, and is 7.23 dB while both are over 25 dB at 6.7 GHz. Further optimization on the transducer design may help improve the impedance matching at any specific operating frequencies in a practical application.
In addition, to achieve higher power-handling capability, the bandpass filters with a 500-μm-thick rotated YIG slab was also presented. The measured transmission coefficient was shown in
The total insertion loss of the bandpass filter's pass band can be estimated as
IL=2·TL+PL+CL+DL+Other Loss (dB) (11)
where PL and TL can be calculated with (8)-(10), in terms of various bias magnetic fields and central resonant frequencies, as shown in
High-power measurements of the bandpass filter were then carried out to investigate the power-handling capability of the nonreciprocal bandpass filters. The schematic of the measurement setup is shown in
The nonreciprocal bandpass filters were then tested with varied bias magnetic fields from 1.1 to 1.9 kOe, which tuned the resonance frequency of the bandpass filter from 5.2 to 7.5 GHz. In order to compare the 1-dB compression point IP1 dB under different bias fields and investigate the maximum bandpass filters tuning range with high power handling, the output transmitted powers were normalized with the input power and the insertion loss of the filters at 0-dBm input power.
Since the power compression level of resonators is proportional to its volume, for a given YIG dimension of L and W, better power-handling capability of the resonator was expected for the bandpass filter with a 500-μm-thick YIG slab. Table III shows a comparison between these two filters with different resonator thickness. At the optimal tuning region, the IP1 dB are all greater than 30 dBm for the thicker YIG slab.
From
There are at least two factors that contribute to these nonlinearity performances: coincidence-limiting effect of ferrite and the premature saturation. The coincidence-limiting effect is related to a subsidiary absorption from coupling between the uniform precession mode and the spin waves with half of the frequency of this mode. The absorption happens below ωM, 4.9 GHz for YIG slabs, where the MSSW devices saturated at a low power level (typically<0 dBm). These effects contribute to the downgrade of IP1 dB at the lower edge of the bandpass filters tuning range. For the MSSW bandpass filters, the closer the operating frequency to, the lower power handling they can achieve. A typical solution for devices intended to operate below 4.9 GHz is to use doped YIG or other ferrites, whose saturation magnetization are smaller than 1750 Gauss.
The premature saturation is related to the instability of susceptibility that arises from nonlinear terms proportional to the exchange and anisotropy energies. The susceptibility first increases and then sharply drops. When the RF power increases to greater than the threshold, the critical field of this threshold power can be estimated as
where ΔH0 is the FMR linewidth of YIG, ΔHk is the spin wave linewidth, defined as ΥΔHk=∂−3 dB, where the −3-dB bandwidth of the resonance with γ is the gyromagnetic coefficient, and ω0 is the FMR frequency related to the external bias field. Therefore, the power-handling capability of MSSW filters is proportional to the bandwidth while inversely proportional to the operating frequency or bias field. This effect contributes to the downgrade of IP1 dB at the upper edge of the tuning range of the bandpass filters. According to (12), the critical field hcritical decreased when the FMR frequencies increased, which led to a lower cutoff input power.
The bandwidth for bandpass filters with 108-μm YIG resonator is around 240 MHz while is around 300 MHz for 500-μm YIG at bias field 1.9 kOe, as shown in
Conventionally, YIG-based bandpass filters are based on its uniform resonance mode, i.e., FMR. Others have reported a MSW filter has 0-dBm power handling with 15-MHz bandwidth at 9 GHz, and that typical MSW filters with 0.2%-0.5% bandwidth can achieve 50-mW power handling (17 dBm). The bandwidth of these filters described herein are around 300 MHz at 6.7 GHz (4.48%), which leads to roughly 10-13 dB increase on IP1 dB. Although the quality factor is lower than other filters based on single resonance modes, the MSSW filter designed based on this concept can achieve a much higher power-handling capability.
Self-heating will be expected during the MSSW propagation and absorption. In real applications, an additional packing technique, e.g., heat sinks, may be applied to dissipate the heat effectively.
A nonreciprocal-band magnetic tunable bandpass filter with a YIG slab has been designed, fabricated, and tested, which is based on an inverted-L-shaped coupling structure loaded with a rotated single-crystal YIG slab. MSSW propagation in the rotated YIG leads to nonreciprocal performance. The tunable resonant frequency of 5.2-7.5 GHz was obtained for the bandpass filter with a magnetic bias field of 1.1-1.9 kOe applied perpendicular to the feed line. At the same time, the bandpass filter acts as a ultra-wideband isolator with more than 20-dB isolation at the passband with insertion loss of 1.6-3 dB. Power-handling capability of over 30 dBm has been demonstrated under room temperature in the filter's tuning range. The demonstrated nonreciprocal magnetically tunable bandpass filters with isolator dual functionality and with high power handling find use in C-band RF front-end and other microwave circuits.
It will be appreciated that while a particular sequence of steps has been shown and described for purposes of explanation, the sequence may be varied in certain respects, or the steps may be combined, while still obtaining the desired configuration. Additionally, modifications to the disclosed embodiment and the invention as claimed are possible and within the scope of this disclosed invention. Further information regarding the invention is found in Wu et al. “Nonreciprocal Tunable Low-Loss Bandpass Filters With Ultra-Wideband Isolation Based on Magnetostatic Surface Wave”, IEEE Trans. Microwave Theory Tech. Vol. 60, No. 12, pp. 3959-3967, December 2012, the content of which are incorporated in its entirety by reference.
This application claims the benefit of priority under 35 U.S.C. §119(e) to copending U.S. Patent Application Ser. No. 61/706,190, filed Sep. 27, 2012, which is incorporated by reference herein in its entirety.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/US13/62438 | 9/27/2013 | WO | 00 |
Number | Date | Country | |
---|---|---|---|
61706190 | Sep 2012 | US |