Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01P—WAVEGUIDES; RESONATORS, LINES, OR OTHER DEVICES OF THE WAVEGUIDE TYPE
  • H01P7/00—Resonators of the waveguide type
  • H01P7/10—Dielectric resonators
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03B—GENERATION OF OSCILLATIONS, DIRECTLY OR BY FREQUENCY-CHANGING, BY CIRCUITS EMPLOYING ACTIVE ELEMENTS WHICH OPERATE IN A NON-SWITCHING MANNER; GENERATION OF NOISE BY SUCH CIRCUITS
  • H03B5/00—Generation of oscillations using amplifier with regenerative feedback from output to input
  • H03B5/18—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising distributed inductance and capacitance
  • H03B5/1864—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element comprising distributed inductance and capacitance the frequency-determining element being a dielectric resonator

Definitions

• the present invention relates to a very high stability microwave oscillator.
• Signal generation for radar, telecommunications, frequency reference applications (such as atomic clocks Cs, Rb, etc.) as well as the evolution of digital systems require very high stability oscillators with increasing phase noise. weaker.
• An oscillator consists of two main elements: a resonator and an active or amplifier element.
• the phase noise of the oscillators is determined by the low frequency noise, the high frequency noise and the non-linearities of the active element, the overvoltage factor Q of the resonator and the coupling circuit between the oscillator and the active element.
• phase noise of the oscillators corresponds in the time domain to the jitter of the clocks which determines the ultimate precision of all the digital signal processing systems and, first of all, of the analog / digital encoders.
• the evolution of analog and digital systems, in particular the increase in speed and dynamics of analog / digital converters, requires a reduction in the phase noise of the oscillators and the time jitter of the clocks.
• reference oscillators are based on volume wave or surface wave acoustic resonators. These resonators are limited, for the most efficient of them, to frequencies of around 1000 MHz (in FBAR technology: “Film Bulk Acoustic Wave”). In addition to their technological limitations (thickness of the resonators), the acoustic resonators respect a fundamental physical law, according to which the product Q * f of the maximum overvoltage Q by the operating frequency f is a characteristic of the material used. This product can be evaluated by the theory of acoustic losses (enharmonic interactions of phonons). Typically, this product is around 10 THz. The overvoltage of FBAR type resonators is then limited to around 10 4 for an operating frequency of 1000 GHz.
• Electromagnetic resonators allow working directly at frequencies of several Ghz, but their quality factor is limited. For example, for conventional dielectric resonators, a product Q * f of 200 THz is obtained, and for sapphire gallery mode resonators, at room temperature, a product Q * f of 2500 THz. The values of phase noise are then close to those of multiplied acoustic sources: typically -120 dBc / Hz is obtained at a few kHz from the carrier for the best oscillators.
• the limit of the current oscillators corresponds to a measurement resolution of the analog / digital encoders of 8 coding bits at a frequency of approximately 1 GHz, which results in a phase jitter less than 0.3 ps.
• the systems currently contemplated would require coders at least 2 GHz with 10-bit resolution, with phase noise below -150 dBc / Hz at a modulation frequency of 1 kHz.
• Such performances can only be obtained with very high overvoltage resonators (Q> 10 6 at 10 GHz for example) associated with oscillator structures making it possible to preserve the intrinsic qualities of the resonators.
• the only known solution to increase the quality factor Q comprises oscillators based on cooled electromagnetic resonators.
• the present invention relates to a microwave oscillator with very high reference stability, of the resonator type, this resonator having only insignificant sensitivity to thermal vibrations and fluctuations.
• the oscillator according to the invention comprises a one-piece dielectric resonator in the form of a straight trunk of a cylinder hollowed out at mid-height along cords of its cross section, so as to leave a central core and two lateral cheeks, the holes being at symmetry of order N, with
• N> 4 at least the plane faces of the cylinder being covered with a superconductive material, the resonator being placed in a cryogenic enclosure and being connected to an amplifier by optimized couplings, the tuning of the resonator being effected by magnetic field and loop phase.
• FIGS. 1 to 3 are perspective views of three different embodiments of an oscillator resonator according to the invention.
• - Figures 4 and 5 are simplified diagrams of structures of resonator microwave oscillators which can be used by the invention
• - Figure 6 is a simplified diagram of a triple enclosure cryogenic system used by the invention.
• One of the essential elements of a microwave oscillator being its resonator, and the stability of this resonator being affected by mechanical deformations, the invention provides for making it differently from the usual embodiment.
• the usual structure of a known resonator generally comprises a cavity in the form of a cylinder trunk closed at its two ends by flat walls of lanthanum aluminate coated on one face with a monocrystalline superconductive material, for example in Y ⁇ Ba 2 Cu 3 O 7 containing the resonator proper with its sapphire support and centering base and two coupling ports to the cavity.
• a monocrystalline superconductive material for example in Y ⁇ Ba 2 Cu 3 O 7 containing the resonator proper with its sapphire support and centering base and two coupling ports to the cavity.
• the invention proposes to produce a monolithic dielectric resonator, generally in the form of a cylinder trunk, suitably hollowed out, with a deposit direct superconductive layers on the two flat faces of the cavity, before their machining.
• the electric field being concentrated on the central core, the degradation of the quality factor by the currents induced in the rest of the structure of the resonator is minimized.
• the material constituting the resonator is advantageously monocrystalline sapphire.
• the body 1 of the resonator is in the form of a diabolo with symmetry of revolution about its axis.
• This body basically has two cheeks 2, 3 in the form of discs connected together by a central core 4 formed integrally with them.
• a superconductive film is deposited on the flat faces 5, 6 of the cheeks 2, 3.
• the structure 7 represented in FIG. 2 is formed from a dielectric block 8 in the form of a straight trunk of a solid cylinder in which four holes 9 are made, the axes of which are situated in a plane perpendicular to the axis of the cylinder, mid -distance between the flat faces of the cylinder trunk.
• the axes of these holes follow a symmetry of order 4 with respect to the axis of the cylinder, so as to leave a large part of the cylindrical wall and a central core.
• a superconductive film 8a, 8b is deposited on the flat faces of the structure 7.
• the structure 10 shown in FIG. 3 is formed, like that of FIG. 2, from a dielectric block 11 in the form of a straight trunk of a solid cylinder in which five holes 12 are made, the axes of which are situated in a plane perpendicular to the axis of the cylinder, midway between the flat faces of the cylinder trunk.
• the axes of these holes follow a symmetry of order 5 with respect to the axis of the cylinder, so as to leave a large part of the cylindrical wall and a central core.
• This structure is generally preferred to that of FIG. 2.
• a superconductive film 11a, 11b is deposited on the planar faces of the structure 10.
• FIGS. 1 to 3 make it possible to very greatly attenuate the phase noise between 1 and 10 kHz, frequencies for which there is a coincidence between the acoustic wavelengths and the dimensions of the resonator cavity.
• the values predicted by Leeson's theory (according to which the quality factors of a cavity with superconductive layers can be estimated by the point of ascent of the colored noise above the thermal noise floor) then become accessible.
• the materials constituting these structures must have low dielectric losses and must be compatible with the deposition of the superconductive layers.
• - Z ⁇ is a Bessel function of second kind, of order i
• the change in volume can be calculated from the Young's modulus and the Poisson's ratio. In the general case, there is no analytical solution, because:
• the resonant frequency is a function of the diameter D of the central core, the height h and the permittivity ⁇ .
• the frequency fluctuations df related to the dimensions of the resonator cavity are given by:
• the structure of the oscillator is optimized by tending to fulfill the following requirements:
• the overvoltage factor (or: quality factor) in charge of the cavity would be substantially equal to half of the empty overvoltage factor.
• the electrical loop comprising the cavity, the amplifier and their connection connections must have a minimum length
• the electrical length is then 2 ⁇ , namely ⁇ due to the amplifier, ⁇ / 2 due to the input coupling of the cavity and ⁇ / 2 due to its output coupling.
• the amplifier must be integrated in the cooled enclosure of the cavity, as specified above.
• the amplifier is preferably in SiGe technology and cooled to very low temperature (critical temperature of the superconductive layers of the cavity). He then presents a very low noise (varying like the inverse of the working frequency).
• the coupling circuit between amplifier and cavity should include a varactor phase adjustment device. - Slaving of the total phase of the oscillation loop to obtain an operation at an optimal point for which the derivative of the phase with respect to the frequency is maximum.
• the output of the oscillator signal is preferably done on a third port of the cavity, as described below with reference to Figure 5.
• This characteristic guarantees a noise floor at -180 dBm / hz by taking a signal filtered by the resonator itself.
• This solution is advantageous as soon as the empty quality factor of the cavity exceeds 10 6 . It is then possible to load the cavity while keeping a high value for the quality factor under load.
• the coupling of this output port can be chosen such that the quality factor under load remains clearly greater than 1/3 of the quality factor when empty.
• FIG. 4 shows a first possibility of producing an oscillator with a cooled cavity.
• the cavity 13 with superconductive layers is cooled to a very low temperature, for example 77K, in an enclosure
• the amplifier device 16 conventionally comprises an amplifier circuit 17 followed by a coupler 18 and an isolator 19, and it comprises two tunable phase shifters 20, 21 connecting the elements 17 to 19 to the cables
• This embodiment requires relatively long connecting cables (they introduce a phase shift of 2k ⁇ , with k much greater than 1).
• the cables have a high temperature gradient. With one end at 72 K and the other at 300 K, these cables can generate noise, and the stability of the oscillator is not excellent.
• the invention proposes to use the structure of FIG. 5.
• the cavity 22 and the amplifier 23 are arranged in the same enclosure 24 cooled, to 77 K for example.
• the amplifier is connected to two coupling sockets 25, 26 of the cavity by very short, which means that the phase shift of 2k ⁇ between the input and the output of the cavity is low, k being minimal.
• the signal output 27 is made by means of a third coupling socket 28.
• the amplifier 23 may have a specific topology suitable for its mounting in the enclosure 24, as close as possible to the cavity, and it is easy to adapt the impedances of the cavity and the amplifier.
• This structure does not include tunable phase shifters, fine adjustment of the phase shift is less easy to achieve than in the case of the structure of Figure 4.
• the preferred structure of the invention is that of Figure 5 because of its many advantages compared to that of Figure 4.
• the invention uses a triple enclosure 29 as shown diagrammatically in FIG. 6.
• This triple enclosure comprises, from the outside to the inside, a first vacuum enclosure 30, of the Dewar vase type, having a role of thermal insulation, containing a second enclosure 31 under low pressure (1 Bar, at ambient temperature, for example) filled with a liquefiable gas or solidifiable at temperature operating (for example nitrogen or argon), this second enclosure containing the third enclosure 32, which is a sealed case containing helium or neon at very low temperature (for example 73K) and which contains the oscillator 33 of the invention.
• a first vacuum enclosure 30, of the Dewar vase type having a role of thermal insulation, containing a second enclosure 31 under low pressure (1 Bar, at ambient temperature, for example) filled with a liquefiable gas or solidifiable at temperature operating (for example nitrogen or argon), this second enclosure containing the third enclosure 32, which is a sealed case containing helium or neon at very low temperature (for example 73K) and which contains the oscill
• the residual pressure of the gas is controlled to ensure precise control of the temperature of the enclosure 32 by evaporation and sublimation of the gas, which condenses into a liquid or solid layer on the external surface of the housing 32.
• gas contained in the enclosure 32 must remain in the gas phase at the temperature prevailing in this enclosure, in order to ensure temperature uniformity throughout this enclosure and to avoid any condensation on the constituents of the oscillator (condensation would result in circuit losses and frequency shift).
• the cooling in the enclosure 32 is ensured with a minimum of vibrations, advantageously by pulsed tube and circulation of the gas (helium or neon gas).
• a thermal bridge is formed between the enclosure 32 and the oscillator using a flexible metallic braid, for example made of copper.
• the oscillator is suspended inside the housing 32 by a suspension system transmitting the minimum possible vibration.
• This system suspension comprises for example, in a manner known per se, suspension cables with springs and anti-resonant weights.

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• Control Of Motors That Do Not Use Commutators (AREA)
• Inductance-Capacitance Distribution Constants And Capacitance-Resistance Oscillators (AREA)

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• 2002
  • 2002-12-31 FR FR0216903A patent/FR2849558B1/fr not_active Expired - Lifetime
• 2003
  • 2003-12-15 AU AU2003299240A patent/AU2003299240A1/en not_active Abandoned
  • 2003-12-15 US US10/540,921 patent/US20060097807A1/en not_active Abandoned
  • 2003-12-15 WO PCT/EP2003/051014 patent/WO2004059831A1/fr not_active Ceased
  • 2003-12-15 EP EP03799573A patent/EP1579565A1/de not_active Withdrawn

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