Abstract:
The invention concerns a tunable cavity resonator that comprises a resonator body ( 2, 3, 4 ) defining a cavity ( 5 ), a tuning plate ( 28 ) whose position with respect to the resonator body ( 2, 3, 4 ) is modifiable and which influences the resonance frequency (ω R ) of the cavity resonator, and an adjustment device ( 22, 26 ) for mechanically changing the position of the tuning plate ( 28 ), which is characterized in that a conversion ratio mechanism ( 18, 20 ) couples the adjustment device ( 22, 26 ) to the tuning plate ( 28 ) in terms of movement and converts a linear excursion (Δx 1 ) generated by the adjustment device ( 22, 26 ), at a predefined ratio (U), into a reduced linear excursion (Δx 2 ) that acts on the tuning plate ( 28 ), the conversion ratio mechanism ( 18, 20 ) comprising a first spring element ( 20 ) whose end toward the adjustment device is deflectable with the linear excursion (Δx 1 ) generated by the adjustment device ( 22, 26 ), and a second spring element ( 18 ) which impinges with an opposing force on the end of the first spring element ( 20 ) remote from the adjustment device.

Description:
FIELD OF THE INVENTION 
     The invention concerns a tunable cavity resonator as defined in the preamble of claim 1. The invention further concerns a tunable microwave oscillator that uses a cavity resonator of such a kind. 
     BACKGROUND OF THE INVENTION 
     Tunable cavity resonators are used, inter alia, in microwave oscillators that are utilized to generate carrier signals in microwave communication. Such oscillators substantially comprise a microwave amplifier that is operated in feedback, and a high-quality cavity resonator that is located in the feedback path of the oscillator and filters out phase noise generated in the amplifier. A microwave oscillator of this kind furthermore uses a mechanical or electrical phase shifter to adjust the phase condition in the feedback path, and a high-frequency coupler to couple out the useful signal (carrier signal). 
     Adjustment of the oscillator frequency is accomplished in two stages: For coarse adjustment, first the resonance frequency of the tunable cavity resonator is modified in suitable fashion. This is done by means of the adjustment device with which the position of the tuning plate with respect to the resonator body is displaced. For fine adjustment of the oscillator frequency, the oscillator frequency is then shifted in controlled fashion within the resonance width of the tuned cavity resonator, using the phase shifter to displace the phase in the feedback path of the oscillator. 
     One difficulty with this type of two-stage toning of an oscillator results from the fact that the maximum frequency excursion achievable by phase adjustment is relatively small, for example only approximately 100 kHz for resonator qualities above 10 4  (i.e. Q&gt;10 4 ). Complete tunability of the microwave oscillator can only be achieved, however, if the minimum frequency change achievable in the context of resonance frequency tuning (i.e. cavity resonator tuning) is less than the aforementioned maximum frequency excursion when varying the phase in the feedback path of the oscillator. To meet this criterion, cavity resonators with an extremely high tuning accuracy are required. 
     It must be considered in this context that as the quality Q of a cavity resonator increases, the requirements in terms of the adjustment accuracy of the tuning mechanism in order to achieve a defined tuning accuracy also increase. 
     In practice, therefore, difficulties often occur in terms of the physical design of the tuning mechanism; and it has been found that the desired high adjustment accuracies, in combination with the necessary vibration resistance and good tuning reproducibility, are not always achieved. 
     The publication entitled “Temperature compensated high-Q dielectric resonators for long term stable low phase noise oscillators,” Proceedings of the 1997 Frequency Control Symposium, I. S. Ghosh et al., pp. 1024-1029, describes a tunable cavity resonator as defined in the preamble of claim 1. This cavity resonator, with a quality Q≈10 5 , meets the tuning accuracy requirements necessary for continuous tunability of a microwave oscillator. 
     DE 1 687 62 discloses an apparatus for adjusting the spacing between a stationary and a movable partition element of a cavity resonator, a lever that is in engagement with the movable partition via a bearing element being arranged rotatably on the stationary partition element. The lever is displaced via a conically tapering segment. The wall of the cavity resonator is thereby moved in order to retune the frequency of the resonator. The linear excursion through which the lever travels at its free end is converted at the wall of the resonator into a reduced linear excursion. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to create a cavity resonator that possesses high adjustment accuracy in terms of its resonance frequency. The intention is, in particular, to make available a cavity resonator that exhibits high quality and nevertheless makes possible complete tunability of a microwave oscillator when used therein. A further purpose of the invention is to create a completely tunable microwave oscillator having a high-quality cavity resonator. 
     The features of claims 1 and 12 are provided in order to achieve the object. The result of the conversion ratio mechanism provided according to the present invention is that upon an actuation of the adjustment device, it is not the linear excursion generated by the adjustment device, but rather a linear excursion reduced with respect thereto, that adjusts the tuning plate. The consequence of this is that the minimum excursion change attainable with the adjustment device is transformed into an even smaller minimum excursion change acting on the tuning plate. As a result, the adjustment accuracy of the tuning plate is increased, compared to the adjustment accuracy of the adjustment device, by an amount equivalent to the predefined ratio of the conversion ratio mechanism. The predefined ratio (i.e. the transmission ratio) is determined by the spring constants of the two spring elements. The use of two spring elements pressing against one another has the advantage that the conversion ratio mechanism operates continuously and in a manner largely free of backlash. 
     In this instance, a particularly preferred variant embodiment is characterized in that the first spring element is formed from at least one cup spring, and the second spring element is implemented by a plate spring that is immobilized at the periphery and impinged upon centrally by the cup spring. A spring mechanism of this kind can be designed with sufficient stiffness to be insensitive to external shock or vibrations. In addition, suitable cup and plate springs can easily be manufactured with the requisite high spring constants. 
     The adjustment device preferably comprises an, in particular, manually actuable mechanical actuating element and a first electromechanical actuating element, in particular a first piezoelement, downstream from the mechanical actuating element. The first electromechanical actuating element makes possible electrical activation of the adjustment device, which is advantageous in particular when the adjustment device is operated in a closed-loop mode for adjustment of the resonance frequency ω R . The electromechanical actuating element can also be used, for example, to compensate for temperature-related drift, and can moreover, within a limited excursion range, eliminate the need for an actuation of the mechanical actuating element. 
     The tuning plate is preferably made of a dielectric material, in particular sapphire. A tuning plate of this kind has very low dielectric losses especially at low temperatures, so that the quality achievable for the cavity resonator (defined as the product of the resonance frequency OR times the quotient of the field energy stored in the resonator and the power dissipation occurring in the resonator) is high (Q≈10 7 ). 
     The positionally adjustable tuning plate according to the present invention can also, in principle, be a wall element (for example the cover wall) of the cavity resonator. A particularly preferred exemplary embodiment of the invention is, however, characterized in that a dielectric element is provided in the resonator body; and that the tuning plate is arranged inside the resonator body at a small distance d from a flat surface of the dielectric element. With a design of this kind, much of the field energy is stored in the dielectric element, and a precise change in the resonance frequency of the cavity resonator can be achieved by means of a change in the position of the tuning plate. 
     When a dielectric element is used, a further variant implementation that is advantageous in terms of design consists in mounting the dielectric element on a displaceable base whose height can be modified by means of a second electromagnetic actuating element, in particular a second piezoelement. It is thereby possible, without great effort, to define a desired nominal or initial distance between the tuning plate and the flat surface of the dielectric element, which can then be finely adjusted in suitable fashion by the adjustment device according to the present invention with downstream conversion ratio mechanism. 
     The invention will be explained below by way of example with reference to an exemplary embodiment, with the aid of the drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic sectioned depiction of a cavity resonator according to the present invention; 
     FIG. 2 shows a block diagram of a microwave oscillator that uses the cavity resonator shown in FIG. 1; and 
     FIG. 3 shows a diagram in which the change in oscillator frequency Δf is depicted as a function of the change in position Δx 2  of the tuning plate. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a cavity resonator  1  of cylindrical design having a resonance frequency W R  in the GHz range. Cavity resonator  1  has a bottom plate  2  in the shape of a circular disk, a cylindrical peripheral wall  3 , and a cover wall  4 . Resonator wall elements  2 ,  3 , and  4  are made of a metal having good electrical conductivity, for example Cu or an HTSL material, and define in their interior a cavity  5 . 
     Bottom plate  2  has, distributed over its circumference, passthrough holes  6  through which pass threaded bolts  7  with which bottom plate  2  is fastened to a bottom flange  8  of peripheral wall  3 . Arranged between bottom plate  2  and flange  8  is a spacer element  9  of predefined thickness in the shape of an annular disk, and above it a displaceable base  10  in the shape of a circular disk. 
     A multi-layer piezoelement  11  is located in the central region between bottom plate  2  and displaceable base  10 . Multi-layer piezoelement  11  has a maximum excursion of a few μm, which can be transferred to displaceable base  10  and brings about a central bulging of the latter. 
     In the central region above multi-layer piezoelement  11 , a dielectric pedestal element  12  that carries a dielectric cylinder  30  is arranged on displaceable base  10 . Dielectric cylinder  30  is made of a dielectric material having a high dielectric constant ∈ (for example, sapphire), and is arranged coaxially with peripheral wall  3  of cavity resonator  1 . 
     A coupling-in antenna  13   a  and a coupling-out antenna  13   b  project through the cylindrical peripheral wall  3  into cavity  5 . Coupling-in and coupling-out antennas  13   a ,  13   b  are each embodied as coaxial cables having coaxial loops configured at the ends. 
     Cover wall  4  of cavity resonator  1  is spaced away from a cover-side flange  15  of peripheral wall  3  by means of a spacer element  14  of predefined thickness in the form of an annular disk, and is secured to cover-side flange  15 , in a manner similar to bottom wall  2 , by way of threaded bolts  17  passing through passthrough holes  16 . 
     A comparatively coarse preadjustment of the resonance frequency ω R  of cavity resonator  1  can be performed by using spacer elements  9 ,  14  with variable thicknesses. 
     A plate spring  18  configured in the form of a thin metal disk is secured at the rim between annular disk-shaped spacer element  14  and cover wall  4 . In its central region, plate spring  18  delimits a cylindrical spring receiving space  19  present in cover wall  4 . In the example depicted here, spring receiving space  19  contains three cup springs  20 , arranged one above another, which are mounted around a central guide element  21  and are braced at the bottom against plate spring  18 . 
     Located above cover wall  4  is a micrometer screw  22  that comprises a screw casing  23  joined immovably to cover wall  4 , and a rotary member  24  guided therein in a fine-pitch thread. Rotary member  24  impinges, with an actuating pin  24   a  protruding at the bottom end, upon the upper end of a plunger  25 , guided in a central bore of screw casing  23 , whose lower end impinges upon a first multi-layer piezoelement  26  that acts on the upper cup spring  20 . 
     When rotary element  24  is displaced, plunger  25  is moved in the axial direction with high adjustment accuracy (for example, 50 μm per revolution). The movement travel is transferred to first multi-layer piezoelement  26  and can be additionally modified, i.e. shortened or lengthened, by it. The linear excursion Δx 1  occurring at the output end of first multi-layer piezoelement  26  acts on the topmost cup spring  20  and compresses it. Cup springs  20  press on plate spring  18  and deflect it in its central region over a deflection travel Δx 2 . Because of the opposing force exerted by plate spring  18 , the output-end deflection travel Δx 2  is smaller than the input-end linear excursion Δx 1 . The reduction in the deflection travel Δx 2  as compared to Δx 1 , is determined by the spring constant k 1  of the cup spring stack and the spring constant k 2  of plate spring  18 . 
     If the spring constants are identical (k 1 =k 2 ), the result is to shorten the movement travel by a factor of 2. 
     A tuning disk  28  is mounted by way of a rod  27  on the side of plate spring  18  facing away from spring receiving space  19 . Tuning disk  28  extends parallel to and at a short distance d from a flat surface  29  of dielectric cylinder  30 . A central deflection ΔX 2  of plate spring  18  toward the bottom end causes tuning disk  28  also to be displaced by a distance Δx 2  so that a previously adjusted distance d between tuning disk  28  and cylindrical body  30  is shortened to d−Δx 2 . 
     FIG. 2 shows, in the form of a block diagram, the general construction of a microwave oscillator that uses cavity resonator  1  depicted in FIG.  1 . An amplifier signal  41  of an amplifier  40  is conveyed to a high-frequency coupler  42 . High-frequency coupler  42  on the one hand couples a useful signal  43  out of amplifier signal  41 , and on the other hand sends amplifier signal  41  on to cavity resonator  1 . The coupling of amplifier signal  41  into cavity resonator  1  is accomplished via input antenna  13   a.    
     An output signal  44  is coupled out of cavity resonator  1  via output antenna  13   b  and conveyed to an electrically or mechanically actuable phase shifter  45  which is provided in order to adjust the phase condition in feedback path  41 ,  42 ,  1 ,  44 ,  45 . The phase-shifted feedback signal  46  generated by phase shifter  45  is fed into amplifier  40 . 
     As already mentioned, the microwave oscillator can be continuously tuned only if cavity resonator  1  achieves a requisite resonance frequency adjustment accuracy Δω R  of approximately 100 kHz or less. It is unfavorable in this context that the tuning slope Δω R /Δx 2  of a cavity resonator increases in proportion to its quality Q. With resonators  1  of comparatively low quality (Q≈10 4 ), a typical tuning slope of 10 kHz/μm is observed. This means that the adjustment accuracy of the tuning mechanism, in terms of the achievable positional accuracy of tuning plate  28 , needs to be only approximately 10 μm in order to achieve the requisite tuning accuracy Δω R  of 100 kHz for the resonance frequency. 
     The tuning slope for a quality Q≈10 7 , on the other hand, is already 10 3  kHz/μm. A quality Q≈10 7  can be achieved, in the context of cavity resonator  1  according to the present invention, by cooling the latter to approximately 77 K, since this allows the dielectric losses occurring in dielectric cylinder  30  for so-called whispering gallery modes to be greatly reduced. In order to achieve continuous tunability of a microwave oscillator with the cooled cavity resonator  1 , the tuning mechanism of cavity resonator  1  must then have an adjustment accuracy of 0.1 μm. 
     Conversion ratio mechanism  18 ,  20  depicted in FIG. 1 makes it possible to achieve such adjustment accuracy when a micrometer screw  22  having an adjustment accuracy of 50 μm per revolution is used, and thus allows implementation of a completely tunable microwave oscillator having a cavity resonator  1  with a quality Q≈10 7 . 
     The high adjustment accuracy of tuning mechanism  22 ,  20 ,  18  is due not only to the reduction according to the present invention in movement travel by way of conversion ratio mechanism  18 ,  20 , but also to the fact that because conversion ratio mechanism  18 ,  20  is constructed from spring elements placed one behind another, practically no backlash occurs in it. This additionally makes possible excellent reproducibility for the adjustment position. 
     A further essential advantage of tuning mechanism  22 ,  20 ,  18  is its mechanical stability and vibration resistance, especially at relatively low excitation frequencies (&lt;1 kHz). This is due not only the aforementioned robust and substantially zero-backlash design of conversion ratio mechanism  18 ,  20 , but also on the one hand to the high natural mechanical frequencies of plate spring  18  and on the other hand to the large forces that must be applied in order to deflect it (for example, k 2 =5000 N/m). An extremely low susceptibility to “microphoning” is thereby achieved, and even if cavity resonator  1  is cooled by means of a commercial miniature cooler  1  [sic], no transfer of cooler vibrations into the resonance frequency spectrum is observed. 
     Preferably the first and second multi-layer piezoelements  26 ,  11  can also be used for electrical adjustment of the resonance frequency ω R . In this context, first multi-layer piezoelement  26  causes a movement of tuning plate  28  relative to the stationary dielectric cylinder  30 , while operation of second multi-layer piezoelement  11  results in a movement of dielectric cylinder  30  relative to the stationary tuning plate  28 . In particular, first multilayer piezoelement  26  placed upstream from conversion ratio mechanism  18 ,  20  makes possible very accurate fine electrical adjustment of resonance frequency ω R  and is thus particularly suitable as an actuating element for regulating the resonance frequency ω R  in a closed-loop mode. 
     FIG. 3 depicts a diagram that elucidates the tuning behavior of the oscillator shown in FIG. 2 under the following exemplary conditions: Cavity resonator  1  is cooled to a temperature of 77° K., and has a dielectric cylinder  30  made of sapphire. A micrometer screw  22  having an excursion of 50 μm per revolution is used, as well as three cup springs  20  and a plate spring  18  that is 1 mm thick (k 2 =5000 N/mm). Tuning plate  28  is made of sapphire and has a thickness of 0.5 mm. Tuning is performed at a frequency of 23 GHz. 
     The Y axis shown on the left side of FIG. 3 depicts the change in oscillator frequency Δf as a function of the linear excursion Δx 2  of tuning plate  28 , plotted on the X axis. A change of 0.75 mm in the linear excursion Δx 2  corresponds to a frequency change of 45 MHz. 
     Under the conditions specified, a minimum mechanical change in the position of tuning plate  28  of Δx 2 (min)&lt;0.2 μm is achieved. FIG. 3 shows that for small distances d&lt;0.3 mm between tuning plate  28  and dielectric cylinder  30 , this corresponds approximately to a minimum change in resonance frequency Δω R (min)=4 kHz. This frequency change attainable by mechanical retuning of cavity resonator  1  is thus much smaller than the maximum frequency variation of approximately 100 kHz that can be effected by phase shifter  45 , i.e. the condition mentioned initially for continuous tunability of the microwave oscillator is easily met. 
     The quality Q of cavity resonator  1 , plotted on the Y axis shown on the right side of FIG. 3, is largely constant over the entire tuning range of the microwave oscillator, and in the example here is Q&gt;2·10 6 . Even during an adjustment operation, practically no degradation occurs in the quality of cavity resonator  1 .