U.S. patent number 3,890,582 [Application Number 05/370,580] was granted by the patent office on 1975-06-17 for floating-ground microwave ferrite isolators.
This patent grant is currently assigned to Addington Laboratories, Inc.. Invention is credited to David Hoy Jeong.
United States Patent |
3,890,582 |
Jeong |
June 17, 1975 |
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.
Inventors: |
Jeong; David Hoy (Palo Alto,
CA) |
Assignee: |
Addington Laboratories, Inc.
(Sunnyvale, CA)
|
Family
ID: |
23460259 |
Appl.
No.: |
05/370,580 |
Filed: |
June 15, 1973 |
Current U.S.
Class: |
333/24.2;
333/1.1 |
Current CPC
Class: |
H01P
1/383 (20130101) |
Current International
Class: |
H01P
1/383 (20060101); H01P 1/32 (20060101); H01p
001/32 () |
Field of
Search: |
;333/1.1,24.1,24.2,24.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gensler; Paul L.
Attorney, Agent or Firm: Limbach, Limbach & Sutton
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.
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
______________________________________
* * * * *