Patent Application
20120319918
The invention relates to the matching of the antenna of a radio device, and it includes both a matching arrangement and a method. The invention is intended especially for small-sized mobile terminals.
Matching the impedance of the antenna of a radio device to the power amplifier of the transmitter feeding the antenna is a normal arrangement in transmission technology. By means of the matching, the radiation power of the antenna can be made as high as possible in proportion to the power of the power amplifier. The poorer the matching of the antenna, the higher the strength of the field reflected from the antenna towards the power amplifier in proportion to the strength of the field propagating towards the antenna. If a certain transmitting power is wanted even though the matching degrades, the gain of the power amplifier has to be raised, which will result in increased current consumption and possibly problems in heating up in the output stage.
The matching of an antenna can degrade for external and internal reasons. If the device approaches some conductive object, the impedance of the antenna changes. Similarly, already the head of a user and the hand, in which the mobile terminal usually is during the connection, can cause a significant change in the impedance. In addition, in case of a multi-band antenna, changing the operating band changes the antenna impedance, which means a change in the matching. For these kind of facts it is favourable to make the antenna matching adaptable in such a way that it varies to be each time conformable to the circumstances. This requires that an adjustable matching circuit is added to the feed circuit of the antenna. Usually the matching circuit is controlled on grounds of the information of the strength of the field reflected from the antenna so that the antenna matching is all the time as good as possible.
In
The antenna matching can never be perfect, so a certain part re of the field ff propagating to the antenna is reflected back. The directional coupler provides two measuring signals: A radio frequency voltage VRE proportional to the reflected field is received from its port P3 and a radio frequency voltage VFF proportional to the propagating field from its port P4. These measuring signals are converted to direct voltages and further to binary digits in the control unit 150. In addition, the band signal BND indicating the current operating band and the power signal PWR proportional to the set value of the transmitting power are led to the control unit. The output signals SET of the control unit are connected to the matching circuit 130, control signals of which they then are.
The component values of the matching circuit 130 are selected by means of the multiple-way switches, which have a certain total number of state combinations. The control unit 150 executes at regular intervals an adjusting process. The interval of the starting moments in the process is e.g. 10 ms. The standing wave ratio, or SWR, of the antenna is obtained from the measuring signals VRE and VFF provided by the directional coupler. The higher the SWR, the poorer the matching. On grounds of the SWR value, the state of the band signal BND and the state of the power signal PWR the control unit chooses a substantially smaller array from the total array of the state combinations of the switches. In the matching process the switches of the matching circuit are in turn set to each of the state combinations, which belong to said smaller array, and the SWR value of the transmitting signal is read in each setting. Finally in the process the control unit sets the switches to the states, the combination of which corresponds to the lowest of the obtained SWR values.
In
Between each switch and the separate conductor SCR of the transmission path there is a circuit LCC, the object of which is usually to function as an ESD (ElectroStatic Discharge) protector for the switch. In addition, the serial capacitor belonging to the LC circuit functions, when needed, as a blocking capacitor preventing the forming of a direct current circuit from the switch control through the conductor SCR.
The branches in the transverse portions of the matching circuit, each branch including a change-over switch and alternative reactive elements, can naturally be also inverted so that the common terminals of the switches are connected to the ground conductor and one end of each reactive element to the separate conductor of the transmission path. One reactive element is then connected between the conductors of the transmission path at a time.
A drawback of the above-described solution is that the linear operating range of the directional coupler, being for the measurement of the antenna's mismatch, is relatively limited. In addition, the directional coupler is located on the transmission path of the transmitting signal, which means a certain extra loss in the transmitter. A drawback is also that the adjusting algorithm is relatively complex regardless of the fact that the number of the switches' state combinations, which are taken into account, is reduced in the early stage of the adjustment. A further drawback of the solution is that it is not suitable for the adjustment of the receiver matching.
An object of the invention is to implement the adaptable antenna matching in a way which reduces the above-mentioned drawbacks. The arrangement according to the invention is characterized in that which is specified in the independent claim 1. The method according to the invention is characterized in that which is specified in the independent claim 12. Some advantageous embodiments of the invention are presented in the dependent claims.
The basic idea of the invention is the following: A capacitive sensor is arranged in the antenna structure for detecting the electric changes in the surroundings of the antenna. The mismatch caused by a change is rectified by means of the signal proportional to the capacitance of the sensor. This capacitance and the frequency range currently in use are input variables of the control unit. The antenna impedance is adjusted by means of a π-shaped reactive matching circuit, the component values of which can be selected from a relatively wide array of the alternatives by means of change-over switches, which are only located in the transverse portions of the matching circuit. The control unit executes an adjusting process at regular intervals, on grounds of the result of which process it selects the combination of the component values of the matching circuit and sets the switches.
An advantage of the invention is that the antenna matching keeps relatively good, although the impedance from the duplexer towards the antenna would strive to change for external reasons or because of a band exchange. Maintaining the impedance results in that the mean efficiency of the transmitter improves, the level of the harmonic frequency components springing up in the power amplifier lowers and the function of the filters in the transmitter becomes more linear. Another advantage of the invention is that no directional coupler and serial adjusting components are needed in the transmission path of the transmitter, in which case the losses of the transmission path decrease and the efficiency of the transmitter improves also for this reason. A further advantage of the invention is that it can be used for the antenna matching also during the receiving. A further advantage of the invention is that the algorithm to be used in the adjusting process is relatively simple and fast compared to the known algorithms.
Below, the invention is described in detail. Reference will be made to the accompanying drawings where:
a,b present an example of the sensor belonging to the arrangement according to the invention in the antenna structure,
Close to a radiator of the antenna there is a capacitive sensor 370. This is connected to a capacitance unit 380, which converts the capacitance CSE of the sensor to a binary signal CAP, the level of which is proportional to said capacitance. The capacitance is measured using a low frequency (e.g. 35 kHz) current fed to it. This capacitance signal CAP is led to the input of the control unit 350. The sensor, the capacitance unit, the control unit and the first matching circuit constitute the matching arrangement according to the invention.
Information about the changes in the surroundings of the antenna is acquired by the sensor. If a conductive and/or dielectric object, such as a finger of the user, comes near to the antenna, the antenna impedance changes. Also the capacitance CSE of the sensor changes for the same reason, and therefore it can be used in the rectification of the antenna matching. In
The outputs SET of the control unit are connected to the first 330 and second 360 matching circuit for selecting reactances in them. The control unit executes at regular intervals the adjusting process pursuant to a certain algorithm, in which process the control of the first matching circuit is determined on grounds of the level, or value, of the capacitance signal CAP and band signal BND. The second matching circuit 360 is primarily controlled on grounds of the band signal BND. When the GSM850 system is exchanged to GSM900 system or vice versa, the antenna's operating band is shifted correspondingly by means of the second matching circuit, the antenna matching being thereby improved.
a and 4b show an example of the sensor belonging to the arrangement according to the invention in the antenna structure.
The sensor 470 consists of the first 471 and the second 472 electrode, which are distinct conductor strips on the outer surface of the antenna frame FRM. The conductor strips are so close to each other that a clearly higher capacitance than different stray capacitances exists between them. A coil L1; L2 is in series with each electrode, between it and a conductor of the line, which connects the sensor to the capacitance unit 380. The impedance of these coils is very high at the radio frequencies. Therefore no radio-frequency currents can be generated in the line between the sensor and capacitance unit 380, and the circuit of the sensor then does not cause losses and change the antenna impedance.
The sensor is located close to the main element of the antenna in the space of its near field. In addition, the sensor is placed in the area, where the electric field of the main element has a minimum at its lower resonance frequency, in which case the sensor degrades the antenna function as little as possible. The area in question is located in the middle part of the longer arm of the main element. In order to avoid a short between the sensor strips and the main element, the middle part 441b of the longer arm of the main element is located on the inner surface of the frame FRM. This middle part joins the starting part 441a and the tail part 441c of the longer arm of the main element through the conductive vias locating close enough to each other. Alternatively, the main element would be wholly located on the outer surface of the frame, and the sensor would be insulated from it by a dielectric layer.
In the example of
The sensor 570 consists of two electrodes, which are in this embodiment parts of the longer arm of the main element 541. The first electrode is the middle part 541b of the longer arm, and the second electrode is the tail part 541c of the longer arm. For this purpose the middle part 541b is galvanically separated from the rest 541a of the main element and from the tail part 541c. However, the middle part is coupled to the rest 541a of the main element by a capacitor C51 and to the tail part by a capacitor C52, the capacitances being e.g. 70 pF. The impedance of these capacitors is then very low (about 2Ω) at the radio frequencies, for which reason the longer arm of the main element is united in the operating band. At the use frequency (35 kHz) of the sensor the impedance of these capacitors is about 20 kΩ, which represents a good separation between the electrodes. The middle part 541b and tail part 541c are located mostly parallelly so that there is a suitable capacitance CSE between them. A coil L1; L2 is in series with each electrode, the impedance of which coils is very high at the radio frequencies. Therefore no radio-frequency currents can be generated in the line between the sensor and capacitance unit, and the circuit of the sensor then does not cause losses and change the antenna impedance.
In this example the sensor is located in the area where the electric field of the main element is relatively strong at its lower resonance frequency. The area of the weak electric field is not so useful here because of the typical location of the user finger during communication.
The first matching circuit is a π-shaped network, which then comprises in order a first trans-verse portion, a longitudinal portion and a second transverse portion. Each transverse portion comprises one change-over switch, and the number of the reactive elements to be chosen by each switch is four. In this case the total number of the state combinations of the first matching circuit is 16. The first reactive element of the first switch SW1 is the capacitor C61, in other words the first change-over terminal of the switch SW1 is connected to the ground conductor of the transmission path, or the signal ground GND, through this capacitor C61. Correspondingly, the second reactive element of the first switch is the capacitor C62, the third ‘reactive element’ is an open circuit representing then a very high reactance, and the fourth reactive element is the coil L61. In series with the coil L61 there is a blocking capacitor CB for breaking the direct current path from the switch control. The capacitance of the blocking capacitors is so high, for example 100 pF, that they constitute almost a short-circuit at the operating frequencies of the antenna. The first reactive element of the second switch SW2 is an open circuit representing then a very high reactance. The second reactive element of the second switch is the capacitor C63, the third reactive element is the capacitor C64 and the fourth reactive element is the coil L62. In series with the coil L62 there is a blocking capacitor CB. The longitudinal portion of the first matching circuit is constituted by the capacitor C6S, in series with the parts of the separate conductor SCR of the transmission path.
Between the common terminal of switch SW1 and the separate conductor SCR there is the capacitor C65, and between the end of this capacitor on the side of the conductor SCR and the ground plane there is the coil L63. Correspondingly, between the common terminal of switch SW2 and the separate conductor SCR there is the capacitor C66, and between the end of this capacitor on the side of the conductor SCR and the ground plane there is the coil L64. The LC circuits C65-L63 and C66-L64 function as ESD protectors for the switches. In addition, the capacitors C65 and C66 function as a blocking capacitor preventing the forming of a direct current circuit from the control of switches SW1 and SW2 to the conductor SCR.
The first switch SW1 is set by the first control signal SET1 and the second switch SW2 is set by the second control signal SET2. These control signals are two-bit binary digits, corresponding to the number of the switching alternatives.
In the second matching circuit 660 there is the third switch SW3 and four alternative reactive elements to be chosen by this switch. The first reactive element is a bare blocking capacitor, which represents at the radio frequencies a short-circuit, or a very low reactance. The second reactive element is the capacitor C67, the third reactive element is an open circuit representing then a very high reactance and the fourth reactive element is the coil L65, in series with which there is a blocking capacitor CB. Between the common terminal of switch SW3 and the grounding point GP of the radiator there is the capacitor C68, and between the end of this capacitor on the side of the grounding point GP and the ground plane there is the coil L66. The circuit C68-L66 functions as an ESD protector for the switch. In addition the capacitor C68 functions as a blocking capacitor preventing the forming of a direct current circuit from the control of switch SW3 to the ground through the radiator.
The third switch SW3 is set by the third control signal SET3, which is in this example a two-bit binary digit.
The memory MEM of the control unit contains i.a. the matching program PRG, which implements the adjusting process of the matching in accordance with a certain algorithm. The process is started again at regular intervals, and the interval of the startings is counted either by software or by a timer circuit being included in the central processing unit 751. Of course, the central processing unit needs in any case a clock signal CLK.
By structure, the control unit can also be a bare hardware logic without any central processing unit proper with software.
The search of the state combinations of the switches in the adjusting process takes place in accordance with a certain algorithm. The algorithm can be based on a table, in which the optimal state combinations corresponding to different values of the input signals have been stored. The input signals are then used to address the memory in which the table is. A research and measurement activity precedes the forming of the table by which activity the sufficient extent of the π-shaped matching circuit, in other words the number of the transverse portions and the number of the alternative reactances in each portion and favourable component values for the reactances, is found out.
In
Curve 91 shows the fluctuation of the reflection coefficient S11 as a function of frequency when the antenna is almost in a free space. Switch SW1 is in state ‘1’ and switch SW2 in state ‘2’. It is seen from the curve that the reflection coefficient varies between the values 6.4 dB and −19.4 dB in the frequency range W1, being about −12 dB on average. Curve 92 shows the fluctuation of the reflection coefficient when a finger of the user is at the antenna on the radiator, and the switches are in the same states as before. It is seen from the curve that the reflection coefficient varies between the values −6.0 dB and −7.0 dB in the frequency range W1, being −6.5 dB on average. Thus the matching has clearly degraded. Curve 93 shows the fluctuation of the reflection coefficient when the finger of the user is still in the same place on the radiator, and the switches of the first matching circuit are set in a new way. Now switch SW1 is in state ‘2’ and switch SW2 in state ‘4’. It is seen from the curve that the reflection coefficient varies between the values −8.3 dB and −16.5 dB in the frequency range W1, being about 13 dB on average. Thus the matching has clearly improved.
In
As mentioned, the second matching circuit 660 is used for improving the matching by tuning the resonance frequency of the antenna on grounds of the value of the band signal BND, when this value changes. When GSM850 is in use (
The nominal impedance of the transmission path is 50Ω. In the case of curve B1 the overall impedance is very close to it in the middle range, the reactive part being small. At the borders of the range the impedance is sligthly inductive. In the case of curve B2 the mismatch is clearly visible, the impedance changing about from the value 28Ω+j33Ω to value 65Ω+j41Ω when moving from the lower border of the range to the higher border. The impedance is then clearly inductive. In the matching case, shown by curve B3, the impedance changes about from the value 43Ω+j17Ω to value 50Ω−j26Ω when moving from the lower border of the range to the higher border and is in the middle range purely resistive, about 60Ω.
The quality of the antenna can be considered also by means of its efficiency. When the frequency range 824-894 MHz of the GSM850 system is chosen, the efficiency of the above-mentioned antenna is on average −3.7 dB in free space. The value 0 dB corresponds to the ideal, or lossless, case. In the mismatch case corresponding to curve 92 in
As it appears from the description of
The arrangement and method according to the invention for matching the antenna of a radio device has been described above. The implementation of the reactive elements of the matching circuit belonging to the arrangement can vary. At least a part of them can be also short planar transmission lines on the surface of a circuit board. The term ‘change-over switch’ covers in this description and claims also the structures, where the reactance is changed by changing the control voltage of a varactor-type capacitive element. The location of the sensor in respect of the radiator can naturally vary. The invention does not limit the structure and type of the antenna proper. The inventive idea can be applied in different ways within the scope defined by the independent claims 1 and 12.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20096101 | Oct 2009 | FI | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FI2010/050805 | 10/15/2010 | WO | 00 | 8/31/2012 |
