zyxwv zyx zyxw zyxwv zyxwvu 3248 IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 5, SEPTEMBER 1989 MAGNETICALLY TUNEABLE OSCILLATORS AND FILTERS H. Tanbakuchj.*, D. Nicholson** , B. Kunz***, W. Ishak*** '*Hewlett-Packard Co., Rohnert Park, CA **Hewlett-Packard Co., Santa Rosa, CA ***Hewlett-Packard Labs, Palo Alto, CA Abstract INPUT Magnetically tuneable oscillators and filters can be made using a number of ferrite materials and varied geometries of the ferrites for the magnetically tuneable elements. This paper will survey current trends and state of the art results for oscillators and filters using: 1. YIG spheres, 2 . YIG films, and 3 . Hexagonal ferrites as the magnetically tuneable elements. * 2 . Y I G SPHERE I Devices with YIG Sphere Tunins Elements YIG tuned filters (YTFs) are capable of covering a broad frequency bandwidth (up to one decade) with excellent power handling capability and tuning linearity. Their performance characteristics are in sharp contrast to the narrow bandwidth, poor power handling capability, and nonlinear tuning of the less expensive varactor tuned filters. Because of these performance advantages, YTFs (both bandpass and notch) are widely used from 5 0 0 MHz to 2 6 . 5 GHz. Fig. 1 shows a schematic diagram of a YIG-tuned filter. It consists of two orthogonal coupling loops [l] with a small YIG sphere centered on the intersection of the loop axes. When the YIG sphere is not magnetized, RF power is not transferred between the loops because there is no interaction between the RF signal and the ferrite (YIG) sphere, and the loops are perpendicular to each other. In the presence of an externally applied DC magnetic field (Ho) in the z-direction, the magnetic dipoles in the YIG sphere align with the DC magnetic field to produce a net magnetization (M) in the sphere. If an RF driving current (say, i exp[jwt]) is applied at the input (loop in the x-z plane), it produces an RF magnetic field along the y-axis, thereby causing the magnetic dipoles in the ferrite to precess around the applied DC magnetic field. The precession frequency is equal to the frequency of the RF input signal, provided it is at or very close to the dipole resonance frequency, Fig. 1 YIG Sphere Tuned Filter in the other direction by 1 8 0 deg. The filtering function is achieved because RF signals deviating from the dipole resonant frequency by more than a small amount do not couple to the YIG sphere. Typical loaded Q-values for YIG-tuned filters range from 1 0 0 to 4 0 0 . YIG tuned filters are extensively used in microwave communication instrumentations. One example of this is a fully integrated front end [ 2 ] for high performance spectrum analyzer (Fig. 2). It contains a three sphere, YIG tuned preselector in which the input sphere, combined with a pin diode MIC, replaces a slow, mechanical relay switch, The third sphere, in conjunction with a GaAs monolithic diode IC, functions as a balanced, fundamental mixer with low conversion loss and high third-order intercept. The LO multiplier converts a 3 . 0 - 6 . 7 GHz input LO to a 3 - 2 2 . 3 GHz output LO. This output is the lrfundamental"signal supplied to the mixer. A fourth YIG sphere is tuned to the LO frequency using the "offset coil". The sphere acts as a discriminator, generating an TO RF FIRST CONVERTOR where Ho is the field strength (in oersteds) of the applied DC field, Ha is the internal anisotropy field (in oersteds) in the YIG, and is the gyromagnetic ratio with the well-known value of 2.8 MHz/oersted. The precessing dipoles create a circularly polarized magnetic field rotating at the RF frequency that couples to the output loop (in the y-z plane) with a 9 0 deg. phase shift. The circuit therefore acts as a gyrator the phase shift in one direction through the YIG-tuned filter differs from the phase shift 321.4 M H IF ~ , , OFFSE1 DC-22 GHz GHz LO LO MULTIPLIER zyxwvuts Fig. 2 Switched Tracking Preselector Mixer 0018-9464/89/0900-3248$01.0001989 IEEE - 3249 zy error voltage signal which is fed back to the magnetic tuning circuitry in order to frequency lock the preselector. This eliminates swept amplitude inaccuracies in the filter caused by the nonlinearity, hystersis, and eddy current delay in the magnetic tuning elements. Fig. 3 shows the center puck of this integrated component. zyxwvutsrq zyxw zyxwvut zyxwvuts zyxwvutsr / & YIG SPHERE YIG Sphere Tuned Oscillators (YTOs) Fig. 4 YIG sphere tuned oscillators (YTOs) are important components of modern communication systems. They are used in many applications that require broad frequency coverage and low noise, such as high performance local oscillators and synthesizers. YIG resonators used in YTOs have excellent linearity and can tune over one decade of bandwidth, in addition to exhibiting high unloaded Q t s (1000 to 5 0 0 0 ) that help to minimize phase noise. T, s,; Fig. 4 shows the equivalent circuit of a spherical YIG resonator with coupling loop, Lo, Ro, and CO can be calculated from well established formulas [ 3 ] . Lc is the inductance of the coupling loop. Currently the most common YTO topology is the negative resistance reflection oscillator. It consists of a YIG resonator connected to an active device, usually a silicon bipolar transistor or a GaAs FET. Negative resistance is produced by the active device through positive feedback. Fig. 5 illustrates this general topology. For oscillation to start, the product of the reflection coefficient of the resonator TL and the reflection coefficient of the input port S11' must be greater than or equal to one. In equation form, (3) Equivalent Circuit for YIG in LOOP 2, 2 1 Fig. 5 General YTO Topology after oscillation has stabilized, the magnitude of TL becomes equal to the magnitude of S11' while the phase of TL (OL) becomes opposite to the phase of Silt (Os). In equation form, zyxwvu (4) eL = es (5) The common-base configuration for silicon bipolars and common-gate configuration for GaAs FETs are the most common topologies for generating negative admittance across the YIG resonator. These configurations are shown in Figures 6a and 6b, respectively. " - 4 b b. COMMON BASE COMMON GATE Fig. Fig. 3 Center Puck A 6 a. 6 Sample YTO Topologies A z 3250 zyxwvutsrqponm zyx zyxwvutsrqp zyxwvutsr Other topologies have been reported. Fig. 6c shows the flsource capacitor" feedback topology [4]. The MESFET transforms the reactive source termination into a capacitive reactance and negative resistance at the gate. Typically a negative resistance is also presented at the drain. Figure 6d shows the topology of oscillators built by Letron et a1 [5] utilizing two coupling loops with separate spheres. The resonant frequencies were slightly offset to realize a capacitive termination at the source and the appropriate inductive impedance for oscillation at the gate. A 3.5 - 14 GHz oscillator was also constructed by coupling two loops to the same sphere as shown in fig. 6e.[6!. Schiebold extended these works by building a 3.5 - 19.5 GHz oscillator also with two loops coupled to the same sphere [7]. The terminating impedances and sphere feedback were optimized along with interloop capacitive coupling and angular spacing. Since the resonator is doubly loaded, phase noise performance is compromised. Although bipolar YTOs have 10 to 15 dB lower phase noise than comparable GaAs FET YTOs, bipolar YTOs have largely been limited to low frequency operation (below 10 GHz) due to the lower fT of bipolar transistors. With the availability of submicron bipolar transistors from NEC (ie NE64700 and NE64800), however, broadband 3-18 GHz bipolar based YTOs have become feasible. I of the transducer width and YIG film width wideband filters, tuning from .3 to 12 GHz have been built with insertion loss of 1634 dB and off resonance isolation >45 dB. 'a6 d. C. e. SOURCE FEEDBACK TOPOLOGIES Fig. 6 Sample YTO Topologies zyxwvuts BIAS FIELD ORlENTATlQN @ Above 18 GHz, YTOs still have to rely on GaAs FETS or GUNN diodes as active devices. Both devices work well up to 40 GHz with GaAs FETs exhibiting better DC to RF conversion efficiency than Gunn diodes. DISPERSION DIAGRAM Msw"p k YIG Film Tuned Devices Thin films of YIG grown by liquid phase epitaxy (LPE) on gadolinium gallium garnet (GGG) substrates are used for nearly all MSW devices presently reported. Three pure MSW modes exist [8-101 depending on the orientation of bias magnetic field relative to the YIG film and the propagation direction. These modes are: magnetostatic surface waves (MSSWs), magnetostatic forward volume waves (MSFVWs) and magnetostatic backward volume (MSBVWs) with frequency limits shown in Fig. 7. All three modes are dispersive, with the dispersion modified by the boundary conditions [11, 121. coupling of microwave circuits to the spin waves in an MSW device is commonly done with short circuited or open circuited microstrip transducers in meander line or grating configuration. The entire MSSW, MSFVW or MSBVW frequency band can be excited by using microstrip transducers as narrow as 10 um. Fig. 7 MSW Filters Filters can be built using MSW devices in either a delay line or a resonator configuration. Narrowband filters built utilizing MSW delay lines have been demonstrated in the 3-7 GHz range with low passband ripple as shown in Fig. 8. The bandwidth of this filter is -30-50 MHz. By careful adjustment zyxwv START STOP Fig. MSW Dispersion 8 3 000000800 GHz 7 0000140W00 GI4z MSW Narrowband Filter 325 1 MSW straight edge resonators (SERs) can be formed by placing a ferrimagnetic resonator cavity on a thin film transducer structure (Fig. 9). The resonant cavity is made of a piece of GGG/YIG cut into a rectangle by a wafer SAW. Short circuited microstrip transducers couple the energy in and out of the resonator, which can use MSSWs or MSFVWs. These waves propagate along the surface of the YIG film and are reflected back at the straight edges resulting in a high Q resonator. A MSSW SER was built which tuned from 1-22 GHz with the performance shown in Fig. 10. Thick YIG films were used to increase the power handling capability and the 1 dB power compression level was above 0 dBm for this device over most of the tuning range. One port SERs using MSSWs and MSFVWs suitable for tuneable oscillator applications have been reported by Kunz et a1 [14]. zy TOP VIEW zyxwvutsrqpo zyxwvutsrqpon zyxwvuts q SIDE VIEW YlOlGGG @J HYSSW GROUND PLANE ' Fig. 9 MSW Straight Edge Resonator Oscillators Two configurations for MSW oscillators have been built to date: the first uses a two port delay line or resonator in a feedback configuration while the second uses a one port resonator in a reflection configuration. The schematic for a two-port feedback oscillator using an MSW delay line is shown in Fig. 11. Oscillations occur at frequencies where the loop gain exceeds unity and the phase shifts around the loop are 2rn. The delay line oscillators exhibit good phase noise and tuning range (4-24 GHz) but have problems with multimoding and modehopping as well as oscillation dropouts over parts of the tuning range. The multimoding can be solved by changing coupling gratings but the oscillation dropouts are generic to the two port feedback configuration as the integer n in the 2an of phase shift around the loop increments to higher numbers with increasing frequency. If the MSW delayline is replaced with an MSW resonator in the feedback configuration you can get rid of the mode hopping problems and also use a smaller magnet. The oscillation dropout problem is not helped by using the resonator however. : am 4: 0 2 zy 4 6 zyxwvutsrq 8 10 12 14 16 18 20 FREQUENCY ( G k ) I 0, MAIN R W N A N C E - SPUmOUSYODE2 4 6 8 10 12 14 16 18 20 FREQUENCY ( G k ) Fig. 10 MSW SER Performance A one port oscillator can be constructed by replacing the sphere in Fig. 6a with a MSW-SER. Kunz et a1 have designed and tested several oscillators in the 1.4-4.0 and 3-9 GHz regions with bipolar transistors as the active device. Tuneable oscillations with no dropouts were achieved with typical phase noise of -105 dBc/Hz @ 10 KHz off the carrier. zyxwvutsrq YSW DELAY LINE DIRECTIDNAL AMPLIFIER Fig. 11 MSW Delay Line Oscillator 3252 zyxwvutsrqponm zyxwvuts zyxwv Hexasonal Ferrite Sphere Tuning MAGNET POLE k Although YIG can be tuned to an arbitrarily high frequency as long as a suitably strong magnetic field can be generated, alloys with low hysterisis and high permeability for magnets saturate at -15K gauss, and generating fields higher than this also begins to consume considerable power. From equation #1 it can be seen that if a ferrite material with a high Ha is available the applied magnetic field needed to tune to high frequencies could be greatly reduced. The hexagonal ferrites are a class of materials like this. There are many different phases and dopings available such that the anisotropy field for these materials can vary from -0 XOe up to >30K Oe [15]. Using hexagonal ferrites, researchers have made one, two, three and four sphere filters [16, 17, 181 as well as tuneable oscillators that operate in the millimeter wave region [19]. At this time all known devices using hexagonal ferrites use spheres, although there there have been several reports of thin film growth of hexagonal ferrites which could be used for MSW devices. zyxw I Fig. 12 :L, Two Sphere Hexagonal Ferrite Filter Hexasonal Ferrite Tuned Filters YIG sphere filters typically have coaxial inputs and outputs with loop coupling (Fig. 1) to the YIG spheres. Hexagonal ferrite bandpass filters have also been built with loop coupling, but the more common method is to use waveguide inputs, outputs and coupling to the spheres like the two sphere filter shown in Fig. 12. 0 40 30 20 10 50 Fig. 13 zyxw zyxwvut 60 70 80 90 100 x Of Band Two Sphere Filter O R 1 zyxwvutsrqp zyxwvutsrq The input and output waveguides are crossed at 90 deg. angles to create a magnetic field mode mismatch and increase off resonance isolation ( O R I ) . Two sphere hexagonal ferrite tuned filters have been built in waveguide bands [18] from A band, 26 1/2-40 GHz, to W band, 75-110 GHz, with typical insertion loss of -6 dB (7-8 dB for 75-110 GHz) and O R 1 as shown in Fig. 13. Magnets used to tune all of these filters were made out of 48% Ni, 51% Fe low hysterisis material. In order to increase the O R I , a four sphere filter was designed and built combining two, two sphere crossed waveguide filters under one magnet assembly. The O R 1 of these four sphere filters is typically >75 d% with typical insertion loss of -10 dB. The response of a four sphere filter displayed at 20 points across the band is shown in Fig. 14 with -6 dB I.L. 57STmr 75.0000.0000 50.00.O00000 Linz Linl Fig. 14 Four Sphere Filter Response Hexasonal Ferrite Tuned Oscillators Oscillators tuned by an hexagonal ferrite sphere were first reported by Lemke [19] with a GaAs Gunn Diode being tuned by a barium ferrite sphere from 62.5-65.7 GHz with -7 mW output power. By utilizing InP Gunn Diodes which can oscillate much more efficiently at mm waves, later researchers [20] produced an oscillator that used an hexagonal ferrite sphere to tune from 40-50 GHz with -10 mW output power as shown in Fig. 15. 2 8- I [COIL-lnP BARIUMI 4 ~ 0 InP (BARIUM FERRITE)' 33 34 ' 35 ' 36 ' I 37 38 _ _ _ - -L-= - , , 30 40 41 42 43 ' 44 45 " I 46 47 48 48 50 FREQUENCY (GHz) Fig. 15 Hexagonal Ferrite Tuned Oscillator Response zyxwvutsr zyxwvutsr zyxwvutsr 3253 Conclusion Magnetically tuneable filters and oscillators can be built throughout the 5 0 0 MHz 1 1 0 GHz range. They can be built with YIG spheres, YIG thin films or hexagonal ferrite spheres as the tuning element to cover broad bandwidths, and they fill a unique role in the world of microwave components. - References r11 P.S. Carter, Jr., "Equivalent Circuit of Orthogonal-Loop Coupled Magnetic Resonance Filters and Bandwidth Narrowing Due to Coupling Inductance*t,vol. MTT-18, no. 2 , pp. 1 0 0 - 1 0 5 , Feb. 1 9 7 0 . [21 H . Tanbakuchi, "A Broadband Tracking YIG [31 Tuned Mixer For a State of the Aqt Spectrum Analyzert1,17th European Microwave Conf., Sept. 1 9 8 7 , pp. 4 8 2 - 4 9 0 . P.M. Ollivier,llMicrowaveYIG-Tuned Transistor Oscillator Amplifier Design:Application to C Band", IEEE Journal of Solid State Circuits, vol. S C - 7 , pp. 5 4 - 6 0 , Feb. 1 9 7 2 . R.T.Oyafuso,IlAn 8 - 1 8 GHZ FET YIG-Tuned Oscillator1v,Proc. of IEEE MTT Symp. 1 9 7 9 , pp. 1 8 3 - 1 8 4 . pp. 1 8 3 - 1 8 4 . [51 Y. Letron et all "Broadband YIG-Tuned FET Oscil1ators1l, Proceedings of the European Microwave Conference, 1 9 7 8 , pp. r71 [81 [ l l ] T.W. O'Keefe and R.W. Patterson, I1Magnetostatic Surface Wave Propogation in Finite Samplesg1,J. Appl. Phys., vol. 4 9 , pp. 4 8 8 6 - 4 8 9 5 , Sept. 1 9 7 8 . [ 1 2 ] J.D. Adam and S.N. Bajpai, "Magneto- static Forward Volume Wave Propogation in YIG Strips", IEEE Trans. Magn. vol. MAG - 1 8 , pp. 1 5 9 8 - 1 6 0 0 , Sept. 1 9 8 2 . [ 1 3 ] W.S. Ishak, "Magnetostatic Wave Technology: A Review", IEEE Proc. , vol. 7 6 , pp. 1 7 1 - 1 8 7 , Feb. 1 9 8 8 . [ 1 4 ] W.E. Kunz, K.W. Chang, and W.S. Ishak, "MSW-SER Based Tuneable Oscillatorsg1, Proc. IEEE Ultrasonics Symp., 1 9 8 6 . [ 1 5 ] G. Winkler and H. Dotsch, "Hexagonal Ferrites at Millimeter Wavelengthsp1, Proc. of 9th Euro. Microwave Conf., pp. 1 3 - 1 9 , 1 9 7 9 . Lemke et all "Magnetically Tuneable Millimeter-Wave Filter with SingleCrystal Barium Ferriten1,Microwaves, Optics, and Acoustics, vol. 3 , pp. 2 5 3 2 5 4 , Nov. 1 9 7 9 . [ 1 6 ] M. [ 1 7 ] J.A. Sweschenikow et all "Bandfilter aus 274-278. Hexaferriten im Mikrowel1enbereichl1, Nachrichtechnik Elektronik, vol. 2 6 , pp. Y. Letron et all Wultioctave FET Oscillators Double Tuned by a Single YIGII, IEEE Int. Solid State Circuit Conf., 1 9 7 9 , pp 1 6 2 - 1 6 3 . 262-264, C.F. 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Nicholson, "Barium Ferrite Tuned-Indium Phosphide Gunn Millimeter Wave Oscillators', Proc. of IEEE MTT Spp., 1 9 8 6 , pp. 1 8 3 - 1 8 6 .