Microwave cavity resonators and fixed geometry probes

Abstract

A fixed-geometry probe for exchanging microwave energy with a cavity resonator is easy to manufacture, reliable, and easily adjustable outside the cavity to select a coupling coefficient. The probe includes a transmission line that enters, circles, and exits the cavity resonator. A first end of the transmission line is outside the cavity resonator for connection to a microwave circuit to exchange microwave energy. A portion of the transmission line's outer conductor(s) is removed within the cavity resonator to form a fixed-geometry radiating element for exchanging microwave energy with the cavity resonator according to a coupling coefficient. The transmission line's outer conductors are connected to the cavity resonator on either side of the radiating element. A second end of the transmission line terminates outside the cavity resonator, with the termination impedance mismatched to the characteristic impedance of the transmission line to create a reflective stub. The coupling coefficient is controlled by the length of the reflective stub and the termination impedance.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority to U.S. Patent Application No. 17 / 887,043, filed August 12, 2022, which is incorporated by reference herein in its entirety.

[0002] The present invention relates to oscillators, particularly sapphire oscillators, and probes for exchanging microwave energy with cavity resonators for use in microwave circuits such as filters. [Background technology]

[0003] 2. Description of Related Art Modern radar and telecommunications systems require microwave frequency signal sources and signal processing systems with stringent performance requirements and spectral purity, specifically subsystems such as oscillators and filters that exhibit these characteristics.

[0004] Cavity resonators, by their very nature, offer the ability to distinguish wanted signals from unwanted ones. The purity and stability of the signal generated is directly related to the resonator used as the frequency determining device and depends on its Q-factor, power handling capability, and its resistance to vibration and temperature-related effects.

[0005] A microwave cavity resonator is typically a closed metallic structure (e.g., a metal cylinder) that resonates at a high amplitude at a specific set frequency called the resonant frequency. When microwave energy passes through the cavity, the resonator acts as a bandpass filter, passing microwave energy at the resonant frequency and blocking other nearby frequencies. The resonant frequency of a cavity resonator and the corresponding electric and magnetic field patterns or modes depend on its dimensions (length, width, height) and the permittivity and permeability of the materials from which it is constructed.

[0006] A piece of dielectric material, often called a "puck," can be placed inside the cavity resonator to improve its Q factor. Common dielectric materials include rutile, various proprietary ceramics based on complex oxides of titanium and rare metal elements, and sapphire. Sapphire is often preferred because it provides the highest Q factor of any known material in the microwave frequency range. The dimensions of the puck are often determined by experiment for a particular material and a defined mode and frequency at a determined temperature, or by solving Maxwell's equations.

[0007] A cavity resonator further contains one or more microwave coupling elements, known as "probes," that penetrate the cavity walls and exchange microwave energy with the field inside the cavity. Each probe couples microwave energy either inside or outside the cavity. A probe typically includes a radiating element inside the cavity and a transmission line for carrying microwave energy to and from the radiating element. The exchange of microwave energy is characterized by a coupling coefficient (energy in / energy out) that ranges from 0 to 1, with 1 being called "critical coupling." When the coefficient is close to zero, most of the microwave energy is reflected. When the coefficient is close to 1, the probe is said to be well-matched, with only a small amount reflected. A well-known formula relates the coupling parameter to the scattering or "S" parameter used to model the behavior of cavity resonators. For additional details regarding S-parameters and coupling coefficients, see James D. Anstie, "A 50 K Dual-Mode Sapphire Oscillator and Whispering Spherical Mode Oscillators," Ph.D. Thesis, University of Western Australia, April 2006.

[0008] The coupling coefficient is highly sensitive to the geometry and dimensions of the probe. Typically, probe design is primarily an empirical tuning exercise until the final geometry and dimension set is determined. The final performance of the probe is generally subject to variations due to manufacturing tolerances of both the probe and the cavity. A commonly used probe is a coaxial cable terminated with a "continuous loop" of wire connecting the inner and outer conductors. Microwave energy is exchanged between the continuous loop (the radiating element) and the cavity. Once the loop dimensions are fixed, the coupling coefficient can be adjusted by changing the penetration depth of the probe into the cavity.

[0009] As shown in FIG. 1 , an embodiment of a cavity resonator 10 includes input and output continuous loop probes 12 and 14 that penetrate the cavity resonator wall. The resonator is suitably a conductive (metal) cylinder 16 with a central post structure 18 (e.g., a pair of opposing posts attached to the top and bottom of the resonator with a gap separating the posts) that holds an annular sapphire puck 20 in the center of a cavity 22 defined by the resonator's inner wall. The cavity resonator and puck are dimensioned to provide the desired resonant frequency. Each probe includes a coaxial cable 24 having an inner conductor 26, a dielectric cladding layer 28, and an outer conductor 30. A continuous loop of wire 32 connects the inner conductor 26 to the outer conductor 30, forming a radiating element. Because the diameter of the continuous loop may be constrained by the diameter of the coaxial cable, the cable "fits" through the hole in the wall unless more complex assembly techniques are used.

[0010] Microwave energy 40 enters the cavity 22 through the coaxial cable 24 of the input probe 12 and is injected into the cavity via the continuous loop of wire 32 that forms the radiating element. The microwave energy entering the cavity generates an electric field (E) 42 and a magnetic field (B) 44 within the cavity 22, inducing current flow in the walls of the cavity resonator. Microwave energy 50 is extracted from the cavity via the continuous loop of wire 32 of the output probe 14. The penetration depth of each probe into the cavity 22 is adjusted 52, 54 to change the coupling coefficient to a desired value by varying the geometry of the probe inside the cavity. Typically, the probes are aligned so that their near-field radiation patterns are aligned with the direction of energy flow into the resonant field to provide a useful coupling level. Summary of the Invention

[0011] The following is a summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description and claim definitions that are presented later.

[0012] The present invention provides a fixed geometry probe for exchanging microwave energy with a cavity resonator that is easier to manufacture, more reliable, and easily adjustable outside the cavity to select a coupling coefficient without changing the fixed geometry of the radiating element inside the cavity.

[0013] The probe includes a transmission line that enters, circles, and exits a microwave cavity resonator. A first end of the transmission line is outside the cavity resonator for connection to a microwave circuit to exchange microwave energy. A portion of the transmission line's outer conductor(s) is removed within the cavity resonator to form a fixed geometry radiating element for exchanging microwave energy with the cavity resonator according to a coupling coefficient. The outer conductors of the transmission line are connected to the cavity resonator on either side of the radiating element. A second end of the transmission line terminates outside the cavity resonator with a termination impedance mismatched to the characteristic impedance of the transmission line to form a reflective stub. The coupling coefficient is controlled by the length of the reflective stub and the termination impedance.

[0014] In different embodiments, the reflection stub may be terminated with an open circuit, a short circuit, or a finite impedance value. In all three cases, the length of the reflection stub may be trimmed externally to the cavity resonator to control the coupling coefficient. For example, the reflection stub may be nominally designed to provide a coupling coefficient near zero and then trimmed to increase the coupling until the desired coefficient is reached. For the short circuit and finite impedance cases, each iteration requires removing the termination impedance, trimming the transmission line, and replacing the termination impedance. Open circuit termination is preferred because it is much simpler.

[0015] In embodiments in which the reflective stub terminates in a finite termination impedance, the coupling coefficient may be selected or adjusted by the value of the finite termination impedance. This value can be adjusted, for example, by replacing the termination impedance with one of different impedance values ​​or by using electrical impedance tuning. In these cases, the length of the reflective stub is fixed.

[0016] In an embodiment, the transmission line is a coaxial cable including an inner conductor, a dielectric layer around the inner conductor, and an outer conductor around the dielectric layer. The coaxial cable enters the cavity through a first hole in the boundary wall of the cavity resonator, forms a U-shaped bend at the radiating element (with the outer conductor removed), and exits through a second hole in the boundary wall. The outer conductor is suitably soldered or brazed to connect it to the boundary wall.

[0017] In an embodiment, the transmission line is a stripline including a planar inner conductor sandwiched between dielectric layers sandwiched between planar outer conductors. The planar inner conductor is patterned to form a U-shaped transmission line. The planar outer conductor is removed inside the cavity resonator to form the radiating element. The outer conductor(s) are appropriately soldered or brazed to connect the conductor(s) to the boundary wall. The stripline may terminate within a set or adjustable termination impedance to control the coupling coefficient. Alternatively, the stripline may transition to a non-stripline transmission line such as microstrip, coplanar waveguide, coaxial cable, or other topology where trimming of the length is more easily achieved to control the coupling coefficient.

[0018] In an embodiment, the sapphire oscillator includes input and output probes configured to inject and extract microwave energy from a cavity resonator including a sapphire puck. The fixed geometry probes may be independently set or adjusted to provide desired input and output coupling coefficients. The input probe is configured to receive amplified microwave energy at a microwave operating frequency, and the output probe is configured to deliver microwave energy at the microwave operating frequency to a microwave coupler configured to send a first portion of the microwave energy into the loop and a second portion to the output. A phase shifter shifts the phase N around the loop to meet the conditions for the sapphire oscillator to oscillate. *The mode-selective filter is configured to select a mode of the cavity resonator at a microwave operating frequency, and the amplifier is configured to provide amplified microwave energy to the cavity resonator and provide sufficient gain for the loop to oscillate.

[0019] These and other features and advantages of the present invention will be apparent to those skilled in the art from the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0020] [Figure 1] As noted above, a cavity resonator is shown that utilizes a continuous loop probe to exchange microwave energy with the cavity resonator. [Figure 2] 1 illustrates an embodiment of a fixed geometry probe formed from a coaxial cable for exchanging microwave energy with a cavity resonator. [Figure 3] 1 shows an embodiment of the probe in which a reflective stub of coaxial cable is terminated as an open circuit, the length of which controls the coupling coefficient. [Figure 4] 1 illustrates an embodiment of a method for trimming the length of a reflective stub to achieve a desired coupling coefficient. [Figure 5] 1 illustrates an embodiment of a probe in which a reflective stub terminates in a termination impedance whose selectable or adjustable value controls the coupling coefficient. [Figure 6] 10 illustrates an embodiment of a method for selecting or adjusting termination impedance to achieve a desired coupling coefficient. [Figure 7] 10 illustrates an embodiment of a probe formed from a stripline transitioning to a non-stripline transmission line for exchanging microwave energy with a cavity resonator. [Figure 8] 1 shows an embodiment of a sapphire oscillator in which a probe is used to both inject and extract microwave energy into and from the cavity resonator. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention provides a fixed geometry probe for exchanging microwave energy with a cavity resonator that is easier to manufacture, more reliable, and easily adjustable outside the cavity to select a coupling coefficient without changing the fixed geometry of the radiating element inside the cavity. The fixed geometry probe is configurable for frequencies in the microwave band between 1 and 50 GHz and can be used in single or dual probe configurations for oscillators, filters, or other microwave circuits.

[0022] 2, in an embodiment, a cavity resonator and probe structure 100 includes a dielectrically loaded cavity resonator 102 and input and output probes 104 and 106 that inject microwave energy 80 into and extract microwave energy 82 from the cavity resonator, respectively. In this particular embodiment, which may be suitable for a whispering-gallery mode configuration, the cavity resonator 102 includes a conductive (metal) cylinder 84 with a central post structure 86 (e.g., a pair of opposing posts attached to the top and bottom of the resonator with a gap separating the posts) that holds an annular sapphire puck 88 in the center of a cavity 90 defined by the resonator's bounding walls 92.

[0023] The input (output) probe 104 (106) includes a transmission line 108 (110), such as a coaxial cable, that enters, circles, and exits the cavity resonator 102. A first end 112 (114) of the transmission line is outside the cavity resonator for connection to a microwave circuit (microwave source) to exchange microwave energy. A portion of the transmission line's outer conductor(s) 120 (122) is removed within the cavity resonator to form a fixed-geometry radiating element 116 (118) for exchanging microwave energy with the cavity resonator according to a coupling coefficient. The transmission line's outer conductor 120 (122) is connected (e.g., by soldering or brazing) to the boundary walls 92 of the cavity resonator 102 on either side of the radiating element. Preferably, the entire outer conductor within the cavity is removed so that the radiating element extends from wall to wall, minimizing / eliminating any additional conductor within the cavity. Any remaining outer conductor may degrade the cavity, complicate the design, or reduce the coupling coefficient. The second end 124 (126) of the transmission line terminates outside the cavity resonator, with a termination impedance 128 (130) mismatching the characteristic impedance Z of the transmission line to form a reflective stub 132 (134). The coupling coefficient is controlled by the length 136 (138) of the reflective stub and the value of the termination impedance 128 (130).

[0024] The reflective stub functions as a reflecting element, but with some phase shift associated with the stub's length and the value of the termination impedance. Thus, there are two components of microwave energy traveling through the radiating element: a component 140 (142) that is exchanged with the microwave source or forwarded by the cavity resonator, and a component 144 (146) that is reflected by the stub. The phase relationship between these components may be such that they are "in phase," resulting in enhanced radiation contributions from each other, resulting in better power transfer into the cavity, or they may be "out of phase," resulting in reduced radiation into the cavity. In the latter case, the signal reflected by the stub exits the probe, increasing the reflection seen at the probe input, as would be expected in a poorly coupled probe.

[0025] In different embodiments, the reflection stub may be terminated with an open circuit, a short circuit, or a finite impedance value. In all three cases, the length of the reflection stub may be trimmed outside the cavity resonator to control the coupling coefficient without changing the fixed geometry of the radiating element within the cavity resonator. For example, the reflection stub may be nominally designed to provide a coupling coefficient near zero and then trimmed to increase the coupling until the desired coefficient is reached. For the short circuit and finite impedance cases, each iteration requires removing the termination impedance, trimming the transmission line, and replacing the termination impedance. Open circuit termination is usually preferred because it is much simpler.

[0026] In embodiments in which the reflective stub terminates in a finite termination impedance, the coupling coefficient may be selected or adjusted by the value of the finite termination impedance. This value can be adjusted, for example, by replacing the termination impedance with one of different impedance values ​​or by using electrical impedance tuning. In these cases, the length of the reflective stub is fixed.

[0027] Whether trimming the length of the reflective stub or changing / adjusting the value of the termination impedance, the geometry of the radiating element within the cavity remains unchanged once the outer conductor is connected to the interior boundary wall of the cavity resonator. Trimming the length of the reflective stub or changing the termination impedance has the effect of adjusting the coupling coefficient by changing the pattern of current flowing into the probe during a cycle of the microwave energy waveform, thereby improving the manufacturability and reliability of the probe structure.

[0028] Microwave energy 80 enters cavity 118 through input probe 104, where it is injected into the cavity via input probe radiating element 116. The entry of microwave energy into the cavity creates an electric field (E) 152 and a magnetic field (B) 154 within cavity 90, inducing currents in the cavity resonator walls and sapphire puck 88. For example, in a whispering gallery mode resonator, the field pattern consists of a series of nodes and antinodes spaced azimuthally around the cavity, depending on the mode order selected. Orders between 5 and 15 are typically used for sapphire-loaded cavity resonators. To visualize the field pattern in one such resonator, at each antinode, the B field circulates in the equatorial plane, the E field circulates in the radiation plane containing the cavity axis, and the induced currents in the surfaces of the cavity walls also circulate coherently with the E field. Microwave energy 82 is extracted from the cavity via the output probe's radiating elements 118. Typically, the probes are aligned so that their near-field radiation patterns are aligned with the direction of energy entry into the resonant field at the probe location.

[0029] In this embodiment, the cavity resonator 102 includes a bottom portion including a circular base, a boundary wall 92 extending from the base the entire length of the cavity, a bottom portion of a post structure 86 extending slightly less than halfway from the base, and a top portion including a lid and a top portion of the post structure 86 extending slightly less than halfway. Holes are formed through the walls within the bottom portion of the cavity resonator. Each probe is inserted through the hole, preferably swiveling inside the cavity to form a U-shaped bend, and exiting the pair holder. An outer conductor is removed within the cavity and connected (e.g., soldered or brazed) to the boundary wall 92 on either side of the radiating element to fix the radiating element geometry. Other configurations and assembly methods will be apparent to those skilled in the art.

[0030] As shown, a pair of input and output probes is typically used to inject and extract microwave energy from the cavity resonator. However, a single probe can be used to inject microwave energy into the cavity resonator, with the reflected component providing the output microwave energy. For example, this type of cavity can be used as a frequency discriminator to control another oscillator or microwave signal source.

[0031] Referring now to FIG. 3 , in an embodiment, probe 200 includes a coaxial cable 202 having an inner conductor 204, a dielectric layer 206, and an outer conductor 208 arranged to provide a characteristic impedance at microwave operating frequencies. A first end 210 of coaxial cable 202 is outside the cavity resonator and configured for connection to a microwave circuit (source) for exchanging microwave energy. The coaxial cable enters the cavity resonator through a hole 211 in a wall 212, makes a turn, and exits the cavity resonator through a hole 213 in wall 212. In some embodiments, the coaxial cable may enter or exit a different wall. The coaxial cable continues to a second end 214 that is outside the cavity resonator. A portion of outer conductor 208 is detached from the coaxial cable within the cavity resonator to form a fixed geometry radiating element 216 for exchanging microwave energy with the cavity resonator according to a coupling coefficient. The outer conductor 208 is connected to the cavity resonator walls 212 on either side of the radiating element, preferably at the cavity's internal boundary walls 215. In this embodiment, the second end 214 of the coaxial cable is terminated in an open circuit that mismatches the characteristic impedance to form a reflective stub 218 that constitutes a length 220 of coaxial cable between the radiating element 216 and the second end 214. The coupling coefficient is controlled by the length 220 of the reflective stub 218.

[0032] Referring now to FIG. 4, the coupling coefficient can be controlled by trimming the length of the coaxial cable, or more generally, the transmission line. This is true whether the reflective stub is terminated with an open circuit, a short circuit, or a finite termination impedance, although an open circuit is a simpler procedure. In step 300, the coaxial cable is inserted through a hole in the wall of the cavity resonator to form a reflective stub of some nominal length. Most commonly, the nominal length can be arbitrary, as long as it is sufficient to span the desired range of coupling coefficient performance, e.g., the full range from 0 to 1, or a portion thereof (step 302). Alternatively, the nominal length can be calculated or determined by experiment to provide a length that approximates the desired coupling coefficient plus some extra length to account for design and manufacturing tolerances (step 304). In another approach, the nominal length can be calculated as the length of the reflective stub to set the desired coupling coefficient to zero or near zero by first trimming the length of the reflective stub (step 305), so that the coefficient always increases.

[0033] Once the coaxial cable is positioned and connected to the walls of the cavity resonator such that the geometry of the radiating element inside the cavity is fixed, the coupling coefficient is measured (step 306). This may be accomplished, for example, by connecting the cavity resonator and probe to a vector network analyzer that measures not only the scattering (S) parameters (S, which is the complex ratio of reflected power to input power, S, which is the ratio of power received at port 2 to power received at port 1, etc.), but also the Q of the coupled cavity. A set of known equations converts the Q and S parameters into coupling coefficients.

[0034] If the measured coefficient is within a specified range of the desired coefficient value (step 308), the process stops (step 310). In some applications, the design equations may be sufficiently well specified and the manufacturing process for the cavity resonator and probe may be sufficiently controlled so that the nominal length (when designed for the desired coupling coefficient) is sufficient without further trimming.

[0035] Otherwise, if the reflective stub terminates in an open circuit, the length of the reflective stub (e.g., the tail of a coaxial cable) is trimmed by some incremental amount (step 312), the coupling coefficient is measured (step 306), and the process is repeated until the desired value of the coupling coefficient is obtained in step 308. This is a very simple process for accurately and reliably adjusting the probe to the desired coupling coefficient without changing the geometry of the radiating element inside the cavity.

[0036] If instead the reflective stub is terminated in a short circuit, for example by placing a shorting cap over the exposed end of the coaxial cable, the shorting cap must be removed (step 314), the length of the reflective stub trimmed (step 316), and the shorting cap replaced (step 318), and this process repeated until the desired coupling coefficient is achieved. Note that the same process applies if the reflective stub is terminated with a fixed, finite impedance. In any case, when operating in the microwave frequency range, repeatedly removing and replacing terminating impedances is generally undesirable, which is why open-circuit termination is preferred.

[0037] Referring now to FIG. 5 , in an embodiment, a probe 400 includes a coaxial cable 402 having an inner conductor 404, a dielectric layer 406, and an outer conductor 408 arranged to provide a characteristic impedance at microwave operating frequencies. A first end 410 of the coaxial cable 402 is outside the cavity resonator and configured for connection to a microwave circuit (source) for exchanging microwave energy. The coaxial cable enters the cavity resonator through a wall 412, makes a turn, and exits the cavity resonator through another wall 412. In some embodiments, the coaxial cable may enter and exit a different wall. The coaxial cable continues to a second end 414 that is outside the cavity resonator. A portion of the outer conductor 408 is from the coaxial cable that resides within the cavity resonator to form a fixed geometry radiating element 416 for exchanging microwave energy with the cavity resonator according to a coupling coefficient. The outer conductor 408 is connected to the cavity resonator walls 412 on either side of the radiating element. In this embodiment, the second end 414 of the coaxial cable is terminated with a finite termination impedance 417 that mismatches the characteristic impedance to form a length 420 of coaxial cable between the radiating element 416 and the second end 414 and a reflective stub 418 that defines the termination impedance 417. The length of the reflective stub is fixed. The coupling coefficient is controlled by setting or adjusting the termination impedance 417.

[0038] Referring now to FIG. 6, the coupling coefficient can be controlled by selecting or adjusting the value of the termination impedance. In step 500, a coaxial cable is inserted through a hole in the wall of the cavity resonator to form a reflective stub of some nominal length. The nominal length is appropriately calculated based on the nominal value of the termination impedance to generate the desired coupling coefficient. In step 502, a termination impedance having a value at or near the nominal value is connected to the reflective stub. The coupling coefficient is measured (step 504) and compared to the desired value of the coupling coefficient (step 506). If the measured coefficient is close enough, the process stops (step 508). Otherwise, the value of the termination impedance is adjusted. In one case, the current termination impedance is disconnected and a different value termination impedance is connected to the reflective stub (step 510). In another case, the value of the termination impedance is adjusted directly (step 512), e.g., an electronically tuned impedance, such as may be provided by a reverse-biased varactor diode, within a microwave circuit on a printed circuit board (PCB) connected to the coaxial cable.

[0039] 7, in another embodiment, a probe 600 is formed as a stripline 602 passing through an aperture 603 in a boundary wall 606 of a cavity resonator 605. The stripline 602 includes a planar inner conductor 604 sandwiched between dielectric layers 607 and 608 and planar outer conductors 610 and 612. The planar inner conductor 604 is patterned to form a U-shaped transmission line. The planar outer conductors 610 and 612 are patterned to remove conductive material inside the cavity to form a radiating element 616 in a U-shaped bend. The planar outer conductors 610 and 612 are connected to the boundary wall 606 on either side of the aperture 603. The stripline may be terminated in a set or adjustable termination impedance to control the coupling coefficient. Alternatively, the stripline may transition to a non-stripline transmission line 618 such as microstrip, coplanar waveguide, coaxial cable, or other topology that can be easily trimmed to control the coupling coefficient. As shown, the microstrip 618 is formed by removing a portion of one of the outer conductor and dielectric layers to expose the planar inner conductor 604, the width of which is adjusted to maintain the characteristic impedance Z0 within the microstrip.

[0040] Referring now to FIG. 8 , a fixed geometry probe can be configured to exchange microwave energy with a cavity resonator within a sapphire oscillator 700. In an embodiment, the sapphire oscillator 700 includes fixed geometry input and output probes 702 and 704 configured to inject microwave energy into and extract microwave energy from a cavity resonator 706 that includes a sapphire puck 708. The fixed geometry probes may be independently set or adjusted to provide desired input and output coupling coefficients, as previously described. The input probe is configured to receive amplified microwave energy 710 at a microwave operating frequency, and the output probe is configured to deliver microwave energy 712 at the microwave operating frequency to a microwave coupler 714, which is configured to send a first portion 716 of the microwave energy into the loop and a second portion 718 to the output. A phase shifter 720 shifts the phase N around the loop to meet the conditions for the sapphire oscillator to oscillate. * 2pi. The mode selection filter 722 is configured to select a mode of the cavity resonator at the microwave operating frequency, and the amplifier 724 is configured to provide amplified microwave energy 710 to the cavity resonator sufficiently to cause the loop to oscillate.

[0041] While several illustrative embodiments of the present invention have been shown and described, numerous variations and alternative embodiments will occur to those skilled in the art. Such variations and alternative embodiments are contemplated and can be made without departing from the spirit and scope of the present invention as defined in the appended claims.

Claims

1. A microwave frequency discriminator element, comprising: a microwave cavity resonator configured to resonate at a microwave operating frequency; a probe including a transmission line having an inner conductor, one or more dielectric layers, and one or more outer conductors arranged to provide a characteristic impedance at the microwave operating frequency; the transmission line has a first end located outside the cavity resonator configured for connection to a microwave circuit, the transmission line entering the cavity resonator, making a turn, exiting the cavity resonator, and continuing to a second end located outside the cavity resonator, a portion of the one or more outer conductors being detached from the transmission line located within the cavity resonator to form a fixed geometric radiating element for injecting microwave energy into the cavity resonator according to a coupling coefficient; the outer conductor is connected to the cavity resonator on both sides of the radiating element, the second end of the transmission line is terminated outside the cavity resonator, and a termination impedance by a reflective stub including a length of the transmission line between the radiating element and the second end is mismatched with the characteristic impedance of the transmission line; A microwave frequency discriminator element, wherein the coupling coefficient is controlled by the length and termination impedance of the reflective stub.

2. 2. The microwave frequency discriminator element of claim 1, wherein the transmission line is a coaxial cable, the coaxial cable entering the cavity through a first hole in a boundary wall of the cavity resonator and exiting through a second hole in the boundary wall, the portion of the coaxial cable defining the radiating element present within the cavity resonator having a U-shaped bend.

3. 2. The microwave frequency discriminator element according to claim 1, wherein the transmission line is a stripline passing through an aperture in a boundary wall of the cavity resonator.

4. 4. The microwave frequency discriminator element of claim 3, wherein the second end of the stripline couples to a non-stripline transmission line having an inner conductor whose length is adjustable to set the coupling coefficient.

5. 2. The microwave frequency discriminator element of claim 1, wherein the termination impedance is either an open circuit or a short circuit, and the length of the reflective stub is selected or trimmed to set the coupling coefficient.

6. 2. The microwave frequency discriminator element of claim 1, wherein the termination impedance is an open circuit, the length of the reflective stub sets the coupling coefficient near zero, and the reflective stub is trimmed to set the coupling coefficient.

7. 2. The microwave frequency discriminator element of claim 1, wherein the value of the termination impedance is selected or adjusted to set the coupling coefficient, and the length of the reflective stub is fixed.

8. 2. The microwave frequency discriminator of claim 1, wherein a first component of microwave energy travels through the radiating element and a second component of microwave energy is reflected by the reflective stub, and the length of the reflective stub or the value of the termination impedance determines a phase relationship between the first and second components to set the coupling coefficient.

9. 1. A method for injecting microwave energy into a cavity resonator, comprising: providing a probe including a transmission line having an inner conductor, one or more dielectric layers, and one or more outer conductors arranged to provide a characteristic impedance at microwave operating frequencies; removing the outer conductor from a central portion of the transmission line to form a radiating element; inserting the probe into the cavity resonator such that the transmission line enters the cavity resonator leaving a first end outside the cavity resonator for connection to a microwave circuit, positions the radiating element inside the cavity resonator to inject microwave energy into the cavity resonator according to a coupling coefficient, and exits the cavity resonator to a second end outside the cavity resonator; connecting outer conductors of the probe to the cavity resonator on either side of the radiating element to fix the geometry of the radiating element; The second end of the transmission line is terminated outside the cavity resonator, and a termination impedance due to a reflective stub including a length of the transmission line between the radiating element and the second end is mismatched with the characteristic impedance of the transmission line; After connecting the probe to the cavity resonator, trimming the length of the reflective stub or adjusting the termination impedance to control the coupling coefficient; The method comprising:

10. the transmission line is a coaxial cable, and the inserting of the probe includes inserting the coaxial cable through a first hole in a boundary wall of the cavity resonator; bending the coaxial cable to have a U-shaped bend at the radiating element; and inserting the coaxial cable through a second hole in the boundary wall to position the reflective stub outside the cavity resonator.

10. The method of claim 9, comprising:

11. 10. The method of claim 9, wherein the transmission line is a stripline, and the inserting the probe includes passing the stripline through an aperture in a bounding wall of the cavity resonator.

12. 12. The method of claim 11, further comprising coupling the second end of the stripline to a non-stripline transmission line having an inner conductor whose length is adjustable to set the coupling coefficient.

13. 10. The method of claim 9, wherein the termination impedance is an open circuit, the length of the reflective stub sets the coupling coefficient near zero, and the reflective stub is trimmed to set the coupling coefficient.

14. 10. The method of claim 9, wherein a first component of microwave energy travels through the radiating element and a second component of microwave energy is reflected by the reflective stub, and the length of the reflective stub or the value of the termination impedance determines a phase relationship between the first and second components to set the coupling coefficient.

15. a microwave cavity resonator including a sapphire puck configured to resonate at a microwave operating frequency; input and output probes configured to inject and extract microwave energy from the cavity resonator, respectively, each of the probes including a transmission line having an inner conductor, one or more dielectric layers, and one or more outer conductors arranged to provide a characteristic impedance at the microwave operating frequency, the transmission line having a first end located outside the cavity resonator, the transmission line entering the cavity resonator, making a turn, exiting the cavity resonator, and continuing to a second end located outside the cavity resonator, a portion of the one or more outer conductors coupled to a microphone according to a coupling coefficient; the input and output probes being detached from the transmission line present within the cavity resonator to form a fixed geometric radiating element for injecting microwave energy into the cavity resonator, the outer conductor being connected to the cavity resonator on both sides of the radiating element, the second end of the transmission line terminating outside the cavity resonator, a termination impedance by a reflective stub including a length of the transmission line between the radiating element and the second end mismatching the characteristic impedance of the transmission line, and the coupling coefficient being controlled by the length and termination impedance of the reflective stub; a first end of the transmission line for the input probe configured to receive amplified microwave energy at the microwave operating frequency; a first end of the transmission line for the output probe configured to deliver microwave energy at the microwave operating frequency to a microwave coupler configured to send a first portion of the microwave energy into a loop and a second portion to an output; The phase is adjusted N around the loop to satisfy the condition for the sapphire oscillator to oscillate. ＊ a phase shifter configured to set the phase to 2pi; a mode selection filter configured to select a mode of the cavity resonator at the microwave operating frequency; an amplifier for supplying the amplified microwave energy to the cavity resonator and providing sufficient gain for the loop to oscillate; A sapphire oscillator comprising:

16. 16. The sapphire oscillator of claim 15, wherein the transmission lines for the input and output probes are coaxial cables that enter the cavity through a respective first hole in the cavity resonator and exit through a respective second hole in the cavity resonator, the portion of the coaxial cable that defines the radiating element present in the cavity resonator has a U-shaped bend, and the second end of the coaxial cable terminates in an open circuit, the length of which is selected or trimmed to set the respective coupling coefficient.

17. 16. The sapphire oscillator of claim 15, wherein the transmission lines for the input and output probes are striplines passing through respective apertures in the cavity resonator, the second end of each stripline coupling to a non-stripline transmission line, the non-stripline transmission line terminating in an open circuit and having an inner conductor whose length is adjustable to set the coupling coefficient.

18. 16. The sapphire oscillator of claim 15, wherein the termination impedance is an open circuit and the length of the reflective stub is trimmed to set the coupling coefficient.

19. 20. The sapphire oscillator of claim 18, wherein the length of the reflective stub sets an initial coupling coefficient near zero, and the reflective stub is trimmed to set the coupling coefficient.

20. 16. The sapphire oscillator of claim 15, wherein a first component of microwave energy travels through the radiating element and a second component of microwave energy is reflected by the reflective stub, and the length of the reflective stub or the value of the termination impedance determines a phase relationship between the first and second components to set the coupling coefficient.

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