Wave transmission lines and networks – Plural channel systems – Nonreciprocal gyromagnetic type
Reexamination Certificate
1999-09-01
2003-01-14
Bettendorf, Justin P. (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Nonreciprocal gyromagnetic type
C333S024200
Reexamination Certificate
active
06507249
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Introduction
This invention pertains generally to microwave devices, and more particularly to non-reciprocal microwave devices, such as isolators and circulators. Non-reciprocal microwave devices are based on electrically insulating magnetic materials, such as the materials generally known as “ferrites”. Their performance can be characterized by the ratio f
max
/f
min
, where f
min
and f
max
are defined as the edges of the frequency band in which the devices have acceptable operating characteristics (typically less than 1 dB insertion loss and more than 15 dB isolation). For the most advanced isolators and circulators available today this ratio is approximately 3:1. The present invention shows how the broadband performance can be improved substantially.
For both isolators and circulators, the bandwidth that has been achieved in practice is generally much smaller than that predicted by the design theories that have been developed for these devices. One reason for the failure of these theories to account satisfactorily for the observed performance is that they assume that the microwave ferrite is in a very uniform magnetic bias field (internal field). Such a very uniform field is difficult to achieve in practice, and is not usually realized in typical isolators and circulators. Another reason is that the theories make no allowance for the excess low-field, low-frequency loss observed in these devices.
2. Review of the Microwave Properties of Magnetic Materials
The theoretical analysis of ferrite microwave devices is generally based on a “constitutive” equation, expressing the relation between rf magnetic flux b vector and rf magnetic field vector h by a tensor equation of the form
b=&mgr;
0
h.
(1)
Here &mgr;
0
is the permeability of vacuum and
the permeability tensor (permeability relative to vacuum)
μ
↔
=
[
μ
-
j
κ
0
j
κ
μ
0
0
0
μ
z
]
.
(
2
)
Here the dc bias field is assumed to be applied in the z-direction and a time dependence proportional to exp(j&ohgr;t) is implied, where &ohgr;=2&pgr;f and f is the signal frequency. The tensor components &mgr; and &kgr; can be calculated from the gyromagnetic equation of motion for the magnetization vector, with the result
&mgr;−&kgr;=1
+f
M
/(
f
H
−f
)
&mgr;+&kgr;=1
+f
M
/(
f
H
+f
)
f
M
=&mgr;
0
&ggr;M
s
, f
H
=&mgr;
0
&ggr;H
int
(3)
where &ggr; is the gyromagnetic ratio, M
s
is the saturation magnetization and H
int
is the internal magnetic field. Losses can be taken into account by assigning an imaginary part j&agr;
G
f to the resonance frequency f
H
, &agr;
G
being the so-called “Gilbert damping parameter”.
The propagation of electromagnetic waves in an unbounded ferrite medium can easily be analyzed on the basis of Eqs. (1-3). Such an analysis shows that, in general, two types of waves or “wave modes” exist for any propagation direction. For propagation orthogonal to the bias field, one of the modes is characterized by an rf magnetic field in the z-direction and an effective permeability equal to &mgr;
z
, whereas the other mode is characterized by an rf magnetic field having x- and y-components, and an effective permeability given by
&mgr;
e
=(&mgr;
2
−&kgr;
2
)/&mgr;. (4)
In the literature this scalar permeability is generally referred to as the “effective” permeability, and this custom is therefore also adopted in the present patent application. It plays an important role in the analysis of edge-mode isolators and stripline/microstrip circulators. It should be kept in mind, however, that the expression given in Eq. (4) generally does not represent an effective permeability for a guided wave in a ferrite substrate. From (3) and (4), the tensor components and the effective permeability can readily be shown to be
&mgr;=[
f
H
(
f
H
+f
M
)−
f
2
]/(
f
H
2
−f
H
2
)
&kgr;=−
f
M
f
/(
f
H
−f
2
)
&mgr;
e
=[(
f
H
+f
M
)
2
−f
2
]/[f
H
(
f
H
+f
M
)−
f
2
] (5)
In the analysis of broadband isolators and circulators, the case in which f
H
is very small compared to f
M
and f is of special significance. If damping is neglected, Eq. (5) easily reduces to
&mgr;=1
&kgr;=
f
M
/f
&mgr;
e
=1−(
f
M
/f
)
2
(6)
under these conditions, which implies that &mgr;
e
is negative for frequencies less than f
M
.
3. Broadband Isolators
The most successful broadband isolators currently available are based on the edge-mode configuration, described in a paper entitled “Reciprocal and Nonreciprocal Modes of Propagation in Ferrite Stripline and =Microstrip Devices” by M. E. Hines [IEEE Trans. MTT-19, pp. 442-451, 1971]. These devices typically include a stripline or microstrip line on a ferrite substrate, which is magnetized normal to its plane. A sheet of resistive material with a predetermined surface resistance is located along the side of the strip conductor, in a plane orthogonal to the strip conductor. Hines describes a simple approximate analysis, which applies to this structure if the strip conductor is much wider than the substrate thickness. Under these conditions, the actual boundary conditions that exist at the edge of the strip conductor can be replaced by so-called “magnetic wall” boundary conditions by way of approximation. This approximate procedure may be justified by the observation that any electric current in the strip conductor can not flow orthogonal to the edge, and hence cannot induce a magnetic field component parallel to the strip conductor. For magnetic wall boundary conditions, the field equations can be solved exactly and simply, as shown by Hines. His analysis shows that the fundamental mode of this structure, with the resistive plane removed, consists of a wave that propagates parallel to the strip conductor and varies exponentially in the transverse direction. Thus the energy carried by the wave is displaced predominantly to one side of the strip conductor. The dispersion relation for these waves can be characterized by a scalar permeability, which turns out to be equal to the diagonal component of the permeability tensor, not the effective permeability of Eq. (4).
The effect of the resistive layer on the propagation characteristics of the edge-mode isolator has also been analyzed by Hines. Because of the field displacement effect mentioned above, the attenuation depends on the direction of propagation. The presence of a resistive layer with a given surface resistance (ohm per square) can be taken into account by imposing the appropriate transverse impedance condition on the rf field. The resultant characteristic equation for the complex propagation constant &bgr; as a function of frequency &ohgr; can be expressed as F (&bgr;,&ohgr;)=0, where F is a relatively simple transcendental function that depends on all relevant device parameters. Solutions for the propagation constant can be constructed by Newton's method for both directions of propagation, and for the dominant mode as well as any higher-order mode. Hines has reported the results of such calculation, taking only the losses due to the resistive layer into account and assuming a homogeneous bias field. The difference in the attenuation constants can be very large when the strip conductor is sufficiently wide.
Hines has also pointed out that, on the basis of the theory he developed, one might expect the edge-mode circulator to work over a virtually unlimited bandwidth if the ferrite is biased to saturation and the internal magnetic bias field is suitably small. In his experiments he obtained a frequency ratio f
max
/f
min
of about 2:1, which was considered very good at the time. Later investigators have improved the bandwidth somewhat and have achieved f
max
/f
min
of approx. 3:1. The discrepancy between the theoretically expected bandwidth and that obtained in prac
LandOfFree
Isolator for a broad frequency band with at least two... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Isolator for a broad frequency band with at least two..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Isolator for a broad frequency band with at least two... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3015174