GB2476035A

GB2476035A - Adaptive adjustment of RF amplifier load impedance to maximize efficiency

Info

Publication number: GB2476035A
Application number: GB0921421A
Authority: GB
Inventors: Michael Gaynor
Original assignee: Antenova Ltd
Current assignee: Antenova Ltd
Priority date: 2009-12-08
Filing date: 2009-12-08
Publication date: 2011-06-15
Anticipated expiration: 2029-12-08
Also published as: GB201602953D0; GB2532901A; GB2532901B; GB2476035B; GB0921421D0

Abstract

A variable impedance matching circuit 104 between the RF power amplifier 102 and the antenna 103 is adjusted to obtain a low DC current drain in the RF amplifier. The DC current drain may be sensed by a resistor 100, or, if the RF signal is pulsed, by a transformer. The arrangement avoids losses which would be present in conventional designs using two matching circuits, one matching the amplifier output to 50 Ohms and the other matching the antenna to 50 Ohms. A wide impedance matching bandwidth may be achieved. The variable impedance matching circuit 104 may use a variable capacitor or inductor, or a capacitor array switched by MEMS devices.

Description

IMPROVEMENTS RELATING TO POWER AMPLIFIERS AND ANTENNAS

[0001] This invention relates to a method and apparatus for improving efficiency in connections between power amplifiers and antennas, in particular but not exclusively in relation to mobile telephone handsets.

BACKGROUND

[0002] The cellular communications industry has seen a significant integration in RF functionality in the last 10 years. A module may now incorporate many semiconductor die and passive components forming a complete system-in-package (SiP). This allows for ease of design into the final application. However, the antenna interface is still a burden to the designer, who must work within severe mechanical constraints imposed by the physical design and layout of the handset. This mechanical design is often restrictive and may not be significantly changed by the antenna designer while trying to fit the antenna in the limited space available. In addition, the designer must provide an effective antenna radiator that may require matching circuitry. The antenna matching circuit provides an input impedance which is as near as practicable to 50+jOO. In practice the input VSWR is typically less than 3:1 over the required operating frequency bands. The matching circuit is usually developed or optimized with the handset in a free space environment. Embodiments of the present invention seek to provide optimal matching between the output impedance of the power amplifier (PA) and the antenna. There is no necessity to use the conventional 500 impedance for matching between the PA and the antenna. A different impedance may be used to enable a more efficient operation of the PA and/or a more effective power transfer to the antenna. This circuit arrangement reduces losses which would be present in conventional designs using two separate matching circuits, i.e. power amplifier match to 500 and antenna match to 500.

[0003] Tuning schemes have been proposed to compensate for the environmental surroundings of the antenna. The objective of these schemes is typically to match any possible antenna to 50+jOO. This results in over design and frequently results in a narrow impedance matching bandwidth.

BRIEF SUMMARY OF THE DISCLOSURE

[0004] Embodiments of the present invention seek to provide a simple scheme to improve power transfer from the power amplifier to the antenna, without requiring or obtaining a perfect or near perfect impedance match.

[0005] According to a first aspect of the present invention, there is provided an apparatus for connecting a radio frequency (RF) power amplifier to an antenna, the apparatus comprising: at least one tunable electronic component having a tuning input; and means for determining or estimating a power efficiency of the RF power amplifier and generating a corresponding signal at a sensing output; wherein the tuning input and the sensing output are configured for connection to a means for tuning the tunable electronic component in response to the determined or estimated power efficiency.

[0006] The apparatus may further comprise the means for tuning the tunable electronic component in response to the determined or estimated power efficiency connected between the sensing output and the tuning input, although in other embodiments, this means is may be provided separately.

[0007] In accordance with a second aspect of the present invention, there is provided an apparatus for connecting a radio frequency (RF) power amplifier to an antenna, the apparatus comprising: at least one tunable electronic component; means for determining or estimating a power efficiency of the RF power amplifier; and means for tuning the at least one tunable electronic component in response to the determined or estimated power efficiency of the RF power amplifier.

[0008] The apparatus operates by adjusting or tuning the at least one tunable electronic component so as to deliver a desired RF power to the antenna while at the same time maintaining high or maximum efficiency of the RF power amplifier.

[0009] In accordance with a third aspect of the present invention, there is provided a method of controlling an impedance matching circuit connected between an output of a radio frequency (RF) power amplifier and an antenna; wherein the impedance matching circuit includes at least one tunable electronic component that is tunable in response to tuning signals applied at a tuning input; the method comprising: electronically determining or estimating a power efficiency of the RF power amplifier and generating a corresponding signal at a sensing output; electronically processing the signal at the sensing output as a parameter for generating or selecting at least one of a plurality of predetermined tuning signals; and applying the at least one predetermined tuning signal at the tuning input, thereby to adjust the impedance matching circuit so as to allow the impedance between the RF power amplifier and the antenna to be actively adjusted in response to the determined or estimated power efficiency of the RF power amplifier.

[0010] The at least one tunable electronic component may be a capacitor that could be implemented in many fashions, for example a MEMS (Micro Electromechanical Switch) device, PIN diodes switching a bank of discrete capacitors, or a solid state integrated device of switches connecting discrete capacitors on the same silicon. Alternatively or in addition, the at least one tunable component may be an inductor. Where a plurality of tunable electronic components is provided, all may be capacitors or all may be inductors, or some may be capacitors with others being inductors.

[0011] The means for tuning the tunable electronic component, for example by controlling a tuning value of the component, may comprise a feedback-based control circuit, for example an analog or a digital feedback circuit. In some embodiments, this control circuit may comprise or be connected to a memory circuit having stored therein a plurality of predetermined levels.

Each output level corresponds to an efficiency target for each output power level setting.

Ideally, these efficiency target output levels are the same for all operating frequencies, but optionally there may be different target levels for each operating frequency or group of frequencies. The means for detection of the power efficiency, the means for controlling the tuning circuit and the interface to the tuning circuit may operate by analog or digital means with analog-to-digital or digital-to-analog conversion being implemented as required. The control functions may optionally be implemented by means of a microprocessor with associated program and memory facilities.

[0012] The at least one tunable electronic component is configured as part of an impedance matching circuit connected between the output of the RF power amplifier and the feed terminal of the antenna. The impedance matching circuit may comprise one or more tunable electronic components as defined herein, the same as each other or different, and in any appropriate topology chosen to provide the required range of impedance transformation at the intended frequencies of operation. Suitable circuit topologies include but are not restricted to L-, Tee-and Pi-networks. Preferably the antenna and the RF power amplifier are physically located close to each another, limiting the length of any interconnecting circuits and transmission lines, but this is not an operational requirement.

[0013] Means for measuring the RF power amplifier DC current may comprise a resistor provided with a means by which the current flowing through it can be measured, for example by determining the potential difference across the resistor. The power efficiency of the RF power amplifier can be calculated approximately from knowledge of the RF power setting and the measured supply current.

[0014] Alternatively, the effective output power of the RF power amplifier can be measured by means of an RF coupling device and an associated detector, preferably located between the output of the matching circuit and an input terminal of the antenna. The RF coupling device may be in the form of a directional coupler sensing the forward power fed to the antenna or a simple voltage probe providing a DC output voltage proportional to the RF voltage on the antenna feed. The power efficiency of the RF power amplifier is optimized by controlling the values of the adjustable components such that the desired forward RF power is obtained with the lowest achievable RF power amplifier DC current.

[0015] In another embodiment, the output power of a harmonic of the operating frequency, typically but not limited to the third harmonic, is measured by way of an RF coupling device.

This RF coupling device may be relatively simple and comprise a narrowband coupler, operating only at the chosen harmonic frequency, or a more complex coupler that measures the power at the fundamental frequency and the harmonic. The magnitude of the selected harmonic may be used to control the linearity of the RF power amplifier under varying environmental conditions by means of adjustments to the matching circuit. Harmonic output control may also adjust other parameters such as the RF power amplifier bias voltage, feedback magnitude and/or phase, pre-distortion. The Applicant has found that the third harmonic power content is directly related to the third order intermodulation or linearity, which affects the adjacent channel splatter performance that is critical for digital modulation schemes. It has been verified by the Applicant that controlling the third order harmonic content will automatically reduce the effects of the adjacent channel splatter.

[0016] In another embodiment, the output power at the fundamental operating frequency and its harmonics may be measured through a single wideband RF coupling device constructed in a manner known to those of ordinary skill in the art. The output of the broadband coupler is fed through filter circuits to separate the fundamental frequency and those harmonics which it is desired to measure.

[0017] In a further embodiment, particularly applicable to an RF power amplifier operating in a pulsed or burst mode such as that associated with TDMA air interface protocols, the RF power amplifier current may be measured through a transformer device connected in series with its DC feed line. Such a transformer preferably comprises two separate inductors, one being connected in series with in the supply feed to the power amplifier, and the other placed in proximity with the first in order to provide the desired amount of mutual coupling between the two inductors.

[0018] One or both of the inductors may be in the form of a conductive circuit trace on a printed circuit board (PCB) or printed wiring board (PWB) on which the device is mounted.

The inductive trace may be of spiral, meander, or other configurations on the surface of the PCB or PWB. The second inductor is placed in proximity to the first, preferably directly over it, so as to provide sufficient mutual coupling. In the event of both inductors composed of conductive traces, the second trace may be within the PCB or PWB on a different metal layer or on the same layer adjacent or intertwined to the first trace. Either the inductor of the inductive circuit trace may be used to feed the power amplifier.

[0019] Embodiments of the present invention may be used with different types of antenna, for example dipoles, monopoles, balanced, PIFA, PILA, patch, plate and/or dielectric antennas, with or without a ground connection.

[0020] Embodiments of the present invention may be used with different types of RF power amplifiers commonly used in mobile telephones and other devices incorporating radio transmitters.

[0021] Embodiments of the present invention provide for the estimation of the efficiency of the associated RF power amplifier through directly measuring its DC supply current. The "power added efficiency" of a system is the RF output power minus the RF input power, all divided by the DC input power. The RF input power is negligible for cellular power amplifiers due to their high gain, so the ratio of the RF output power at the antenna to the DC input power at the RF power amplifier provides a good measure of the RF power amplifier efficiency. In many circumstances, the output power of the RF power amplifier is known by a software-controlled power setting means and in this case does not need to be measured directly. In these circumstances, the efficiency can be optimized under active conditions using the only the RF power amplifier DC current drain as a metric.

[0022] Since maximum efficiency is used as the matching circuit optimization criterion, a circuit arrangement of embodiments of the invention does not require a directional coupler to be provided at the input to the matching network in order to measure the input impedance of the antenna. Existing schemes employ directional couplers to measure forward and reflected power and phase information in order to enable computation of the impedance at the input to the antenna matching circuit. The theory of such schemes is that if the input VSWR is minimised, the performance of the combination of the RF power amplifier and the antenna should improve.

[0023] In some embodiments of the present invention, the efficiency of the RF power amplifier is directly optimized and the antenna impedance is ignored. A forward-power directional coupler may optionally be used to monitor the forward power at the antenna feed terminal and a wideband coupler for third order harmonics may optionally be provided to enable the use of adaptive tuning to achieve efficiency and linearity improvement. However, this coupler does not need to provide for the measurement of both forward and reflected power or to provide phase information. These less onerous requirements simplify its design and calibration. In some embodiments, the coupler may be required only to measure the third harmonic power or voltage at the antenna, resulting in a smaller physical device.

[0024] The use of RF power amplifier efficiency as the tuning criterion as provided by this invention may also result in the achievement of a wider effective operating bandwidth. With conventional schemes an attempt is made to optimize the matching network for a near perfect 500 input impedance typically resulting in very narrow band impedance match. This may be acceptable for narrow band modulation schemes like GSM, but it is not acceptable for wider bandwidth modulation schemes like WiMAX, WLAN, etc. The proposed approach has been shown to provide a well matched condition over a 10% bandwidth, while allowing for a system that is physically smaller and of lower cost than existing, typically 1% or less bandwidth, solutions.

[0025] The proposed scheme requires a method by which the current consumption of the power amplifier can be measured. The easiest way to accomplish this is by measuring the voltage drop across a resistor placed in series with the current which it is desired to measure.

The power dissipation of this series resistor can be reduced by using a resistor of small ohmic value. A lOmO resistor in series with the DC supply with 1A current drain will dissipate 10mW.

Existing schemes that use a dual directional coupler to measure impedance also cause power loss. The power dissipation under the scheme of this new invention, using current drain as a metric to adjust the matching circuit, has less impact on overall efficiency than an RF coupler device with 0.1-0.3dB of RF loss. A coupler with 0.1dB loss would dissipate 45mW of RF power for a 2W incident signal. In addition, a 50% efficient power amplifier that is typically utilized in the industry, would require an increase in DC input power to the RF power amplifier of twice this, i.e. 90mW to offset the RF power loss.

Operation of the control circuit [0026] A variety of control circuit operation methods are possible. These have different complexities and costs and offer a greater or lesser extent of optimization of the efficiency of the RF power amplifier.

[0027] The lowest complexity scheme is a simple open-loop embodiment where the optimum tuning control settings corresponding to each power level and to each frequency band or sub-band are stored in memory. These are used to set the values for the tuneable components independent of its operating conditions. There is no continuous feedback adjustment. A simple system of this kind does not require the provision of a current measuring circuit.

[0028] In a further more complex embodiment, the setting of the tuneable components is controlled by an analog feedback loop such as that shown in Figure 1. Here the current or efficiency target values corresponding to the required power level are stored in a look-up table, memory element 108 in the Figure. This desired target current is compared with the actual current by means of a comparator and the values of the tuned components are adjusted until the desired target current or a lesser value is obtained. Alternatively, this scheme may be embodied in a product delivered with only the current detection output and the tuning control input for connection to external circuitry. In this scenario, the feedback circuitry and associated control algorithms would be incorporated by another party, for example a handset manufacturer.

[0029] In a further embodiment the forward RF power at the input of the antenna is monitored using a directional coupler. This embodiment works the same as above with the exception of the power control level setting. In this embodiment, the power control setting could be used as above to reference a memory location with the same operation as already discussed. In addition, the power control loop is a separate loop and the control voltage to this loop could be used directly with a scaling circuit for the current target voltages. This scaling could be an amplifier that takes the control voltage for the power setting and amplifies it linearly or non-linearly. In addition, the scaling circuit could also be a resistive type circuit. Another implementation could use the output of the power control loop comparator voltage or a combination of any of the power control signals to control the matching circuit loop dynamics.

Such an embodiment may be appropriate for systems which already employ closed loop power control (for example mobile CDMA handsets) because in such a case enhanced performance is obtained for little extra cost.

[0030] The sensing and control circuits of the present invention may be implemented with analog or digital circuit techniques, or by mixtures of both.

[0031] Embodiments of the invention may find utility in various wireless applications with separate matching circuitry between a power amplifier and an antenna. Embodiments of the invention seek to provide a fully integrated system for improved performance with minimal impact to cost and size.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: FIGURE 1 is a circuit diagram of a first embodiment of the present invention; FIGURE 2 is a circuit diagram of a second embodiment of the present invention; and FIGURE 3 is a circuit diagram of a third embodiment of the present invention.

DETAILED DESCRIPTION

[0033] Figure 1 illustrates an embodiment of the invention incorporating an analog control scheme. A DC supply to a power amplifier (PA) 102 passes through a series resistor 100.

The voltage drop across this resistor 100 is the input voltage to a DC amplifier 101 whose output voltage Vsense is proportional to the supply current to the PA 102. This Vsense voltage is applied to a first input of a differential integrating amplifier 106. In response to an input signal Plevel, an electronic memory circuit 108 provides an output signal which generates a DC voltage by means of a digital-to-analog converter 107. This signal is the current or efficiency target value discussed in the Brief Summary of the Disclosure. The signal is applied to a second input of the differential integrating amplifier 106 through a divider circuit comprising resistors 120, 121. The differential integrating amplifier 106, which is configured as a comparator circuit, compares the current drain signal, Vsense, to this target current drain for the set power level and drives the tuneable component 104, in the matching circuitry 105, to achieve a current drain equal to or less than the desired target value from a look up table stored in memory circuit 108.

[0034] Figure 2 shows a further embodiment which makes use of a digital feedback circuit 109 rather than the analog feedback circuit 106. The digital feedback embodiment depicted in Figure 2 operates in generally the same way as the analog feedback embodiment of Figure 1 except that the feedback algorithm is processed digitally. Provision of a microprocessor 108 offers freedom in developing the feedback algorithm. This algorithm typically comprises logic statements and conditional statements in accordance with the known art of digital control systems. Typically, the microprocessor 108 uses internal memory and develops the Plevel signal in Figure 2 internally. However, the microprocessor 108 could instead be a small dedicated microprocessor that communicates with external memory and an external Plevel signal. In this digital feedback implementation, the signal Vsense is converted to a digital signal through an analog to digital converter 116. In addition, the control signal to the at least one tuneable electronic component 104 in the output matching circuit 105 may be converted to an analog signal through a digital to analog converter 107. In alternative embodiments using digitally controlled tuneable electronic components this conversion is not required and the microprocessor 108 is programmed to output signals in the form required to control the tuneable electronic components 104 directly.

[0035] Figure 3 shows a simplified third embodiment of the present invention. Operation of this embodiment is identical to that described by reference to Figure 1, but only a DC current sense output, Vsense, and a tuning control voltage input, Vcontroi are provided. In this embodiment the control circuits and functions are provided externally.

[0036] Throughout this description the term "tunable" is used to indicate an electronic component which is capable of the adjustment of its electrical circuit value by means of an external stimulus. The control interface to such a component may be digital or analog. The tunable component may comprise a single device such as a capacitor made with Barium Strontium Titanate (BST) or other tunable dielectric material, an array of devices such as MEMS-switched capacitors or a component such as a variable capacitor or inductor together with a mechanical drive means.

[0037] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0038] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0039] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

CLAIMS: 1. An apparatus for connecting a radio frequency (RF) power amplifier to an antenna, the apparatus comprising: an impedance matching circuit having an input for connection to an output of the RF power amplifier and an output for connection to the antenna; the impedance matching circuit including at least one tunable electronic component having a tuning input; and means for determining or estimating a power efficiency of the RF power amplifier and generating a corresponding signal at a sensing output; wherein the tuning input and the sensing output are configured for connection to a means for tuning the tunable electronic component in response to the determined or estimated power efficiency.
2 An apparatus as claimed in claim 1, further comprising the means for tuning the tunable electronic component in response to the determined or estimated power efficiency connected between the sensing output and the tuning input.
3. Apparatus for connecting a radio frequency (RF) power amplifier to an antenna, the apparatus comprising: an impedance matching circuit having an input for connection to an output of the RF power amplifier and an output for connection to the antenna; the impedance matching circuit including at least one tunable electronic component; means for determining or estimating a power efficiency of the RF power amplifier; and means for tuning the at least one tunable electronic component in response to the determined or estimated power efficiency of the RF power amplifier.
4. An apparatus as claimed in any preceding claim, wherein the at least one tunable electronic component is or includes a capacitor.
5. An apparatus as claimed in any preceding claim, wherein the at least one tunable electronic component is or includes an inductor.
6. An apparatus as claimed in any preceding claim, wherein the means for determining or estimating a power efficiency of the RF power amplifier comprises means for measuring a current drawn by the RF power amplifier.
7. An apparatus as claimed in claim 6, wherein the means for measuring a current drawn by the RF power amplifier comprises a resistor.
8. An apparatus as claimed in claim 7, wherein the resistor is a sensing resistor connectable in series with a power supply to the power amplifier
9. An apparatus as claimed in claim 7 or 8, further comprising a differential amplifier configured to measure a potential difference across the resistor and to generate the signal at the sensing output.
10. An apparatus as claimed in any one of claims 1 to 5, wherein the means for determining or estimating a power efficiency of the RF power amplifier comprises an RF coupling device to measure an RF power fed to the antenna.
11. An apparatus as claimed in claim 10, wherein the apparatus has an operating frequency, the operating frequency having a fundamental frequency and at least a third order harmonic frequency.
12. An apparatus as claimed in claim 11, wherein the RF coupling device is a narrowband coupler configured to couple only at the third order harmonic frequency.
13. An apparatus as claimed in claim 11, wherein the RF coupling device is a wideband coupler configured to couple at the fundamental frequency and the third order harmonic frequency.
14. An apparatus as claimed in any one of claims 1 to 5, wherein the means for determining or estimating a power efficiency of the RF power amplifier comprises a single transformer device.
15. An apparatus as claimed in any one of claims 1 to 5, wherein the means for determining or estimating a power efficiency of the RF power amplifier comprises first and second discrete inductors, the first inductor being connectable in series with a power supply to the power amplifier, and the second inductor being located sufficiently close to the first inductor so as to allow the second inductor to couple with the first and to generate the signal at the sensing output.
16. An apparatus as claimed in claim 15, wherein either one or both of the inductors is configured as a circuit trace on a printed circuit board on which components of the apparatus are mounted.
17. An apparatus as claimed in any preceding claim, wherein the means for tuning the tunable electronic component comprises an analog feedback circuit.
18. An apparatus as claimed in any one of claims 1 to 16, wherein the means for tuning the tunable electronic component comprises a digital feedback circuit.
19. An apparatus as claimed in claim 18, wherein the digital feedback circuit includes or is connected to a microprocessor.
20. An apparatus as claimed in any one of claims 17 to 19, wherein the feedback circuit is provided with an additional input for a power level signal and means for adjusting the operation of the feedback circuit in response to changes in the power level signal.
21. An apparatus as claimed in claim 20, further comprising a memory device having a plurality of memory locations corresponding to and selectable by different levels of the power level signal, the memory locations storing different preset values, and wherein the preset value output from a selected memory location is used by the feedback circuit as a parameter for tuning the tunable electronic component.
22. An apparatus as claimed in claim 2 or claim 3 or any one of claims 4 to 21 depending from claim 2 or claim 3, wherein the means for determining or estimating a power efficiency of the RF power amplifier and the means for tuning the tunable electronic component comprise digital and/or analog circuits.
23. A method of controlling an impedance matching circuit connected between an output of a radio frequency (RF) power amplifier and an antenna; wherein the impedance matching circuit includes at least one tunable electronic component that is tunable in response to tuning signals applied at a tuning input; the method comprising: electronically determining or estimating a power efficiency of the RF power amplifier and generating a corresponding signal at a sensing output; electronically processing the signal at the sensing output as a parameter for generating or selecting at least one of a plurality of predetermined tuning signals; and applying the at least one predetermined tuning signal at the tuning input, thereby to adjust the impedance matching circuit so as to allow the impedance between the RF power amplifier and the antenna to be actively adjusted in response to the determined or estimated power efficiency of the RF power amplifier.
24. A mobile electronic communications device comprising an apparatus as claimed in any one of claims 1 to 22
25. An apparatus for connecting a radio frequency (RF) power amplifier to an antenna substantially as hereinbefore described with reference to or as shown in the accompanying drawings.