Tuneable RF filters and methods thereof

Field of the invention

This invention generally relates to making radio frequency (RF) and/or microwave filters tuneable across a wide range at low cost. Making filters tuneable across a wide range involves the features:

a. Make the resonator structure tuneable across a wide range; b. Ensure a frequency independent coupling mechanism between resonators; c. Provide a tuning mechanism; d. Provide a mechanism to ensure that the filter performance is according to specifications.

This invention relates to point c. and d. and is applied to point a. and b.

RF band pass filters are used in all communications systems for selecting the communication frequency of interest and deselecting all frequencies potentially disturbing communication.

Having tuneable RF filters makes it potentially possible to use the same filter for multiple frequency standards or using the same filter for multiple sub-band channelizations. Being able to use the same filter for multiple applications saves cost and reduces inventory. In addition these filters might possibly reduce the volume of the equipment.

Tuneable filters are currently only having limited success due to the electrical performance constraints of existing methods and cost constrain of others.

More particularly, the invention relates to a filter adapted to be arranged in a radio-system.

Background of the invention

A RF filter can be realized in a multiple number of ways. The two most used realization methods for wireless and broadband infrastructure products are combline filters or filters implementing dielectric resonators. Both of these technologies are relatively bulky mechanical filters that are labour intensive in relation to assembly and tuning.

Performance parameters of filters currently used for mobile and wireless infrastructure are achieved by adjusting screws. The screws are used to adjust the resonant frequency of each resonator in the filter and the coupling between adjacent resonators. Multiple resonators coupled together constitute a filter. In order to achieve a desired filter response for a given filter, each resonator will have to be tuned relatively to the other resonators.

This tuning process is done manually due to the complex nature of tuning. It is however possible to perform this tuning by electrical actuators as for example DC-stepper motors. Either by using one DC-stepper motor pr. screw or by using some kind of gearing arrangement/toothed rack that moves several screws simultaneously.

To achieve the desired performance of the filter it is necessary to monitor the characteristics by the use of expensive measurement equipment. During production the filter is tuned to meet performance requirements at one specific centre frequency. It is hereafter assumed that the frequency characteristics can be moved linearly across frequency to meet performance characteristics at other centre frequencies by actuating screws with the motor.

When trying to make the filter electrically adjustable with DC-stepper-motors (DSM), a number of problems are faced:

- Since a filter is very sensitive it can be very hard to make the DSM sufficiently repeatable. This might require implementation of encoders that will add cost.

- DSMs are fairly expensive and expensive to implement. This makes it hard to meet customer expectations to cost, since cost is a primary parameter when adopting new technologies.

- DSMs can potentially only be used with filters having a reduced number of resonators. This prevents the technology to be applied to transceiver front- end filters.

- DSMs need manual brakes to lock position during power-off.

- DSMs are not sound free.

- DSMs face serious corrosion problems in harsh environments when not being operated for a longer period. This leads to poor reliability.

- DSMs are relatively large and heavy.

- The tuning screws need to be positioned very accurately. To achieve sufficient accuracy with DSMs, it's necessary to implement a gearing mechanism which is adding to cost.

- Commercially available motors are somehow limited by the necessary build- in height.

If the resonator is operated in a non-linear region it can be complex to move the filter response because all resonators potentially might need different actuations.

Summary

Disclosed is a radio frequency filter comprising a cavity member and a tuning element defining a cavity, the tuning element being adapted to tune a resonance frequency of the cavity; wherein the filter further comprises a piezoelectric motor adapted to displace the tuning element so as to electrically tune the filter.

Consequently, it is an advantage that the filter can be electrically tuned, since manual tuning therefore will be unnecessary. In addition it will be possible to change the center frequency and characteristics of the filter or duplexer after delivery and after installation.

The use of a piezomotor as an actuator for tuning the filter has a number of advantages:

- Fewer parts which provides easier integration;

- Small size and thereby low weight;

- Automatically breaks when power is removed and thereby no extra break is needed; - Better reliability compared to a DSM due to the materials and number of parts;

- Opportunity to perform extremely fine positioning without gearing;

- Silent operation;

- Non-magnetic (EMC-free); - Can be integrated on a printed circuit board (PCB).

Since piezomotors do not need any mechanical brake, piezomotors automatically brake, when current is switched off. The advantage of this is that it makes piezomotors more reliable and easier to control. Furthermore there are potentially no movable parts in piezomotors, which makes piezomotors more stable and less fragile.

The piezomotor can also be realized at a cost that makes it less expensive than DC-motors. Cost is a key issue for companies requesting filters.

In some embodiments one piezomotor actuates one or more tuning elements.

It is an advantage that one piezomotor can actuate one or more tuning elements, because if the motor is powerful enough, only one motor is needed, whereas if the motors are not so powerful, more motors is implemented.

In some embodiments the tuning is performed by actuating an alumina bar/screw.

An advantage of this is that frequency can be changed by means of alumines which have advantageous friction characteristics when used in piezomotors. A further advantage is that alumines have low electrical loss.

In some embodiments the piezomotor is a low profile motor. It is an advantage that the motor is a low profile motor, because no extra space for a moving piston is necessary.

In some embodiments the filter comprises one or more ceramic resonators with one or more ceramic disk tuners. It is an advantage to use ceramic filters, since ceramic filters have a high Q-factor compared to e.g. combline filters.

In some embodiments the piezomotor is a surface mounted device (SMD mounted) or an integrated part of the PCB. An advantage of this is that, when the piezomotor is realized in a SMD-mountable version, it is minimizing manual work in implementing the solution.

In some embodiments the PCB is an integral part of the piezomotor construction acting as a stator in the motor. Thus the piezomotor may be realized as a part of the PCB print.

It is an advantage that this allows for realizing rotational motors directly at the PCB at low cost. The realization is as well very compact.

In some embodiments the filter is adapted to be arranged in a radio-system. In case the filter is a part of a radio-system of which the applicant Radiocomp is providing the complete radio design as is the case when realizing Remote Radio Heads (RRH), the receiver in the radio and the digital platform in the RRH can potentially be used to replace the measurement equipment used for production. In this case it is potentially possible to ensure completely logical tuning of the filter by implementation of algorithms which at any time can monitor the exact performance of the unit.

The advantages of using the radio receiver and digital platform for tuning of the filter as well as closed-loop control are:

- Removes need for manually tuning in production; - Makes it possible to ensure performance at all times;

- Makes it possible to implement filter with higher complexity;

- Doesn't need calibration.

Further embodiments are disclosed in the dependent claims.

The present invention relates to different aspects described above and in the following, and corresponding methods, devices, and/or product means, each yielding one or more of the benefits and advantages described in connection with the first mentioned aspect, and each having one or more embodiments corresponding to the embodiments described in connection with the first mentioned aspect and/or disclosed in the appended claims.

Brief description of the drawings

The above and/or additional objects, features and advantages of the present invention, will be further elucidated by the following illustrative and non- limiting detailed description of embodiments of the present invention, with reference to the appended drawings, wherein:

Fig. 1 shows cavity filter applications.

Fig. 2 shows a potential implementation of a tuneable filter.

Fig. 3 shows a filter placed in a remote radio head radio-system.

Detailed description

In the following description, reference is made to the accompanying figures, which show by way of illustration how the invention may be practiced.

FIG. 1a shows a cavity filter application using ceramic resonators (9) with ceramic disk tuners (10). The ceramic resonator is placed on an alumina support (8) and the ceramic disk is held by an alumina rod (5). Preferably, alumina is used since it has low loss at RF frequencies.

The ceramic resonator (9) is placed in a cavity with a conducting surface (1 ). The cavity is closed by a cover with a conducting surface (2). The dimensions of (1 ), (8), (9) and (10) determines the resonant frequency of the cavity. It is understood that cavity also can be implemented without a cover.

To adjust the frequency of the cavity the tuner (10) can be moved by actuating the alumina rod (5). The rod is actuated by a piezoelectric motor (4). A piezoelectric motor is typically using piezo crystals to actuate an alumina bar or a screw. The piezoelectric motor (4) is placed on a printed circuit board (PCB) (3) where drivers, control and other electrical functions are placed. The PCB may thus be an integral part of the piezomotor construction acting as a stator in the motor. Thus the piezomotor may be realized as a part of the PCB print. The PCB (3) is placed on top of the cover (2).

Alternatively the PCB (3) can be screwed directly onto the cavity (1 ) without the use of a cover.

It is understood that alternatively or additionally the piezomotor can be fixed on a tuning cover of the filter.

FIG. 1 b shows a cavity combline resonator tuned by a dielectric bar. The resonator type is called a combline. In FIG. 1 b a combline resonator (7) is shown. The resonant frequency of the system is determined by the diameter/width of the cavity (1 ), the length and diameter of the hollow combline resonator (7) and the position of the alumina tuner (5). In a preferred embodiment the alumina tuner is made of metal. However, it is understood that the alumina tuner could be made of any suitable material.

In FIG. 1 a and FIG. 1 b a piezomotor (4) is shown. The motor is shown to be a low profile motor. However, it is understood that the motors could be any kind of piezomotors.

The motors have a connection to a PCB (3) for control and driving. The motors can be SMD mounted. Alternatively, the motors can be mounted in a mechanical fixture and connected to the PCB with a wire and connector. However, it is understood that the motors can be mounted in any suitable way.

To realize a filter one or more of the resonators, as described above, are coupled together.

If a filter, as described above, is used in a radio-system it is, as previously mentioned, possible to use this radio-system and the associated digital platform to perform tuning of the filter.

FIG. 2 shows a potential implementation of a tuneable filter.

The unit contains two filters. Each filter has several resonators. For each resonator a piezomotor is implemented. The piezomotor is driven from a motor driver which interfaces to a controller that controls all operations. It is understood that the unit may contain any number of filters, and that each filter can contain any number of resonators and piezomotors.

FiG. 3 shows a filter placed in a radio-system as realized in Radiocomp ^' s remote radio heads.

In FIG. 3 the electrical adjustable filters are denoted BPF. FIG. 3 shows a dual-channel system which contains two BPF's. In order to perform electrical adjustments of the filters in this system the following actions are taken:

a. In the high speed digital platform (HSDU) a digital pilot signal is generated. b. This signal is converted to an analogue signal in the D/A- converter.

c. The analogue signal is up-converted to the frequency of interest, amplified and fed through the filter. d. At the output of the filter a directional coupler captures a part of the signal and feeds it back to the receiver. e. In the receiver the signal that is fed back is down-converted and digitized in the A/D converter. f. The digital signal is fed to an algorithm in the control system of the HSDU. g. This algorithm calculates the positioning of the motor elements in the filter and performs the actuation.

In order to perform this regulation it is essential to build an electrical model of the filter that is incorporated into the algorithm that predicts the necessary movement of each of the actuators for a given centre frequency of the filter.

Although some embodiments have been described and shown in detail, the invention is not restricted to them, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilised and structural and functional modifications may be made without departing from the scope of the present invention.

In device claims enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers,

steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.