GB2161653A - Microwave device - Google Patents

Abstract

A microwave device including a ferrimagnetic resonator, e.g. a YIG-tuned oscillator, includes a resonator heating means. The resonator, e.g. a YIG crystal, 17 is supported in a magnetic field in a resonant cavity by a dielectric rod 20 cantilevered from a support 19 in which it may be axially rotatable to align the crystal with the field. The heating means may take the form of a semiconductor integrated circuit 25 of the type including a controlled heater for temperature stabilisation, and is supported in contact with the cantilevered portion of the rod, by thermally conductive adhesive or by an intermediate metal body giving a greater thermal contact area with the rod. Apart from smaller dimensions, because the heat is transferred more efficiently to the crystal rather than the support 19 (unlike conventional heaters which are in support 19), the low current i.c. heater is adequate, and the heater current is obtainable from internal oscillator circuitry without the need for a separate supply. <IMAGE>

Description

SPECIFICATION Microwave device This invention relates to microwave devices and in particular to devices tuned by ferrimagnetic material.

Microwave devices are known tuned to resonance at predetermined or controllably variable frequencies by means of a ferrimagnetic resonator such as a spherical crystal of yttrium iron garnet (YIG). YIG is one of many ferrimagnetic materials which may be employed in this way and for convenience the following description will in general refer to YIG as exemplifying such material.

Similarly many types of microwave circuits employ ferrimagnetic resonators, typically oscillators and filters and also for convenience the following description will be directed to an oscillator.

In a so-called YIG tuned oscillator an electrical oscillator circuit employing semiconductor devices is tuned in respect of oscillation frequency by a high-Q resonator formed by a crystal of yttrium iron garnet (YIG) disposed in a substantially uniform magnetic field.

The YIG material is used because it offers a resonant frequency which is substantially a linear function of the magnetic field strength and is tunable over a range of frequencies by variation of field strength. The field may be constant and unidirectional for a particular resonant frequency or may vary, for example as a saw-tooth, to give a frequency sweep. Alternatively, the field may alternate sinusoidally producing a frequency modulated envelope on the oscillator output frequency. The resonant frequency is however subject to drift which is a function of alignment of the crystal structure in the magnetic field. The degree of drift with crystal orientation is strongly temperature dependent and alignment to avoid frequency drift takes both parameters into account.

The YIG crystal is usually produced is spherical form and is supported in the magnetic field by a dielectric rod the longitudinal axis of which it is mounted so that rotation of the rod about the axis varies the alignment of the crystal in the field. Whilst such crystal alignment minimises frequency drift with temperature it is not obviated and to further stabilise the temperature effects the crystal is maintained at a substantially constant temperature, in excess of the ambient device operating temperatures likely to be encountered, by means of heat conducted along the supporting dieletric rod from a heating element located adjacent the other end.

It has been found with existing designs of heating means that considerably more heat is lost into the device structure than is conducted along the crystal rod and this leads at least to an initial device warmup time which may be excessive for some applications and requires a more powerful heater than might be expected.

The physical dimensions and electrical requirements of existing heating means require them to be mounted as far as possible from the resonant circuit and be supplied with heating current separately from other components of the device. This impedes miniaturisation of the device and complicates the factors which have to be taken into consideration by a user.

It is an object of the present invention to provide a microwave device including ferrimagnetic resonator heating means which mitigates the above outlined drawbacks of known designs.

According to a first aspect of the present invention a tuned microwave device includes electromagnet means operable to define a uniform magnetic field in a cavity between pole pieces thereof, a ferrimagnetic resonator disposed in the cavity, resonator support means comprising a cantilever formed by a rod arrangement supported at one end and at the other end extending into the cavity, the resonator being attached to said other end of the rod assembly on the said longitudinal rod axis, said rod arrangement including resonator heating means, including a heating element, located between said supported end and the resonator adapted to make thermal contact with the cantilevered portion of the rod arrangement extending to the resonator.

Preferably the heating means includes a semiconductor die incorporating an integrated circuit heating element.

According to a second aspect of the present invention a method of temperature stabilising a ferrimagnetic resonator of a microwave device, in which the resonator is supported in a magnetic field by a cantilevered rod arrangement, comprises mounting adjacent the cantilevered portion heating means, having a heating element, making good thermal contact between the die and the cantilevered portion and passing an electric current through the heating element.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a simplified sectional elevation through a VIG-tuned oscillator device according to the present invention showing the YIG crystal and its support and heating means, Figure 2 is a plan view of a portion of the device of Figure 1 illustrating particularly the YIG crystal, its support and heating means, Figures 3(a) and 3(b) are a sectional elevation taken along the linesll--ll and Ill-Ill respectively of Figure 2 showing constructional details of the YIG crystal heating means, Figure 4 is a plan view similar to Figure 2 but of a YlG-tuned oscillator device including known form of crystal heating means, Figures 5(a) to (d) show sectional elevation, similar to Figure 3(b) showing variants on the form of YIG crystal heating means, Figure 6 shows a cross-sectional elevation through yet another form of heating means, Figures 7(a) and 7(b) show cross-sectional elevations of modifications of the form of heating means shown in Figure 3(b), Figures 8(a) to 8(c) show sectional elevations through variants of yet another form of the heating means, Figures 8(d) to 8(f) are cross-sectional elevations along lines VIll-VIll through the body in the variants of Figures 8(b) and 8(c) illustrating in each case variants on the shape of the body, and Figure 9 is a sectional elevation through yet another form of heating means.

Referring to Figure 1 a YlG-tuned microwave oscillator comprises a base portion 10 and a lid 11, the lid containing an electromagnetic coil 12 wound around a central core 13. The base and lid are formed of ferromagnetic material such as iron and the lid engages the base to form a magnetic circuit for the electromagnet, the central core 13 being tapered at its end and providing a magnetic pole piece spaced from the base to define a gap 14 across which a uniform unidirectional magnetic field is produced when a direct current is passed through the coil 12.

The base supports a ceramic or alumina substrate 15 which contains the electronic components necessary to form the oscillator and connections are made thereto by pins 16 extending through the base and electrically insulated therefrom. The substrate 15 and gap 14 define an isolation cavity in which the r.f. elements of the resonant circuit are contained.

The oscillator tuning element comprises a ferrimagnetic resonator in the form of a polished spherical YIG crystal disposed in the gap 14. YIG crystal support means comprises a rod arrangement 18 supported at one end thereof in a mounting block 19 carried by the base 10. The rod arrangement 18 has a cantilevered portion comprising a rod 20 of low dielectric constant material, such as sapphire, held by a sleeve 21 which passes through a bore in the mounting block 19. The sleeve 21 has a slot 22 or like tool engaging means at the end thereof whereby the sleeve and rod can be rotated about their common longitudinal axis.

As stated, the rod 20 forms a cantilever and the end remote from the mounting block 19 extends into the gap 14. The spherical YIG crystal 17 is bonded to the end of the rod coaxiallywith the longitudinal axis thereof so that the crystal structure can be orientated in the magnetic field through rotation of the rod.

In order to mitigate the effects of oscillator drift with crystal orientation, which is also a function of crystal temperature, the crystal is heated to a temperature in excess of the range of ambient temperatures in which the oscillator device is required to work.

In accordance with the present invention heating means 24 comprises a semiconductor die 25 bonded to a body 26 of similar plan area supported by, and bonded to, the cantilevered portion of the rod arrangement, that is, between the mounting block 19 and the crystal 23. The die incorporates an integrated circuit heating element to which current is supplied by way of flying leads 27.

As it is normal practice to hermetically seal the device the semiconductor die is not encapsulated separately. Whilst the principal benefit of this is the minimisation of size and mass of the heater it may also serve to minimise any degree of stray capacitance caused by encapsulation materials.

The heating means 24 is shown in greater detail in Figures 2 and 3. Figure 2 is a plan view of that portion of the device of Figure 1 including the rod arragement 18 and Figures 3(a) and (b) are sectional views along the lines Il-Il and Ill-Ill respectively.

Like reference numbers are employed for corresponding parts.

The semiconductor integrated circuit die 25 may be a specially manufactured device but conveniently comprises a portion of a commercially available integrated circuit device incorporating such a heater. One example of such a device is precision reference zenertype LM199 manufactured by National Semiconductor Corporation.

It will be appreciated that in such a device the reference components have temperaturedependent characteristics and to stabilise their operating conditions the integrated circuit includes a resistive heating element and associated control components to maintain the die at a constant operating temperature. It will also be appreciated that the heating element, because it is intended to produce heat substantially only for the die, draws very little current, of the order of a few milliamps.

Referring also to Figures 3(a) and 3(b), the semiconductor die 25 is bonded to a body 26 of metal, such as silicon or copper, offering good thermal conductivity by a bonding material having good mechanical and thermal transfer properties, such as solder.

The body 26 is recessed on the face opposite to that carrying the die the recess being in the form of an open groove 28 having a profile corresponding to that of the rod 20. The body 26 sits on the rod like a saddle with the supporting rod being located in the groove 28 and secured by an adhesive material shown at 29. The adhesive material fills any interstices between the rod and groove wall providing thermal coupling without the necessity for close dimensional tolerances between the rod and the groove. Asilver loaded epoxy provides suitable thermal and mechanical properties.

The rod 20 is dimensionally stable and has a low dielectric constant so as not to introduce stray capacitance into the tuned circuit. A suitable material is sapphire which has a low but adequate thermal conductivity for the transfer of heat generated in the semiconductor die to the YIG crystal.

The particularly advantageous features of the heater structure and the advantages they offer will be better understood in relation to a conventional form of heating arrangement shown in the plan view of Figure 4.

The device structure is similar to that shown in Figure 2 in that a spherical YIG crystal 30 is supported between pole pieces of an electromagnetic field generator (not shown) and adjacent an oscillator circuit (not shown) formed on substrate 31 by a rod 32 of dielectric material formed as a cantilever extending from a mounting block 33 attached to the device base 34. The rod is attached coaxialiy to a metal sleeve 35 which is rotatable in the block by adjusting slot 36.

The heating arrangement for the YIG crystal comprises a resistive heater 37 mounted also in the block 33 and formed as a coil wound around the sleeve 35. Conductors 38 connect the heating coil to supply pins 39 electrically and physically separated from similar pins supplying other circuit components of the device.

It will be appreciated that the mounting block 33 must be suitably dimensioned to carry the heating coil in addition to the sleeve 35 and the heating coil must also be dimensioned and electrically rated to provide heat not only to the rod and crystal but also the mounting block. Furthermore the block 33 is coupled thermally to the ferromagnetic base 34 of the device and heat continually lost therefrom requires replenishment by the heating coil. Once the block temperature has stabilised its thermal capacity helps to provide stability against fluctuations in heat generated by the coil but at the expense of a relatively long warm-up time (of the order of minutes) whilst the block is heated and a correspondingly highly rated heater to accommodate the losses.Furthermore the current drawn by the heating coil to accommodate heat losses and shorten warm-up time is large in comparison with other currents associated with the device and a separate supply is called for to isolate any electromagnetic coupling.

In designing a microwave frequency device resonator with such a heating arrangement care must be taken to minimise any effect of the heater as well as its supply leads on the high frequency circuit and in general the larger and more powerful the heating coil the further away from the frequency sensitive components it, and, therefore, the mounting block should be, limiting the dimensions of the overall device.

On the other hand the heating arrangement of the present invention described with reference to Figures 1 to 3, by being mounted on the rod 20 is required to produce less heat. As described above the material of rod 20 is not a good thermal conductor and although heat is conducted also to the mounting block it is conducted along the rod towards the YIG crystal without the mounting block 19 having to be heated first. This obviates the long warm-up time associated with a 'heated' mounting block and as the conductivity of the rod limits the rate at which heat can be extracted from the semiconductor die by the mounting block establishes the viability of such a low current heat source.

The semiconductor heating element with its low current requirement and temperature regulation integrated into the circuit on the die may be connected internally of the device across the supply terminals for other circuitry of the device appearing transparent to the user, that is, provision does not have to be made in design of the circuit in which the device is used for a separate supply to a YIG heating means.

It will be appreciated also that not only is the heating means physically smaller per se, but because it is coupled directly to the cantilevered rod the mounting block 19 may also be made smaller than in prior designs. Furthermore, any requirement for the mounting block to be mounted distant from the crystal, as when carrying a large heating components is removed and the whole device may be made smaller. Although a small reduction in dimensions and weight may not appear significant it will be appreciated that microwave devices of this nature are frequently employed in flying vehicles where every saving in size, weight and operating power is significant.

Having established the significance of coupling the heating means to the rod and forming it of corresponding small dimensions, particularly of the dimensions of the semiconductor die disciosed, the constructional features of embodiments various embodiments and forms of heating means should now be appreciated more readily.

Referring again ,to Figure 3 it will be seen that the use of a recessed body intermediate the heated die 25 and rod 20 provides maximum thermal contact area with the fiat die for transfer of heat therefrom and permits a corresponding contact area between the recess wall and the curved rod surface for transfer of heat thereto, as well as providing mechanical protection for the fragile die.

As an alternative to securing the body to the rod wall by adhesive it may be held in contact by mechanical means such as the resilient clips 40 shown in the cross-sectional elevation of Figure 5(a).

The recess in the body may be other than the groove described above. Figure 5(b) shows a crosssectional elevation similar to Figure 3(b) but in which the body 41 has a recess in the form of a through-aperture 42 and is threaded onto the rod from one end before being secured thereto.

It will be appreciated that apart from any manufacturing benefits obtained by having the integrated circuit contact pads uppermost as illustrated in the above Figures there is no restriction on the orientation of the heating die with respect to the rod. In any one of the aforementioned embodiments the body with the die attached may be displaced about the longitudinal axis.

Alternatively, in the embodiment shown in Figure 5(b) the die may be secured to one of the other faces of the body as indicated by broken lines 43. To facilitate this the body may have a cross-section other than rectangular and may have a larger number of faces as shown in Figure 5(c).

Figure 5(d) shows an arrangement in which a through aperture is effected by securing to each other two bodies 44,45 each having a suitably grooved face.

It will be appreciated that if desired more than one semiconductor die including a heating element may be employed. These may readily be disposed about different faces of multiple-faced bodies as shown in Figures 5(b) to (d) and/or a plurality of die carrying bodies may be disposed along the rod.

The requirement for thermal coupling between the body and the rod is conveniently achieved when they are bonded by adhesive material by the gapfilling nature of the adhesive but when a mechanical means of attachment is employed a layer of a suitable thermally conductive paste or like material is preferably interposed between the body and the rod.

One feature of mechanical, rather than adhesive, coupling and a thermally conductive paste layer is that it permits relative rotation between the rod 20 and the body after it is assembled thereon, the purpose of which will be discussed hereinafter.

In all of the above described embodiments the heater assembly whether fixed or rotatable in relation to the rod is supported by the rod. Figure 6 shows in cross-sectional elevation an embodiment in which a die-carrying body 46 is connected to adjacent circuit points 47 by supply leads 48. The leads not only provide electrical connection to the integrated circuit but also mechanically support the heating arrangement which thermally contacts the rod by way of thermally conductive paste layer 49, so that the rod may undergo rotation relative to the heating means.

In ali of the above described embodiments the heating means makes thermal contact with the rod by conforming to the profile thereof.

Figure 7(a) shows in cross-sectional elevation an embodiment of heating means in which the rod 50 has one, or more, 'fiats' or faces 51 formed around it and along part of its length and to which the body, without a groove, is attached either by adhesive or mechanically. The surface of the rod is effectively profiled to conform to that of the body.

It will be appreciated that the area of the, or each, 'flat' may be reduced (or the number increased) until the area of contact between the body and rod is theoretically reduced to a line contact at 52.

However as shown in Figure 7(b) the presence of a meniscus 53 of thermally conductive adhesive used to secure the body to the rod (or of thermally conductive paste where the body is mechanically coupled) extends the effective area of contact and this may be sufficient without the need for a flat face formed on the rod.

It will be appreciated that in the embodiments shown in Figure 7 the body 26 is somewhat redundant and the die 25 may instead be bonded directly to the rod.

In all of the above described embodiments of heating means in accordance with the present invention the cantilevered portion of the rod arrangement comprises a unitary rod structure, heat being transferred from the heated die to the rod material by way of contact with a peripheral wall.

Figure 8(a) is a sectional elevation through an alternative embodiment of heating means in which the cantilevered portion of the rod arrangement is in two parts 54,55 joined co-axially by way of an interposed die 56 (corresponding to the die 25 described above). Adhesive material 57 provides mechanical support and thermal coupling.

In an alternative form shown in Figures 8(b) and 8(c) the rod parts may be joined by conductive body 58, 58' to which the die is bonded as described above.

Forming the cantilevered portion in two parts clearly demands precise axial alignment between the parts to facilitate orientation of the YIG crystal by rotational adjustment of the rod arrangement but it also enables the parts to be formed of different materials. For example, the part 54 between the heating element and the YIG crystal is chosen for its dielectric properties and 'good' thermal conductivity, whereas the part 55 between the heating element and the mounting block is chosen principally for its poor thermal conductivity and with less emphasis on its dielectric properties.

Where the heating element includes a conductive body 58 or 58' as shown in Figures 8(b) and 8(c) the body may extend axially of the rod arrangement to a greater or lesser extent than the die as shown by the Figures. The cross-sectional profile may be in general be polygonal giving a choice of one or more faces (depending on the relative dimensions of the body and die) to which the die may be bonded ranging from triangular as shown in Figure 8(d) giving maximum area of contact to circular as shown in Figure 8(e) making a line contact with the die but achieving a greaterthermal coupling area by means of a meniscus formed by the securing material. A compromise is shown in Figure 8(f) in which the body 58/58' has a hexagonal section providing at least one flat face to which a die is attached with an area of contact providing adequate thermal contact.

The embodiments described above with reference to Figures 8(b) to 8(f) in which a body 58 is aligned with, and forms part of, the rod assembly depends upon the strength and thermal conductivity of the adhesive bonds 57.

In an alternative arrangement shown in the sectional elevation of Figure 9, the body 59 is similar to that of 41 shown in Figure 5(b) with blind recesses 60 (or possibly a through aperture) into which the component parts 54 and 55 of the rod assembly locate. The body profile may of course have any of the profiles suggested by Figures 8(d) to (f).

As stated in the introduction, one of the alignment operations on a YIG oscillator is the orientation of the YIG crystal in the magnetic field by axial rotation of the rod arrangement.

As stated above with the heating arrangement fixed to the rod such rotation would cause the final orientation of the die to be unpredictable. From a heat-providing aspect this in itself is not a problem but it will be appreciated that large rotation cannot be accommodated after the fine supply leads are bonded, unfavourable orientations may make later bonding of leads difficult or impossible and crystal alignment would have to be effected without the heating means operational.

A preferred assembly procedure for such a construction is for the oscillator device to be assembled without the heating arrangement fixed to the rod and for the crystal to be aligned whilst the oscillator is in an environment having an ambient temperature substantially equivalent to the temperature at which the YIG crystal is to be maintained by the heating means. The heating means is then secured to the cantilevered portion of the rod arrangement in a convenient orientation and the supply leads bonded to the appropriate pads of the semiconductor die and on the adjacent substrate. Sufficient play is provided in the supply leads to permit limited roational adjustments of rod arrangement after the device is fully assembled.

This assembly procedure is particularly applicable to the embodiments of heating means described and shown in Figures 3,5,7 and Figures 8(b) to 8(f) and Figure 9. Where the body is mounted on the rod by mechanical means rather than adhesive, the heating means may be fitted to the rod prior to rotational alignment of the rod and then repositioned to enable electrical connections to be made.

The arrangement described with reference to Figure 6 on the other hand permits such alignment of the rod arrangement after the heating means is fully assembled although of the above assembly procedue may still be used.

With the constructions shown in Figure 8 the die or body joining the two rod parts must of course be included in the construction of the rod assembly and provision made in the case of Figure 8(a), for lead bonding irrespective of the final orientation. The embodiments of Figure 8(b) and (c) and Figure 9 favour mounting and lead-bonding to the die after initial alignment of the crystal.

With all of the embodiments described above, which list should not be considered exhaustive of all possible variations, it will be seen that such a heating means is compact. For instance, the semiconductor die, even when including components intended for other purposes, possibly within the device, but not used in respect of providing heat as required by the invention, typically measures of the order of 2 mm x 2 mm and the total thickness of the body and die combination may be made typically less than 2 mm.

Even with the additional body, the presence of a recess in the embodiments shown in Figure 1-3, 5 and 6 or its inclusion in the rod assembly as shown in Figure 8(b) to (f) can result in an actual projection of the heating means above the rod assemblv of less than 1 mm.

The heating means has hereinbefore been described consistently in the form of a semiconductor die incorporating an integrated circuit heating element in order to clearly identify the ancillary thermal conduction and mounting variants. It will be appreciated that subject to the dimensional limitations imposed upon the heating means by the chosen mounting arrangements the heating element may be formed by other than an integrated circuit contained in a semiconductor die, such as a miniature thin film or thick film resistive track on a substrate or a miniature wound coil of highly resistive wire.

It will be appreciated that the technique described herein of heating the YIG crystal is directly applicable to resonators of other ferrimagnetic materials which are inherently temperature sensitive and may of course be employed in devices other than oscillators which include such resonators.

Claims (25)

1. A tuned microwave device including electromagnet means operable to define a uniform magnetic field in a cavity between pole pieces thereof, a ferrimagnetic resonator disposed in the cavity, resonator support means comprising a cantilever formed by a rod arrangement supported at one end and at the other end extending into the cavity, the resonator being attached to said other end of the rod assembly on the said longitudinal rod axis, said rod arrangement including resonator heating means, including a heating element, located between said supported end and the resonator adapted to make thermal contact with the cantilevered portion of the rod arrangement extending to the resonator.

2. A device as claimed in claim 1 in which the heating means is carried by the rod assembly.

3. A device as claimed in claim 1 in which the heating element is part of a temperature controlled circuit.

4. A device as claimed in any one of claims 1 to 3 in which the heating means includes a semiconductor die incorporating an integrated circuit heating element.

5. A device as claimed in claim 4 when dependent on claim 3 in which the temperature controlled circuit is formed as an integrated circuit of the semiconductor die.

6. A device as claimed in claim 4 or claim 5 in which the integrated circuit is electrically connected in parallel with other device circuitry to receive the same operating voltage and draws a current much lower than said other device circuitry.

7. A device as claimed in any one of claims 4 to 6 in which the semiconductor die is bonded directly to the cantilevered position of the rod arrangement by an adhesive material forming also a heat transmission path.

8. A device as claimed in any one of claims 4 to 6 in which the die is bonded to a thermally conductive body said body being in thermal contact with the cantilevered portion of the rod arrangement.

9. A device as claimed in claim 8 in which the body is recessed, said recess containing at least a part of the cantilevered portion of the rod arrangement and providing a heat transmission path by way of the recess wall.

10. A device as claimed in claim 9 in which the recess is in the form of an open groove said body being attached to the surface of the cantilevered portion such that at least a part thereof is contained in the groove.

11. A device as claimed in claim 10 in which in which the groove profile is substantially identical with that of the cantilevered portion of the rod arrangement contained therein.

12. A device as claimed in claim 10 or claim 11 in which the body includes mechanical clamping means arranged to secure the body in contact with the surface of the cantilevered portion of the rod arrangement.

13. A device as claimed in any one of claims 8 to 11 in which the body is secured in position by adhesive.

14. A device as claimed in claim 7 or claim 13 in which the adhesive is a silver-loaded epoxy.

15. A device as claimed in any one of claims 8 to 14 in which the body is formed of metal.

16. A device as claimed in claim 15 in which the metal is silicon.

17. A device as claimed in claim 15 or claim 16 in which the die is soldered to the body.

18. A device as claimed in any one of the preceding claims in which the cantilevered portion of the rod arrangement is formed of sapphire.

19. A device as claimed in any one of the preceding claims in which the ferrimagnetic resonator is a YIG crystal.

20. Atuned microwave device substantially as herein described with reference to and as shown in, Figures 1 to 3 or any one of the variants in Figure 5 to 9 of the accompanying drawings.

21. A method of temperature stabilising a ferrimagnetic resonator of a microwave device, in which the resonator is supported in a magnetic field by a cantilevered rod arrangement, comprising mounting adjacent the cantilevered portion heating means, having a heating element, making good thermal contact between the die and the cantilevered portion and passing an electric current through the heating element.

22. A method as claimed in claim 21 comprising forming the heating means from a semiconductor die, said heating element being formed as an integrated circuit thereof.

23. A method as claimed in claim 22 including interposing a thermally conductive body between the semiconductor die means and the cantilevered portion of the rod assembly, and providing a recess in said body arranged to conform with the wall of the cantilevered portion of the rod assembly so as to increase the area of contact therebetween.

24. A method as claimed in any one of claims 21 to 23 in which the heating means and, where appropriate, the body is secured to the cantilevered portion of the rod arrangement by a thermally conductive adhesive.

25. A method of temperature stabilising substantially as herein described with reference to and as shown in Figure 1 to 3 or in any of the parts of Figure 5 to 9 of the accompanying drawings.