US3531747A

US3531747A - Tunable inductor

Info

Publication number: US3531747A
Application number: US806047A
Authority: US
Inventors: Vernon E Dunn; Roy W Roberts Jr
Original assignee: Melabs
Current assignee: Melabs
Priority date: 1969-03-07
Filing date: 1969-03-07
Publication date: 1970-09-29
Anticipated expiration: 1987-09-29

Description

Sept. 29, 1970 v. E. DUNN EI'AL TUNABLE INDUCTOR Original Filed Oct. 6, 1966 42 3s F/G. 4

INVENTORS VERNON E. DUNN ROY W. ROBERTS JR BIAS MAGNETIC FIELD-H United States Patent 015cc 3,531,747 TUNABLE INDUCTOR Vernon E. Dunn, Palo Alto, and Roy W. Roberts, Jr., Mountain View, Calif., assignors to MELABS, Palo Alto, Calif., a corporation of California Continuation of application Ser. No. 584,877, Oct. 6, 1966. This application Mar. 7, 1969, Ser. No. 806,047 Int. Cl. Hlf 21/08 11.5. Cl. 336-110 2 Claims ABSTRACT OF THE DISCLOSURE A reciprocal low-loss tunable inductor including a coil coupled to a body of ferrite material which is biased above ferromagnetic resonance with a variable magnetic field to tune the inductor.

This application is a continuation of application Ser. No. 584,877 filed Oct. 6, 1966, now abandoned.

This invention relates generally to a tunable inductor and more particularly to a tunable inductor which can be tuned mechanically or electrically.

Tunable inductors have been known in the prior art. In general, such inductors depend upon the variation of incremental permeability of ferrites or other magnetic material as a biasing magnetic field changes the ferrite from an unsaturated to a saturated state. The magnetic material forms the core for an A-C or signal winding whose inductance is varied by the incremental variation in the permeability of the core.

One type of tunable inductor has the biasing magnetic field applied in the same direction as the RF signal or A-C field. In effect then, the RF or A-C field is added directly to the bias field. If the bias field is not large in comparison to the RF field, it is modulated by the RF field producing such objectionable nonlinear effects as generation of signals at harmonic frequencies. The loss in the inductor is also increased.

Another type of tunable inductor includes a toroidal core which carries the signal winding. The winding covers only portions of the core. A biasing magnetic field is applied to otherportions of the core to vary the incremental permeability at such portions. Since the entire magnetic material is not subjected to the magnetic bias field, there is only a small change in incremental permeability.

Devices of the prior art have operated with the ferrite in an unsaturated or partially saturated state. It is a well known characteristic of unsaturated ferrites that at certain frequencies, whichdepends upon the material and the biasing field, the losses increase rapidly as a result of domain wall motion and ferromagnetic resonance.

In general, the prior art tunable inductors have been useful over a relatively narrow range of frequencies, have low power handling capacities, have been relatively large and bulky, and have had relatively high losses at high frequencies of operation.

It is an object of the present invention to provide a tunable inductor forhigh frequency operation having a wide tuning range.

It'is another object of the invention to provide a tunable inductor for 'high frequency operation having low losses.

It is another object of the invention to provide a tunable inductance which can handle large amounts of RF power with low loss and with small or negligible nonlinear effects.

It'is still a further object of the present invention to provide a tunable inductor which has small size for a given value of inductance and unloaded quality factor Q.

It is still a further object of the present invention to provide a tunable inductor inwhich RF fields associated with the core are located partially or completely in a fer- 3,531,747 Patented Sept. 29, 1970 rite medium in such a way that the RF fields are predominantly orthogonal to a biasing field through the same portion of the core, with the operation of the inductor based upon the interaction of the RF fields with the precessing magnetization of the saturated ferrite.

It is still another object of the present invention to provide a tunable inductor in which the bias field is applied perpendicular to the RF fields and of sufficient strength to saturate the ferrite and bias it so that the ferromagnetic resonance frequency is greater than the signal frequency.

The foregoing and other objects of the invention will become more clearly apparent from the following description when taken in conjunction with the accompanying drawing.

Referring to the drawing:

FIG. 1 is a perspective view of an inductor in accordance with the present invention;

FIG. 2 is a perspective view of another inductor in accordance with the invention;

FIG. 3 is a perspective view of a mutually coupled inductor in accordance with the present invention;

FIG. 4 is a schematic perspective view of an electrically tunable inductor in accordance with the invention;

FIG. 5 is a schematic view of a mechanically tunable inductor in accordance with the invention; and

FIG. 6 is a curve showing the permeability as a function of D-C magnetic field.

As previously described, operation of the device of the present invention is based on the interaction of the signal or RF field with the precessing magnetization of saturated ferrites. The permeability in an infinite ferrite medium, saturated by a D-C magnetic field, of magnitude H in the Z direction is given by the Polder tensor (Lax and Button, Microwave Ferrites and Ferrimagnetics, Mc Graw-Hill Book Co., New York, 1962).

w w w, w 2

ww (T e (3) w ='yH =frequency of ferromagnetic resonance (4) (a 'y4'rrM w angular frequency of RF fields (6) 'y=1.76 107 radians/sec. oersted (7) Where p. is permeability K is orthogonal permeability to is frequency H is magnetic field 7 is gyromagnetic ratio m=is the magnetic moment per unit volume. I

From this expression, it will be seen that the RF fields perpendicular to the D-C field encounter a permeability which is a function of the applied D-C biasing field H.

I The permeability holds to very high frequencies and is biasing close to resonance is limited by the dissipation 1 associated with the ferrite in the resonance region. Mathematic ally, the effect of loss is to make t and K become complex. The quality factor Q, can be defined as the a minimum tolerable Q establishes a maximum value for 11..

In principle, it is possible to bias to a low loss region below resonance to achieve a permeability which varies cludes serially connected

windings

36 and 37. A permanent magnet 39 may be employed to apply additional bias fields to the

pole pieces

33 and 34 via

bridging members

41 and 42 which complete the DC magnetic circuit. As a result, permanent magnet 39 provides a fixed bias field while the

coils

36 and 37 provide a variable bias field to vary the permeability.

In FIG. '5 there is shown a mechanically tunable inductor. The inductor includes an inductance member 32. A first fixed steel pole piece 46 is disposed on one face of a magnet 47 and in contact with inductor 32. A nonmagnetic spacer 48 maintains the tunable inductor and magnetic pole piece in position. A second magnet 49 inwith bias field. In practice, however, with VHF and UHF frequencies and bias fields small-enough to operate belorw resonance the ferrite remains unsaturated and it may become lossy and may not behave in accordance with the theory established above.

Referring particularly to FIG. 6, the variation in permeability with variation in H is given with ferromagnetic resonance indicated by line-13.'It is seen that the permeability varies with biasing magnetic fields considerably below resonance, which is the region invwhich prior art apparatus operated. In the region defined by-the dotted lines 11 and 12, as resonance is approached, it is seen that the permability begins to increase rapidly. However, as pointed out above, the losses may be considerable. As the magnetic field H is increased toward resonance, the losses increase further. Above the resonance condition, to the right of dotted line 13, the losses arereduced. Itis seen that in this region the permeabiliy changes appreciably with changes in H.

The tunable inductor of the present invention operates eluding a pole piece 51 is carried by a threaded screw 52 which is 'threadably received by an iron pole piece 53. Rotation of the screw 52 will bring the' pole piece 51 towards and away from the ferrite 32 thereby increasing or decreasing the-reluctance path 54 and varying the D-C magnetic field. The return path for the D-C fields is, of course, through the steel pole piece which threadably receives ihG SCIBWT. Thus, there is provided mechanicaltubing for the inductor.

Tunable inductors of the type described may be used as circuit elements in various filter applications such as in tunable band pass filters and tunable band rejection filters. They can also be used in tunable matching circuits for matching transmitters to antennas. They can provide 7 a load impedance which can be tuned to give the best above resonance where the ferrite material presents relatively low losses and a wide tuning range.

Referring to FIG. 1, there is shown a simple realization of a tunable inductor in accordance with the invention. The inductor includes a ferrite wafer or body 16 disposed within a signal or RF coil 17 so that the fields in the coil are run partially through the ferrite material in the direction of the axis of the coil and substantially parallel to the spaced faces 18 and 19 of the wafer 16. A magnetic bias field H is applied in the direction of the arrows 21 that are perpendcular to the RF fields. The bias field is of sufficient strength to bias the ferrite material above resonance.

As a result of the foregoing, the inductance then varies substantially as shown by the portion of the curve 22 to the right of dotted line 13, FIG. 6. In this region, the incremental permeability can be controlled by the applied biasing magnetic field 21.

The device shown in FIG. 2 is similar to that in FIG. 1 with

ferrite wafers

23 and 24 added to sandwich the coil 17 and wafer 16 therebetween to provide a substantially complete path for the RF fields to close upon themselves. In the embodiment of FIG. 1, there is fixed inductance (air path) in series with the tunable inductance (ferrite path) and hence there is a degradation inthe tuning range. The embodiment of FIG. 2 provides an improved tunability since the paths for the RF fields are ther, however, the winding 27 is coupled to the winding 17 whereby there is provided mutual tunable inductance between the same to give a tunable mutual inductance.

Referring to FIG. 4, an inductor of the type shown in FIG. 2 and identified as 32 is shown disposed between the

pole pieces

33 and 34 of an electromagnet which inmatch between a load and its connected apparatus.

Thus, there has been provided an improved tunable inductor which can be used at high frequencies, which provides tuning over' a relatively broad band of frequencies, and which has low losses.

We claim; I V i 1. 'A tunable inductor including a body of ferrite material, a coil coupled to said body of material so that the magnetic fields associated with the electrical signal carried by the coil are at least partially within the ferrite material, said body of ferrite material being disposed within the coil, additional bodies of ferrite material on each side thereof to sandwich the ferrite body and the coil therebetween, and means for applying a biasing magnetic field through said ferrite material in a direction such that the signal and biased magnetic fields are predominantly orthogonal to one another.

2. A variableinductor including a body of magnetic material, a coil coupled to said body of material so that the magnetic fields associated with the electrical signal carried by the coil are at least partially within the magnetic material, said body of magnetic material being disposed within the coil, additional bodies of magnetic material disposed adjacent to said body of magnetic material on each side thereof to sandwich the body of material and the coil therebetween, and means for applying a' variable biasing magnetic field to said magnetic material which biases the magnetic material above ferromagnetic resonance throughout the range of variation of the bias field to vary the inductance in response to variations of the biasing magnetic field.

I References Cited UNITED STATES PATENTS