GB2537676A

GB2537676A - Switch architecture for antenna matching circuits

Info

Publication number: GB2537676A
Application number: GB1506994.1A
Authority: GB
Inventors: Hu Sampson; Wang Zhengpeng; Thind Surinder
Original assignee: Smart Antenna Technologies Ltd
Current assignee: Smart Antenna Technologies Ltd
Priority date: 2015-04-24
Filing date: 2015-04-24
Publication date: 2016-10-26
Anticipated expiration: 2035-04-24
Also published as: GB2537676B; GB201506994D0

Abstract

An antenna matching network for use between an antenna 1 and a signal port 2 comprising a plurality of matching circuits connected in parallel 3, each matching circuit independently isolatable through use of switches 4 and 5 on either side of each matching circuit, where the plurality of matching circuits is divided into two individually isolatable groups 9 and 10, and each group is connected in series with a pre-matching circuit 12. Resistors that are connected to ground may be provided either side of the group switches 11 and also on either side of the switches between the matching circuits 3 and the signal port 2.

Description

SWITCH ARCHITECTURE FOR ANTENNA MATCHING CIRCUITS

[0001] This invention relates to matching circuits for switchable antennas. In particular, but not exclusively, the invention relates to a matching circuit architecture for a reconfigurable multiple-input multiple-output (MIMO) antenna for use in a portable electronic device such as a mobile phone handset, laptop, tablet, femtocell, wireless router or other radio communications device.

BACKGROUND

[0002] A conventional switchable antenna normally employs parallel switchable matching circuits as shown in Figure 1. The antenna 1 is connected to an antenna feeding port 2 by a network comprising a plurality of parallel matching circuits 3 that can be selectively switched in line with switches 4, 5. Each matching circuit 3 is optimised for operation at a different frequency so as to match the antenna 1 to a standard 500 at the feeding port 2.

Each matching circuit will typically comprise some combination of capacitors and inductors (for passive matching), or may include negative impedance converters or operational amplifiers (for active or non-Foster matching). This switchable circuit works quite well while the input impedance of the antenna is high. However, when the input impedance of the antenna is low, the performance of the switches becomes more critical. The switches 5 that are in the off state will have a serious energy leakage problem, especially when many off state switches are in parallel. Their individual capacitances, Coft, will add together, resulting in a high total capacitance and a high leakage of energy. The total efficiency of the whole system will therefore be low.

[0003] Figure 2 shows a more detailed schematic of a conventional switch architecture for a MIMO antenna 1 with eight switchable parallel passive matching circuits 3. In this arrangement, each matching circuit 3 comprises first and second inductors 6, 7 and a capacitor 8.

BRIEF SUMMARY OF THE DISCLOSURE

[0004] Viewed from a first aspect, there is provided a switchable matching network for matching an antenna to a signal port at a predetermined impedance, the matching network comprising a plurality of matching circuits connected in parallel, each matching circuit being independently isolatable by way of first and second switches on either side of each matching circuit, wherein the plurality of matching circuits is divided into at least two independently isolatable groups, each group being connected in series with a pre-matching circuit between the antenna and the signal port.

[0005] By dividing the matching circuits into two or more groups, it is possible to switch out the groups that are not required for impedance matching at a given frequency, thereby reducing the total overall capacitance and associated energy leakage in the network.

[0006] Advantageously, at least some of the pre-matching circuits are additionally each provided with at least one group switch so as to allow the groups of matching circuits to be selectively connected to the antenna.

[0007] An improvement in overall efficiency is obtained by reducing the energy leakage in the parallel switches connected to the antenna. The switches are lossy in their off state due to leakage caused by the Ca capacitance. In the novel architecture of present embodiments, the number of switches directly connected in parallel to the antenna is reduced, thereby reducing the signal loss caused by switch leakage. Moreover, the insertion loss caused by the switches is also reduced.

[0008] For example, eight matching circuits may be arranged as two groups of four, or four groups of two. Twelve matching circuits may be arranged as two groups of six, three groups of four, four groups of three or six groups of two.

[0009] The pre-matching circuit for each group is used to improve the input impedance of the antenna so as to decrease the losses in the matching circuits between the pre-matching circuit and the signal port. The pre-matching circuits may be passive matching circuits comprising a predetermined capacitance and inductance. The main insertion loss of the overall matching network is caused by the switches. By providing a pre-matching circuit to improve the input impedance of the antenna, the insertion losses resulting from the switches are significantly reduced. Another important advantage obtained by some embodiments is reduced voltage on the first and second switches. In the Figure 1 and 2 arrangements, there is a relatively high voltage across each matching circuit between the first and second switches, and this means that each matching circuit requires higher specification and hence more expensive components (such as capacitors and inductors). In present embodiments, by dividing the matching circuits into two or more groups and providing a switch and a pre-matching circuit for each group, the voltage across each matching circuit between the first and second switches is reduced. This means that lower specification and hence cheaper components can be used.

[0010] Additional protection may be implemented by providing high-value resistors on either side of the group switches, and also on either side of the switches between the matching circuits and the signal port, the resistors being connected to ground. In some embodiments, the high-value resistors may have resistances over 10004.

[0011] The order in which the pre-matching circuits and the group switches are arranged between the antenna and the matching circuits can also have an effect on overall efficiency. If some or all of the pre-matching circuits are located between the antenna and the group switches, the impedance seen by the group switches is improved, and therefore the insertion losses of the group switches will be reduced.

[0012] In some embodiments, it is possible to omit the group switch for one of the pre-matching circuits without significantly affecting the overall matching performance of the matching network.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: Figure 1 shows an outline of a conventional matching network; Figure 2 shows a more detailed schematic of the matching network of Figure 1; Figure 3 shows an outline of a first embodiment; Figure 4 shows a more detailed schematic of the embodiment of Figure 3; Figure 5 shows a detailed schematic of a second embodiment provided with protective resistors; Figure 6 shows an outline of a third embodiment; Figure 7 shows a more detailed schematic of the embodiment of Figure 6; Figure 8 shows an outline of a fourth embodiment; and Figure 9 shows a more detailed schematic of the embodiment of Figure 8.

DETAILED DESCRIPTION

[0014] A first embodiment is shown in outline in Figure 3, and in more detail in Figure 4. An antenna 1 is connected to an antenna feeding port 2 by a network comprising a plurality of parallel matching circuits 3 that can be selectively switched in line with switches 4, 5. Each matching circuit 3 is optimised for operation at a different frequency so as to match the antenna 1 to a standard 500 at the feeding port 2. Each matching circuit will typically comprise some combination of capacitors and inductors (for passive matching), or may include negative impedance converters or operational amplifiers (for active or non-Foster matching). Figure 4 shows a specific embodiment with passive matching circuits 3, comprising inductors 6, 7 and capacitors 8, although it will be understood that other appropriate matching circuit arrangements may be used.

[0015] In accordance with present aspects, the matching circuits 3 are divided into at least two groups 9, 10, each group being independently isolatable by way of group switches 11. Each group 9, 10 is provided with a pre-matching circuit 12 between the respective group 9, 10 and its group switch. The pre-matching circuit 12 and group switch 11 are located between each group 9, 10 and the antenna 1. Two groups 9, 10 are explicitly shown in Figure 3, although it will be understood that more than two groups 9, 10 may be provided. The provision of several pre-matching circuits 12 helps to cover the whole operating frequency band. This is because a single pre-matching circuit 12 is not typically able to improve wide band antenna input impedance.

[0016] Indeed, Figure 4 shows an arrangement with three groups, 9, 10 and 13, where groups 9 and 10 each comprise three matching circuits 3, and group 13 comprises two matching circuits 3. In the arrangement of Figure 4, each matching circuit comprises an appropriate combination of inductors and capacitors of predetermined values so as to provide the necessary impedance matching at the relevant frequency. By switching between the matching circuits 3, it is possible to select the matching circuit 3 best suited for any given frequency signal that is being passed between the antenna 1 and the signal port 2.

[0017] Furthermore, each pre-matching circuit 12 in this specific arrangement comprises an inductor 14 and a capacitor 15 of appropriate values. By operating the group switches 11, it is possible to select only the group 9, 10 or 13 that contains the matching circuit 3 for the frequency for which impedance matching is required at any given time. In this way, only leakage or lossiness due to the switches and matching circuits in any particular group 9, 10, 13 are of relevance, the switches and matching circuits in the other groups 9, 10, 13 no longer being of concern. This results in an improvement in overall efficiency. For instance, a typical antenna input impedance may be quite low, for example with a real part of only 20. If the Ron of a switch is 20, the total efficiency will be lower than -3dB.

Furthermore, the Coif of the switches may also cause low total efficiency. The pre-matching circuits 12 are introduced to improve the antenna input impedance and to reduce the losses due to the switches.

[0018] In order to match the impedance of the antenna 1 to one frequency, all three switches 11, 4, 5 in the related branch will be switched on and all the other switches will be switched off. For example, to switch the matching network to the lowest frequency, only one of the three group switches 11 will be switched on, and only the switches 4, 5 on either side of the lowest frequency matching circuit 3 will be switched on. A chassis antenna is used in this simulation. Table 1 compares the total efficiency simulation results for the matching networks of Figures 2 and 4. It can be observed from Table 1 that the total efficiency is improved at all the frequency points. The maximum improvement in the total efficiency is nearly 3 dB. The improvement in the total efficiency comes from reducing energy leakage in the parallel switches connected to the antenna 1. The switches are lossy in the off state due to leakage caused by the Cott capacitance. In the novel architecture of the Figure 4 embodiment, there are fewer switches in parallel connected to the antenna 1 (only three group switches 11, rather than eight switches 4), and this will reduce the signal loss caused by switch leakage. The insertion loss of the switches itself is also reduced.

Table 1: Total efficiency comparison Port Frequency Total Efficiency Total Efficiency Efficiency (MHz) (Figure 2) (Figure 4) improvement A 800 -9 dB -7.1 dB 1.9 dB B 1000 -7.2 dB -4.4 dB 2.8 dB C 1500 -2.9 dB -2.7 dB 0.2 dB D 2000 -4.2 dB -3.2 dB 1.0 dB E 2500 -1.7 dB -1.8 dB -0.1 dB F 3000 -4.9 dB -3.2 dB 1.7 dB G 4000 -6.2 dB -5.2 dB 1.0 dB H 5500 -9.3 dB -6.4 dB 2.9 dB Another important improvement of the new matching network is reduced voltage on the switches. The switches 4, 5 of the matching network shown in Figures 1 and 2 suffer very high voltages. This will increase the price of the whole matching network, since more expensive components will be required. The switches 4, 5 of the matching network shown in Figures 3 and 4 need only handle a much lower voltage. This will help to reduce the price of the matching network. Table 2 sets out the voltages on the switches in the matching network architecture of Figure 2, and Table 3 sets out the voltages on the switches in the matching network architecture of Figure 4. The voltages are measured at the GSM frequency band, the output power being 30dBmW.

Table 2: The voltage on the switches Port Voltage on the Voltage on the switch 4 close to switch 5 close to the antenna (V) the Tx/Rx (V) A 2.8 2.0 B 31.1 9.9 C 31.8 10.5 D 29 10.4 E 31.4 10.1 F 31.8 10.4 G 31.1 10.5 H 30.5 10.5 Table 3: The voltage on the switches Switch Voltage on the switch in the first stage (V) (group switches 11) Voltage on the switch in the Second stage (V) (switches 4) Voltage on the switch in the Second stage (V) (switches 5) 1 (On state) 2.6 1.7 1.9 2 (Off state) 33.3 4.3 10.3 3 (Off state) 34 4.4 9.8 4 (Off state) 0.9 9.6 (Off state) 0.8 9.4 6 (Off state) 1.3 9.8 7 (Off state) 2.4 9.6 8 (Off state) 2.2 10.4 [0019] Figure 5 shows an embodiment similar to that of Figure 4, but with the addition of large resistors 16, 17, 18, 19 (each with a resistance of several thousand ohms) on either side of the group switches 11 and on either side of the switches 5 between the matching circuits 3 and the signal port 2, the resistors 16, 17, 18, 19 being connected to ground. These large resistors 16, 17 can help to protect the entire network from large voltages.

[0020] Figures 6 and 7 show an embodiment similar to that of Figure 4, but with the order of the pre-matching circuit 12 and group switch 11 reversed for the first group 9 of matching circuits 3. By locating the pre-matching circuit 12 between the antenna 1 and the group switch 11, the impedance seen by the group switch 11 is improved, and the insertion loss of the group switch 11 is reduced. In some embodiments, the order of the pre-matching circuit 12 and the group switch 11 may be reversed for two, but not all, of the groups of matching circuits, but it has been found that reversing the order for all of the groups of matching circuits results in a deterioration in matching circuit performance due to interface problems between the pre-matching circuits 12.

[0021] Figures 8 and 9 show an embodiment similar to that of Figure 4, but with one of the group switches 11 omitted entirely. This can help to reduce costs by cutting the 15 number of components, without significantly affecting the impedance matching performance of the network as a whole.

[0022] Simulation results have shown a total efficiency improvement of around 1 to 2dB over the arrangement of Figure 2.

[0023] 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.

[0024] 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.

[0025] 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. A switchable matching network for matching an antenna to a signal port at a predetermined impedance, the matching network comprising a plurality of matching circuits connected in parallel, each matching circuit being independently isolatable by way of first and second switches on either side of each matching circuit, wherein the plurality of matching circuits is divided into at least two independently isolatable groups, each group being connected in series with a pre-matching circuit between the antenna and the signal port.
2. A network as claimed in claim 1, wherein at least some of the pre-matching circuits are additionally each provided with a group switch so as to allow the groups of matching circuits to be selectively connected to the antenna.
3. A network as claimed in claim 1, wherein all of the pre-matching circuits are additionally each provided with a group switch so as to allow the groups of matching circuits to be selectively connected to the antenna.
4. A network as claimed in any preceding claim, further comprising resistors on either side of the group switches, and also on either side of the switches between the matching circuits and the signal port, the resistors being connected to ground.
5. A network as claimed in claim 4, wherein the resistors have resistances of at least 10000.
6. A network as claimed in any preceding claim, wherein at least one of the pre-matching circuits is located between its associated group switch and the antenna.
7. A network as claimed in claim 2 or any one of claims 3 to 6 depending from claim 2, wherein at least one of the pre-matching circuits is not provided with an associated group switch.
8. A switchable matching network for matching an antenna to a signal port at a predetermined impedance, substantially as hereinbefore described with reference to or as shown in Figures 3 to 9 of the accompanying drawings.