WO2012066452A1

WO2012066452A1 - Adjustable frequency-determining element

Info

Publication number: WO2012066452A1
Application number: PCT/IB2011/054973
Authority: WO
Inventors: Floris Maria Hermansz Crompvoets; Paulus Thomas Maria Van Zeijl
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2010-11-15
Filing date: 2011-11-08
Publication date: 2012-05-24

Abstract

The present invention relates to a frequency- determining element which is made of an electro - active material (20) sandwiched between electrodes (10) so as to provide tuning for optimal signal transmission and/or signal reception. At least one electrode pattern is arranged on the surface of the electro - active polymer (20) so as to change size or shape when the electro - active polymer (20) changes size or shape.

Description

Adjustable frequency-determining element

FIELD OF THE INVENTION

The present invention generally relates to an adjustable frequency-determining element for tuning a transmitter, receiver, transceiver or amplifier to a desired frequency band or load.

BACKGROUND OF THE INVENTION

For wireless communications, only a few narrow frequency bands are available and thus often crowded by different wireless standards and/or protocols. Accordingly, there is a need to provide wireless electronics whose parameters can be adapted.

A. Mahanfar et al, "Smart antennas using electro-active polymers for deformable parasitic elements", ELECTRONICS LETTERS, Vol. 44, No. 19, 11^th September 2008, discloses deformable smart materials which can be used as parasitic elements to alter the antenna pattern of smart antennas. A new actuation technology, made of an electro-active polymer (EAP) is introduced, which has the potential to implement mechanical motion at low cost and power. Among the large variety of smart materials as described for example in R. Smith, "Smart Material Systems: Model Development", SIAM, 2005, EAPs are often investigated for artificial muscles as described e.g. in Y. Bar-Cohen, "Electroactive polymer (EAP) actuators as artificial muscles - reality, potential and challenges", SPIE Press, PM136, March 2004, 2^nd edition.

In the above-mentioned smart antennas a pre-fluorinated polymer with platinum surface electrodes is suggested as EAP and offers large deformation at low direct current (DC) voltage. The proposed EAP approach effectively allows replacement of much RF circuitry and signal processing modules present in a conventional antenna array by simple low cost and low power smart materials. A deformable conducted strip has a voltage- controlled slant angle and its close proximity to an antenna monopole reconfigures the antenna pattern. However, the changing proximity of the parasitic element affects antenna impedance and a mismatch loss can compromise improvements in signal-to-noise-plus- interference gained by the beam forming. This mismatch must be compensated by a matching circuit adapted to specific different slant angles.

Conventionally, tuning of electric circuits in communication devices can be achieved by providing mechanically controlled or voltage- or current-controllable reactance elements, which are independently controllable to tune bandwidth and/or center frequency of oscillators, filters, load matching circuits and the like.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a tuning element which enables flexible tuning of electric circuits in communication devices to desired frequency bands and loads.

This object is achieved by a frequency-determining element as claimed in claim 1.

Accordingly, an active polymer made from a non-conducting polymer sandwiched between two electrodes is provided, which can change its shape or size based on an attractive force achieved by a charge difference accumulated on the two electrodes by application of a voltage. Due to conservation of volume, the change of size or shape of the elastic polymer leads to a change in its surface area and thus to a change of the stretchable pattern of the at least one conductive electrode. Thus, the change of the size or shape of the elastic polymer can be used to adapt or tune an electric circuit (e.g. a filter circuit, a matching circuit or an oscillator circuit) to an optimal reception and/or transmission frequency or frequency band used in a communication system. The length or shape of the elastic polymer changes when a voltage is applied on it.

According to a first aspect, the stretchable pattern may comprise a stretchable conductive polymer. Thereby, a change of the length or shape of the electrode pattern can be achieved when the voltage is applied thereto.

According to a second aspect, the stretchable pattern may comprise percolated conducting particles. Such percolated conducting particles allow the electrode pattern to stretch while maintaining a good electric conduction.

According to a third aspect which can be combined with any one of the above first and second aspects, the elastic polymer may be pre-stretched in at least one of its dimensions. This facilitates the expansion in a specific direction. According to a fourth aspect which can be combined with any of the above first to third aspect, the elastic polymer may be made of at least one of a silicone, an acrylate and derivates thereof. Thus, a low elasticity modulus, a high resistance to electrical breakdown and a possibility of pre-stretching can be provided.

According to a fifth aspect which can be combined with any of the above first to fourth aspects, the at least two electrodes may be made of a metal, carbon or a conductive polymer. According to a specific example, the at least two electrodes may comprise graphene sheets or carbon clusters. These materials allow sufficient stretchability of the electrode pattern. As a specific example of a conductive polymer, polyaniline could be used as material for the at least two electrodes.

According to a sixth aspect which can be combined with any of the above first to fifth aspects, the stretchable pattern may comprise an undulated pattern. Thereby, a change of length of the electrode pattern can be achieved, which could be advantageously used for example to change the length of an adaptive antenna. As an example, the undulated pattern may be made from an embossed polymer film with a conductive thin film deposited thereon. This is advantageous especially for metallic films as they cannot be stretched as easy as polymeric films.

According to a seventh aspect which can be combined with any of the above first to sixth aspects, the stretchable pattern may be shaped in a form of a coil. Thereby, a tunable inductance can be achieved.

In general, the proposed frequency-determining element can be structured as a tunable antenna or tunable reactance which could be used for a filter circuit, a matching circuit or an oscillator circuit.

Furthermore, the frequency-determining element according to the above aspects can be provided in a transmitter, receiver, or transceiver circuit for tuning to a desired frequency, or in an amplifier for tuning to a desired frequency band.

Various types of reactances or tunable antennas can be achieved by modifying the at least one electrode pattern to achieve a tunable capacitance or other types of tunable inductances or antennas.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings: Fig. 1 shows a schematic presentation of a basic principle of an electro-active polymer;

Fig. 2 shows a schematic side view of an antenna zigzag pattern according to a first embodiment;

Fig. 3 shows a schematic presentation of an adaptive coil pattern according to a second embodiment; and

Fig. 4 shows a schematic block diagram of a transceiver with various tuning options according to a third embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

According to the following embodiments, an elastic polymer, such as an electro-active polymer is used for controlling the shape and size of at least one electrode pattern provided on the surface of the elastic polymer based on a tuning voltage applied across at least two electrodes between which the elastic polymer is sandwiched.

Thus, the elastic polymer which is made from a non-conducting polymer sandwiched between two compliant electrodes can be used to control the size and shape of the at least one electrode pattern so as to achieve a tunable reactance or a tunable antenna which can be used in filter circuits, oscillator circuits, matching circuits, antenna circuits, antennas, and the like. When a voltage is applied over the at least two electrodes, opposite charges accumulate on the electrodes resulting in an attractive force between them. The elastic polymer prevents the electrodes from crashing into each other and hence delivers the stability for the system. Due to a conversion of volume, a decrease in distance between the electrodes results in an increase in surface area and vice versa.

Fig. 1 shows a schematic diagram of the basic principle of the elastic polymer. Adding a charge Q to the compliant electrodes 10 results in an attractive force between the electrodes 10. This force is counterbalanced by the elastic dielectric polymer 20, so that the thickness z varies by dz as a function of a variance dQ of the charge Q applied to the electrodes 10. This variation of the thickness results in a variation dA of the surface area A of the compliant electrodes 10. It is clear that Fig. 1 only shows a simplified embodiment of the electro-active polymer, while any kind of shapes and electrode arrangements can be implemented to achieve different deformations and changes of size and/or shape.

Examples of the elastic polymer 20 may be dielectric elastomers which can produce large strains and may include silicones, acrylic elastomers and other other materials such as poly[Styrene-b-(Ethylene-co-Butylene)-b-Styrene] (SEBS), polyurethanes, or polyvinyl chloride (PVC). Such silicones are polymers that include silicone together with at least one of carbon, hydrogen, oxygen and other chemical elements. Other forms may include at least one of silicon oil, silicon grease, silicon rubber and silicon resin. Acrylic elastomer is a general term for a type of synthetic rubber whose main component is acrylic acid alkyl ester (ethyl or butyl ester). An acrylate polymer belongs to a group of polymers which could be referred to generally as plastics. They are noted for their transparency and resistance to breakage and elasticity and are also commonly known as acrylics or polyacrylates. Typical acrylate monomers used to form acrylate polymers are acrylic acid, methyl methacrylate and acrylonitrile.

The electro-active polymer or dielectric elastomer actuator shown in Fig. 1 can be interpreted as a compliant capacitor wherein the elastic polymer 20 forms an elastomeric film coated on both sides with the electrodes 10. The electrodes 10 can be connected to an electric circuit. By applying a voltage, an electrostatic pressure acts. Due to the mechanical compression the elastic polymer 20 contracts in the thickness direction and expands in the film plane directions. The elastic polymer 20 may move back to its original position when the voltage across the electrodes 10 is removed. The electrostatic pressure arises from the Coulomb forces acting between the electrodes 10. Therefore the electrodes 10 squeeze the elastic polymer 20. The equivalent electromechanical pressure is twice the electrostatic pressure and depends on the vacuum permittivity, the dielectric constant of the polymer and the thickness of the elastic polymer 20.

A possibility to enhance the electric breakdown strength is to pre-stretch the elastic polymer 20 mechanically. Further reasons for pre-stretching the elastic polymer 20 are to decrease the thickness of the elastic polymer 20, so that a lower voltage can be applied to obtain the same electrostatic pressure. Additionally, the prestrain avoids compressive stresses in the plane directions of the elastic polymer 20, which might be responsible for failure.

Several different types of electrodes can be used, such as for example graphite powder, silicon oil/graphite mixtures, gold or silver electrodes, etc. However, metal electrodes should be undulated in order to be able to stretch. The electrodes 10 should be conductive and compliant. The compliance of the electrodes 10 is important in order that the elastic polymer 20 is not constrained mechanically in its elongation by the electrodes 10.

In the following, different embodiments for applying the above principle of Fig. 1 to obtain different controllable antennas or reactance elements and their implementation in different electric circuits are described. Fig. 2 shows a schematic side view of an antenna pattern with top and bottom zigzag or undulated patterns forming electrodes.

The antenna of Fig. 2 can change length in order to adapt to an optimal reception and transmission frequency. More specifically, it comprises an elastic polymer 20 (electro-active polymer) that changes length or shape when a voltage is applied thereto. A wobbling or zigzag shape of an electrode pattern 30 makes it possible for the electrode pattern 30 to stretch along with the elastic polymer 20 when it stretches upon application of a voltage.

Antenna reception and transmission will work fine as long as the amplitude d of the zigzag pattern is substantially smaller than the wavelength of the electromagnetic waves (typically some centimeters for radio frequency (RF) applications) which is to be received or transmitted. Shorter zigzag wavelengths and larger zigzag amplitudes d result in bigger changes of the antenna length L.

The elastic polymer 20 may be made from silicones (e.g. CF 19-2186) or acrylates (e.g. VHB4910) which can stretch more than 100% to thereby allow doubling the antenna length L.

The electrode patterns 30 will act as the antenna and therefore have to conduct well. Suitable electrode materials may be metal (e.g. silver or gold), carbon materials (such as graphine sheets and carbon clusters) or conducting polymers such as for instance polyaniline.

The electrode zigzag pattern 30 may be obtained by embossing a polymer film with the desired zigzag pattern. A thin productive film (e.g. silver) may be vapor deposited onto this embossed zigzag pattern. A voltage can then be applied over the silver electrodes 30, so that the antenna length L changes due to the deformation of the elastic polymer 20.

Consequently, the tunable antenna element shown in Fig. 2 can be used as a frequency-determining element which can be controlled to fine tune a reception and/or transmission frequency for optimal signal transmission and signal reception.

Fig. 3 shows a tunable frequency-determining element according to a second embodiment. Here, the principle of Fig. 1 is applied to achieve a controllable reactance element. More specifically, an electromagnetic coil and thus a tunable inductance is obtained.

An electrode pattern 32 of Fig. 3 has a coil shape that can change length in order to adapt to an optimal reception and transmission frequency. A change in coil length changes the induction of the coil. Again, the coil-shaped electrode pattern 32 changes length or shape when a voltage is applied to the elastic polymer (not shown in Fig. 3) on the surface of which the electrode 32 is applied or deposited. The coil-shaped electrode pattern 32 may comprise percolated conducting particles 40 that allow the electrode 32 to stretch while maintaining a good electrical conduction. As an alternative, a stretchable conducting polymer may be used in the coil- shaped electrode pattern 32. Of course, other materials, as mentioned above, could be used as well.

When a voltage is applied between the electrode 32 and an opposite electrode (not shown) on the other side of the elastic polymer, the resulting actuation makes the surface of the elastic polymer larger and hence the length of the coil-shaped electrode pattern 32. This leads to a change in the induction of the coil. The crossed arrows in Fig. 3 show stretching dimensions of the surface of the underlying elastic polymer.

Again, silicon materials (such as VHB4910) and acryline materials (such as CF19-2186) can be used due to their high stretchability of more than 100%.

Additionally, the elastic polymer may be pre-stretched in the two dimensions shown in Fig. 3, to thereby facilitate expansion of the coil in a radial direction.

Of course, other stretchable electrode patterns can be used to obtain desirable tunable reactance elements or antennas.

Fig. 4 shows a schematic block diagram of a transceiver circuit with various controllable circuit parts in which at least one of the frequency-determining elements described above can be implemented.

The transceiver circuit of Fig. 4 consists of a transmission branch including blocks 51 to 54 and a reception branch including blocks 56 to 59. Either the transmission branch or the reception branch is switched to the antenna by a suitable RF switch 50. The antenna element may be a tunable antenna obtained by or comprising a tunable antenna element according to the above first embodiment. Both transmission and reception branches are connected to a transceiver (TRX) board responsible for analog-to-digital and digital-to- analog conversion, respectfully, conversion between base band and intermediate frequency band, filtering, etc.

In the transmission branch, an intermediate frequency (IF) signal generated by the TRX board 55 is supplied to a first mixer circuit 54 which is adapted to mix the IF signal with an oscillator signal received from a controllable transmission oscillator 53. The first mixer circuit 54 serves to convert the IF signal into the transmission frequency or RF range. Then, the RF signal is amplified by a tunable power amplifier 52 and supplied to a tunable transmission filter 51 which may be used as harmonic filter for example. In the reception branch, the received signal from the RF switch 50 is supplied to a tunable reception filter 59 which may be used as a pre-selector for pre-selecting a desired frequency range. Then, the received signal is applied to a tunable low noise amplifier (LNA) 58 for pre-amplifying the received signal and supplying it to a second mixer circuit 56, where it is mixed with an oscillation signal of a tunable reception oscillator 57 to be converted to the IF band and then supplied to the TRX board 55.

It is noted that the tunable elements of Fig. 4 may at least partially be tuned by using one of the above tunable frequency-determining elements of the first and second embodiments. In the tunable amplifiers 52 and 58, a tunable reactance based on an electro- active polymer can be used for providing impedance matching. As an example, the power amplifier 51 provides maximum efficiency (e.g. maximum battery life-time in a cellular phone or the like) if used with a proper load impedance. The real part of the load impedance for optimum efficiency is defined by the load-line while the imaginary part of the load impedance should be equal to zero. By modifying the reactance of the tunable frequency- determining element (e.g. length of an inductor), the reactance value can be changed and the power amplifier 52 can be tuned to its optimum efficiency point by providing the tunable reactance in a corresponding matching circuit. The same principle can be used in the reception branch, where an improved receiver performance can be obtained when the LNA

58 is tuned to the desired frequency band. Hence, the use of tunable inductances and/or capacitances as proposed in the second embodiment can fulfill this tuning requirement.

The required control voltage can be generated by a control circuit provided in the transceiver and may be controlled for example based on the quality of the received base band signal.

The first and second mixer circuits 54 and 56 may comprise two or more mixers for different signal components (e.g. an in-phase (I) and quadrature phase (Q) components), e.g., to realize single-sideband frequency conversion.

It is noted that the above reception branch may be provided in a separate receiver and that the above transmission branch may be provided in a separate transmitter.

In a similar manner as described above, the proposed tunable frequency- determining element may be provided as a tunable reactance in either one of the filters 51 and

59 or the oscillators 53 and 57. In the filters 51 and 59, the tunable reactance may be used in at least one resonance circuit which determines at least one of the bandwidth and center frequency. In the oscillators 53 and 57, the tunable reactance may be used as frequency- determining element in the respective oscillator circuits. In summary, a frequency-determining element has been described, which is made of an electro-active material sandwiched between electrodes so as to provide tuning for optimal signal transmission and/or signal reception. At least one electrode pattern is arranged on the surface of the electro-active polymer so as to change size or shape when the electro- active polymer changes size or shape.

The above embodiments can be used for example in radio and/or wireless systems based on standards such as Multi-band General System for Mobile communication (GSM), 3^rd Generation (3G), Bluetooth, wireless local area networks (WLAN, 802.11 a/b/g/n), ultra wide band (UWB, 3 - 10 GHz), Zigbee, 60GHz wireless-HDMI and others.

While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments and can be used for all kinds of tunable antenna elements, inductances, capacitances or other frequency-determining elements. Additionally, the frequency- determining element may comprise several elastic polymers and more than two electrodes or more than one elastic electrode pattern, so that desired tuning behaviors or more complex reactive circuits or antenna structures can be implemented.

Variation to the disclosed embodiments can be understood and effected by those skilled in the art, from a study of the drawings, the disclosure and the appended claims. In the claims, the wording "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality of elements or steps. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope thereof.

The present invention relates to a frequency-determining element which is made of an electro-active material sandwiched between electrodes so as to provide tuning for optimal signal transmission and/or signal reception. At least one electrode pattern is arranged on the surface of the electro-active polymer so as to change size or shape when the electro- active polymer changes size or shape.

Claims

CLAIMS:

1. A frequency-determining element for tuning an electric circuit, said element comprising:

- at least two conductive electrodes (10); and

- at least one non-conductive elastic polymer (20) sandwiched between said at least two conductive electrodes (10);

- wherein at least one of said at least two conductive electrodes is arranged in a stretchable pattern (30, 32) on a surface of said elastic polymer (20), so that the size or shape of said stretchable pattern (30, 32) changes when the size or shape of said elastic polymer (20) changes due to a force generated by a voltage applied over said at least two electrodes (10). 2. The element according to claim 1, wherein said stretchable pattern (30,

32) comprises a stretchable conductive polymer.

3. The element according to claim 1, wherein said stretchable pattern (30, 32) comprises percolated particles (40).

4. The element according to claim 1, wherein said elastic polymer (20) is pre-stretched in at least one of its dimensions.

5. The element according to claim 1, wherein said elastic polymer (20) is made of a silicone or an acrylate.

6. The element according to claim 1, wherein said stretchable pattern (30, 32) is made of metal, carbon or a conductive polymer.

7. The element according to claim 6, wherein said stretchable pattern (30,

32) comprises graphene sheets or carbon clusters or carbon nanotubes.

8. The element according to claim 6, wherein said stretchable pattern (30, 32) is made of polyaniline.

9. The element according to claim 1, wherein said stretchable pattern comprises an undulated pattern (30).

10. The element according to claim 9, wherein said undulated pattern (30) is made from an embossed polymer film with a conductive thin film deposited thereon.

11. The element according to claim 1, wherein said stretchable pattern (32) is shaped in the form of a coil.

12. The element according to claim 1, wherein said frequency-determining element is used as a tunable reactance.

13. The element according to claim 1, wherein said frequency-determining element is a tunable antenna.

14. A transceiver, transmitter or receiver circuit comprising a frequency- determining element according to any one of claims 1 to 13 for tuning to a desired frequency band.

15. An amplifier (52, 58) comprising a frequency-determining element according to any one of claims 1 to 13 for tuning to a desired load impedance.