An electrically tunable oscillator

TECHNICAL FIELD

The present invention relates to an oscillator device comprising an amplifier unit and a waveguide cavity resonator. The waveguide cavity resonator comprises a first tuning element arranged for mounting in the waveguide cavity resonator. The waveguide cavity resonator has a first cavity length between two opposing inner wails of the waveguide cavity, where a resonance frequency of the waveguide cavity resonator is dependent on the first cavity length.

BACKGROUND

Oscillators are used for delivering a signal with a predetermined frequency, which may be adjustable. However, all oscillators that are set to a certain frequency tend to vary siightly around said frequency. This variation is known as phase noise. in order to achieve low phase noise in an oscillator, it is well known that one of the main contributing parameters is the losses of the resonator, measured by its so-called O value, where a high Q means tow losses and low phase noise. Especially for a Voltage Controlled Oscillator (VCO) where an electrical tuning element is coupled to the resonator, it is very difficult to acquire a low phase noise. There exist a vast number of different technologies for realizing an osciiiator. Basically, an oscillator is formed by an amplifier that is coupled to a resonator, where the resonator normally incorporates the tuning element and where the degree of coupling of the tuning element to the resonator determines the relative tuning bandwidth. Normal ranges for the tuning vary from single percentages to octave bandwidths,

A resonator can be built from microstrip or stripline structures on a substrate. It can also be built from discrete LC components, dielectric resonators, waveguide cavities or variants of these. The tuning element can be a varactor diode, ferroelectric material or some other variable reactance structure. The total Q of a resonator structure depends on the combined resistive losses of the respective components. in all existing resonator structures, the common problem is that as soon as a tuning element is coupled to a resonator such as a cavity, the losses of the tuning element will lower the Q and thereby the phase noise will be increased. The tighter the coupling between the tuning element and the resonator is, the wider bandwidths may be obtained, but also the more losses occur, and then the phase noise is increased

A secondary problem with most tuning devices is the limited breakdown voltage which prevents the use of higher power within the resonator as another means of reducing the phase noise.

There is thus a need for an enhanced oscillator device, where a large bandwidth may be obtained without having an increased phase noise. SUMMARY The object of the present invention is to provide an enhanced osciliator device, where a large bandwidth may be obtained without having an increased phase noise. This object is achieved by means of an oscillator device comprising an amplifier unit and a waveguide cavity resonator. The waveguide cavity resonator comprises a first tuning element arranged for mounting in the waveguide cavity resonator. The waveguide cavity resonator has a first cavity length between two opposing inner wails of the waveguide cavity, where a resonance frequency of the waveguide cavity resonator is dependent on the first cavity length. The first tuning element is arranged for altering the first cavity length between at least two values such that the resonance frequency of the waveguide cavity resonator is adjustable between at least two corresponding magnitudes.

According to an example, the waveguide cavity resonator comprises a first inner wall, a second inner wall, a third inner wall and a fourth inner wall. The first inner wali and the second inner wall are facing each other and are separated by the first cavity length. The third inner wall and the fourth inner wall are facing each other and are separated by a second cavity length. A fifth inner wall and a sixth inner wall are mounted to at least one of said first inner wall, second inner wall, third inner wall and fourth inner wali. The fifth inner wall and the sixth inner wall are facing each other and are separated by a third cavity length such that a cavity is formed and is limited by the inner walls, where the inner walls are constituted by respective main surfaces and are electrically conducting.

According to another example, the waveguide cavity resonator is in the form of a surface-mountabie waveguide cavity resonator, which is arranged to be mounted to a printed circuit board (PCB) such that a metallization on the PCB constitutes the first inner wall. According to another example, the first tuning element comprises a nonconducting laminate on which at least one row of switches is placed. The switches are electrically openable and closabie and are arranged to constitute an electrically conducting connection between opposing inner walls. The switches may be of the type Micro Electro Mechanical Systems (MEMS).

According to another example, the amplifier unit is either positioned inside the cavity or outside the cavity.

According to another example, the oscillator device comprises a second tuning element which for example may be constituted by at least one varactor diode. Preferably, the electromagnetic coupling between the first tuning element and the waveguide cavity resonator exceeds the electromagnetic coupling between the second tuning element and the waveguide cavity resonator.

Other examples are evident from the dependent claims, A number of advantages are obtained by means of the present invention, for example:

high Q-factor,

low loss, and

low phase noise for an electrically tunable oscillator

- increased dynamic range

lowered sensitivity to external and internal noise

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described more in detail with reference to the appended drawings, where: Figure 1 shows a schematic perspective view of an oscillator according to the present invention;

Figure 2 shows a schematic presentation of an amp!ifier used in the oscillator according to the present invention;

Figure 3 shows a simplified view of a substrate with a MEMS structure as used in the present invention; Figure 4 shows the different states of each switch of a MEMS structure as used in the present invention;

Figure 5 shows a schematic top view, side view and sectional view of a waveguide cavity resonator wit MEMS structures in a first open state;

Figure 6 shows a schematic top view, side view and sectional view of a waveguide cavity resonator with MEMS structures in a second closed state;

Figure 7 shows a simplified view of a surface-mounted waveguide cavity resonator with the MEMS structure;

Figure 8 shows a sectional perspective view of the item shown in Figure

7;

Figure 9 shows a sectional side view of the item shown in Figure 7;

Figure 10 shows a simplified view of an oscillator according to the present invention whic comprises a cylindrical waveguide cavity resonator: and Figure 11 shows a sectional side view of the cylindrical waveguide cavity resonator shown in Figure 10.

DETAILED DESCRIPTION

With reference to Figure 1 there is an oscillator device 1 which comprises an amplifier unit 2 and a waveguide cavity resonator 3.

The waveguide cavity resonator 3 comprises a first inner wail 5, a second inner wall 6, a third inner wall 7 a fourth inner wall 8, a fifth inner wail 9 and a sixth inner wall 10,

For reasons of clarity, only the inner walls are indicated in figure 1 and also in Figure 3, Figure 4, Figure 7, Figure 8 and Figure 10, although, in practice, the walls have a certain thickness with an outer side and an inner side and may be referred to as walls as well as inner walls in the description, but it should be understood that in this context, it is electrically conducting surfaces of the inner walls that define the cavities in question as will be discussed below. The first inner wall 5 and the second inner wall 6 are facing each other and are separated by a first cavity length a. The third inner wall 7 and the fourth inner wall 8 are facing each other and are separated by a second cavity length b. The fifth inner wall 9 is mounted to the second inner wail 6, the third inner wall 7 and the fourth inner wall 8, white the sixth inner wall 10 is mounted to the first inner wall 5, the second inner wall 6, third inner wall 7 and fourth inner wall 8.

In this way, the fifth inner wall 9 leaves an opening 21 against the first inner wall 5. The fifth inner wall 9 and the sixth inner 10 wall are facing each other and are separated by a third cavity length L such that a cavity 1 1 is formed and is limited by the inner walls 5. 6, 7, 8, 9, 10. These inner walls 5, 8, 7, 8, 9 _f 10 are constituted by respective main surfaces and are electrically conducting,

With reference to Figure 2 _; the amplifier unit 2 comprises a transistor unit 22 which is biased from a bias port 23 via a radio frequency (RF) blocking inductor 24. The transistor unit 22 has an input and an output, the input being connected to a first direct current (DC) blocking capacitor 25 and the output being connected to an output port 26 via a second DC blocking capacitor 27. The first DC blocking capacitor 25 is connected to a first varactor diode 20a which is connected to a second grounded varactor diode 20b via a directionai coupier 28. The directional coupier 28 is in turn connected to a resonator coupling port 29. A varactor tuning port 30 is connected to the directional coupler 28 such that a control voltage may be applied to the varactor diodes 20a, 20b from the varactor tuning port 30.

The varactor diodes 20a, 20b constitute a second tuning element, and the amplifier unit 2 constitutes a so-called negative resistance amplifier.

The amplifier unit 2 is positioned outside the cavity 11 and the resonator coupling port 29 is connected to the cavity 11 by means of a probe 19 that enters the cavity 1 1 via the opening 21 in the fifth wall 9. The waveguide cavit resonator 3 and the amplifier unit 2 are here shown mounted to a base 40 which may be constituted by any suitable material. The waveguide cavity resonator 3 and the ampiifier unit 2 may be mounted to the base 40 in many ways, for example by means of gluing or screwing. Soldering of surface-mounted components is also a possibility that will be discussed more later in the description. According to the present invention, with reference to Figure 1 , the waveguide cavity resonator 3 further comprises a first tuning element 4 which is arranged for altering the second cavity length a between a first value a _eff i and a second value such that a resonance frequency f _r of the waveguide cavity resonator 3 is adjustable between two corresponding magnitudes.

The size of the waveguide cavity resonator 3 is thus altered in steps between two values, which results in different resonance frequencies. In order to achieve this, the first tuning element 4 comprises three rows 13, 14, 5 of switches 16. The switches 16 are electrically openable and c!osable and are arranged to constitute an electrically conducting connection between the third inner wall 7 and the fourth inner wall 8, The switches 16 are of the type Micro Electro Mechanical Systems {MEMS), the switches being cantilever switches 16 and are positioned a MEMS substrate 12. The MEMS substrate 12 serves as a carrier material, and is mounted on the first inner wall 5. A magnification 31 of a part of the first tuning element 4 with the MEMS switches 16 is shown in a magnifying circle. in Figure 3, one MEMS structure forming the first tuning element 4 is shown with the 3 rows 13, 14, 15 of switches 16. The switches 16 are electrically controlled by means of bias voltages, which are applied at certain bias inputs 31. A common ground pad 32 is used. When no bias voltage is applied, the switches 16 in a corresponding row 13, 14, 15 are open, and when voltage is applied, the switches 16 in a corresponding row 13, 14, 15 will be closed.

This is illustrated in detail in Figure 4, where, in a first state where bias voltage is applied, one shown switch 16 is ciosed, and can be regarded as equivalent to a resistor R, In a second state where no bias voltage is applied, the shown switch 16 is open, and can be regarded as equivalent to a capacitor C. How the tuning is performed by means of the MEMS structure 1 will now be explained more in detail with reference to Figure 5 and Figure 6. where each of these Figures shows a simplified top view, side view and sectional view of the waveguide cavity resonator 3.

As shown, the first tuning element 4 is mounted to the first inner wall. The substrate 8 comprises a conducting frame 33 with vias 34, In this way, a good electrical connection between the rows 13, 14, 15 of switches 16 and the surrounding third inner wall 7 and fourth inner wall 8 is ensured. in Figure 5, the switches 16 are opened, and in this state the first tuning element 4 does not affect the original electrical dimension of the waveguide cavity resonator 3, the first length a having the first value a _eff i - in Figure 8, the switches 18 are closed such that each row 13, 14, 15 constitutes a electrically conducting connection between the third inner wail 7 and the fourth inner wall 8 via the conducting frame 33. In this way, the electrical dimension of the second length a is altered from the first value a _eff i to the second value a _e «2, which here is a minimum value.

In Figure 6 _: the electric dimension of the cavity 1 1 is not defined only by the inner walls 5, 6, 7, 8, 9, 10, but also of an artificial wall which primarily is constituted by the rows 13, 14, 15 of closed switches 16 that constitute electrically conducting connections. The artificial wall confines the electric field and makes the cavity 1 electrically smaller which makes the resonance frequency higher than when the switches 16 are opened.

When the switches 18 are opened again, as shown in Figure 5, the field will be confined by the inner wails 5, 6, 7, 8, 9, 10 only. The waveguide cavity resonator 3 will be electrically larger, which makes the resonance frequency lower than when the switches 16 are closed. An example of an oscillator device 1 ' with a surface-mounted waveguide cavity resonator will now be described with reference to Figure 7, showing a view of a surface-mounted waveguide cavity resonator 3\ Figure 8, showing a sectional perspective view of the waveguide cavity resonator 3' in Figure 7, and Figure 9, showing a sectional side view of the waveguide cavity resonator 3' in Figure 7.

The waveguide cavity resonator 3' is in the form of a Surface ountabie Waveguide (SMW) part 35 mounted to a printed circuit board (PCB) 17 such that a metaiiization 18 on the PCB 17 constitutes the first inner wail 5'. The second inner wall 6', the third inner wall 7\ the fourth inner wall 8', the fifth inner wall 9' and the sixth inner wall 10' are all formed in the SMW part 35. As before, the first tuning element 4 is positioned on the first inner wall 5' which here thus is constituted by the metaiiization 18 on the PCB 17. The metaiiization may either only be present at the position for the first inner wall 5', or cover a larger area on the PCB 17. As before, the amplifier unit 2 is positioned outside the cavity 11', and is connected to the cavity 11' by means of a probe 19 that enters the cavity 11' via the opening 21 ^* in the fifth wall 9'.

Preferably the SMW part 35 and the first tuning element 4 are soldered to the metallization 18 on the PCB 17, and will be biased from the PCB 17 where the bias connection (not shown) is placed in an internal layer of the PCB 1 . The MEMS substrate 12 thus comprises a solderab!e part on the side facing away from the rows of switches 13, 14, 15 such as a copper ground plane. It is conceivable the soldering to the metallization 18 only is made by means of the vias 34 shown in Figure 5 and Figure 6. For ail examples, it is not necessary that there is a second tuning element 20a, 20b, but when there is a second tuning element 20a, 20b element, the electromagnetic coupling between the first tuning element 4 and the waveguide cavity resonator 3 is relativefy strong while the electromagnetic coupling between the second tuning element 20a, 20b and the waveguide cavity resonator 3 is relatively weak. In general, the electromagnetic coupling between the first tuning element 4 and the waveguide cavity resonator 3 exceeds the electromagnetic coupling between the second tuning element 20a, 20b and the waveguide cavity resonator 3.

The reiativeiy weak electromagnetic coupling between the second tuning element 20a, 20b and the waveguide cavity resonator 3 will have a negligible impact on the total resonator Q. The first tuning element 4 thus provides a reiativeiy coarse adjustment of the oscillator device 1 , and the second tuning element 20a ₍ 20b thus provides a relatively fine adjustment of the oscillator device 1.

The waveguide cavity resonator may have other shapes where the present invention still is applicable. An example of an oscillator device 1 " with a cylindrical waveguide cavity resonator 3" will now be described with reference to Figure 10, showing a view of such an oscillator device 1", and Figure 11 , showing a sectional side view of the cylindrical waveguide cavity resonator 3" shown in Figure 7.

The cylindrical waveguide cavity resonator 3" is mounted to a PC8 17 or any other suitable supporting layer, for example by means of gluing or soldering.

The cylindrical waveguide cavity resonator 3" comprises a first circular wall 37 and a second circular wall 38, the circular walls being positioned at the opposing ends of a circular pipe part 39, the circular pipe part 39 having a diameter that corresponds to a first cavity length a". A first tuning element 4" is positioned on the inner side of the circular pipe part 39, such that it follows the curvature of the circular pipe part 39. As in the previous examples, the amplifier unit 2 is positioned outside the cavity 1 1", and is connected to the cavity 11" by means of a probe 19 that enters the cavity 1 1 " via an opening 21" in the first circular wall.

In this example, the inner diameter of the circular pipe part 39, the first cavity length a", is changed when the switches of first tuning eiement 4" are opened and closed as described in the previous examples. Here, the inner diameter is changed between a first value a _e * _f i ' ' to a second value as illustrated in Figure 11. The arrows indicating the diameter are slightly displaced for practical reasons, but it should be understood that it is the diameter that is meant to be indicated,

Generally, the waveguide cavity resonators 3, 3', 3" which may be used for the present invention thus have a first cavity length a, a" between two opposing inner walls 7, 8; 37, 38 of the waveguide cavity resonator 3, 3', 3", where a resonance frequency f _r of the waveguide cavity resonator 3, 3 , 3" is dependent on the first cavity length a, a".

The present invention is not limited to the examples discussed above, but may vary freely within the scope of the appended claims. For example, more than one of the disciosed first tuning element 4 may be used, a number of tuning elements may be either piaced on different wails in the waveguide cavity resonator 3' or stacked, allowing tuning in several steps. It is possible to use any number of tuning elements along one short side or both short sides of the waveguide cavity resonator 3'. Each tuning element has been described as a MEMS structure based on cantilever switches, but other electrically controllable structures that are able to create an electrically conducting connection between two opposing inner walls, such that a virtual wali is created, may be used. Generally, a!! such structures constitute a tuning element, each tuning element being electrically controllable. The basic idea of the present invention lies in positioning an electrically controllable tuning element in a cavity resonator, preferably where the electrical field is weak and the magnetic field is strong.

Preferably, in the examples above, the second length a exceeds the first length b.

Where MEMS structures are used, the number of rows and the general constitution of the MEMS structure are only given as an example and may of course vary. There may be any suitable switch arrangement constituting such a MEMS structure. There does not have to be any vias or conducting frame, but there has to be an electrical connection via the rows 13, 14, 15 of switches 18 and the surrounding third inner wall 7 and fourth inner wail 8 when the switches 16 are closed Also, a MEMS structure does not have to be mounted against any one of the inner wails 5, 6, 7, 8, 9, 10, but there may be a distance between each MEMS structure and the corresponding inner wail, it is, however, important that there is an electrical contact between each MEMS structure and the third inner wall 7 and fourth inner wall 8, or other corresponding wails in another configuration, such that an electrical connection via the rows 13, 14, 15 of switches 16 and the surrounding third inner wall 7 and the fourth inner wall 8 when the switches 16 are closed,

The amplifier unit 2 may be positioned inside the cavity 11 as well, and it may even be placed on the MEMS substrate 12 or using the MEMS substrate 12 as a carrier material. The coupling to the cavity 11 may be performed in any suitable way, either by electrical or magnetically coupled probes to achieve the desired feedback, providing a stepped voltage controlled oscillator, for example by using a bond wire or en etched conductor on the amplifier unit 2.

The amplifier unit 2 may be made in many suitable ways, the one described with reference to Figure 2 is only one example of how an amplifier unit that can be used for the present invention may be constituted. For example, the varactor diodes 20a, 20b may be omitted or placed on a separate part or on the MEMS substrate 12.

The amplifier unit 2 is formed on a substrate 36, either in the form of MMIC or with discrete components mounted to the substrate. The substrate may then be any suitable type of PCB. in general, the second tuning element may be constituted by one component or several components, where each such component may be the discussed varactor diode, an adjustable capacitor such as a MEMS capacitor or an inductor.

By means of the present invention, a high Q is obtained for a large tuning range resulting in a low phase noise.

A MEMS switch is not sensitive to a high RF voltage swing, which makes it possible to use a very high voltage in the amplifier and thereby increase the dynamic range and further improve the phase noise. For example, the use of GaN, which is an upcoming semiconductor microwave material, could be advantageous because of high breakdown voltages. As the continuous analog tuning range is relatively small, the sensitivity to externally and internally generated noise is effectively eliminated.