ANTENNA IMPEDANCE MATCHING USING NEGATIVE IMPEDANCE CONVERTER AND PRE- AND POST-MATCHING NETWORKS

Abstract

There is disclosed a matching network for connecting an electrically small antenna to an RF source or load. The matching network includes a negative impedance converter, a pre-matching network for connecting the negative impedance converter to the antenna and a post-matching network for connecting the negative impedance converter to the RF source or load. The pre-matching network comprises a combination of capacitors and/or inductors to transform both a real part and an imaginary part of an impedance of the antenna. The negative impedance converter is configured to cancel the transformed imaginary part of the impedance of the antenna. The post-matching network comprises a combination of capacitors and/or inductors to transform a residual real part of the impedance of the antenna to match an impedance of the RF source or load. There is also disclosed an antenna system comprising a plurality of antenna radiating elements each having an associated feed, at least one of the feeds being connected to an RF source or load by way of an active matching circuit comprising a pre-matching network, a negative impedance converter and a post-matching network.

Claims

1. A matching network for connecting an electrically small antenna to an RF source or load, the matching network comprising a negative impedance converter, a pre-matching network for connecting the negative impedance converter to the antenna and a post-matching network for connecting the negative impedance converter to the RF source or load, wherein the pre-matching network comprises a combination of capacitors and/or inductors to transform both a real part and an imaginary part of an impedance of the antenna, the negative impedance converter is configured substantially to cancel the transformed imaginary part of the impedance of the antenna, and wherein the post-matching network comprises a combination of capacitors and/or inductors to transform a residual real part of the impedance of the antenna to match an impedance of the RF source or load.

2. The matching network as claimed in claim 1, wherein the pre-matching network comprises at least one tuneable element.

3. The matching network as claimed in claim 2, wherein the at least one tuneable element is a tuneable or switchable capacitor.

4. The matching network as claimed in claim 1, wherein the negative impedance converter comprises at least one tuneable element.

5. The matching network as claimed in claim 4, wherein the at least one tuneable element is a tuneable or switchable capacitor.

6. The matching network as claimed in claim 1, wherein the post-matching network comprises at least one tuneable element.

7. The matching network as claimed in claim 6, wherein the at least one tuneable element is a tuneable or switchable capacitor.

8. The matching network as claimed in claim 1, wherein the pre-matching network is configured to transform an in-band real part of the antenna impedance to a higher level.

9. The matching network as claimed in claim 1, wherein the pre-matching network is configured to transform an in-band imaginary part of the antenna impedance to a lower level.

10. The matching network as claimed in claim 9, wherein the pre-matching network is configured to transform an in-band imaginary part of the antenna impedance to zero or substantially zero.

11. The matching network as claimed in claim 9, wherein the negative impedance converter is configured substantially to cancel the transformed imaginary part of the antenna impedance at an operational frequency or frequency band.

12. The matching network as claimed in claim 1, wherein the post-matching network is configured to transform a residual real part of the transformed antenna impedance to match an impedance of the RF source or load.

13. The matching network as claimed in claim 1, wherein the pre-matching network is configured to keep a real part of the transformed antenna impedance substantially flat or constant across an operational frequency band.

14. The matching network as claimed in claim 1, wherein the pre-matching network is configured so that an imaginary part of the transformed antenna impedance has a zero crossing frequency in an operational frequency band.

15. The matching network as claimed in claim 1, further comprising a system controller for tuning or switching the network or components thereof.

16. An antenna system comprising a plurality of antenna radiating elements each having an associated feed, at least one of the feeds being connected to an RF source or load by way of an active matching circuit comprising a pre-matching network, a negative impedance converter and a post-matching network; wherein the pre-matching network comprises a combination of capacitors and/or inductors to transform both a real part and an imaginary part of an impedance of the respective antenna feed.

17. The system of claim 16, wherein the RF source or load comprises at least one transceiver port.

18. The system of claim 16, wherein the RF source or load comprises at least one transmitter port.

19. The system of claim 16, wherein the RF source or load comprises at least one receiver port.

20. The system of claim 16, wherein the negative impedance converter is configured substantially to cancel the transformed imaginary part of the impedance of the respective antenna feed.

21. The system of claim 20, wherein the post-matching network comprises a combination of capacitors and/or inductors to transform a residual real part of the impedance of the antenna feed to match an impedance of the RF source or load.

22. The system of claim 16, wherein the pre-matching networks are configured to decouple the antenna radiating elements over the frequency bands of interest at any given time.

23. The system of claim 16, wherein all of the feeds are connected to the RF source or load by way of a respective active matching circuit comprising a negative impedance converter.

24. The system of claim 16, wherein at least one of the feeds is connected to the RF source or load by way of a passive matching circuit that does not include a negative impedance converter.

25. The system of claim 17, wherein the matching circuits are all connected to a single port.

26. The system of claim 17, wherein the matching circuits are all connected to different ports.

27. The system of claim 16, wherein each of the radiating antenna elements and their associated matching circuits are configured to operate in a predetermined continuous frequency band.

28. The system of claim 16, wherein the radiating antenna elements are sized differently to each other and/or have different electrical sizes.

29. (canceled)

30. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

[0055] FIG. 1 shows an electrically small antenna connected to a 50 ohm signal port;

[0056] FIG. 2 shows the antenna of FIG. 1 represented as an equivalent series connected resistor, capacitor and inductor;

[0057] FIG. 3 shows the arrangement of FIG. 2 provided with a passive impedance matching network, together with a plot of reactance against angular frequency;

[0058] FIG. 4 shows the arrangement of FIG. 2 provided with a non-Foster matching network comprising a negative capacitance, together with a plot of reactance against angular frequency;

[0059] FIG. 5 illustrates an antenna circumscribed by a sphere of radius a;

[0060] FIG. 6 is a schematic of a conventional Linvill-type negative impedance converter (NIC);

[0061] FIG. 7 shows a conventional NIC arrangement for matching an antenna to a transceiver;

[0062] FIG. 8 shows a matching network including a pre-matching network, an NIC and a post-matching network in accordance with the present disclosure;

[0063] FIG. 9 shows the main parts of the network of FIG. 8;

[0064] FIG. 10 shows one implementation of the network of FIG. 9;

[0065] FIG. 11 shows a detail of the NIC circuit of FIG. 10;

[0066] FIG. 12 shows a detail of the pre-matching circuit of FIG. 10;

[0067] FIG. 13 shows a plot of matching performance for the embodiment of FIG. 10 with a first set of capacitor values;

[0068] FIG. 14 is a plot showing the change of impedance values with frequency;

[0069] FIG. 15 shows the 3.sup.rd order intermodulation distortion (IMD3);

[0070] FIG. 16 shows a plot of matching performance for the embodiment of FIG. 10 with a second set of capacitor values;

[0071] FIG. 17 is a plot showing the change of impedance values with frequency;

[0072] FIG. 18 shows the 3.sup.rd order intermodulation distortion (IMD3);

[0073] FIG. 19 shows an outline schematic of an embodiment of the disclosure;

[0074] FIG. 20 shows the arrangement of FIG. 19 without a pre-matching circuit;

[0075] FIG. 21 shows the return loss for the arrangements of FIGS. 19 and 20;

[0076] FIG. 22 is a Smith chart for the arrangements of FIGS. 19 and 20;

[0077] FIG. 23 is a schematic outline of a first embodiment of the second aspect;

[0078] FIG. 24 is a schematic outline of a second embodiment of the second aspect;

[0079] FIG. 25 is a variation of the embodiment of FIG. 23;

[0080] FIG. 26 is a variation of the embodiment of FIG. 24;

[0081] FIG. 27 is a more detailed schematic of a variation of the first embodiment of the second aspect;

[0082] FIG. 28 shows the return loss at each transceiver port of the embodiment of FIG. 27;

[0083] FIG. 29 shows the total efficiency of the embodiment of FIG. 27;

[0084] FIG. 30 is a more detailed schematic of a variation of the second embodiment of the second aspect;

[0085] FIG. 31 shows the return loss at the single transceiver port of the embodiment of FIG. 30; and

[0086] FIG. 32 shows the total efficiency of the embodiment of FIG. 30;

DETAILED DESCRIPTION

[0087] FIG. 8 shows in schematic outline an embodiment of the present disclosure. An electrically small antenna 1 is connected to an RF transceiver 2 by way of a negative impedance converter (NIC) 3. A pre-matching network 4 is connected between the NIC 3 and the antenna 1, while a post-matching circuit 5 is connected between the NIC 3 and the transceiver 2. In preferred embodiments, one or more of the NIC 3, the pre-matching network 4 and the post-matching network 5 includes tuneable or switchable components such as tuneable or switchable capacitors or inductors. A system controller 29, for example a microprocessor or integrated circuit, is provided to control the transceiver 2 and the tuneable or switchable components in the NIC 3, pre-matching network 4 and/or post-matching network 5 by way of control and/or programming lines 30.

[0088] FIG. 9 shows the impedance transforming components of the embodiment of FIG. 8 in order to illustrate more clearly the desired function of each component. The antenna 1 has an antenna radiation resistance R.sub.ant and an antenna reactance X.sub.ant. The pre-matching network 4 is configured to transform the antenna radiation resistance R.sub.ant to a transformed antenna radiation resistance R.sub.a, and to transform the antenna reactance X.sub.ant to a transformed antenna reactance X.sub.t. The antenna impedance may be considered as R.sub.ant (the real part) plus X.sub.ant (the imaginary part). The pre-matching network 4 is configured to transform X.sub.ant to a value X.sub.t that is zero or close to zero. Advantageously, the pre-matching network 4 will transform R.sub.ant to a higher value R.sub.a. This is because the overall RF power efficiency of the NIC 3 is proportional to R.sub.a, and for any given R.sub.a will be at a maximum when X.sub.t is zero. The post-matching circuit 5 is configured to transform the real part of the impedance at the output of the NIC 3 to match the impedance of the transceiver port 2, which is typically 50.

[0089] Ideally, the NIC 3 is further configured to cancel the induced ohm loss resistance RI1 of the pre-matching network 3 and the induced ohm loss resistance RI2 of the post-matching network 4.

[0090] A specific implementation of the arrangement of FIG. 9 is shown in FIG. 10. The implementation comprises an antenna 1, a pre-matching network 4, an NIC circuit 3, a post-matching circuit 5 and a transceiver port 2.

[0091] FIG. 11 shows the NIC circuit 3 of FIG. 10 in more detail. The NIC 3 is a Linvill-type NIC comprising first and second transistors 8, 9 connected in a cross-over configuration as will be understood by those skilled in the art. A resistor 10, inductor 11 and switchable or tuneable capacitor 12 between the collectors or drains of transistors 8, 9. The negative impedance presented by the NIC circuit 3 can be adjusted by adjusting the capacitor 12. A parallel resistor-capacitor bank 13 is connected across the NIC 3 to provide an additional parallel passive impedance adjustment network, and an optional further parallel resistor-capacitor bank 14 may be connected in series with the first bank 13.

[0092] FIG. 12 shows the pre-matching network 4 of FIG. 10 in more detail. A switchable or tuneable capacitor 15 is provided so as to allow tuning.

[0093] It will be noted that the embodiment shown in FIG. 10 has only two switchable or tuneable capacitors 12, 15.

[0094] One exemplary set of results will now be described. The entire matching network of FIG. 10 is tuned to have a substantially zero reactance at around 900 MHz by setting the tuneable capacitor 12 in the NIC 3 to 0.34 pF, and the tuneable capacitor 15 in the pre-matching network 4 to 0.77 pF. The matching performance of this implementation is shown in FIG. 13, the impedances in FIG. 14 and the linearity in FIG. 15. The power efficiency at 900 MHz is found to be around 20%.

[0095] After tuning the capacitor 12 to 0.42 pF and the capacitor 15 to 1.17 pF, the matching performance as shown in FIG. 16 covers almost the same frequency band, and the impedance as shown in FIG. 17 has a different zero reactance frequency of 800 MHz. The linearity at 800 MHz is shown in FIG. 18, and the power efficiency is found to be 18.2%.

[0096] As well as improving power efficiency, embodiments of the present disclosure are effective in reducing noise. FIG. 19 shows a test arrangement comprising an antenna 1, a pre-matching network 4, and NIC 3, a post-matching network 5 and a 500 measurement port 16. FIG. 20 shows a comparative test arrangement similar to that of FIG. 19, but without a pre-matching network 4.

[0097] FIG. 21 is a return loss plot demonstrating that the arrangement of FIG. 19, with the pre-matching network 4, has a noise figure at 800 MHz of just 1.285 dB, in contrast to the noise figure of 4.932 dB for the arrangement of FIG. 20. Thus, an improvement in noise of approximately 3.8 dB is obtained, as well as an improvement in antenna efficiency.

[0098] FIG. 22 is a Smith chart showing the noise circle of the NIC 3. It can be seen the antenna with the pre-matching circuit 4 lies between NF=1 dB to 2 dB, which that of the arrangement without the pre-matching circuit 4 lies just on the NF=5 dB circle. Improving the impedance match between the antenna 1 and the NIC 3 helps to reduce noise.

[0099] FIG. 23 shows a schematic outline of a first embodiment of the second aspect, in which a compound antenna comprising antenna radiating elements 21A, 21B and 21C has multiple feeds 22A, 22B and 22C. Feed 22A is connected to a transceiver port 23A by way of an active matching circuit comprising a pre-matching network 24A, a negative impedance converter network 25A and a post-matching network 26A. Feed 22B is connected to a transceiver port 23B by way of an active matching circuit comprising a pre-matching network 24B, a negative impedance converter network 25B and a post-matching network 26B. Feed 22C is connected to a transceiver port 23C by way of a passive matching circuit comprising a pre-matching network 24C and a post-matching network 26C. Transceiver port 23A is configured to handle a first frequency band A, transceiver port 23B is configured to handle a second frequency band B, and transceiver port 23C is configured to handle a third frequency band C. It will be appreciated that additional frequency bands can be accommodated by adding further antenna radiating elements, transceiver ports and matching circuits. While all embodiments will have at least one active branch comprising a pre-matching network, an NIC network and a post-matching network, some embodiments will comprise just active branches, and others may have one or more passive branches.

[0100] The antenna radiating elements 21A, 21B and 21C, which will generally be close together, for example in a mobile handset or other portable device, will tend to couple with each other during operation. In order to address this problem, the pre-matching networks 24 (and, in some embodiments, the post-matching networks 26) are configured to selectively decouple the matching circuits or branches across frequency bands of interest. In other words, coupling between antenna radiating elements, which is often unavoidable, can surprisingly be made unproblematic by appropriate configuration of the pre-matching networks 24 and, in some embodiments, the post-matching networks 26.

[0101] The pre-matching networks 24 have two functions. One is to decouple the multi-feed antenna 21 over all of the interesting frequency bands. Typically, after decoupling, the input impedance after the pre-matching network 24 in one branch can be independent of the circuits connected after the pre-matching networks 24 in the other branches. The other function of pre-matching network 24 is to transform the antenna impedance to a proper level so that the NIC network 25 can cancel the transformed reactance. Typically, the real part of the antenna impedance should be transformed to a higher, relatively flat level across the relevant frequency band, and the imaginary part of the antenna impedance should be transformed so that it increases monotonically from negative to positive across the relevant frequency band. The post-matching network 26 also has two functions. One is to match the impedance after cancellation by the NIC 25 (in an active branch) or the impedance after transformation (in a passive branch) to the impedance of the transceiver port 23 (normally 50 ohms). The other is to decouple different branches when all branches are connected to a single transceiver port 23, as shown in FIG. 24.

[0102] FIG. 24 shows an alternative embodiment, with like parts labelled as for FIG. 23. The embodiment of FIG. 24 is similar to that of FIG. 23, except that all of the branches connect to a single transceiver port 23, rather than to separate transceiver ports 23A, 23B and 23C.

[0103] FIG. 25 shows a specific implementation of the embodiment of FIG. 23 to cover low, middle and high frequency bands, with parts being labelled as for FIG. 23. Antenna radiating element 21B and its associated matching circuitry 24B, 25B, 26B are configured for operation in a middle frequency band. Antenna radiating element 21C and its associated matching circuitry 24C, 26C are configured for operation in a high frequency band. Antenna radiating element 21A has the largest size, with antenna radiating element 21B having a middle size and antenna radiating element 21C having the smallest size. The input impedance of each antenna radiating element 21A, 21B, 21C is optimised for its respective frequency band by appropriate adjustment of the pre- and post-matching networks 24, 26. It will be noted that a mixture of active branches with NIC components 25 and passive branches with no NIC components may be provided in order to help fulfil desired bandwidth requirements.

[0104] Similarly, FIG. 26 shows a specific implementation of the embodiment of FIG. 24 to cover low, middle and high frequency bands, with parts being labelled as for FIG. 24.

[0105] FIG. 27 shows a more detailed implementation of the embodiment of FIG. 23, comprising a multi-port NIC-based impedance matching circuit for a multi-feed antenna to cover multiple bands, with parts being labelled as in FIG. 23. The embodiment of FIG. 27 comprises first and second active branches, with no passive branch. Although the antenna radiating elements 21A, 21B are shown as a single component, this is merely a consequence of circuit diagram conventions. The multi-feed antenna will physically have different antenna radiating elements 21A, 21B.

[0106] FIG. 28 shows the return loss at each transceiver port 23A, 23B of the FIG. 27 embodiment. It can be seen that transceiver port 23A can cover the LTE low band (700 MHz-960 MHz), and transceiver port 23B can cover the GNSS band and the LTE middle and high bands (1.56 GHz-2.7 GHz). The isolation between the two transceiver ports 23A, 23B is mostly lower than 18 dB. FIG. 29 shows the total efficiency of the antenna system over the two continuous wide frequency bands after matching.

[0107] FIG. 30 shows a more detailed implementation of the embodiment of FIG. 24, comprising a single port NIC-based impedance matching circuit for a multi-feed antenna to cover multiple bands, with parts being labelled as in FIG. 24. The embodiment of FIG. 30 comprises first and second active branches, with no passive branch.

[0108] FIG. 31 shows the return loss at the transceiver port 23 of the FIG. 30 embodiment. It can be seen that the single transceiver port 23 can cover the LTE low band (700 MHz-960 MHz), GNSS band and the LTE middle and high bands (1.56 GHz-2.7 GHz) simultaneously. FIG. 32 shows the total efficiency of the antenna system over the two continuous wide frequency bands after matching.

[0109] 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.

[0110] 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.

[0111] 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.