Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Reexamination Certificate
1999-04-01
2001-07-03
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
C333S219000, C333S219200
Reexamination Certificate
active
06255918
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to structures for high frequency fundamental resonators, particularly for frequency sources and for filters. More particularly, the invention relates to ferrite oscillators and filters, such as yttrium-iron-garnet (Y
3
Fe
5
O
12
or YIG) oscillators and filters.
Ferrite resonators are a favored type of resonator for both oscillator and filter applications because of their typically high resonant frequency (about 1 GHz to about 100 GHz), wide tuning range (typically over one octave), linear tuning characteristics, and spectral purity (high Q factor). A ferrite element may be used in a resonator structure in several ways.
A preferred type of ferrite element is a highly polished sphere of single-crystal material. A spherical shape provides a boundary condition that approximates an infinite volume of ferrite material (thus allowing uniform and predictable resonant modes), and a highly polished surface minimizes surface scattering, and improves the resonant quality of the sphere. The sphere is typically placed in a magnetic circuit, such as within a gap between two magnetic pole faces, that applies a magnetic field sufficient to magnetically saturate the sphere and initiate resonance. A loop or loops couple to the sphere and may transfer resonant energy into or out of the sphere.
Some conventional oscillators attach a ferrite sphere to a long, slender mounting rod, which is inserted through the body of the magnetic circuit to hold the sphere in the air gap near the coupling loops. Most rods are made of electrically non-conductive material near the sphere to avoid eddy currents in the rod affecting the electromagnetic field pattern near the sphere. Some rods have been fabricated out of sapphire or alumina. Other rods use a non-conductive tip in an otherwise metal rod. Such tips have been made of beryllia (BeO), in addition to alumina (Al
2
O
3
) and sapphire tips. These non-conductive materials are often chosen for their relatively high thermal conductivity.
The resonant frequency of a ferrite element may drift with temperature. The amount of frequency drift of a YIG sphere over temperature, for example, depends on the crystallographic orientation of the sphere to the applied magnetic field. Certain orientations will exhibit a positive frequency drift with increasing temperature, and others will exhibit a negative frequency drift with increasing temperature. Between these two regions lie thermally-compensated (TC) axes, as is known in the art, where the resonant frequency does not drift with temperature. These TC axes are usually a desirable orientation, hence provision is typically made to rotate the sphere-rod assembly within the resonant structure to align a TC axis with the applied field.
Achieving the ideal, or near ideal, TC solution angle typically involves orienting the sphere along a known axis prior to mounting the sphere on a rod. Orienting the sphere and/or subsequent mounting of the sphere usually requires specialized equipment and knowledge. One crystallographic orientation commonly used for this purpose is to align the sphere so that a [110] axis is normal to the axis of mechanical rotation, and thus a (110) plane may be normal to the applied field.
This alignment can be very difficult to achieve, and some ferrite resonator applications couple significant power to the sphere, thereby heating it. For these reasons, some conventional designs have sought to remove heat from the sphere via its mounting rod to reduce temperature effects. However, providing a heat conduit down the mounting rod may create a thermal gradient across the sphere, altering its uniform resonant characteristics and producing other temperature effects.
Conventional designs typically clamp the rod
101
to the body of the magnetic circuit after aligning a sphere
114
, which may be attached to the rod
101
with adhesive
116
, in the resonant circuit, as shown in FIG.
1
A. This may create a cantilever member supporting the sphere
114
, which may be susceptible to mechanical vibration. The cantilevered sphere may move differentially from the loop
115
that is mounted on a substrate
111
, and induce a mechanical resonance that appears as phase jumps or frequency breaks offset from the tuned frequency of the structure, as shown in
FIG. 1B
, or as an instantaneous change in the resonant frequency. Such vibration-induced responses are highly undesirable in both oscillator and filter applications. For example, a vibration-induced phase jump may pull a phase-locked oscillator outside of its phase-locked loop bandwidth, thereby losing the desired output until phase lock is re-established.
One approach to avoid differential motion between a ferrite sphere and its coupling loop is to glue or otherwise fix the sphere under the loop in the magnetic circuit. This approach does not allow in situ TC axis alignment, however, and therefore these structures may exhibit higher thermal drift. Gluing a sphere in a resonator structure also makes it impractical to swap ferrite spheres in and out of the resonator structure. Spheres may have inclusions or other defects that are not apparent until the sphere is placed in the resonant structure, where the defects cause power holes, crossing resonant modes, or other problems. Often, swapping the sphere would destroy the microcircuit and coupling loop.
Therefore, it is desirable to reduce the differential motion between a ferrite sphere and associated coupling structures in a resonant circuit. It is also desirable to accomplish this in a manner that allows ferrite spheres to be exchanged in a resonant structure without damage to other components of the resonant structure, and to provide a resonant structure less sensitive to thermal variations.
SUMMARY OF THE INVENTION
According to the invention, a resonating element is mounted on a mechanically stiff support structure to reduce differential motion between the resonating element and a coupling element. The support structure allows controllable insertion, removal, and rotation of the resonating element with respect to the RF coupling element. The support structure may be mechanically coupled to the RF coupling element to further reduce differential motion between the resonating element and the RF coupling element.
In a specific embodiment, a support structure for a ferrite-based resonator comprises a stiff mounting rod supporting a YIG sphere that is mechanically coupled through a mounting bracket to a substrate that supports an RF coupling loop. This support may be placed within a magnetic circuit, such as within an air gap of an electromagnet, to provide a broadband resonant circuit suitable for oscillators or filters, for example. The mounting rod has a bearing surface which allows it and the sphere to be rotated within the magnetic circuit assembly, and allows changing the ferrite sphere. The structure reduces the differential movement between the coupling loop and the ferrite sphere that may arise from vibration and degrade resonator performance.
The mounting rod is preferably made from an electrically and thermally non-conductive material, such as plastic. The substrate may further include a hybrid microcircuit patterned on its surface which may include active devices, such as GaAs Darlington ICs. A thermally nonconductive mounting rod reduces heat flow to or from the sphere down the mounting rod, thus resulting in a more isothermal sphere than conventional designs. A chip heater may be added to the assembly to uniformly control the assembly temperature above the ambient temperature. The small size and thermal mass of the assembly makes the use of a chip heater practical.
The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings.
REFERENCES:
patent: 4247837 (1981-01-01), Mezak et al.
patent: 4283691 (1981-08-01), Burgoon
patent: 4480238 (1984-10-01), Iwasaki
patent: 4651116 (1987-03-01), Schloemann
patent: 4758800 (1988-07-01), DiBiase et al.
patent: 4988959 (1991-01-01), Khanna
London Christopher L.
Parrott Ronald A.
Allen Kenneth R.
Nguyen Patricia T.
Pascal Robert
Townsend and Townsend / and Crew LLP
Verticom, Inc.
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