GB2531520A - Antenna impedance matching with negative impedance converters - Google Patents

Antenna impedance matching with negative impedance converters Download PDF

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Publication number
GB2531520A
GB2531520A GB1418563.1A GB201418563A GB2531520A GB 2531520 A GB2531520 A GB 2531520A GB 201418563 A GB201418563 A GB 201418563A GB 2531520 A GB2531520 A GB 2531520A
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transceiver
network
negative impedance
antenna
nic
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GB1418563.1A
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GB201418563D0 (en
GB2531520B (en
Inventor
Hu Sampson
Tade Oluwabunmi
Thind Surinder
Wan Liang
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Smart Antenna Technologies Ltd
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Smart Antenna Technologies Ltd
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Priority to GB1418563.1A priority Critical patent/GB2531520B/en
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Classifications

    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03HIMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H11/00Networks using active elements
    • H03H11/02Multiple-port networks
    • H03H11/04Frequency selective two-port networks
    • H03H11/10Frequency selective two-port networks using negative impedance converters
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01QANTENNAS, i.e. RADIO AERIALS
    • H01Q5/00Arrangements for simultaneous operation of antennas on two or more different wavebands, e.g. dual-band or multi-band arrangements
    • H01Q5/30Arrangements for providing operation on different wavebands
    • H01Q5/307Individual or coupled radiating elements, each element being fed in an unspecified way
    • H01Q5/314Individual or coupled radiating elements, each element being fed in an unspecified way using frequency dependent circuits or components, e.g. trap circuits or capacitors
    • H01Q5/335Individual or coupled radiating elements, each element being fed in an unspecified way using frequency dependent circuits or components, e.g. trap circuits or capacitors at the feed, e.g. for impedance matching
    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03HIMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H11/00Networks using active elements
    • H03H11/02Multiple-port networks
    • H03H11/28Impedance matching networks
    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03HIMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H11/00Networks using active elements
    • H03H11/02Multiple-port networks
    • H03H11/28Impedance matching networks
    • H03H11/30Automatic matching of source impedance to load impedance
    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03HIMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H11/00Networks using active elements
    • H03H11/02Multiple-port networks
    • H03H11/40Impedance converters
    • H03H11/44Negative impedance converters
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B1/00Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/02Transmitters
    • H04B1/04Circuits
    • H04B1/0458Arrangements for matching and coupling between power amplifier and antenna or between amplifying stages
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B1/00Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/38Transceivers, i.e. devices in which transmitter and receiver form a structural unit and in which at least one part is used for functions of transmitting and receiving
    • H04B1/40Circuits
    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03HIMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H11/00Networks using active elements
    • H03H11/46One-port networks
    • H03H11/52One-port networks simulating negative resistances
    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03HIMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H7/00Multiple-port networks comprising only passive electrical elements as network components
    • H03H7/38Impedance-matching networks
    • H03H7/40Automatic matching of load impedance to source impedance
    • HELECTRICITY
    • H03BASIC ELECTRONIC CIRCUITRY
    • H03HIMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
    • H03H7/00Multiple-port networks comprising only passive electrical elements as network components
    • H03H7/46Networks for connecting several sources or loads, working on different frequencies or frequency bands, to a common load or source
    • H03H7/463Duplexers
    • H03H7/465Duplexers having variable circuit topology, e.g. including switches

Abstract

There is disclosed an active impedance matching network for an electrically-small antenna. The network may include a plurality of negative impedance converters (NICs) 1-4 individually connectable between a transceiver 6 and the antenna 5. Each of the NICs is respectively configured for optimum performance at a respective different power output of the transceiver. Switches 200-203, 100-103 are used to select the appropriate NIC, based on the transmit power. Alternatively, the network may include a negative impedance converter (10, Fig. 9) with a bias voltage (11, Fig. 9) which is controlled based on the power output of the transceiver.

Description

ANTENNA IMPEDANCE MATCHING WITH NEGATIVE IMPEDANCE CONVERTERS
[0001] This invention relates to antenna impedance matching circuits or networks making use of negative impedance converters.
BACKGROUND
[0002] Electrically small antennas can be generally classified as TM (transverse magnetic) and TE (transverse electric) mode antennas. For a TM mode small antenna, which is widely used in wireless communication systems, the input impedance is considerably reactive with a small real part. It is therefore critical to match the antenna to the receiver or transmitter to maximise the total efficiency in the frequency range of interest.
[0003] Normally, an electrically small TM mode antenna can be characterised by or represented as a series connected combination of a resistor, a capacitor and an inductor. Figures 1 and 2 show that, at low frequencies, the reactance can equally be represented by a series connected capacitor and inductor, with the capacitor playing a dominant role in the reactance. The resistor represents the resistance of the radiating element of the antenna [0004] There are two different ways to match a highly reactive antenna of this type. One approach is conventional passive matching, where a large series inductor Lek is placed between the antenna and the signal port as a necessary component. However, the resistive loss that is introduced by the inductor Lek dramatically degrades the total efficiency. In fact, even with lossless inductors, the match is effective over only extremely small instant bandwidths because the reactive part of the electrically small antenna cannot be neutralised over a broad frequency band with passive components (the real part of the impedance is much smaller than the imaginary part). This is illustrated in Figure 3.
[0005] The other approach uses an NIC (negative impedance converter) to create a negative capacitor, which can then be configured to cancel the reactance of the antenna as much as possible. This is a type of non-Foster impedance matching, and is illustrated in Figure 4.
[0006] There is a relationship between antenna size and the realisable bandwidth as defined by the Chu limit (Chu, L. J.; "Physical limitations of omni-directional antennas"; Journal Applied Physics 19: 1163-1175; December 1948). The Chu limit gives the relationship between the radius of the circle that completely circumscribes an antenna and the Q of the antenna. However, McLean (McLean, J. S.; "A re-examination of the fundamental limits on the radiation Q of electrically small antennas"; IEEE Transactions on Antennas and Propagations; Vol. 44; No. 5; pp. 672-676; May 1996) redefined how the Q of an antenna should be calculated, and this is given in equation 1.1: (k.a) where k is the wave number and a is the radius of a sphere that completely circumscribes the antenna as shown in Figure 5.
[0007] McLean's equation is a derivation from the original Chu limits equations. There has also been much research into ways of improving the gain of an antenna through the use of matching networks, but this is also bounded by the Harrington limits (Harrington, R. F.; "Effect of antenna size on gain, bandwidth and efficiency"; Journal of Research of the National Bureau of Standards -D. Radio Propagation; vol. 64D; p. 12; 29th June 1959) on antennas as given in: G = ( kajA T2k3 (1.2) The Chu limit can be related to the antenna bandwidth by rewriting the Q of the antenna as shown in equation 1.3: Q = (1.3) where f, is the antenna centre frequency at resonance and Af is the bandwidth of the 15 antenna.
[0008] Comparing equation 1.1 with equation 1.3, it can be seen that reducing the radius of the sphere which translates to a physical reduction in the antenna size, the antenna bandwidth also reduces. The reduction in size means that the antenna radiation resistance also reduces, and this in turn leads to a reduction the antenna efficiency. From equation 1.2, it is clear that antenna gain is also proportional to the antenna size a.
[0009] These two fundamental limits on the antenna make it difficult to provide a small antenna with a low Q (wideband). However, more and more devices these days require smaller antennas and there is need for these antennas to still have wide usable bandwidths.
[0010] Passive matching networks help to match antennas, but because they involve resonating the reactive part of the antenna with passive elements, they only give a good match at specific frequencies. Away from the specific frequency, the antenna return loss decreases. This necessitates the use of multiple or reconfigurable matching networks to cover wide frequency bands. However, using non-Foster elements could help provide continuous wideband matching because unlike Foster elements, the slope of the reactance versus frequency of a non-Foster element is always negative as shown in Figure 4.
[0011] With these properties, non-Foster elements are able to cancel out completely the reactance of other elements and antennas because of the difference in slope and direction of rotation on the Smith chart.
[0012] One implementation of non-Foster elements is through the use of NICs (negative impedance converters). NICs were first proposed by Linvill (Linvill, J.G.; "Transistor negative-impedance converters"; Proc. IRE; vol 41, pp 725-729; 1953). The Linvill NIC consists of two transistors connected in a common base configuration. The reactance to be inverted is connected between the two collectors and the base of one transistor is connected to the collector of the other transistor in the form of a feedback path. The emitters form the two ports of the NIC. The circuit schematic of the NIC is shown in Figure 6.
[0013] It is known from US6121940 to provide a broadband impedance matching circuit employing active circuits featuring non-Foster reactance behaviour, such as negative capacitor circuits and negative inductor circuits, to achieve broadband impedance matching for electrically small antennas. The circuit includes multiple negative elements, which can result in instability. In addition, there can be a noise mismatch between the source antenna and the subsequent active circuits, which can result in a high noise figure in receive mode. Finally, high voltages are easily induced at the antenna input terminal, which can be problematic, and the power requirement is relatively large.
[0014] It is also known (Sussman-Fort, S. E. and Rudish, R. M.; "Non-Foster Impedance Matching of Electrically-Small Antennas"; IEEE Transactions on Antennas and Propagation; vol. 57; pp. 2230-2241; 2009) to implement a non-Foster impedance matching circuit with a single active NIC device. However, it is difficult to optimise the single NIC device to provide the ideal negative capacitance and/or inductance. In addition, relatively high losses, instability and reflection may be induced by the single active NIC.
[0015] NICs offer useful features when used for matching antennas to transceiver RF modules. As NICs are active matching circuits, they will consume power, and the amount of power consumed will depend on the maximum power to be transmitted. A more conventional design of NIC is usually based on satisfying the maximum transmitter power bias condition and then using the NIC with the same bias for lower transmitter powers.
This will work, but the amount of battery power consumed at lower transmit powers in a handset, tablet or other mobile device is wasteful. A conventional NIC setup is shown in Figure 7. The NIC is located between the antenna and the RF module (Tx/Rx).
[0016] The parameters that are optimised for NIC are: a) Compression point (Ride) b) Third order intercept point/IMD3 c) Noise figure d) Antenna efficiency e) Antenna matched bandwidth f) Antenna return IossNSWR [0017] For the maximum transmitter power case, a high bias current is required to meet the linearity requirement for 3GPP/4G LTE applications -a typical NIC bias current from a 3V battery could be as high as 484mA. This may satisfy the linearity requirement at maximum transmitter power (24dBm for LTE), but for lower transmit powers this is clearly wasteful.
BRIEF SUMMARY OF THE DISCLOSURE
[0018] Viewed from a first aspect, there is provided an active impedance matching network for an electrically-small antenna, the network comprising a plurality of negative impedance converters individually connectable between a transceiver and the antenna, wherein the negative impedance converters are respectively configured for optimum performance at respective different power outputs of the transceiver.
[0019] At a minimum, there may be provided two negative impedance converters (NICs), one NIC being configured for good performance at high power outputs, and the other being configured for good performance at low power outputs.
[0020] Preferably, three, four, five or more NICs are provided. In some embodiments, one of the NICs is configured for optimum performance at a maximum power output of the transceiver, another is configured for optimum performance at a minimum power output of the transceiver, and the other NICs are configured for optimum performance at power levels between the maximum and minimum power outputs of the transceiver.
[0021] The NICs may be connectable by way of switches. The switches may be operated by a digital controller such as a microprocessor, field programmable gate array (FGPA), a PIC, a digital signal processor (DSP), an application-specific integrated circuit (ASIC) or the like. It is preferred to be able to isolate each NIC completely from both the antenna and the transceiver when the NIC is not in use, and each NIC may thus be connected to the transceiver by way of a first switch, and to the antenna by way of a second switch. In order to connect a given NIC, the switch on each side of the NIC must be switched on.
[0022] The transceiver may be connected to a baseband processor, which in turn is connected to the digital controller. The digital controller can monitor the power output of the transceiver by way of the baseband processor and operate the switches accordingly.
[0023] Alternatively, the baseband processor may be configured to operate the switches directly, thus removing the need for a separate digital controller. Depending on application and resource availability, the digital controller may be integrated with the baseband processor in this case.
[0024] Embodiments thus far described may be suitable for implementations where transmit and receive functions take place at the same time, for example in frequency-division duplexing (FDD) of 4G/LTE.
[0025] In other implementations, for example where time-division duplexing (TDD) is used, transmit and receive functions do not take place simultaneously. In these embodiments, a separate receive mode NIC is connected when the transceiver is in receive mode, with the other NICs switched out. When the transceiver is in transmit mode, the receive mode NIC is switched out, and the appropriate NIC for the transmit power level is switched in.
[0026] Viewed from a second aspect, there is provided an active impedance matching network for an electrically-small antenna, the network comprising a negative impedance converter connectable between a transceiver and the antenna, wherein the negative impedance converter has a bias voltage connection, and wherein the negative impedance converter is adjustable for different power outputs of the transceiver by applying different bias voltages to the bias voltage connection.
[0027] The different bias voltages may be applied by way of an array of individually operable switches under the control of a digital controller and/or a baseband processor. The bias voltages may be derived from a power source, for example a battery, and the switches may switch in different resistors or other components so as to allow a range of different bias voltages to be applied to the NIC in accordance with the power output of the transceiver.
[0028] Some embodiments of the second aspect may be cheaper to implement than some embodiments of the first aspect because fewer NICs are required.
[0029] As with the first aspect, embodiments of the second aspect may be adapted for operation where the transceiver does not transmit and receive simultaneously. This may be done by providing a separate receive mode NIC that can be switched in when the transceiver is in receive mode, and which may have its own bias voltage applied to a bias voltage connection. Alternatively, a single NIC may be used for both transmit and receive functions, with an additional switch allowing the NIC to be selectively connected to the receive input of the transceiver and the transmit output of the transceiver.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: Figure 1 shows an electrically small antenna connected to a 50 ohm signal port; Figure 2 shows the antenna of Figure 1 represented as an equivalent series connected resistor, capacitor and inductor; Figure 3 shows the arrangement of Figure 2 provided with a passive impedance matching network, together with a plot of reactance against angular frequency; Figure 4 shows the arrangement of Figure 2 provided with a non-Foster matching network comprising a negative capacitance, together with a plot of reactance against angular frequency; Figure 5 illustrates an antenna circumscribed by a sphere of radius a; Figure 6a is a schematic of a negative impedance converter (NIC) employing bipolar junction transistors; Figure 6b is a schematic of a negative impedance converter (NIC) employing field effect transistors; Figure 7 shows a conventional NIC arrangement for matching an antenna to a transceiver; Figure 8 shows a first embodiment of the present application with multiple NICs; Figure 9 shows a second embodiment of the present application with a single NIC that is adjustable by applying a bias voltage; Figure 10 shows a third embodiment of the present application with multiple NICs; Figure 11 shows a fourth embodiment of the present application with a transmit mode NIC that is adjustable by applying a bias voltage and a receive mode NIC; Figure 12 shows a fifth embodiment of the present application with a single NIC that is adjustable by applying a bias voltage; and Figure 13 shows a sixth embodiment of the present application with multiple NICs.
DETAILED DESCRIPTION
[0031] A first embodiment is shown in Figure 8, comprising a network of NICs 1, 2, 3, 4, each NIC being optimised for a particular transmitter power level in terms of linearity, noise figure, antenna efficiency, antenna matching bandwidth and return loss. The NICs 1, 2, 3, 4 are arranged in parallel between an antenna 5 and an RF transceiver 6. The network of NICs may be controlled directly from a baseband processor 7 of a mobile device, or by way of an interface device 8 such as a microcontroller, PIC, FPGA or ASIC. The choice of NIC 1, 2, 3, 4 depends on the power level set by the baseband processor 7. Where an interface device 8 is used, the baseband processor 7 may convey the power level setting in terms of an absolute power level or as a range of transmit powers. The interface device 7 can then make a decision as to which NIC 1, 2, 3, 4 is to be used.
[0032] As shown in Figure 8, each NIC 1, 2, 3, 4 is provided with a first switch 100, 101, 102, 103 on the transceiver side, and a second switch 200, 201, 202, 203 on the antenna side. A particular NIC is selected by closing the corresponding switches. For example, for transmissions at maximum power, switches 100 and 200 are closed, while the other switches are held open.
[0033] In one particular example, practical power settings could be arranged as: a) Maximum power -equal to or above 21dBm b) High power -between 21dBm and 15dBm c) Medium Power -between 15 and 10dBm d) Low power -10dBm and below These power ranges are not fixed and could be adjusted to suit different application scenarios.
[0034] An alternative embodiment is shown in Figure 9. Instead of a plurality of NICs, only a single NIC 10 is employed. However, the NIC 10 has a bias voltage connection 11 to which a bias voltage can be applied. Changing the bias voltage allows the NIC 10 to be adjusted to match the transmit power level condition from the transceiver 6 in the RF module.
[0035] The bias voltage applied to the NIC 10 is selected by closing the corresponding switch 300, 301, 302, 303. The switches 300, 301, 302, 303 may be controlled directly by the baseband processor 7, or by way of an interface device 8 such as microcontroller, PIC, FPGA or an ASIC. The bias voltages BV are derived from a power source such as a battery 12, and the bias levels are calculated to optimise NIC 10 performance at a chosen transmitter power level. Table 1 below outlines an exemplary scheme, with other NIC 10 parameters also optimised for a given transmit power level and bias voltage. One benefit of this embodiment over the multiple NIC embodiment is cost, it being cheaper to implement a single NIC then multiple NICs.
[0036]
Table 1
NIC Parameter Unit Max Power High Power Mid Power Low Power P1dB dBm 25 to 19 18 to 11 10 to 6 7 and below IMD3 dB 50.6 51.7 23.75 27.85 Noise figure (best) dB 3.2 3.2 0.9 0.9 Antenna dB -0.5 -0.5 -2 -2 Efficiency (best) Return Loss dB -10 -10 -8 -6 Bandwidth MHz 200 200 200 200 DC power 242mA and 3V 62mA and 3V 5.57mA and 3V 2.62mA and 3V Bias Voltage V BV1 BV2 BV3 BV4 [0037] The embodiments thus far described are applicable to FDD applications of 4G/LTE where transmit and receive functions occur simultaneously. In the case where TDD operation is required, the matching networks have to be modified to take this into account.
[0038] Figure 10 shows a modification of the Figure 8 embodiment with an additional NIC 13 that is optimised for RF signals received by the antenna 5. NIC 13 is provided with first and second switches 104, 204, and these switches are closed during receive operation, the other switches 100-103 and 200-203 being kept open. To further optimise the design, switch 104 can be eliminated and NIC 13 can be connected directly to the receive input of the transceiver RF module 6.
[0039] To implement TDD in the arrangement with a single NIC 10 with a bias voltage connection, the embodiment of Figure 9 is may be modified as shown in Figure 11. An additional NIC 14 dedicated to the receive mode is provided, the additional NIC 14 having first and second switches 104, 204. In this embodiment, the transmit NIC 10 is also provided with first and second switches 105, 205 so as to allow the network to switch between NIC 14 and NIC 10 depending on whether the system is in receive mode or transmit mode. To simplify the design, switches 104 and 205 could be omitted, the NIC 14 connected directly to the receive input of the transceiver 6 and the NIC 10 connected directly to the antenna 5.
[0040] Alternatively, as shown in Figure 12, a single adjustable NIC 10 is shared between transmit and receive modes, but an additional switch 400 allows the NIC 10 to be selectively connected to transmit and receive ports of the transceiver RF module 6. A receive bias voltage is applied to the NIC 10 by way of switch 304 when the NIC 10 is in receive mode. It may be possible to simplify the circuit further by omitting switch 304 and using the bias voltage for the lowest power transmit setting as the bias voltage for receive mode.
[0041] Figure 13 shows a further development of the embodiment of Figure 8 adapted for TDD operation. There is provided a network of NICs 1, 2, 3, 4, each NIC being optimised for a particular transmitter power level in terms of linearity, noise figure, antenna efficiency, antenna matching bandwidth and return loss. The NICs 1, 2, 3, 4 are arranged in parallel between an antenna 5 and an RF transceiver 6. The network of NICs may be controlled directly from a baseband processor 7 of a mobile device, or by way of an interface device 8 such as a microcontroller, PIC, FPGA or ASIC. The choice of NIC 1, 2, 3, 4 depends on the power level set by the baseband processor 7. Where an interface device 8 is used, the baseband processor 7 may convey the power level setting in terms of an absolute power level or as a range of transmit powers. The interface device 7 can then make a decision as to which NIC 1, 2, 3, 4 is to be used.
[0042] An additional switch 400 allows the NICs 1, 2, 3, 4 to be selectively connected to transmit and receive ports of the transceiver RF module 6.
[0043] As shown in Figure 13, each NIC 1, 2, 3, 4 is provided with a first switch 100, 101, 102, 103 on the transceiver side, and a second switch 200, 201, 202, 203 on the antenna side. A particular NIC is selected by closing the corresponding switches. For example, for transmissions at maximum power, switches 100 and 200 are closed, while the other switches are held open, and the additional switch 400 is switched to connect the maximum power NIC 1 to the transmit port of the RF module 6. To save on battery power consumption, the lowest transmit power NIC 4 (NIC Low P) could be used for receive operation.
[0044] 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.
[0045] 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.
[0046] 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 (16)

  1. CLAIMS1. An active impedance matching network for an electrically-small antenna, the network comprising a plurality of negative impedance converters individually connectable between a transceiver and the antenna, wherein the negative impedance converters are respectively configured for optimum performance at respective different power outputs of the transceiver.
  2. 2. A network as claimed in claim 1, comprising two negative impedance converters, one negative impedance converters being configured for good performance at high power outputs, and the other being configured for good performance at low power outputs.
  3. 3. A network as claimed in claim 1, comprising at least three negative impedance converters. 15
  4. 4. A network as claimed in claim 3, wherein a first negative impedance converter is configured for optimum performance at a maximum power output of the transceiver, a second negative impedance converter is configured for optimum performance at a minimum power output of the transceiver, and the other negative impedance converters are configured for optimum performance at power levels between the maximum and minimum power outputs of the transceiver.
  5. 5. A network as claimed in any preceding claim, wherein the negative impedance converters are connectable by way of switches.
  6. 6. A network as claimed in claim 5, wherein the switches are operated by a digital controller.
  7. 7. A network as claimed in claim 5 or 6, wherein each negative impedance converter is connected to the transceiver by way of a first switch, and to the antenna by way of a second switch.
  8. 8. A network as claimed in any one of claims 5 to 7, wherein the transceiver is connected to a baseband processor, which in turn is connected to the digital controller.
  9. 9. A network as claimed in any preceding claim, further comprising a receive mode negative impedance converter, the receive mode negative impedance converter being connected between the transceiver and the antenna when the transceiver is in receive mode, with the other negative impedance converters being disconnected when the transceiver is in receive mode.
  10. 10. An active impedance matching network for an electrically-small antenna, the network comprising a negative impedance converter connectable between a transceiver and the antenna, wherein the negative impedance converter has a bias voltage connection, and wherein the negative impedance converter is adjustable for different power outputs of the transceiver by applying different bias voltages to the bias voltage connection.
  11. 11. A network as claimed in claim 10, wherein the different bias voltages are applied by way of an array of individually operable switches under the control of a digital controller and/or a baseband processor.
  12. 12. A network as claimed in claim 11, wherein the bias voltages are derived from a power source, and wherein the switches are operable to switch in different components so as to allow a range of different bias voltages to be applied to the negative impedance converter in accordance with the power output of the transceiver.
  13. 13. A network as claimed in any one of claims 10 to 12, further comprising a receive mode negative impedance converter, the receive mode negative impedance converter being connected between the transceiver and the antenna when the transceiver is in receive mode, with the other negative impedance converters being disconnected when the transceiver is in receive mode.
  14. 14. A network as claimed in claim 13, wherein the receive mode negative impedance converter has a bias voltage connection.
  15. 15. A network as claimed in any one of claims 10 to 12, further comprising an additional switch allowing the negative impedance converter to be selectively connected to a receive input of the transceiver and a transmit output of the transceiver.
  16. 16. An active impedance matching network for an electrically-small antenna substantially as hereinbefore described with reference to or as shown in Figures 8 to 12 of the accompanying drawings.
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