Floating-ground microwave ferrite isolators

Abstract

A plurality of floating-ground microwave isolators, each with a different center frequency and overlapping bandwidths are serially connected, and a variable capacitance is connected between the floating-ground of each and ground, magnetic field adjusting elements are used to adjust the magnetic fields of each isolator, and a common magnetic shield covers the magnetic field areas of the isolators.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a type of broadband microwave isolator applicable primarily in the VHF region.

Three ways of achieving a wideband isolator in the VHF region are commonly used. One type is the below-resonance technique where the ferrimagnetic material has a magnetic bias field which is less than that required for resonance.

A second type is the above-resonance technique where the magnetic bias field is greater than that required for resonance. A third type is also biased above resonance but differs from the conventional above-resonance isolator in its circuitry, size, tuning and matching elements which are lumped constant components. Below-resonance isolators are characterized by their large size and large sensitivity to temperature changes. Bandwidths for this variety of device may reach an octave (67 percent) but operating frequencies do not extend much below five hundred MHz. Above-resonance isolators are smaller and more temperature stable than the below-resonance variety but the bandwidths are not as large. Conventional varieties of lumped element isolators behave somewhat like the above-resonance type except that they are smaller yet and their losses are higher.

A practical method of achieving a desired bandwidth with reasonable size is a technique of connecting in series two or more isolators of different center frequencies but with overlapping band-edges. These isolators must have low insertion loss. They must also have a low VSWR (voltage standing wave ratio). Both the low insertion loss and VSWR must occur not only within the frequency range where their isolation is high but also outside of their high isolation band. The previously described isolators do not exhibit these characteristics.

One type of isolator having the desired response is a special lumped element kind which has its ground plane not in direct contact with the main chassis ground. This "floating ground" type of structure is connected from its ground plane to the main ground through a series resonant network. This type of isolator is described in a publication entitled "Design of a New Broad-Band Isolator" by Yoshihiro Konishi and Norio Hoshino, IEEE Transactions on Microwave Theory and Techniques, March, 1971.

Because this type of isolator is not at ground potential and is connected to the center conductor of a TEM line system, it behaves as any other part of the center conductor of a TEM line system in that the series inductance and shunt capacitance to ground need to be consistent with the rest of the transmission line system to sustain a constant impedance without requiring the use of a matching transformer. When constructing an isolator such as this, the irregular surfaces make the line characteristics at this place extremely difficult to calculate. Without this information the design of these units could be very complex and/or inaccurate.

Another characteristic of isolators operating in this "floating ground" mode is their great sensitivity to changes in magnetic field amplitude and distribution. Since a reduction in size is generally desired, these isolators are usually placed in a common enclosure. Unless they are thoroughly shielded magnetically, the interaction of the magnetic fields between units can cause both amplitude and distribution changes in the magnetic field patterns which deteriorate the overall performance. Magnetic shielding of individual devices sensitive to magnetic fields is a common practice but the separate shielding of individual isolators in a multi-isolator enclosure is more costly in construction, space, and insertion loss than an integrated unit with a common shield.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an improved broadband microwave isolator.

Another object of the invention is to provide means for adjusting and optimizing the impedance of a floating-ground isolator.

Another object of the invention is to provide for the adjustment, compensation, and optimization of magnetic fields of isolators in close proximity.

Another object of the invention is to provide a magnetic shield which also serves as an element in the magnetic structure of series-connected floating-ground microwave isolators.

These objectives are achieved in accordance with this invention in one example by a broadband microwave isolator which includes a TEM mode transmission line with one or more isolators connected to the center conductor and operating in the "floating-ground" mode. A variable capacitance is connected between isolator ground plates and the transmission line or chassis ground of each isolator. This variable capacitor may be a conventional variable capacitor or any other mechanical or electrical device which causes a change in the capacitance between the isolator ground and the transmission line ground. This enables the impedance of each isolator to be finely adjusted to match the impedance of the TEM transmission line.

Where a number of isolators are spaced closely together, there is frequently disadvantageous interaction of their magnetic fields as explained previously. Thus, in accordance with another aspect of the present invention, means are provided to optimize the characteristics of the individual isolator's magnetic fields. Shims or discs are provided adjacent to one of the surfaces of one or both of the isolator magnets to alter the D.C. magnetic field through the isolator.

For example, if the magnetic field through the isolator is too strong, a disc made of a non-magnetic material is placed adjacent one or both of the isolator magnets which, typically, sandwich a pair of ferrite discs enclosing the lumped circuit element.

If the magnetic field is non-uniform, i.e., the strength of the field is a function of the position along the isolator, the non-magnetic disc is tapered so that the distance between the magnets varies along the length of each and consequently the magnetic field can be compensated so that it is uniform throughout.

If it is desired to increase the magnetic field, a small disc of a magnetic material is placed on the outside surface of one or both of the magnets. This need not cover the entire surface of the magnet and its size is selected to provide desired increase in the magnetic field strength. The position of this disc can also be used to compensate or adjust the magnetic field.

To prevent interaction of magnetic fields from external sources which can cause deterioration of the overall performance of the isolator, a common magnetic shielding, spaced apart from the individual isolators, and covering the magnetic field areas of the isolators is provided in accordance with the present invention. This shield reduces the reluctance of a magnetic path in the vicinity of the magnets forming individual isolators and provides magnetic shielding. This shield may serve as the transmission line ground, or if the transmission line ground is of a non-magnetic material, it may be external to the transmission line ground.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a broadband microwave isolator in accordance with the present invention.

FIG. 2 is a cross-sectional illustration of a floating-ground type microwave isolator in accordance with the present invention.

FIG. 3 is a diagrammatic illustration of a cascaded pair of isolators of the type illustrated in FIG. 2 in accordance with another aspect of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a broadband microwave isolator circuit 10 in accordance with the present invention. Broadband isolator 10 includes first and second individual isolators 12 and 14. Each of these isolators is of the floating-ground type as explained previously.

Isolator 12 has a center point frequency of 262.5 MHz and an isolator bandwidth of 225 to 300 MHz. Isolator 14 has a center frequency of 350 MHz and a bandwidth of 300 to 400 MHz. By cascading the two isolators 12 and 14 as will be described subsequently, the resulting bandwidth of the broadband isolator 10 is extended to a range of 225 to 400 MHz with acceptable insertion loss.

As will be described in more detail with respect to FIG. 2, each of the isolators 12 and 14 includes a lumped element Y circulator element of the type well known to those skilled in the art. For example, one such lumped circuit is described in an article entitled "Lumped Circuit Y Circulator" by Yoshihiro Konishi, IEEE Transactions on Microwave Theory and Techniques, November, 1965. The lumped circuit of isolator 12 has three terminals, 16, 18 and 20. Similarly, isolator 14 has three terminals, 22, 24 and 26.

As described in the Konishi et al. article of March 1971, referred to above, isolator 12 includes a termination circuit 28 connected between the terminal 18 and the common floating-ground 30. Similarly, a termination circuit 32 is connected between terminal 24 and the floating-ground 34 of isolator 14. Terminating circuit 28 includes an absorbing or terminal resistance R1 and a series resonant bandwidth broadening network comprising an inductor L1 and a variable capacitor C1.

A second series resonant network of isolator 12 includes an inductor L2 and a variable capacitor C2 connected between the floating-ground 30 and the actual or transmission line ground 36.

The center frequency of isolator 12 is determined by the values of capacitances C3 and C4 connected across terminal 18 and floating-ground 30, capacitors C5 and C6 connected across terminal 16 and floating-ground 30 and capacitors C7 and C8 connected across terminal 20 and floating-ground 30.

Isolator 14 is very similar to isolator 12. Termination circuit 32 includes a terminal resistance R2 and a first series resonant bandwidth broadening network forming a part of the termination circuit 32, comprising an inductor L3 and a variable capacitor C9. Capacitors C10, C11 and C12 are used to adjust the center of frequency of isolator 14. A second series resonant network, comprising inductor L4 and capacitor C13, is connected between the floating-ground 34 and the transmission ground 36.

Input terminal 16 of isolator 12 is connected to the center terminal 38 of a coaxial line 40 over which microwave signals are sent. Of course, any suitable TEM transmission line could be used in lieu of the coaxial line 40. The microwave signals are circulated after passing through terminal 16 by the lumped element Y circulator in the well-known manner so that they exit through terminal 20, which is connected to the center terminal 42 of a connecting coaxial line 43. The input terminal 22 of the isolator 14 connected to the central terminal 42 of the coaxial line 43 and signals passing through terminal 22 are circulated and exit from the terminal 26 to the center of terminal 44 of an output coaxial line 46.

Signals which attempt to pass in the "wrong" or opposite direction, i.e. into the coaxial line 46, pass through terminal 26 and are circulated to and through the terminal 24 and into the terminating network 32. This network effectively eliminates such backward traveling signals in the isolation frequency range of isolator 14. Similarly, the termination circuit 28 of isolator 12 eliminates backward passing signals within the isolation bandwidth of isolator 12.

In accordance with the invention, isolator 12 is provided with a variable capacitance C14 connected between the floating-ground 30 and the transmission ground 36 and isolator 14 is provided with variable capacitor C15 connected between floating-ground 34 and transmission line ground 36. As described previously, by adjusting the capacitances of these two capacitors, it is possible to match the impedance of the individual isolators 12 and 14 with the impedance of the transmission line 40.

FIG. 2 is a cross-sectional view of floating-ground isolator 12 of FIG. 1. A lumped element Y circulator circuit 48 of the type described in the Konishi publication of November 1965 referred to previously, is terminated by the terminals 16, 18 and 20, and is sandwiched between a pair of thin, circular ferrite slabs 50 and 52. A d.c. magnetic field is provided across the sandwiched combination by means of a pair of disc-shaped magnets 54 and 56. The floating-ground plane 30 is also located between the magnet 56 and the ferrite slab 52. The variable impedance C14 for matching the impedance of isolator 12 with that of the transmission line 40 is shown connected between the floating-ground plane 30 and the transmission line ground 36.

If it is found that the magnetic field strength is insufficient for optimum performance of the isolator 12, then a magnetic field adjusting disc 58 is provided on the outside surface of one of the magnets. The disc 58 is made out of a magnetic material such as iron or steel and has the effect of increasing the field across the ferrites 52 and 50 and the lumped circuit element 48. The field adjusting disc 58 may be smaller in size than the magnet 44. Further, by varying the position of disc 58 on the surface of the magnet, the magnetic field across the ferrite/lumped element combination can be compensated for or adjusted. By trying different sized discs 58 at different locations on the magnet surface, an optimum magnetic field strength can be achieved.

If the field strength is too great, then a field adjusting disc 60 is provided either between the magnet 54 and ferrite 50 or the magnet 56 and ferrite 52. Field adjusting disc 60 is of a non-magnetic material such as aluminum or brass. The thickness of the disc 60 is determined by the amount of the decrease of the magnetic field required.

Where the magnetic field is unequal or asymmetrical across the ferrite-lumped circuit combination, a field adjusting shim 62 having a tapered cross-section is inserted between one or both of the magnets. The tapered cross-section has the effect of increasing the distance between the respective magnets according to the thickness of the shim at any given point, so that the magnetic field becomes weaker as the thickest portion of the shim 62 is approached. Once again, the amount of the taper and thickness of the shim 62 are determined empirically.

FIG. 3 shows a pair of cascaded isolataors 12 and 14 suitable for being encased in a single, multi-isolator chassis or enclosure (not shown). Rather than shielding individual ones of the isolators 12 and 14, a common shield is provided for both isolators. Shield 64 is spaced apart from the individual isolators 12 and 14 and covers the magnetic field areas of each. This shield reduces the reluctance of the magnetic path between isolators and thereby reduces the size requirement of the magnets forming the isolators. Additionally, the shield provides magnetic shielding to prevent deterioration of the isolator performance due to outside sources of stray magnetic energy.

In the embodiment of FIG. 3, the shield 64 is external to the transmission line ground 36. For example, if the transmission line ground is provided by a chassis of a non-magnetic material such as aluminum, the magnetic shield 64 can comprise a layer of a magnetic material surrounding the chassis. However, in accordance with another aspect of the invention, the shield 64 can also serve as the transmission line ground 36. For example, the magnetic material can be used to form the chassis or enclosure for the cascaded isolators.

In the embodiment described in FIGS. 1-3, two isolators are shown connected in series. However, it should be understood that the present invention is applicable either to a single isolator as well as to an isolator comprising more than two individual isolators serially connected.

As explained previously, serially connecting conventional lumped element isolators of the non-floating ground variety do not provide successful wide band isolation because of unacceptable amounts of insertion loss which results. However, there are situations where such conventional isolators can be cascaded, not for the purpose of increasing bandwidth, but, for example, to increase backward loss. In these situations, the magnetic field adjusting elements and the magnetic shield of the present invention are equally as useful as with the floating-ground isolators described herein.

While the foregoing describes the invention in sufficient detail to enable one skilled in the art to duplicate it, the following circuit parameters and circuit performance data is taken from one actual embodiment of the invention.

TABLE I ______________________________________ Circuit 10 Parameters ______________________________________ L1 84 nanohenries L2 32 do. L3 48 do. L4 40 do. C1 2-10 pf C2 1-10 pf C3 3-20 pf C4 10 pf C5 3-20 pf C6 10 pf C7 3-20 pf C8 10 pf C9 2-10 pf C10 3-20 pf C11 3-20 pf C12 3-20 pf C13 1-10 pf C14 1-10 pf C15 1-10 pf R1 58 ohms R2 60 ohms ______________________________________

TABLE II ______________________________________ Circuit 10 Performance Specifications ______________________________________ Isolation 16-18 db. minimum Insertion loss 1.5 - 1.6 db. maximum VSWR 1.5:1 - 1.7:1 Isolation bandwidth 225 MHz - 400 MHz ______________________________________

Claims

I claim:

1. A broadband microwave isolator comprising:

a. a plurality of individual floating-ground ferrite-type isolators connected in series, each having a different center frequency and wherein the bandwidths of individual isolators having adjacent center frequencies are overlapping; and wherein each of said individual floating-ground isolators comprises:

1. a lumped circuit element;

2. a pair of ferrite discs sandwiching said lumped circuit element;

3. a floating-ground plane element; and

4. means for providing a d.c. magnetic field across said sandwiched lumped circuit element and said floating-ground plane element;

b. a TEM mode transmission line for providing microwave signals to said plurality of isolators;

c. a variable capacitance for each individual isolator connected between the isolated floating-ground and the transmission line ground for providing means for adjusting the capacitance therebetween; and

d. including means for compensating for unequal magnetic fields across said lumped circuit element and said floating-ground plane element comprising a tapered nonmagnetic shim located between said magnetic field means and said ferrite discs.

2. A broadband microwave isolator as in claim 1 including a single magnetic shield common to all of said plurality of individual isolators and covering the magnetic field areas thereof and wherein said shield serves additionally as the transmission line ground.

3. A microwave isolator comprising:

a. a lumped circuit element;

b. a pair of ferrite discs sandwiching said lumped circuit element;

c. a floating-ground plane element;

d. a pair of magnets aligned with said sandwiched lumped circuit element and said floating-ground plane element to provide a d.c. magnetic field thereacross; and

e. means adjacent to at least one of said magnets for adjusting the magnetic field across said sandwiched lumped circuit element, and

f. wherein said adjusting means includes means for compensating for unequal magnetic fields thereacross comprising a tapered non-magnetic shim located between at least one of said magnets and said ferrite discs.