Abstract:
An oscillator comprising an oscillator body having a waveguide and a substantially cylindrical bore intersecting the waveguide, and a package comprising a heat sink having a substantially cylindrical portion and an oscillatory semiconductor device supported at one end of the heat sink. The diameter of the cylindrical portion of the heat sink in relation to the diameter of the bore is such that the heat sink is supported in the bore by one of an interference fit and a tight sliding fit, with the semiconductor device being located in the waveguide. The tight sliding fit or interference fit includes a layer of soft deformable material.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an oscillator and in particular, although not exclusively, to a Gunn diode oscillator susceptible to automated mass production. The present invention relates also to a method of assembling oscillators and to a method of tuning oscillators. 
     DESCRIPTION OF THE PRIOR ART 
     Gunn diode oscillators are important components of radar systems. They are formed from gallium arsenide substrates which are processed in a particular manner to form devices known as Gunn diodes. A Gunn diode is incorporated into an oscillator package to form an oscillator which can generate microwave frequency electromagnetic radiation. Application of a dc voltage across a Gunn diode causes high frequency electron pulses across junctions within it, which pulses cause an oscillating electric field to be set up in the vicinity of the Gunn diode. The Gunn diode is located in, and supported by, a metallic oscillator body which defines a waveguide, supports other components of the oscillator, and uses the electric field to generate microwave frequency electromagnetic radiation. A prior art bias-tuned, second harmonic, radial mode oscillator is shown in section in FIG.  1 . 
     In FIG. 1, a Gunn diode oscillator comprises generally a body  10  in which is defined an elongate waveguide  11  of rectangular cross-section, a bore  12  containing an RF choke  13 , and a composite bore  14  on the opposite side of the waveguide to the bore. A Gunn diode chip  15  is ultrasonically scrubbed onto the top surface of a gold-plated, first heat sink  16 . The Gunn diode  15  is enclosed by an annular, electrically-insulating tube  17 , which is made from alumina, and an electrically-conductive lid  18 . A gold-plated radial disk  19  is held in contact with the lid  18  by application of a constant spring force on an electrically conducting-connector  20 , which passes along the central axis of the RF choke  13 . The surface of the Gunn diode  15  closest to the RF choke  13  is electrically connected to the connector  20  by way of a wire bond or Maltese cross connection  21  made to the junction of the alumina tube  17  and the lid  18 . 
     The RF choke  13  comprises alternate high impedance and low impedance sections. In this example, the low impedance section is a brass disk  22 , and the high impedance section is of air  23 . The thickness of each of the sections  22  and  23  is equal to three quarters of the wavelength of the electromagnetic radiation to be generated. Further low and high impedance sections  22  and  23  may be incorporated in the choke  13 . The connector  20 , and thus the radial disk  19 , is electrically insulated from the oscillator body  10  by a layer of plastics material (not shown) which is wrapped around the substantially cylindrical form of the choke  13 . The surface of the Gunn diode  15  furthest from the RF choke  13  is electrically and thermally connected to the oscillator body  10  by the first heat sink  16  and a second heat sink  24 . The Gunn diode oscillator can be activated to generate microwave frequency electromagnetic radiation by application of a dc voltage across the oscillator body  10  and the electrical connector  20 . 
     A short circuit  25 , in the form of an elongate rectangular cross section metallic element, is movable axially along the waveguide  11 . Movement of the short circuit  25  relative to the position of the radial disk  19  and Gunn diode  15  causes variation of the output power of the oscillator, as will be appreciated by those skilled in the art. Microwave frequency electromagnetic energy is radiated in the direction of the arrow. 
     The first heat sink  16  is connected to the second heat sink  24  by the mating of a threaded axial bore  26  of the second heat sink  24  and a screw thread  27  formed on the primary surface of the first heat sink  16 . This connection is made by rotation of the first heat sink  16  relative to the second heat sink  24 , achieved by rotation of a screwdriver or the like having a blade inserted in a groove  28  formed in the end of the heat sink  16  furthest from the RF choke  13 , to screw the heat sinks  16  and  24  together. Rotation of these components is made until the surface of a flange  29  of the first heat sink  16  which is furthest from the RF choke  13  is in firm contact with a surface of the second heat sink  24  which is closest to the choke  13 . A reasonable degree of thermal and electrical contact is thus made between the first and second heat sinks  16  and  24 . 
     The second heat sink  24 , and thus the first heat sink  16 , is supported in the oscillator body  10  by the clamping action of an annular screw  30 . The screw  30  has a thread  31  on its outer surface which mates with a threaded bore  32  of the composite bore  14 . The second heat sink  24  is fixed relative to the body  10  by virtue of frictional forces at the washer shaped contact areas  35  and  36 . A flange  29  of the first heat sink  16  and a part of the second heat sink  24  which is adjacent the flange  29  extend through a bore  33  linking the bore  32  with the waveguide  11  so that the Gunn diode  15  is supported in and located in the waveguide  11 . 
     Because it is difficult to predict what effect the mating of the threads  26  and  27  will have on the concentricity of the outer cylindrical surface of the second heat sink  24  and the central axis of the first heat sink  16 , it is usual to provide the bore  33  with a diameter 3 or 4% larger than the outer diameter of the flange  29 . It has been found that the unscrewing of the annular screw  30  and subsequent re-screwing can cause the operating characteristics of the Gunn diode oscillator to change. In addition to a significant change in the level of the output power, a frequency change of up to 2% can occur as a result of such a re-assembly. This obviously introduces some undesirable variables into the characteristics that can be expected on oscillator assembly, even when tight component tolerances can be achieved. This frequency pulling and the changing of the output power has been thought to be caused by different alignment of the bond wires or Maltese cross connections  21  with respect to the axis of the waveguide  11 . However, the inventor has found that the frequency pulling and output power change caused by the re-assembly is the result largely of the outer surface of the flange  29 , and thus the outer surface of the part of the second heat sink  24  which is closest to the choke  13 , being either more or less concentric with the bore  33  than previously. The concentricity has a bearing on the shape of the air gap between the outer surface of the heat sink  24  and the bore  33  and particularly on the minimum distance between the bore  33  and the outer surface of the heat sink  24 . The inventor has performed experiments which show that the amount of frequency pulling is dependent on the amount of offset from concentric of the bore  33  and the outer surface of the flange  29  of the heat sink  16 . 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention, there is provided an oscillator for generating microwave frequency radiation comprising: 
     an oscillator body having formed therein a waveguide and a substantially cylindrical bore intersecting the waveguide; and 
     a package comprising a heat sink having a substantially cylindrical portion and an oscillatory semiconductor device supported at one end of the heat sink; 
     in which the diameter of the cylindrical portion of the heat sink in relation to the diameter of the bore is such that the heat sink is supported in the bore by one of a) an interference fit, and b) a tight sliding fit, with the semiconductor device being located in the waveguide. 
     This oscillator is not susceptible to variations in frequency and output power which can occur due to variations in concentricity of the heat sink with the bore in which it is supported. This oscillator is also likely to be cheaper to produce than the prior art oscillators because it is not necessary to form the heat sink package with a threaded outer surface, or to form a second heat sink with a threaded bore. This oscillator is also likely to have greater surface area contact, because of the lack of threaded connections and because contact between the heat sink and the body can be made for all or much of the cylindrical outer surface of the heat sink, than the prior art oscillators. This increased contact will improve the thermal and electrical conductivity properties and is likely to improve the operating characteristics of the oscillator. This oscillator is more readily susceptible to automated mass production than the prior art oscillators because the controlled insertion of the package into the bore in the body is easily achievable by a robotic tool. Also, because the possibility that the package will move relative to the body may be substantially reduced, the possibility that the characteristics of the oscillator will change as a result of a mechanical knock or thermal cycling may also be reduced. 
     The interference or tight sliding fit preferably includes a layer of soft deformable material. The soft deformable material layer provides the advantage of lubricating the interference fitting of the package and the bore. The package may be barrel plated with soft gold to provide the layer. The plating may for example be 15 μm thick. 
     A surface of the heat sink adjacent the diode is preferably substantially flush with or slightly proud of a surface of the waveguide. This feature may ease mass production of the oscillator in that protrusion of the heat sink into the waveguide can be detected optically or mechanically, the detection being used to control the insertion of the package into the body. 
     The bore may be formed by the reaming of a rough bore formed in the body. This allows the diameter of the bore to be very accurately determined, thus assisting the interference fit. The rough bore may be formed by die casting the body. Cost savings can be achieved by allowing the body to be die cast in this way, prior art second harmonic oscillator bodies not being thought to be susceptible to die casting. 
     The body may comprise a second bore on the opposite side of the first bore to the waveguide, the second bore being threaded to allow a screw cap to retain the package within the oscillator body. This feature eliminates any possibility that the Gunn diode package will be dislodged by the spring force of the RF choke. 
     In accordance with a second aspect of the present invention, there is provided a method of assembling an oscillator for generating microwave frequency radiation, the oscillator comprising: 
     an oscillator body having formed therein a waveguide and a substantially cylindrical bore intersecting the waveguide; and 
     a package comprising: 
     a heat sink having a substantially cylindrical portion, and 
     an oscillatory semiconductor device supported at one end of the heat sink; 
     in which the diameter of the cylindrical portion of the heat sink is larger than the diameter of the bore, 
     the method comprising inserting the package into the bore such that the package is retained by one of a) an interference fit, and b) a tight sliding fit, with the semiconductor device being located in the waveguide. 
     The first heat sink  16  is connected to the second heat sink  24  by the mating of a threaded axial bore  26  of the second heat sink and a screw thread  27  formed on the primary surface of the first heat sink. The second heat sink  24 , and thus the first heat sink  16 , is supported in the oscillator body  10  by the clamping action of an annular screw  30 . The screw  30  has a thread  31  on its outer surface, which thread mates with a threaded bore  32  of the composite bore  14 . A flange  29  of the first heat sink  16  and a part of the second heat sink  24  which is adjacent to the flange  29  extend through a bore  33  linking the bore  32  with the waveguide  11 , so that the Gunn diode  15  is supported in, and located by the waveguide  11 . 
     DESCRIPTION OF THE PRIOR ART 
     In setting up the oscillator so that it has the desired characteristics, the oscillator is assembled so that it has the structure thus far described. A short circuit  50  is inserted manually into the waveguide  11  by manipulation of a rod  51  which is temporarily screwed into a bore formed in an end of the short circuit which is furthest from the Gunn diode  15 . The short circuit  50  comprises a machined brass component which has a rectangular cross-section of dimensions sufficiently less than the dimensions of the cross-section of the waveguide  11  to allow the short circuit to be moved along the longitudinal axis of the waveguide without encountering significant mechanical resistance from the walls of the waveguide. 
     The displacement of the face of the short circuit  50  closest to the Gunn diode  15  along the waveguide  11  determines the output power of the oscillator in the relationship illustrated in FIG.  2 . The curve  80  relates to a Gunn diode voltage of 6.5V, and the curve  81  relates to a Gunn diode voltage of 4.5V. The short circuit  50  is moved toward the Gunn diode  15 , from a position approximately three quarters of a wavelength of the radiation from the position of the Gunn diode by manipulation of the rod  51  until the output power of the oscillator is equal to a desired value. A desired value will typically be 50 mW, which is about two thirds of the maximum output power. When the position of the short circuit  50  is such that the output power level is at the desired power level, a screw  52  is rotated to lock the short circuit in place. The rod  51  may then be unscrewed and removed from the short circuit  50 . 
     However, it has been found that the output power of the oscillator can vary quite considerably from the relationship shown in FIG. 2, and that there may be a number of different output power levels which can be measured for a single given displacement of the short circuit  50 . Also, it has been found that the output power level can change as a result of the screw  52  being used to lock the short circuit  50  in position in the waveguide  11 . 
     Such variation of the output power is undesirable, and is likely to hinder the setting up process if the process is automated. Automated setting up is complicated also by the fact that the rod  51  and the screw  52  will require separate robotic tools to operate at the same time. 
     SUMMARY OF THE INVENTION 
     In accordance with a third aspect of the present invention, there is provided an oscillator for generating microwave frequency radiation, the oscillator comprising: 
     an oscillator body having formed therein a waveguide; 
     an oscillatory semiconductor device supported in the waveguide; and 
     a short circuit having a skin of soft metal on at least one outer surface thereof; 
     the short circuit having dimensions selected such that the skin allows the short circuit to be supported in the waveguide by one of a) a tight sliding fit, and b) an interference fit. 
     This invention arose, in part, as a result of determining that the power level variations in the above described prior art oscillator is caused by movement of the short circuit  50  other than longitudinally in, and particularly vertically in, the waveguide  11 ; and the location of, and extent of, electrical contact between the oscillator body  10 , the short circuit  50  and the screw  52 . 
     An oscillator constructed in accordance with this aspect of the present invention will not experience variations in output power level caused by movement of the short circuit, other than in a longitudinal direction, because the short circuit is firmly supported in the waveguide by the tight sliding or interference fit. Automated oscillator set-up may be facilitated, both by the reduced number of robotic tools needed for simultaneous operation, and particularly by the avoidance of power level variations. An oscillator constructed in accordance with this aspect of the present invention may also be more mechanically rugged, in that a tight sliding or interference fit may be more resistant to mechanical knocks than the lock-screw arrangement of the prior art. Mechanical integrity is of particular importance where correct operation of the oscillator is important for safety, such as in, for example, automotive autonomous cruise control applications. Here, the oscillator is likely to be subjected to wide temperature variations and to mechanical vibration. 
     The cost of the components for an oscillator constructed in accordance with this invention is likely to be lower than the cost of the components of the prior art oscillator described above, as there is no longer the need for a threaded bore to support the screw  52 , or for the screw  52  itself. Also, a circular cross-section short circuit is likely to be less expensive to manufacture than the prior art rectangular cross-section short circuit especially so in volume production. 
     The transition between the first and section portions of the waveguide is preferably a step transition. Such an arrangement may be easier to machine than other possible arrangements, which may therefore reduce manufacturing costs. 
     The transition is preferably at a position which is chosen to provide the oscillator with a smooth power tuning characteristic. 
     In accordance with a fourth aspect of the present invention, there is provided a method of assembling an oscillator for generating microwave frequency radiation, the oscillator comprising: an oscillator body having formed therein a waveguide, an oscillatory semiconductor device supported in the waveguide; and a short circuit having a skin of soft metal on at least one outer surface thereof; in which the cross-sectional dimensions of the short circuit with the skin are larger than the cross-sectional dimensions of the waveguide, the method comprising inserting the short circuit into the waveguide such that the short circuit is retained in the waveguide by one of a) an interference fit, and b) a tight sliding fit. 
     DESCRIPTION OF THE PRIOR ART 
     Although coarse tuning of the oscillator frequency is performed prior to the output power of the oscillator being set, fine tuning is performed afterward. Where the oscillator is to be used in an automotive autonomous cruise control application, it may be desired for the oscillator to be swept from 76.1 GHz to 76.9 GHz. Such a sweep may be obtained by ramping the dc voltage applied to the Gunn diode from 4.5 volts to 6.0 volts. It is common with such oscillators to find that, due to variations in the mechanical arrangement of the package  16 ,  17 ,  18  and variations in the GaAs Gunn diode itself, the frequency of radiation obtained from a 6 volt supply can vary by up to 3 GHz either side of the desired frequency. 
     Coarse tuning is achieved by the replacement of the RF choke and radial disk assembly with an assembly having a disk of an appropriate diameter. During the coarse tuning, the oscillator is set up so that a frequency between 77 and 78 GHz is obtained when 6.0 volts is applied across the Gunn diode. An inventory of, typically, seven choke and disk assemblies, each of different disk diameter, will be necessary to obtain a coarse-tuned frequency sufficiently near to the desired frequency to allow fine tuning. Fine tuning is achieved by insertion of a frequency tuning probe into the volume between the radial disk  19  and the heat sink  16 . The presence of the frequency tuning probe in this volume, because the material from which it is made has different dielectric properties to the air in the volume, disturbs the electric field between the radial disk  19  and the heat sink  16 . The disturbance of the electric field causes the oscillation frequency of the radial mode oscillator, and thus the frequency of the radiation generated, to change. The extent of the change in frequency is dependent on, in particular, the extent of protrusion of the probe into the volume, the dimensions of the probe and the material used to make the probe. 
     It has been known for a probe made from either metal or dielectric, having a cylindrical form, and having a diameter of 0.2 to 0.3 mm, to be used to tune oscillators of the type described. Such a probe allows a reduction of the oscillator frequency as the probe is moved towards the tube  17 . Because such probes act directly on the radial mode oscillator, the operating frequency is hypersensitive to their position. Large amounts of frequency change are often obtained from a relatively small movement of the probe, which can make fine tuning of the oscillator frequency difficult. Further problems are experienced because the small size of the probes make them difficult to manufacture, handle and support. 
     It has been found also that the presence of the probe in the radial mode oscillator can cause unpredictable fluctuations in the output power of the oscillator. This clearly is undesirable. 
     SUMMARY OF THE INVENTION 
     In accordance with a fifth aspect of the present invention, there is provided an oscillator for generating microwave frequency radiation the oscillator comprising: 
     an oscillator body; 
     a waveguide formed in the oscillator body; 
     an oscillator package supporting an oscillatory semiconductor device in the waveguide, the oscillator package providing mechanical protection for the oscillatory semiconductor device; 
     a conduit formed in the oscillator body, the conduit intersecting the waveguide; 
     a choke assembly supported in the conduit; 
     a radial disk which is mechanically connected to the choke assembly and which is urged by the choke assembly to be in contact with the oscillator package in the waveguide; and 
     a frequency tuning probe which is arranged to be movable into the coaxial resonator volume between the radial disk and the choke assembly to alter the frequency of the radiation generated by the oscillator. 
     In accordance with a sixth aspect of the present invention, there is provided a method of tuning an oscillator for generating microwave frequency radiation, the oscillator comprising: an oscillator body; a waveguide formed in the oscillator body; an oscillator package supporting an oscillatory semiconductor device in the waveguide, the oscillator package providing mechanical protection for the oscillatory semiconductor device; a conduit formed in the oscillator body, the conduit intersecting the waveguide; a choke assembly supported in the conduit; a radial disk which is mechanically connected to the choke assembly and which is urged by the choke assembly to be in contact with the package in the waveguide, the method comprising moving a frequency tuning probe into the volume between the radial disk and the choke assembly. 
     Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 shows a prior art Gunn diode oscillator; 
     FIG. 2 shows how the output power of the FIG. 1 oscillator varies with short circuit displacement; 
     FIG. 3 shows an oscillator in accordance with the present invention in the form of a Gunn diode oscillator; 
     FIG. 4 shows an orthogonal view of the FIG. 3 oscillator; and 
     FIG. 5 shows how the output power of the FIG.  3  and FIG. 4 oscillator varies with short circuit displacement; and 
     FIG. 6 shows the relationship between connector length and oscillator frequency; 
     FIG. 7 shows the relationship between tuning and probe protusion into resonator. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 3 and 4, in which reference numerals have been retained for like elements, the Gunn diode oscillator comprises a heat sink  40  having a substantially cylindrical main body  40  and a tail piece  42 , the main body being interference fitted in a bore  41  of the oscillator body  10 . The heat sink  40 ,  42  with the Gunn diode chip  15 , the alumina tube  17  and the lid  18  together form a Gunn diode package which provides mechanical protection for the Gunn diode chip. The tail piece  42  is cylindrical and has a diameter equal to half of the diameter of the main body  40 . The tail piece  42  is axially aligned with the main body  40 . The edge  43  of the main body  40  closest to the RF choke  13  is rounded or chamfered, so as to ease entry of the heat sink  40 ,  42  into the bore  41 , and to prevent swarf being pushed into the waveguide  11  during assembly. Insertion of the heat sink  40 ,  42  into the bore  41  is made by a robotic tool which grasps the tail piece  42  and forces it towards the RF choke  13 . The robotic tool is controlled to discontinue application of this force when it is detected that the uppermost surface of the main body  40  has extended into the waveguide  11 . This can be achieved using conventional optical or mechanical techniques. 
     In producing the Gunn diode oscillator, the oscillator body  10  is die cast in an alloy of zinc. The die cast oscillator body  10  includes a cylindrical bore having a diameter which is less than the diameter of the bore  41 . This rough bore is then reamed to produce the bore  41 . In this way, the bore  41  can be provided with dimensions which are very accurately predictable and controllable. The heat sink  40 ,  42  can then be inserted into the oscillator body  10  without further processing of the bore  41 . 
     The heat sink  40 ,  42  is machined from a single piece of copper, and gold plated to a depth of 15 μm using a barrel plating process. Lower depth plating may also be used. Barrel plating is preferred because it is low cost and it deposits an even thickness of soft gold on the surfaces of the main body  40  which are of particular importance, i.e. the cylindrical outside surface which contacts the bore  41  when installed in the oscillator body  10 . In the barrel plating process it is also necessary to deposit a layer of soft gold on the surface of the alumina tube  17  which is closest to the RF choke  13 . This is advantageous in that it provides a good surface on which to attach the bond wires or Maltese cross connection  21 . 
     The oscillator further comprises a screw cap  46  which mates with a threaded second bore  47  of the oscillator body  10 . A surface  48  of the screw cap  46  connects with that surface  44  of the main body  40  which is closest to the tail piece  42  with a force which is determined by the final torque applied to a tool, and thus to a groove  49  of the screw cap  46 , which is used to screw the screw cap into the oscillator body  10 . Although the screw cap  46  is not essential, its presence may be desirable where the connector  20  and  68  of the RF choke  13  is spring loaded to apply a reasonably high force onto the package, or where there is a possibility that the heat sink  40 ,  42  will move relative to the oscillator body  10  for other reasons. The screw cap  46  need not be provided if the outer diameter of the main body  40  is sufficiently larger than the diameter of the bore  41  that the force required to dislodge the main body  40  in the oscillator body  10  will not be experienced under normal circumstances. 
     In an alternative embodiment (not shown), the screw cap is replaced by a supporting shoulder in the body, and the Gunn heat sink  40  is inserted from above. 
     Where the oscillator is to operate in the frequency band of 76.1 to 76.9 GHz, the waveguide  11  will typically have a cross-section of 3.10 mm by 1.55 mm. With this frequency band, the oscillator body  10  is formed with a rough bore which has a diameter of 2.85 mm and which intersects the waveguide  11  on its largest side, and which is perpendicular to the axis of the waveguide  11 . This bore is then reamed to form the bore  41  with a diameter of no less than 2.910 mm and no greater than 2.915 mm. In this way, a heat sink having a cylindrical main body  40  with a diameter anywhere in the region of 2.91 mm to 2.93 mm, including the gold plating, will form a good interference or tight sliding fit when inserted into the bore  41  of the oscillator body  10 . The primary requirement of this interference or tight sliding fit is that the air gap between the heat sink  40 ,  42  and the oscillator body  10 , which is present in the prior art devices, is avoided. Although it is not essential, it is preferred that the tight sliding or interference fit prevents the heat sink  40 ,  42  from moving relative to the oscillator body  10 , except with the application of a large force onto the heat sink  40 ,  42 . 
     As shown in FIG. 3, the waveguide  11  has a step transition  60  from the rectangular form that exists in the vicinity of the Gunn diode  15  to a substantially circular bore  61 . The step transition  60  acts as a capacitive reactance. Its position thus influences the output power curve. The position of the transition is chosen so as to allow smooth and continuous power adjustment by a short circuit  62 . The position depends on the type of bias interconnection  21 . In this embodiment, the transition is three quarters of the wavelength of the microwaves, or 3.9 mm. 
     The bore  61  is formed by machining a rough bore which is die cast in the body  10 . The diameter of the bore  61  is chosen so that the wavelength is the same in both the rectangular and circular waveguide sections. 
     The bore  61  is formed by machining the oscillator body  10  to a diameter of between 3.500 mm and 3.505 mm. The short circuit  62  is an annealed copper element having an outside diameter of between 3.49 and 3.50 mm. This short circuit  62  is plated with a skin of soft tin to a depth of about 15 mm to bring the outside dimension of the short circuit to between 3.52 and 3.54 mm. The skin of tin acts as a dry lubricant when sandwiched between the harder metals of the body  10  and the short circuit  62 , which assists entry of the short circuit into the bore  61 . A chamfered edge  72  provided on the forward end of the short circuit  62  allows surplus tin to be displaced to the rear of the short circuit  62 , and not towards the Gunn diode  15 . The short circuit  62  is thus supported in the bore  61  by a tight sliding or interference fit, and thus is provided with a good and continuous electrical connection to the oscillator body  10 . 
     FIG. 5 shows the relationship between short circuit displacement and oscillator output power. The curve  82  relates to a Gunn diode potential of 4.5 volts, and the curve  83  relates to a potential of 6.5 volts. These curves relate to the case where the interconnection  21  is a wire bond connection. Importantly, it will be noted that each curve  82  and  83  is monotonic between maxima and minima. Accurate setting of the output power of the oscillator is easily achievable, because the short circuit  62  is self supporting in the bore  61 . Thus, no locking screw is required, and the short circuit  62  is unable to move in the waveguide, other than by application of a force to a rod  63 , which is used to set the output power. Even removal of the rod  63 , when the output power has been set, is not able to cause movement of the short circuit  62 . 
     The oscillator further comprises a frequency tuning probe  65  which extends through a bore  67  drilled in the body  10  which bore, if extrapolated, intersects the longitudinal axis of the choke assembly  13  at right-angles. The frequency tuning probe  65  is thus movable into the volume between the radial disk  19  and the choke assembly  13  by manipulation of its rear end. Because the distance between the radial disk  19  and the face  66  of the choke assembly  13  which is closest to the Gunn diode  15  is significantly greater than the distance between the radial disk  19  and the heat sink  40 , the probe  65  is able to have a larger diameter than the prior art frequency tuning probes. This means that the probe  65  can be stiffer, easier to manufacture, and easier to secure positively in the oscillator body  10 . 
     The face  66  of the choke  13 , the uppermost surface of the disk  19  and the conductive connector  68  together form a co-axial resonator. The resonant frequency of this co-axial resonator is dependent in part on the length of the connector  68 . The co-axial resonator is magnetically coupled to the radial mode oscillator formed between the disk  19  and the heatsink  40 ,  42 . Thus, adjustment of the resonant frequency of the co-axial resonator, by the insertion of the frequency tuning probe  65  into it, varies the oscillation frequency of the radial mode oscillator. 
     The face  66  is recessed from the form of the waveguide  11 , which thereby provides a longer co-axial resonator than would be provided otherwise. This feature has an affect on the resonant frequency of the co-axial resonator, which is dependent on the length of the connector  68 , the affect being to reduce the frequency. The relationship between the length of the connector  68  and the oscillator frequency is shown by a curve  84  in FIG.  6 . Significantly, the increased length of the connector allows a larger diameter frequency tuning probe  65  to be used which, in turn, allows more tuning of the oscillator than is possible with a smaller diameter probe. The amount of tuning possible depends also on the extent of protrusion of the probe  65  into the co-axial resonator, and the material from which the probe is made. This is illustrated by the curve  85  in FIG.  7 . 
     The probe comprises a forward section  65  which has cross-sectional dimensions selected for altering the frequency of the co-axial resonator, and a rear section  69 . The rear section  69  has a larger cross-sectional diameter than the forward section  65 . The forward section  65  is supported in the bore  67 , whilst the rear section  69  is supported in a larger diameter bore  71 . 
     The rear section  69  of the frequency tuning probe  65  is provided with surface features which allow it to be supported in the bore  71  in such a way that movement of the frequency tuning probe  65 , which would of course affect the frequency of the oscillator output signal, is prevented. Particularly, these surface features comprise three longitudinally-aligned grooves  70  spaced regularly around the circumference of the radial cross-section of the probe  65 . The grooves  70  are substantially triangular in cross-section, which thereby allow material from the body  10  to be displaced into them when the probe  65  is forced into the bore  71 . The rear end of the probe  69 , therefore, bites into the bore as it is pushed into the waveguide during oscillator tuning. The probe  65  is thus less likely to be movable accidentally after tuning of the oscillator is complete. 
     In an alternative embodiment, not shown, the probe  65  is a constant diameter cylindrical probe which is plated with a soft metal such as silver, tin or gold. Here, the probe is supported in a simple bore, rather than a composite bore, by an interference or tight sliding fit provided by the soft metal plating. This probe is thus of simpler construction, and may provide an improved electrical connection with the oscillator body  10 . 
     Instead of the body being die cast, it may be machined from aluminium or any other suitable material. The steps necessary to machine the body will be apparent to the skilled person. 
     Although the embodiment relates to a Gunn diode oscillator, it will be appreciated that the present invention is also applicable to Impatt diode oscillators and the like.