GB2502787A

GB2502787A - Adaptive Antenna Impedance Matching

Info

Publication number: GB2502787A
Application number: GB1209981.8A
Authority: GB
Inventors: Surinder Singh Thind
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 2012-06-06
Filing date: 2012-06-06
Publication date: 2013-12-11
Anticipated expiration: 2032-06-06
Also published as: GB201209981D0; GB2502787B; KR20130137085A

Abstract

An apparatus and method compensates for an antenna impedance mismatch using a pre-determined indicator, which can be a below-threshold signal-to-noise ratio (SNR) in conjunction with a below-threshold SNR rate of change and an above-threshold rate of SNR decrease, comprises a mismatch detection module and an antenna tuning module. A below-threshold received signal strength indication (RSSI), or an above-threshold RSSI in conjunction with a below-threshold SNR, can also be a mismatch indicator. In the event of a mismatch, an antenna tuning circuit increases or decreases the frequency by a pre-determined increment S507, determines if matching is improved S508 and either continues to increase or decrease the frequency by said increment in the event of improved matching S509, or reverses the sign of the increment if matching is not improved S510. Alternatively, an apparatus 600 and method compensates for impedance mismatch of an aerial, comprising a differential amplifier 619 comparing first and second voltages derived from an antenna input and a power amplifier 620 respectively, the difference between these being proportional to impedance mismatch, and an antenna tuning module 602. A processing module and a low pass filter may also be provided.

Description

Adaptive Antenna Impedance Matching

Field of the Invention

The present invention relates to adaptive antenna impedance matching. More particularly, the present invention rethtes to detecting an indicator of an impedance mismatch, such as a predetermined signal-to-noise ratio condition or a vokage difference across an inductor, and tuning the antenna to compensate for the mismatch.

Background of the Invention

io In devices which communicate wirelessly through an antenna, for example mobile telephone handsets, performance can easily be degraded by an antenna impedance mismatch. An antenna impedance mismatch occurs when the antenna impedance is altered by stray capacitance introduced by nearby objects. For example, the antenna impedance can be altered when the handset is placed near a metallic object or when a i user holds the handset close to their face or body. An impedance mismatch can cause problems both when the antenna is used as a transmitter and when the antenna is used as a receiver. An impedance mismatch during transmission results in signal loss, which in turn leads to excess battery consumption since the device's power amplifier (PA) output has to be increased to overcome this signal loss. The excess power is dissipated as heat, causing the handset temperature to increase. Similarly, when the device is acting as a receiver, the antenna sensitivity is reduced which results in a reduction in device range and therefore the coverage of service.

Accordingly, there is a need to minimise an antenna impedance mismatch experienced by a device during use. A well matched antenna may only suffer a fraction of a decibel (dB) coupling loss, whereas losses in a badly matched antenna maybe as high as a few dB, e.g. 2 to 3dB or more. An existing s&ution inv&ves directly measuring the return loss (RL) in a transmitted signal and tuning the antenna to increase the RL. However, this approach has the drawback that a portion of the transmitted signal has to be coupled off and monitored to detect the transmitted signal power, reducing the transmission signal strength. Also, this method is not suitable for use when the antenna is being used as a receiver, since the received signal strength is too low for the return loss to be detectable and so the RL cannot be directly measured.

3 The invention is made in this context.

Srnnmary of the Invenlion According to the present invention, there is provided apparatus for compensating for an antenna impedance mismatch, the apparatus comprising an antenna mismatch detection module arranged to obtain information about a signal-to-noise ratio SNR of a signal received by the antenna, and determine that an impedance mismatch exists if the obtained information indicates that a predetermined condition indicative of an impedance mismatch is met, and an antenna tuning module controllable to tune the antenna to compensate for the impedance mismatch. I0

The antenna mismatch detection moduk may be arranged to determine that the predetermined condition is met if a rate of change of the SNR of the received signal over time is below a predetermined threshold rate of change, and/or a magnitude of the SNR of the received signal is below a first predetermined threshold SNR, and/or the SNR of the received signa' has decreased by at least a predetermined amount over a predetermined time period.

The antenna mismatch detection module may be arranged to determine that the predetermined condition is met if a received signal strength indicator RSSI of the received signal is below a predetermined threshold RSSI.

If the RSSI of the received signal is above the predetermined threshold RSSI, the antenna mismatch detection module can be arranged to determine that the predetermined condition is still met if a magnitude of the SNR of the received signal is below a second predetermined threshold SNR.

The antenna tuning module may be arranged to compensate for the impedance mismatch by tuning the antenna by a first predetermined frequency increment.

After tuning the antenna by the first predetermined frequency increment, the antenna mismatch detection module can be arranged to determine whether the SNR of the received signal has increased, wherein if it is determined that the SNR has increased, the antenna mismatch detection module may be arranged to control the antenna tuning module to repeatedly tune the antenna in the same direction as the first predetermined frequency increment until no further increase in the SNR is obtained, and wherein if it is determined that the SNR has not increased, the antenna mismatch detection module maybe arranged to control the antenna tuning module to tune the antenna by a second predetermined frequency increment opposite in sign to the first predetermined frequency increment.

After tuning the antenna by the second predetermined frequency increment, the antenna mismatch detection module can be arranged to determine whether the SNR of the received signal has increased, wherein if it is determined that the SNR has increased, the antenna mismatch detection module may be arranged to control the antenna tuning module to repeatedly tune the antenna in the same direction as the io second predetermined frequency increment until no further increase in the SNR is obtained, and wherein if it is determined that the SNR has not increased, the antenna mismatch detection module may be arranged not to control the antenna tuning module to apply any further tuning to the antenna, unless a new impedance mismatch is subsequently detected.

The antenna mismatch detection module may be arranged to periodical'y check, when repeatedly tuning the antenna, whether the SNR has decreased to a predetermined acceptable SNR level, and to stop tuning the antenna if it is determined that the predetermined acceptable SNR level has been obtained.

The antenna tuning module may comprise a tuning circuit connected to an input of the antenna, the tuning circuit including a variable capacitor arranged such that a tuning voltage can be applied to a terminal of the variable capacitor to tune the antenna impedance by controlling the electrical reactance of the tuning circuit.

The tuning circuit may further include a capacitor or an inductor having a first terminal connected to the antenna input and a second terminal connected to the terminal of the variable capacitor that is arranged to receive the tuning voltage.

According to the present invention there is also provided apparatus for compensating for an antenna impedance mismatch, the apparatus comprising an antenna mismatch detection module comprising a differential amplifier arranged to detect a first vokage derived from a voltage at an antenna input and a second voltage derived from a voltage at a power amplifier PA output, the antenna input and power amplifier output being connected by an inductor, and output a signal indicating an impedance mismatch if the first and second voltages are different, said output signal being proportional to a voltage difference between the first and second voltages, and an antenna tuning module controllable to tune the antenna to compensate for the impedance mismatch.

The antenna tuning mochile may comprise a tuning circuit connected to an input of the antenna, the tuning circuit including a variable capacitor arranged such that a tuning voltage can be applied to a terminal of the variable capacitor to tune the antenna impedance by controlling the electrical reactance of the tuning circuit, and a gain of the differential amplifier may be selected so that the output signal can be applied to the terminal of the variable capacitor as the tuning voltage. I0

The antenna mismatch detection moduk may further comprise a bridge circuit comprising a first capacitor connected to a node connecting the inductor and the antenna input, a first diode connected between the first capacitor and a node connected to the first input of the differential amplifier, such that a direct current DC voltage is applied to the first input, a second capacitor connected between a reference voltage and the node connecting the first diode and the first input, a third capacitor connected to a node connecting the inductor and the PA output, a second diode connected between the third capacitor and a node connected to the second input of the differential amplifier, such that a direct current DC voltage is applied to the second input, and a fourth capacitor connected between the reference voltage and the node connecting the second diode and the second input.

The first and third capacitors may have the same capacitance as each other and the second and fourth capacitors may have the same capacitance as each other, such that when the first voltage and the second voltage are the same, the voltage level of the signal output by the differential amplifier is zero.

The apparatus may further comprise a processing module arranged to receive the differential amplifier output signa', obtain a tuning correction to be applied to the antenna based on the output signal, and control the antenna tuning module to tune the antenna to app'y the tuning correction.

The processing module may be arranged to obtain the tuning correction by controfling the antenna tuning module to tune the antenna by a first predetermined frequency increment and determine that the impedance mismatch has been reduced after tuning the antenna by the first predetermined frequency increment if the magnitude of the output signal has reduced, wherein if the impedance mismatch has been rednced, the processing module may be arranged to control the antenna tnning module to repeatedly tune the antenna in the same direction as the first predetermined frequency increment until no further reduction in the impedance mismatch is obtained, and use the currently tuned value as the tuning correction, and wherein if the impedance mismatch has not been reduced, the processing module may be arranged to control the antenna tuning module to repeatedly tune the antenna in the opposite direction to the first predetermined frequency increment until no further reduction in the impedance mismatch is obtained, and use the currently tuned value as the tuning correction. I0

The processing module may be arranged to periodicafly check, when repeatedly tuning the antenna in the same or opposite direction as the first frequency increment, whether the output signal has decreased to a level indicating an acceptable impedance mismatch, and to stop tuning the antenna if it is determined that the acceptable impedance mismatch has been obtained and use the currenfly tuned value as the tuning correction.

The apparatus may further comprise a signal conditioning module arranged to low-pass filter the differential amplifier output signal to remove high-frequency noise.

According to the present invention there is further provided a method of compensating for an antenna impedance mismatch, the method comprising obtaining information about a signal-to-noise ratio SNR of a signal received by the antenna, determining that an impedance mismatch exists if the obtained information indicates that a predetermined condition indicative of an impedance mismatch is met, and tuning the antenna to compensate for the impedance mismatch.

It may be determined that the predetermined condition is met if a rate of change of the SNR of the reccived signal over time is below a predetermined threshold rate of change, and/or a magnitude of the SNR of the received signal is below a first predetermined thresh&d SNR, and/or the SNR of the received signal has decreased by at least a predetermined amount over a predetermined time period.

It may be determined that the predetermined condition is met if a received signal strength indicator RSSI of the received signal is below a predetermined threshold RSSI.

If the RSSI of the received signal is above the predetermined threshold RSSI, it may be determined that the predetermined condition is still met if a magnitude of the SNR of the received signal is below a second predetermined threshoki SNR.

Tuning the antenna to compensate for the impedance mismatch may comprise tuning the antenna by a first predetermined frequency increment.

After tuning the antenna by the first predetermined frequency increment, the method may further comprise determining whether the SNR of the received signal has io increased, and if it is determined that the SNR has increased, repeatedly tuning the antenna in the same direction as the first predetermined frequency increment until no further increase in the SNR is obtained, or repeatedly tuning the antenna in the same direction as the first predetermined frequency increment until a predetermined acceptable SNR is obtained, or if it is determined that the SNR has not increased, tuning the antenna by a second predetermined frequency increment opposite in sign to the first predetermined frequency increment.

After tuning the antenna by the second predetermined frequency increment, the method may further comprise determining whether the SNR of the received signal has increased, and if it is determined that the SNR has increased, repeatedly tuning the antenna in the same direction as the second predetermined frequency increment until no further increase in the SNR is obtained, or repeatedly tuning the antenna in the same direction as the second predetermined frequency increment until a predetermined acceptable SNR is obtained, or if it is determined that the SNR has not increased, not applying any further tuning to the antenna unless a new impedance mismatch is subsequently detected.

Tuning the antenna may comprise applying a tuning voltage to a terminal of a variable capacitor to tune the antenna impedance by controlling the ciectrical reactance of a tuning circuit including the variable capacitor.

A computer-readable storage medium may be arranged to store a computer program which, when executed by a processor, performs the method.

According to the present invention there is yet further provided a method of compensating for an impedance mismatch of an antenna based on an output signal of a differential amplifier arranged to detect a first voltage derived from a voltage at an antenna input and a second voltage derived from a voltage at a power amplifier PA output, the antenna input and power amplifier output being connected by an inductor, such that the output signal indicates an impedance mismatch if the first and second voltages are different and is proportional to a voltage difference between the first and second voltages, the method comprising receiving the output signal, obtaining a tuning correction to be applied to the antenna based on the output signal, and controlling an antenna tuning module to tune the antenna to apply the tuning correction.

zo The tuning correction may be obtained by controlling the antenna tuning module to tune the antenna by a first predetermined frequency increment, determining that the impedance mismatch has been reduced after tuning the antenna by the first predetermined frequency increment if the magnitude of the differential output signal has reduced, if the impedance mismatch has been reduced, controlling the antenna i tuning module to repeatedly tune the antenna in the same direction as the first predetermined frequency increment until no further reduction in the impedance mismatch is obtained, and obtaining the currently tuned value as the tuning correction, and if the impedance mismatch has not been reduced, controlling the antenna tuning module to repeatedly tune the antenna in the opposite direction to the first predetermined frequency increment until no further reduction in the impedance mismatch is obtained, and obtaining the currently tuned value as the tuning correction.

The method may further comprise periodically checking, when repeatedly tuning the antenna in the same or opposite direction as the first frequency increment, whether the output signal has decreased to a level indicating an acceptable impedance mismatch, and stopping tuning the antenna if it is determined that the acceptable impedance mismatch has been obtained, and obtaining the currently tuned value as the tuning correction.

A computer-readable storage medium may be arranged to store a computer program which, when executed by a processor, causes the processor to perform the method.

Brief Description of the Drawings

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures tA and iB illustrate apparatus for compensating for an impedance mismatch of an antenna, according to an embodiment of the present invention; Figure 2 illustrates the apparatus of Fig. 1 in more detail; Figure illustrates apparatus for compensating for an impedance mismatch of an antenna based on the signal-to-noise ratio (SNR) of a received signal, according to an embodiment of the present invention; Figure 4 illustrates a method of compensating for an impedance mismatch of an antenna based on the signal-to-noise ratio (SNR) of a received signal, according to an embodiment of the present invention; io Figures 5A and 5B illustrate a method of compensating for an impedance mismatch by monitoring the SNR and a received signifi strength indicator (RSSI) of a received signa', according to an embodiment of the present invention; Figure 6 illustrates apparatus for compensating for an impedance mismatch of an antenna based on an inductor voltage, according to an embodiment of the present invention; Figure 7 illustrates apparatus for compensating for an impedance mismatch of an antenna based on an inductor vollage, according to an embodiment of the present invention; Figure 8 illustrates a method of compensating for an impedance mismatch of an antenna based on an inductor voltage, according to an embodiment of the present invention; Figure 9 illustrates apparatus for compensating for an antenna impedance mismatch including a signal conditioning module, according to an embodiment of the present invention; Figure 10 illustrates apparatus for compensating for an antenna impedance mismatch including a signal conditioning module, according to an embodiment of the present invention; and Figures 11 to 14 illustrate alternative antenna tuning modules, according to embodiments of the present invention.

Detailed Description

Referring now to Figs. iA and iB, an apparatus for compensating for an impedance mismatch of an antenna is illustrated, according to an embodiment of the present invention. The apparatus can be referred to as an adaptive antenna matching module, since the purpose of the apparatus is to apply adaptive impedance matching to the antenna to compensate for a detected impedance mismatch. An impedance mismatch may arise, for example, due to the proximity of a user's body to the antenna while a user is holding the mobile device, or due to proximity of metallic objects.

As shown in Figs. iA and iB, a device includes the adaptive antenna matching modifle 100 connected between an antenna 110 and a receiver/transmitter (RX/TX) module 120. The RX/TX module 120 can include a duplexer to allow simultaneous transmission and reception by the antenna 110. Alternatively, the RX/TX module 120 can switch between receiving and transmitting modes so that at a given point in time the antenna 110 is either receiving or transmitting. Furthermore, in some Jo embodiments of the present invention the adaptive antenna matching modide 100 may be used in a device which on'y transmits, or which on'y receives, i.e. in which a dedicated transmitter or receiver module is provided instead of the dual RX/TX module 120.

The effect of an antenna impedance mismatch is to detune the antenna so that the antenna circuit is resonant at a different frequency to the intended frequency. When the antenna is detuned as a resifit of an impedance mismatch, the return toss increases for signals at the desired frequency, i.e. the frequency to which the antenna is supposed to be tuned. The return loss (RL) is defined as the forward signal power (Pr) divided by the reflected signal power (PR). The return loss will be high when a good impedance match is achieved, as only a small fraction of the forward power will be reflected. When the antenna 110 is used to transmit signals, as in Fig. iA, the forward power is the power sent from the RX/TX module 120 to the antenna 110. In this case, the reflected power returns to a power amplifier (PA) of the device and is dissipated as heat. On the other hand, when the antenna 110 is used to receive signals, as in Fig. iB, the forward power is the received signal power sent from the antenna 110 to the RX/TX module 120. In this case, the reflected power does not reach the receiver but is instead dissipated as heat, e.g. in the antenna 110 or in intermediate components.

The adaptive antenna matching module ioo is arranged to monitor signa's received or transmitted by the antenna 110, and detect a condition indicative of an impedance mismatch of the antenna 110. Embodiments of the present invention can detect the impedance mismatch without having to direcfly measure the forward and reflected signal power. In one embodiment the adaptive antenna matching module 100 monitors the signal-to-noise ratio (SNR) of a received signal and detects when an SNR condition indicative of an antenna mismatch occurs. Examples of SNR conditions that -10-can indicate an impedance mismatch include a large decrease in SNR, or a reduction in SNR without rapid variations that could indicate multipath effects. In another embodiment the adaptive antenna matching module 100 monitors the voltage across an inductor connected between the RX/TX modde 120, a voltage difference across the inductor being indicative of an impedance mismatch.

The adaptive antenna matching module 100 is also arranged to respond to the detected impedance mismatch by tuning the antenna to a different frequency. Specifically, the adaptive antenna matching module 100 can monitor the condition that indicated the io impedance mismatch while tuning the antenna to a higher or thwer frequency to attempt to obtain an improvement in the condition. In this way the antenna can be tuned to compensate for the detuning that has occurred, for example due to the proximity of an object to the antenna, and an improved impedance match of the antenna can be achieved.

The adaptive antenna matching module ioo is illustrated in more detail in Fig. 2.

Specificafly, the adaptive antenna matching module 100 comprises a mismatch detection module 201 and an antenna tuning module 202. The mismatch detection module 201 is arranged to monitor the received or transmitted signals to detect a predetermined condition indicative of an impedance mismatch of the antenna 110, as described above. If an impedance mismatch occurs, the mismatch detection module 201 is arranged to output a signal indicating the impedance mismatch to the antenna tuning module 202, which responds to the signal by tuning the antenna 110. Various approaches are possible for tuning the antenna 110 and will be described later.

Referring now to Fig. 3, apparatus for compensating for an impedance mismatch of an antenna based on the signal-to-noise ratio (SNR) of a received signal is illustrated, according to an embodiment of the present invention. Like the adaptive antenna matching module 100 of Figs. iA, iB and 2, the apparatus 300 in the present embodiment is connected between an antenna 310 and a receiving module (not shown in Fig. 3). The apparatus 300 includes a mismatch detection module 301 comprising an SNR measurement module 301-1 arranged to measure an SNR of the signal received by the antenna 310, and a processing module 301-2 arranged to receive the SNR measurement from the SNR measurement module 301-1. However, in another embodiment the SNR measurement module 301-1 may be omitted and the processing module 301-2 may obtain information about the SNR from another source. The -11 -apparatus 300 further includes an antenna tuning module 302 that can be controlled by the processing module 301-1 to tune the antenna 310 to compensate for the mismatch.

A method that can be employed by the processing module 301-2 to compensate for an impedance mismatch is illustrated in Fig. 4, according to an embodiment of the present invention. In the first step 5401, the processing module obtains SNR information. For instance, the processing module can periodically receive an SNR measurement from the SNR measurement module, e.g. receive an updated measurement every 1 io millisecond (ms). The skilled person will appreciate that this interval is merely exemplary and other time periods may be chosen instead of 1 ms. Preferably the obtained SNR information should enable the processing module to monitor time-variant behaviour of the SNR, such as a rate-of-change of the SNR or a total increase/decrease in SNR over a predetermined time period. To achieve this, the processing module or the SNR measurement module can store information about SNR values of the received signal over a period of time.

Next, in step S402, the processing module checks whether the obtained information indicates that a predetermined SNR condition indicative of an impedance mismatch has been met, i.e. has occurred. Examples of SNR conditions that can indicate an impedance mismatch will be described later. If the obtained information does not indicate an impedance mismatch, no antenna tuning is required and the process returns to step S4o1 to continue to monitor the received signal SNR. On the other hand, if the obtained information indicates that an impedance mismatch has occurred, then the processing module proceeds to tune the antenna in step 5403. Specifically, the processing module controls the antenna tuning module to tune the antenna to compensate for the impedance mismatch. Preferably the processing module uses a trial-and-error approach to identify an optimum tuning adjustment to be applied, in order to compensate for the mismatch. That is, the processing modific can identi' a tuning direction, i.e. positive or negative, that results in an improvement in the condition that indicated the mismatch, and can continue to tune the antenna in this direction until no further improvement is observed.

However, other approaches to tuning the antenna are also possible. For example, an impedance mismatch resulting from the handset being held in a particular way may be identified by a characteristic SNR variation, e.g. a characteristic sudden decrease in -12 -SNR by a specific amount. An appropriate tuning adjustment to compensate for this impedance mismatch condition could be stored, and when the characteristic SNR variation is detected the processing module could simply apply this predetermined tuning adjustment without using trial-and-error. In some embodiments the processing module could subsequently check whether the applied adjustment has worked by checking whether the SNR condition has improved.

Referring now to Fig. 5, a method of compensating for an impedance mismatch by monitoring the SNR and a received signal strength indicator (RSSI) of a received signal Jo is illustrated, according to an embodiment of the present invention. Like the method of Fig. 4, the method of the present embodiment could be executed by the processing module of Fig. 3. In the first step S5o1, SNR information about the received signal is obtained. In the present embodiment, this step also includes obtaining information about the RSSI of the received signal. Then, in step S5o2, it is checked whether the is obtained information indicates that the SNR changing rapidly, i.e. whether the rate-of-change is above a predetermined thresh&d rate-of-change. A high rate of change can indicate mukipath effects, which cannot be compensated for by tuning the antenna.

Therefore if the rate of change is above the predetermined threshold, it is assumed that the variation cannot be corrected for and the method returns to step Soi and continues to monitor the SNR and RSSI in case a subsequent condition indicative of a mismatch occurs. This ensures that processing time and power is not wasted unnecessarily by attempting to correct a condition that cannot be corrected at the handset.

On the other hand, if the rate of change of SNR is low enough that it cannot be attributed to multipath effects, i.e. is below the threshold rate-of-change, it may be possible to improve the SNR by tuning the antenna, and so the process continues to step S5o3. Instep S5o3 it is checked whether the magnitude of the SNR is above a prcdctcrmincd threshold magnitude, for example 13 dB. If the SNR is above the o thresh&d magnitude then no correction is required as the SNR is still high enough for the signal to be reliably received, and the process returns to step S5o1 to monitor the SNR and RSSI. If however the SNR is below the threshold magnitude then the signal quality is degraded unacceptably and the process continues to step S5o4. Here, it is checked whether a large SNR decrease has occurred, i.e. whether the SNR magnitude has decreased by a predetermined amount, e.g. 1 dB, in a predetermined time period. If there has not been a large SNR decrease the process returns to step Soi and continues -13 -to monitor the SNR and RSSI. However, if there has been a large decrease then the process proceeds to step S5o5.

Instep S5o5 it is checked whether the RSSI is high, i.e. above a predetermined threshold RSSI. If the RSSI is high this may indicate that an impedance mismatch is not responsible for the detected change in SNR, md the process proceeds to step S5o6 where it is checked whether the SNR is low, i.e. below another predetermined threshold magnitude. Here, the threshold applied at step S5o6 is lower than the threshold applied at step S5o3. In some embodiments step So6 can be omitted as the previous io check of SNR at step S5o3 may be sufficient.

If the SNR is determined to be low in step S5o6, then it is assumed that the variation in SNR is due to interference and not an impedance mismatch, so the process returns to step S5o1 and continues to monitor the SNR and RSSI. On the other hand, if the SNR i5 is rdativeb' high then it is assumed that the SNR variation is due to an impedance mismatch, so the process continues to step S5o7 and attempts to tune the antenna in order to improve the SNR. Simfiarly, if the RSSI is determined to be low in step 5505 then an impedance mismatch is assumed and the process proceeds directly to step 5507 to attempt to tune the antenna.

It should be noted that in other embodiments one or more of the checks shown in Fig. 5A can be omitted. For instance, depending on the handset design some of the conditions checked in the present embodiment may be a more reliable indicator of an antenna mismatch than others, and the method can be adapted accordingly. In some embodiments only one of the checks may be applied, for example a processing module may only check the rate of change of SNR at step 5502, and if the rate of change is low it may assume that the change is due to an impedance mismatch and attempt to correct the mismatch by tuning the antenna. Also, in some embodiments the step of checking the RSSI may be omitted and the processing module may only obtain information o about the SNR.

Continuing now with reference to Fig. B, in step S5o7 the processing mod&e controls the antenna tuning modu'e to tune the antenna by a predetermined frequency increment. In the present embodiment this is a positive frequency increment, such that the antenna is tuned to a higher frequency, but in other embodiments a negative frequency increment could be used instead. Then, in step 5508 updated SNR information is obtained and it is checked whether the SNR has improved in comparison to the SNR value before the antenna was tuned by the predetermined frequency increment. If the SNRhas improved then in step 8509 the processing module continues to tune the antenna in the same frequency direction as the increment appfled in step S5o7, until no further SNR improvement is obtained. In the present embodiment the antenna is repeatedly tuned by applying the same frequency increment as in step 8507, i.e. +Af, but in other embodiments an increment having the same sign but a different magnitude could be used in step 8509.

On the other hand, if in step 8508 it is determined that tuning the antenna by the predetermined frequency increment did not improve the SNR, then in step Sio the antenna is tuned in an opposite direction to the increment applied in step 8508. In the present embodiment this is done by tuning by the same magnitude in the opposite direction, i.e. -AL but in other embodiments a different step size could be used.

Then, in step S511, updated SNR information is obtained and it is checked whether the SNR has improved after the tuning applied in step 8510. If the SNR has improved then in step S512 the antenna is repeatedly tuned in the same direction as in step 8510 until no further improvement is obtained. On the other hand, if no improvement is observed in step Sii then it is determined that the SNR variation is not the result of antenna detuning due to an impedance mismatch, and the method proceeds to step 8513 and applies no further tuning. After the process completes in step 8509, 8512 or 8513, the processing module can return to step Soi and continue to monitor the SNR and RSSI in case another mismatch is detected at a later stage.

Methods such as those shown in Figs. 5A and B can allow an impedance mismatch to be compensated for without having to directly measure the forward and return signal powers. As such, these methods are suitable for use when an antenna is being used to receive signals, in comparison to prior art methods which require the RL to be directly measured and can only be used to correct impedance mismatches in a transmission mode.

Referring now to Fig. 6, apparatus for compensating for an impedance mismatch of an antenna based on an inductor voltage is illustrated, according to an embodiment of the present invention. The apparatus 600 is connected between the output of a power amplifier (PA) 620 and the input of the antenna, and includes an antenna mismatch -15 -detection module 601 and an antenna tuning module 602 similar to the apparatus of Fig. 2. As shown in Fig. 6, the antenna mismatch detection module 601 in the present embodiment includes a detection circuit arranged to detect when a voltage difference is developed across an inductor 610 connected between the PA output and antenna input.

Any inductor maybe used, i.e. the inductor 610 may be one which is already present in the transmission circuit, or maybe provided solely for the purpose of detecting an impedance mismatch.

In more detail, the antenna mismatch detection circuit 6oi includes a differential io ampUfier (diff amp) 619 arranged to detect a first voltage derived from a voltage at a first node 611 that connects the PA 620 output to the inductor 610, and a second voltage derived from a voltage at a second node 612 that connects the inductor 6io to the antenna. In the present embodiment the first and second voltages are derived using a bridge circuit. The bridge circuit includes a first capacitor 613 connected to the first node 61i, a first diode 614 connected to the first capacitor 613, and a second capacitor 6i connected between the first diode 614 and a reference v&tage, in this case ground.

The first voltage detected by the diff amp 619 is the voltage between the first diode 614 and the second capacitor 615. The bridge circuit further includes a third capacitor 616 connected to the second node 612, a second diode 617 connected to the third capacitor 6i6, and a fourth capacitor 6i8 connected between the second diode 617 and the reference voltage. The second voltage detected by the diff amp 619 is the voltage between the second diode 617 and the fourth capacitor 618. The first and second diodes 614, 617 are fast-switching radio frequency (RF) diodes, which convert RF voltages in the transmission signal path to direct current (DC) voltages that can be detected by the diff amp 619.

Preferably, the capacitors and diode used to derive the first voltage should have the same values as those used to derive the second voltage. This ensures that when the voltage across the inductor 610 is zero, which will occur when the antenna impedance is o perfectly matched, the output VD of the diff amp 619 will also be zero. If there is an impedance mismatch, a vohage difference will develop across the inductor 61o, with the result that the first and second vokages become different and the diff amp 619 outputs a signal V1, proportional to the voltage difference. The duff amp 619 therefore provides an output signal V1 that indicates an impedance mismatch of the antenna, i.e. if Vu has a non-zero value. Also, the signal Vu is proportional to the voltage across the inductor 610, which in turn depends on the extent of impedance mismatch. The value of Vu therefore indicates the extent of the impethmce mismatch.

As shown in Fig. 6, the diff amp 619 output signal V11 is input directly to the antenna tuning module 602 to tune the antenna. Various tuning circuits are described later, and in the present embodiment the diff amp 619 output signal Vu is applied directly to an input of the tuning circuit 602 as a tuning voltage, to change the reactance of the tuning circuit. The gain of the diff amp 619 is selected to be suitable for the particular tuning circuit used, such that for a given impedance, the duff amp 619 output signal Vu io has a magnitude that resu ts in the tuning circuit reactance being adjusted by the appropriate amount to compensate for the impedance mismatch.

As described above, the apparatus of Fig. 6 is preferably used when the antenna is being used to transmit a signal, to ensure that the signal power is sufficient to give a measurable voltage difference across the inductor. However, it may be possible for similar embodiments to be used in a receive signal path, for example if the differentia' ampUfier is sufficienfly sensitive or if an additional amplifier is provided to amplify weak RX signals to a power level at which detection is possible.

Referring now to Fig. 7, apparatus for compensating for an impedance mismatch of an antenna based on an inductor voltage is illustrated, according to an embodiment of the present invention. The apparatus 700 is similar in many respects to the apparatus 6oo of Fig. 6, and in particular includes an antenna mismatch detection module 701 similar to that of Fig. 6. As such a detailed explanation will be omitted here, to maintain brevity. However, unlike the apparatus 6oo of Fig. 6, in the present embodiment the diff amp output signal Vu is sent to a processing module 703 which controls the antenna tuning module 702. In this embodiment, the gain of the duff amp does not have to be selected according to the tuning circuit 702, as the diff amp output signal Vu is not applied directly to the tuning circuit 702.

When the output signa' \TD from the antenna mismatch detection mod&e 701 has a non-zero value, i.e. indicates an impedance mismatch, the processing modde 703 is arranged to apply a tuning algorithm in order to obtain a tuning correction that can compensate at least partly for the impedance mismatch. The processing module 703 is arranged to output a tuning voltage to the tuning circuit 702 to apply the tuning -17-correction. This approach is flexible in that the processor can easily alter the tuning voltage to suit particular conditions.

A tuning method suitable for use by the processing module 703 of Fig. 7 is iflustrated in Fig. 8, according to an embodiment of the present invention. In the first step S8oi, the processing module monitors the diff amp output signal Vp. For example, the processing module may check the value of the output signal Vu every 1 ms, although other time intervals could be used if appropriate. Then, in step S8o2, the processing module checks whether the current value of the output signal Vu is indicative of a low io RL. Here, various approaches are possible. In one embodiment any non-zero value of Vt) could be taken to indicate a ow RL which could be the result of an impedance mismatch. However, in the present embodiment the processing module is arranged to compare the current value of Vu against a predetermine threshold value, i.e. a threshold voltage. The threshold voltage can be determined during calibration, as an output is vokage of the diff amp that corresponds sufficiently high impedance mismatch that compensation is required to ensure acceptable performance. If the current vahie of Vt) is rethtiv&y low, i.e. below the threshold voltage, the process returns to step S8o1 and continues to monitor the output signal Vu in case a worse impedance mismatch occurs at a later stage.

However, if in step S8o2 it is determined that the output signal Vu has a current value above the threshold voltage, then in step 5803 the processing module attempts to tune the antenna to correct for an impedance mismatch and obtain an improved, i.e. lower, value of the output signal Vu. Specifically, in step S8o3 the processing module controls the antenna tuning module to tune the antenna by a predetermined frequency increment, by setting the tuning voltage provided to the antenna tuning module to a suitable level. In the present embodiment this is a positive frequency increment, such that the antenna is tuned to a higher frequency, but in other embodiments a negative frequency increment could be used instead. Then, in step S8o4 an updated value of the output signal Vu is obtained and it is checked whether the value of Vu has decreased in magnitude in comparison to the value before the antenna was tuned by the predetermined frequency increment. A decrease indicates that the vokage across the inductor has dropped, and the antenna mismatch has improved. If the value of Vu has decreased then in step S8o5 the processing module continues to tune the antenna in the same frequency direction as the increment applied in step 8803, until no further improvement in the output signal Vu, i.e. no further decrease, is obtained. In the -18-present embodiment the antenna is repeatedly tuned by applying the same frequency increment as in step 8803, i.e. +Af, but in other embodiments an increment having the same sign but a different magnitude could be used in step 8805.

On the other hand, if in step 8804 it is determined that tuning the antenna by the predetermined frequency increment did not improve the value of output signal Vu, then in step 8806 the antenna is tuned in an opposite direction to the increment applied in step 8803. In the present embodiment this is done by tuning by the same magnitude in the opposite direction, i.e. -Af, but in other embodiments a different step size could be io used.

Then, in step 8807, an updated vahie of the output signal V is obtained and it is checked whether the value of Vu has decreased in magnitude in comparison to the vathe before the antenna was tuned in step 8806. If the value of Vu has decreased then in is step 8808 the processing module continues to tune the antenna in the same frequency direction as the increment appBed in step 8806, until no further improvement in the output signal Vu, i.e. no further decrease, is obtained. On the other hand, if no improvement is observed in step S807 then it is determined that the variation in Vu is not the result of antenna detuning due to an impedance mismatch, and the method proceeds to step S8og and applies no further tuning. After the process completes in step 8805, S8o8 or 8809, the processing module can return to step S8oi and continue to monitor the output signal level Vu in case another mismatch is detected at a later stage.

By following a method such as the one shown in Fig. 8, the processing module can respond to changes in the diff amp output signal Vu by tuning the antenna to find an optimum tuning correction that can compensate for the impedance mismatch. In cases where the change in Vu is not the result of an impedance mismatch, this can also be determined and hence unnecessary tuning of the antenna can be avoided.

Also, although in steps S8o and 8808 of the present embodiment the antenna is tuned until no further improvement is obtained, in other embodiments the antenna may only be repeatedly tuned until an acceptably low value of Vu is obtained, i.e. a value b&ow a thresh&d level that indicates an acceptable evel of mismatch. This can avoid wasting time and power unnecessarily tuning the antenna when no further improvement is required.

Referring now to Fig. 9, apparatus for compensating for an antenna impedance mismatch including a signal conditioning module is illustrated according to an embodiment of the present invention. The apparatus 900 is similar to that shown in Fig. 7, and includes an antenna mismatch detection module 901, an antenna tuning module 902, and a processing module 903. However, the apparatus 900 of the present embodiment further includes a signal conditioning module 904 connected between the diff amp output of the antenna mismatch detection module 901 and the input of the processing module 903. The signal conditioning module is arranged as a low-pass io filter, to remove any high frequency noise that may be present in the diff amp output signal VD and also to increase the attack time to prevent any transient signa's from affecting the antenna tuning. Although one particular low-pass filter circuit is illustrated in Fig. 9, the skilled person will appreciate that other types of low-pass filters could be used and the present invention is not limited to the particular design shown in is Fig. 9.

A similar signal conditioning module can a'so be used in embodiments in which the diff amp output signal is directly applied to the antenna tuning module as a tuning voltage.

An example of such an embodiment is shown in Fig. 10, in which apparatus 1000 for compensating for an impedance mismatch comprises an antenna mismatch detection module 1001, an antenna tuning module 1002, and a signal conditioning module 1004.

Referring now to Figs. 11 to 14, various alternative antenna tuning modules are illustrated, according to embodiments of the present invention. In general, an antenna tuning module operates by varying the inductance or capacitance of one or more elements in a tuning circuit, to alter the reactance of the circuit and hence tune the antenna to be resonant at a different frequency. In the embodiments of Figs. ii to 14 a variable capacitor, also referred to as a varactor diode, is used as these are relatively inexpensive and compact. The capacitance of the varactor can be controlled by adjusting the voltage across the varactor. However, in other embodiments a variable inductor co&d be used as well as, or instead of, a varactor.

In the embodiment of Fig. ii, the antenna tuning module 1102 is connected to the input of the antenna 1110 by an inductor 1120. The inductor 1120 provides a fixed antenna match. Such inductors are often provided in mobile devices, but are not essential.

Therefore the inductor 1120 of Fig. 11 can be omitted in some embodiments, i.e. if a fixed antenna match is not required.

As shown in Fig. ii, the antenna tuning module 1102 includes a varactor 1103 connected between the TX/RX signal line, i.e. the line connecting the RX/TX module and the antenna, and ground. The varactor diode 1103 is arranged to be reverse-biased, with the anode connected to ground whilst the cathode is connected to the antenna input. In other embodiments a reference voltage plane other than ground could be used, and the orientation of the varactor could be reversed if required, i.e. if a high io reference vokage is used.

Also, as shown in Fig. ii the tuning voltage VT is applied to the cathode of the varactor, allowing the voltage across the varactor to be controlled in order to tune the varactor capacitance and hence tune the antenna to a different frequency. The tuning voltage V1 could be provided by a processing module such as those shown in Figs. 3, 7 or 9, or could be a diff amp output voltage as shown in Figs. 6 or 10. The tuning voltage yr may or may not be subject to signal conditioning before being supplied to the antenna tuning module 1100. In the present embodiment, the tuning voltage VT is applied to the varactor via a resistor 1104 and an inductor 1105. The inductor 1105 is used to block RF signals from coupling away from the varactor 1103. A capacitor no6 is also connected between ground and a common node of the resistor 1104 and inductor 1105. The capacitor 1106 and resistor 1104 together act as an RC low pass filter to prevent noise in the tuning voltage VT from reaching the varactor cathode, and also act as a current limiter.

In the embodiment of Fig. 12, a capacitive tuning circuit is illustrated. This antenna tuning circuit 1202 includes a varactor 1203 arranged in a similar manner to that of Fig. ii, except that a capacitor 1206 is connected between the varactor 1203 and the antenna input. The tuning v&tage Vr is then applied directly to the common node connecting the varaetor 1203 and the capacitor 1206, ahhough in some embodiments additional filtering could be used e.g. an RC filter as shown in Fig. ii. A capacitance Cc of the capacitor 1206 is arranged to be much larger than the varactor 1203 capacitance Cv, to reduce the effect of large changes in the varactor capacitance Cv. This embodiment is useful when sensitive tuning is required.

Figure 13 illustrates a capacitive/inductive tuning circuit 1302, including a varactor 1303 connected between ground and the antenna input, and an inductor 1305 connected between the varactor 1303 and the antenna input. The tuning voltage VT is applied to a common node connecting the varactor 1303 and inductor 1305. The tuning impedance of the circuit in Fig. 13 can be capacitive or inductive depending on the relative inductance/capacitance values of the inductor 1305 and the varactor 1303.

Finally, a further embodiment of a tuning circuit is illustrated in Fig. 14. This antenna tuning module 1402 includes a varactor 1403 and capacitor 1406 connected in series io between ground and the antenna input as in Fig. 12, but with the addition of an inductor 1405 connected to the common node connecting the varactor 1403 and capacitor 1406. The tuning voltage V1 is applied via the inductor 1405 to block RF signals coupling away from the varactor 1403.

Any of the antenna tuning modules illustrated in Figs. 11 to 14, or any other suitable tuning circuit, maybe used in any of the adaptive antenna matching modules described above with reference to Figs. 1 to 10.

Whilst certain embodiments of the present invention have been described above, the skilled person will understand that many variations and modifications are possible without departing from the scope of the invention as defined in the accompanying claims. -22-

Claims

Claims 1. Apparatus for compensating for an antenna impedance mismatch, the apparatus comprising: an antenna mismatch detection module arranged to obtain information about a signal-to-noise ratio SNR of a signal received by the antenna, and determine that an impedance mismatch exists if the obtained information indicates that a predetermined condition indicative of an impedance mismatch is met; and an antenna tuning module controllable to tune the antenna to compensate for io the impedance mismatch.
2. The apparatus of claim 1, wherein the antenna mismatch detection module is arranged to determine that the predetermined condition is met if: a rate of change of the SNR of the received signal over time is below a predetermined threshold rate of change; and/or a magnitude of the SNR of the received signal is below a first predetermined threshcild SNR; and/or the SNR of the received signal has decreased by at least a predetermined amount over a predetermined time period.
3. The apparatus of claim 1 or 2, wherein the antenna mismatch detection module is arranged to determine that the predetermined condition is met if a received signal strength indicator RSSI of the received signal is below a predetermined threshold RSSI.
4. The apparatus of claim 3, wherein if the RSSI of the received signal is above the predetermined threshold RSSI, the antenna mismatch detection module is arranged to determine that the predetermined condition is still met if a magnitude of the SNR of the received signal is below a second predetermined threshold SNR.
30. The apparatus of any one of the preceding daims, wherein the antenna tuning module is arranged to compensate for the impedance mismatch by tuning the antenna by a first predetermined frequency increment.
6. The apparatus of claim 5, wherein after tuning the antenna by the first predetermined frequency increment, the antenna mismatch detection module is arranged to determine whether the SNR of the received signal has increased, -23 -wherein if it is determined that the SNR has increased, the antenna mismatch detection module is arranged to control the antenna tuning module to repeatedly tune the antenna in the same direction as the first predetermined frequency increment until no further increase in the SNR is obtained, and wherein if it is determined that the SNR has not increased, the antenna mismatch detection module is arranged to control the antenna tuning module to tune the antenna by a second predetermined frequency increment opposite in sign to the first predetermined frequency increment.

Jo
7. The apparatus of claim 6, wherein after tuning the antenna by the second predetermined frequency increment, the antenna mismatch detection module is arranged to determine whether the SNR of the received signal has increased, wherein if it is determined that the SNR has increased, the antenna mismatch detection module is arranged to control the antenna tuning module to repeatedly tune the antenna in the same direction as the second predetermined frequency increment until no further increase in the SNR is obtained, and wherein if it is determined that the SNR has not increased, the antenna mismatch detection module is arranged not to control the antenna tuning module to apply any further tuning to the antenna, unless a new impedance mismatch is subsequently detected.
8. The apparatus of claim 6 or 7, wherein the antenna mismatch detection module is arranged to periodically check, when repeatedly tuning the antenna, whether the SNR has decreased to a predetermined acceptable SNR level, and to stop tuning the antenna if it is determined that the predetermined acceptable SNR level has been obtained.
9. The apparatus of any one of the preceding claims, wherein the antenna tuning module comprises a tuning circuit connected to an input of the antenna, the tuning ciretut including a variable capacitor arranged such that a tuning voltage can be applied to a terminal of the variable capacitor to tune the antenna impedance by controlling the electrical reactance of the tuning circuit.
10. The apparatus of claim 9, wherein the tuning circuit further includes a capacitor or an inductor having a first terminal connected to the antenna input and a second -24 -terminal connected to the terminal of the variable capacitor that is arranged to receive the tuning voltage.
11. Apparatus for compensating for an antenna impedance mismatch, the apparatus comprising: an antenna mismatch detection module comprising a differential amplifier arranged to detect a first voltage derived from a voltage at an antenna input and a second voltage derived from a voltage at a power amplifier PA output, the antenna input and power amplifier output being connected by an inductor, and output a signal io indicating an impedance mismatch if the first and second voltages are different, said output signa' being proportional to a vollage difference between the first and second voltages; and an antenna tuning module controllable to tune the antenna to compensate for the impedance mismatch.
12. The apparatus of daim ii, wherein the antenna tuning module comprises a tuning circuit connected to an input of the antenna, the tuning circuit induding a variable capacitor arranged such that a tuning voltage can be applied to a terminal of the variable capacitor to tune the antenna impedance by controlling the electrical reactance of the tuning circuit, and wherein a gain of the differential amplifier is selected so that the output signal can be applied to the terminal of the variable capacitor as the tuning voltage.
13. The apparatus of claim 11 or 12, wherein the antenna mismatch detection module further comprises a bridge circuit comprising: a first capacitor connected to a node connecting the inductor and the antenna input; a first diode connected between the first capacitor and a node connected to the first input of the differential amplifier, such that a direct current DC voltage is applied o to the first input; a second capacitor connected between a reference v&tage and the node connecting the first diode and the first input; a third capacitor connected to a node connecting the inductor and the PA output; -25 -a second diode connected between the third capacitor and a node connected to the second input of the differential amplifier, such that a direct current DC voltage is applied to the second input; and a fourth capacitor connected between the reference voltage and the node connecting the second diode and the second input.
14. The apparatus of claim 13, wherein the first and third capacitors have the same capacitance as each other and the second and fourth capacitors have the same capacitance as each other, such that when the first voltage and the second voltage are io the same, the voltage level of the signal output by the differential amplifier is zero.
15. The apparatus of claim ii, further comprising: a processing module arranged to receive the differential amplifier output signal, obtain a tuning correction to be applied to the antenna based on the output signal, and contr& the antenna tuning module to tune the antenna to apply the tuning correction.
16. The apparatus of daim 15, wherein the processing module is arranged to obtain the tuning correction by controlling the antenna tuning module to tune the antenna by a first predetermined frequency increment and determine that the impedance mismatch has been reduced after tuning the antenna by the first predetermined frequency increment if the magnitude of the output signal has reduced, wherein if the impedance mismatch has been reduced, the processing module is arranged to control the antenna tuning module to repeatedly tune the antenna in the same direction as the first predetermined frequency increment until no further reduction in the impedance mismatch is obtained, and use the currently tuned value as the tuning correction, and wherein if the impedance mismatch has not been reduced, the processing module is arranged to control the antenna tuning module to repeatedly tune the antenna in the opposite direction to the first predetermined frequency increment until no further reduction in the impedance mismatch is obtained, and use the currently tuned value as the tuning correction.
17. The apparatus of daim 16, wherein the processing module is arranged to periodically check, when repeatedly tuning the antenna in the same or opposite direction as the first frequency increment, whether the output signal has decreased to a level indicating an acceptable impedance mismatch, and to stop tuning the antenna if it -26 -is determined that the acceptable impedance mismatch has been obtained and use the currently tuned value as the tuning correction.
i8. The apparatus of any one of daims 11 to 17, further comprising: a signal conditioning module arranged to low-pass filter the differential amplifier output signal to remove high-frequency noise.
19. A method of compensating for an antenna impedance mismatch, the method comprising: Jo obtaining information about a signal-to-noise ratio SNR of a signal received by the antenna; determining that an impedance mismatch exists if the obtained information indicates that a predetermined condition indicative of an impedance mismatch is met; and tuning the antenna to compensate for the impedance mismatch.
20. The method of claim 19, wherein it is determined that the predetermined condition is met if: a rate of change of the SNR of the received signal over time is below a predetermined threshold rate of change; and/or a magnitude of the SNR of the received signal is below a first predetermined threshold SNR; and/or the SNR of the received signal has decreased by at least a predetermined amount over a predetermined time period.
21. The method of claim 19 or 20, wherein it is determined that the predetermined condition is met if a received signal strength indicator RSSI of the received signal is below a predetermined threshold RSSI.
22. The method of claim 21, wherein if the RSSI of the received signal is above the predetermined threshold RSSI, it is determined that the predetermined condition is still met if a magnitude of the SNR of the received signal is bdow a second predetermined threshold SNR. -27-23. The method of any one of claims 19 to 22, wherein tuning the antenna to compensate for the impedance mismatch comprises tuning the antenna by a first predetermined frequency increment.24. The method of claim 23, wherein after tuning the antenna by the first predetermined frequency increment, the method further comprises: determining whether the SNR of the received signal has increased; and if it is determined that the SNR has increased, repeatedly tuning the antenna in the same direction as the first predetermined frequency increment until no further increase in the SNR is obtained, or repeatedly tuning the antenna in the same direction as the first predetermined frequency increment until a predetermined acceptable SNR is obtained; or if it is determined that the SNR has not increased, tuning the antenna by a second predetermined frequency increment opposite in sign to the first predetermined frequency increment.25. The method of claim 24, wherein after tuning the antenna by the second predetermined frequency increment, the method further comprises: determining whether the SNR of the received signal has increased; and if it is determined that the SNR has increased, repeatedly tuning the antenna in the same direction as the second predetermined frequency increment until no further increase in the SNR is obtained, or repeatedly tuning the antenna in the same direction as the second predetermined frequency increment until a predetermined acceptable SNR is obtained; or if it is determined that the SNR has not increased, not applying any further tuning to the antenna unless a new impedance mismatch is subsequently detected.26. The method of any one of claims 19 to 25, wherein tuning the antenna comprises applying a tuning voltage to a terminal of a variable capacitor to tune the antenna impedance by controlling the electrical reactance of a tuning circuit including the variable capacitor.27. A computer-readable storage medium arranged to store a computer program which, when executed by a processor, performs the method according to any one of claims 19 to 26.28. A method of compensating for an impedance mismatch of an antenna based on an output signal of a differential amplifier arranged to detect a first voltage derived from a vokage at an antenna input and a second voltage derived from a voltage at a power amplifier PA output, the antenna input and power amplifier output being connected by an inductor, such that the output signal indicates an impedance mismatch if the first and second voltages are different and is proportional to a voltage difference between the first and second voltages, the method comprising: receiving the output signal; obtaining a tuning correction to be applied to the antenna based on the output io signal; and controlling an antenna tuning mod&e to tune the antenna to apply the tuning correction.29. The method of claim 28, wherein the tuning correction is obtained by: controlling the antenna tuning modtile to tune the antenna by a first predetermined frequency increment; determining that the impedance mismatch has been reduced after tuning the antenna by the first predetermined frequency increment if the magnitude of the differential output signal has reduced; if the impedance mismatch has been reduced, controlling the antenna tuning module to repeatedly tune the antenna in the same direction as the first predetermined frequency increment until no further reduction in the impedance mismatch is obtained, and obtaining the currently tuned value as the tuning correction; and if the impedance mismatch has not been reduced, controlling the antenna tuning module to repeatedly tune the antenna in the opposite direction to the first predetermined frequency increment until no further reduction in the impedance mismatch is obtained, and obtaining the currently tuned value as the tuning correction.30. The method of claim 29, further comprising: periodically checking, when repeatedly tuning the antenna in the same or opposite direction as the first frequency increment, whether the output signal has decreased to a lev& indicating an acceptable impedance mismatch; and stopping tuning the antenna if it is determined that the acceptable impedance mismatch has been obtained, and obtaining the currently tuned value as the tuning correction.31. A computer-readable storage medium arranged to store a computer program which, when executed by a processor, causes the processor to perform the method according to claim 28,29 or 30.