Radio frequency antenna tuner block

Abstract

A radio frequency (RF) antenna tuner block (1000) for individually tuning RF antennas (1010) comprises a plurality of selectively switchable tuning elements (1020) The tuner block further comprises an RF common mode choke (1030) providing an interface between the tuner block (1000) and the RF antennas (1010), and the RF common mode choke (103) is configured to reduce stray currents in the tuning elements of the tuner block. The RF common mode choke comprises two conductors (1032) wound around a common core (1034). The RF common mode choke is positioned between the RF antennas (1010) and input terminals (1022) of the tuner block.

Description

Radio Frequency Antenna Tuner BlockCross-Reference to Related Applications

[0001] The present application claims priority from Australian Provisional Patent Application No 2023903284 filed on 13 October 2023, the contents of which are incorporated herein by reference.Technical Field

[0002] The present disclosure broadly relates to radio frequency (RF) antenna tuner blocks for tuning RF antennas.Background

[0003] RFID cabinets have shelves with RFID antennas incorporated in the shelves. Each shelf in the cabinet has one or more antennas. The antennas need to be tuned for the relevant frequency range of operation, and each antenna is normally manually and individually tuned using a tuning circuit associated with each antenna.

[0004] To individually tune RFID antennas various electronic components can be used to adjust the resonance frequency of each antenna to the relevant frequency range of operation. The specific components depend on the design and requirements of the RFID system. This type of tuning circuit is designed to adjust and optimise the impedance or resonance of an antenna or RF circuit. The specific configuration of a tuning network can vary depending on the requirements of the system and the antenna design. However, a tuning network typically includes components such as capacitors, inductors, and resistors. Figure 1 shows a simple example of a series tuned circuit 100. The tuning capacitor C is adjusted to resonate with inductor L. In some circuits, the tuning capacitor can be split or distributed around the coil for improved operation as shown in Fig 24 of US patent no. 5,258,766.

[0005] The tuner block consists of a number of tuning sub-blocks connected in series. Each tuning sub-block in turn consists of an RF block element, a switching element and a tuning element. The purpose of the RF block element is to provide a high impedance to the RF signal and at the same time to allow the DC control signal to pass through. In normal operation, some of the tuning sub-blocks are electrically turned on and others off by the RFID reader to optimise the RF signal going to the antenna.

[0006] The inventors have found that sometimes the tuner block does not operate optimally. If an RF antenna system with multiple RF antennas connected to an RF reader is not functioning properly, or if it appears that the tuning circuit for the antennas is malfunctioning, there can be various potential reasons for the issue. The cause may, for example, relate to an impedance mismatch between the antennas and the RF reader, individual components within the tuning circuit, environmental factors, such as electromagnetic interference (EMI) or physical obstacles, problems with the cables connecting the antennas to the reader, inadequate or unstable power supply to the antennas and tuning circuit, the physical placement and orientation of the antennas, a mismatch in operating frequencies, connector issues, or antenna damage.

[0007] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.Summary

[0008] The inventors have discovered that some of the RF blocks in the tuning block overheat, reaching up to about 90 degrees Celsius. Overheating of tuning blocks or tuning block components is not a known or common phenomenon. The inventors found that the antenna resistance was higher than expected when the tuning block overheated. Further investigation showed that some RF signals found their way back to the transmitter source, bypassing the antenna loop via the RF block element. The inventorsdetermined that the current IR lost in the RF block heats up the element. Furthermore, the field strength measured was lower than expected due to the same reason.

[0009] In the first place, the inventors investigated to determine the source of the unexpected heat. The inventors discovered that, because the tuning circuit is connected to the RFID antennas via several cables that are tightly wound together, the tuning circuits suffer from mutual inductance (i.e., unwanted mutual inductance between various electronic components on the circuit board as well as the cables). This causes the components of the tuning circuit to heat up, especially the inductors in the tuning circuit.

[0010] Following the discovery of the heat and determining its source, the inventors found a solution to the problem.

[0011] The inventors have found that they can use a common mode choke. The solution to the tuning elements overheating is to add an inline common mode choke to force the differential RF signal to enter and exit through the choke. This ensures that the RF signal follows the designated signal path, and that it travels to the designated antenna pairs and tuning elements and returns through the choke.

[0012] A common mode choke is a passive electromagnetic device that permits the passage of the desired electrical signals through a conductor while filtering out unwanted common mode current, for example resulting from stray mutual inductance. The choke has the effect of forcing the ingoing and outgoing current to / from the circuit to be equal, consequently current dissipating as heat in the circuit is reduced.

[0013] Some embodiments may include a differential choke.

[0014] Described herein is an improved antenna tuning circuit, generally applicable to tuned coils and in particular to tuned coils used for generating a magnetic field. In particular, the tuning block described herein is suitable for controlling magnetic fieldsused for radio frequency communication, and more specifically in the high frequency band, e.g., at 13.56MHz.

[0015] In one aspect there is provided a radio frequency (RF) antenna tuner block for individually tuning RF antennas, the tuner block comprising: a plurality of selectively switchable tuning elements; wherein the tuner block further comprises: an RF common mode choke providing an interface between the tuner block and the RF antennas, and configured to reduce stray currents in the tuning elements of the tuner block.

[0016] As used herein, the term “common mode choke” is understood to mean a choke used to attenuate unwanted common-mode signals, consisting of two or more windings with a common core.

[0017] Common-mode (CM) noise is when undesired signals occur on a pair of conductors. Common-mode chokes, also known as common-mode inductors, are designed with two or more coils of wire wound around a single magnetic core. Each winding is connected in series with one of the conductors. This arrangement causes the magnetic fields generated by the wires to combine, resulting in high impedance for the noise signal. The intended differential mode signal can easily pass through the coil, while the common mode noise component is either reduced significantly or completely blocked due to the resulting high reactance of the inductor. The common mode choke itself maintains a low electrical resistance, minimizing power loss during this process.

[0018] The RF common mode choke may comprise two conductors wound around a common core, and the RF common mode choke may be positioned between the RF antennas and input terminals of the tuner block.

[0019] The common mode choke of the tuner block may be connected to the RF antennas via at least one RF bus.

[0020] The RF antennas may comprise pairs of antennas in figure-8 configurations.

[0021] The tuner block may comprise a series of tuning sub-blocks each comprising one or more of: an RF block element, a switching element, and a tuning element.

[0022] The common mode choke of the tuner block may be positioned between an active coil that is conducting an electrical signal, and the tuner block.

[0023] The tuner block may comprise at least one differential choke and / or at least one common mode choke.

[0024] These and other aspects and features will now become apparent to those skilled in the art upon review of the following description of specific non-limiting embodiments in conjunction with the accompanying drawings.Brief Description of Drawings

[0025] The detailed description of illustrative (non-limiting) embodiments will be more fully appreciated when taken in conjunction with the accompanying drawings in which:

[0026] Figure l is a circuit diagram of a series tuned circuit.

[0027] Figure 2A is a circuit diagram of a single turn coil with a tuning circuit.

[0028] Figure 2B is a circuit diagram of a double turn coil with a tuning circuit.

[0029] Figure 2C is a circuit diagram of a multi-turn coil with a tuning circuit.

[0030] Figure 3A is a circuit diagram of a tuning sub-block.

[0031] Figure 3B is a representation of the tuning sub-block of Figure 3 A.

[0032] Figure 4 illustrates an embodiment of a tuning block consisting of three tuning sub-blocks with three PIN diode switches and six control lines.

[0033] Figure 5 is a schematic representation of an antenna coil array with a tuning block.

[0034] Figure 6 is a schematic representation of an embodiment of a tuning circuit.

[0035] Figure 7 is a schematic representation of an embodiment of an antenna array with a tuning block.

[0036] Figure 8 is a schematic representation illustrating the common mode current caused at a tuning circuit by the control lines of the circuit.

[0037] Figure 9 is a schematic representation illustrating the antenna current imbalance caused by the common mode current.

[0038] Figure 10 is a schematic representation of an embodiment of a novel tuning block having a common mode choke.

[0039] Figure 11 is a schematic representation illustrating operation and components of an embodiment of a common mode choke.

[0040] Figure 12 is a schematic representation of an embodiment of an antenna array with a novel tuning block that includes a common mode choke.

[0041] Figure 13 is a schematic representation of a prior art switched tuning block.

[0042] Figure 14 is a schematic representation of a novel switched tuning block that includes a common mode choke.

[0043] Figure 15 is a schematic representation of an embodiment of an antenna array with an associated tuning block.

[0044] Figure 16 is a schematic representation of the tuning sub-blocks and tuning elements of an embodiment of a tuning block.

[0045] Figure 17 is a schematic representation of an embodiment of an antenna array with an associated novel tuning block that includes a common mode choke.

[0046] In the drawings, like reference numerals designate similar parts.

[0047] The drawings are not necessarily to scale and may be illustrated by phantom lines, diagrammatic representations and fragmentary views. In certain instances, details that are not necessary for an understanding of the embodiments or that render other details difficult to perceive may have been omitted.Detailed Description

[0048] . Reference will now be made in detail to various non-limiting embodiments of RF antenna tuner blocks for tuning RF antennas. It should be understood that other non-limiting embodiment(s), modifications and equivalents will be evident to one of ordinary skill in the art in view of the non-limiting embodiment s) disclosed herein and that these variants should be considered to be within scope of the appended claims.

[0049] Furthermore, it will be recognized by one of ordinary skill in the art that certain structural and operational details of the non-limiting embodiment(s) discussed hereafter may be modified or omitted (i.e. non-essential) altogether. In other instances, well known methods, procedures, and components have not been described in detail.What is the problem?

[0050] Referring to Figure 2A of the drawings, a single turn coil 210 is shown with the tuning capacitor cancelling the voltage in the coil as shown. In Figure 2B a multiturn coil 220 is shown, and in Figure 2C a 3-turn coil 230 is shown with the middle capacitor 232 variable to adjust the antenna’s tuning. Tracing the voltages around the 3- tum loop we see that each capacitor has +V / 2 and -V / 2 on its terminals. It is also possible to replace the tuning capacitor with a PIN diode tuner where the PIN diode is used as an RF switch to connect and disconnect the tuning capacitors to make adigitally controllable tuning capacitor. A mechanical switch such as a relay could also be used, however for high-speed adjustment, PEST diode switches are preferred. In some cases, the PIN diode switch uses one or more inductors (e.g., an inductor on either side of the PIN diode) for delivering DC bias voltage and current to the PIN diode while the RF signal is blocked and is not able to pass through the inductors.

[0051] Figure 3 A of the drawings illustrates one example of a tuning block 310. The transistor QI shown in Figure 3 A acts as a switch to turn on the diode bias current which is set by the low voltage LV and resistor Rl. When QI is OFF the diode is reverse biased by the high voltage HT and resistor R2. The choke inductors L prevent RF currents from flowing in the bias circuits. The DC blocking capacitors isolate the bias voltages and currents from the rest of the tuner circuits. The isolated tuner has two control lines 322 carrying the bias current and voltages to the PIN diode. The complete tuner will have at least two control lines for each PIN diode switch, as can be understood with reference to the alternative representation 320 of the tuning block 310 as shown in Figure 3B of the drawings.

[0052] As used herein, the term “choke” refers to a single choke used for RF signal suppression. In electrical circuits a choke is used to eliminate RF signals, blocking the high-frequency alternating current (AC) noise while letting the direct current (DC) or lower frequency AC pass. This type of choke is typically an inductor, consisting of wire wound into a coil around a core.

[0053] “Common mode chokes”, on the other hand (also known as common-mode inductors), are designed with two coils of wire wound around a single magnetic core. Each winding is connected in series with one of the conductor wires carrying a differential signal. This arrangement causes the magnetic fields generated by the differential signals to subtract, resulting in a very low impedance for the differential signal. This arrangement also causes the magnetic field generated by common-mode signals to combine, resulting in a high impedance for the common mode signal. The intended differential component can easily pass through the common mode choke, while the common mode component is either reduced significantly or completelyblocked due to the resulting high reactance of the inductor. The common mode choke itself maintains a low electrical resistance, minimising power loss during this process.

[0054] Three PIN diode switches will have six control lines as illustrated for the tuning block 400 shown in Figure 4, and four PIN diode switches will have eight lines. These control lines will pass across the antenna array between the tuner circuit location and the location of the control circuits with the LV and HT circuits.

[0055] This can be understood with reference to the antenna coil array with tuning block circuit 500 as shown in Figure 5 of the drawings. A bus 502 of control lines is shown between the controls 504 and the tuning circuit 506. When the array is active, one or more coils in the array will produce a strong local magnetic field. This field will induce voltages into adjacent conductors and high voltages can be induced onto the control lines. The control lines are bunched tightly to minimise the differential voltage between the lines, however the common mode voltage on the lines is not reduced by bunching the lines. The common mode voltage results in a high RF voltage applied across the RF choke inductors which can lead to RF currents being forced through the choke inductors in the antenna tuner. The terms “tuner” and “tuner block” are used interchangeably.

[0056] The inventors have found that the current (a common mode current), if sufficiently large, causes heating of the choke inductors and an imbalance in the active coils’ current. The “active coil” is understood to mean the coil that is conducting an electrical signal. This heating effect was not known in the prior art for this type of circuit.

[0057] Referring to the tuning circuit 600 in Figure 6 of the drawings, the tuner choke inductors 602 are sized for the voltage expected on the tuning capacitor. When transistor QI is on, both points A and B in the circuit have a "low" impedance. The choke’s L value is chosen to minimise the RF current 1R resulting from voltage VAC.

[0058] Provided I « a>2LC, the current 1R will be small. Typical values for an HF coil are I = 2A, C = 250pF, L = 22uH therefore 1R = 25mA which is very small, being about l% ofl.

[0059] The situation with the common mode voltage induced in the control lines is different and much higher. The voltage induced in the control lines can be several hundred volts. Typical measured values of 200V are observed when the control lines run near an active coil. Consider, for example, the case for a coil with a current I as shown in the circuit 700 of Figure 7. As can be understood with reference to Figure 8 of the drawings, if we consider the coil with current I in isolation, then all of the choke inductors 800 have a common mode voltage HV and the current Ic caused by this in the chokes will beHV i-C ~ ~ 7 j(i)L

[0060] Typical values for ic are given by HV = 200V, L = 22uH therefore ic = 107mA.

[0061] For all inductors the current ic is in phase and adds together. So for N inductors, N ic = Ic is the total common mode current injected into the tuner by the control lines. Hence:N = 6 then Ic = 640mA for 3 PIN diodes; andN = 8 then Ic = 850mA for 4 PIN diodes.

[0062] Note the choke current in caused by the tuning capacitor voltage VAC was only 25mA.

[0063] The choke current caused by the common mode voltage HV on each control line is 103mA. This is four times larger and will cause sixteen times more heat dissipation in the inductor, leading to significant heating of the choke inductors and power loss in the antenna array system. The inventors have discovered that the extra heating comes from the magnetic field of the active coil and appears as a higher lossand higher coil resistance. This is undesirable as extra transmitter power must be delivered to the antenna array coils. Also, the extra loss and heating will vary depending on the physical relationship and / or distance between the active coil and the control lines.

[0064] Another problem is an imbalance in the active coil current and asymmetry in the magnetic field produced by the active coil(s). This is undesirable because the generation of proper 3D magnetic fields requires the active antenna coil currents to be uniform and equal their expected designed values.

[0065] The mechanism of coil current asymmetry may be understood with reference to the circuit 900 illustrated in Figure 9 of the drawings. Due to Kirchhoff s current law the sum of currents into the tuner must be zero. If the control lines inject current Ic into the tuner, then the RF current must become imbalanced by the exact same current Ic.

[0066] Ic+Ii-l2=0. If Ic 0 then Ii + I2 and the coil current either side of the tuner will be imbalanced. The split of Ic between the coil sides will depend on the relative impedance of each coil side. Assuming the coil is highly symmetric and the current Ic splits equally then Ii —> Ii-Ic / 2 and I2 —> I2 + Ic / 2.

[0067] For the case where Ii = I2 = 2A and Ic = 850mA (4 PIN diodes) then Ii = 2A — 1.15A and I2 = 2A — 2.85 A, which is a significant difference in the current in the two halves of the coil.

[0068] The inventors have discovered that it is this large current difference that causes asymmetry in the magnetic field with associated undesirable effects in performance.What is the solution?

[0069] To solve this previously unknown and undefined problem, the inventors have invented a new tuner circuit that includes additional components that reduce the asymmetry described above, thereby improving the performance of the tuning circuit.

[0070] The inventors have found that the problem can be addressed by forcing the RF coil currents that enter and exit the tuner to be equal, and this is done by using a high- performance common mode choke at the input terminals of the tuner.

[0071] Figure 10 of the drawings shows a radio frequency (RF) antenna tuner block 1000 for individually tuning RF antennas 1010. The tuner block comprises a plurality of selectively switchable tuning elements 1020. The tuner block 1000 also comprises an RF common mode choke 1030 that provides an interface between the tuner block 1000 and the RF antennas 1010. The RF common mode choke is positioned between the RF antennas 1010 and input terminals 1022 of the tuner block. This RF common mode choke 1030 is configured to reduce stray currents in the tuning elements of the tuner block. This improves the symmetry of the magnetic field by controlling the current in the tuning circuit, thereby avoiding excess or stray currents, and ensuring that the components of the tuning block do not overheat.

[0072] The RF common mode choke may comprise two conductors wound around a common core. As shown in Figure 11, the common mode choke 1030 consists of two windings 1032 around a common core 1034. The differential RF current travels in opposite directions and the flux subtracts or cancels out so that the field does not oppose the normal mode signal. This arrangement forces the RF signal to follow the designated signal path and not other stray paths.

[0073] The common mode choke 1030 forces Ii to be the same as I2 and hence Ic = 0. Consequently there are no common mode currents Ic in the PIN diode chokes and the heating of these chokes caused by these currents Ic is reduced or avoided altogether, and the extra coil losses and increased coil resistance due to these losses are alsoreduced or eliminated. The coil resistance and the losses are minimised, and the symmetry of the magnetic field is ensured.

[0074] As shown in Figure 12 of the drawings, a common mode choke is added to the tuner, and is positioned between the active coil 1012 and the tuner.

[0075] In contrast to the prior art tuner 1300 shown in Figure 13 where A and B are the connection points of the tuner to the active coil, the common mode choke 1030 of the novel tuning block is positioned between the active coil (A and B) and the tuning block elements 1400 resulting in a new tuner circuit as illustrated in Figure 14 of the drawings.

[0076] The common mode choke of the tuner block may be connected to the RF antennas via at least one RF bus. The RF antennas may comprise pairs of antennas in figure-8 configurations. An exemplary embodiment of a multiplexed antenna 1500 is shown in Figure 15 of the drawings. The multiplexed antenna consists of RF buses 1502 that connect selectable N pairs of loop antennas 1504 to a central adaptive tuning block 1506. The selectable antenna loops are grouped in pairs to create figure-8 antenna structures 1508. This figure-8 type structure produces low emission patterns in the far field. The advantages of using RF buses 1502 include being able to use a single tuner block to service any N pairs of loops and the same impedance being provided for every pair of loop antennas.

[0077] As can be seen in Figure 15, the RF signal enters through the top RF bus and is routed through a selected antenna pair to exit through the lower RF bus. The RF signal enters through the top RF bus and is routed through the upper selected antenna pair to exit through the second RF bus. The RF signal then travels into the tuner block and out through the 3rd RF bus to the lower same selected antenna pair. Lastly, the RF signal exits the lower antenna pair through the lower RF bus and returns to the transmitter source. The RF signal travels the same length, independent of the active selected antenna pair.

[0078] The tuner block may comprise a series of tuning sub-blocks each comprising one or more of: an RF block element, a switching element, and a tuning element. Referring now to Figure 16 of the drawings, the tuner block consists of a series of tuning sub-blocks 1600 connected in series. Each tuning sub-block in turn has one or more RF block elements 1602, switching elements 1604 and / or tuning elements 1606. The purpose of the RF block element is to provide high impedance to the RF signal but to allow the DC control signal to pass through. In normal operation, some of the tuning blocks are electrically turned on and others off by the reader in order to optimise the RF signal going to the antenna. To improve the performance of the tuner block and to reduce stray currents (and the resulting heating of components in the tuner block such as the choke inductors), an inline common mode choke 1700 is provided as illustrated in Figure 17 of the drawings. The common mode choke has the effect of forcing the differential RF signal that enters and exits the tuner block via the choke to be substantially the same (equal) in both directions. This ensures that the RF signal follows the designated signal path, that is travels to the designated antenna pairs and tuning elements, and returns through the choke.

[0079] The inventors were able to confirm that with the common mode choke in place, the RF block elements operate up to 30 degrees cooler, and that the antenna resistance value is the expected value during operation. This means that the field generated by the antenna pairs are identical. The differential RF current travels in opposite directions and the flux subtracts or cancels out so that the field does not oppose the normal mode signal. The new tuning block arrangement forces the RF signal to follow the designated signal path and ensures that the RF signal does not follow other stray paths.

[0080] It is noted that the foregoing has outlined some of the more pertinent nonlimiting embodiments. It will be clear to those skilled in the art that modifications to the disclosed non-limiting embodiment(s) can be effected without departing from the spirit and scope thereof. As such, the described non-limiting embodiment(s) ought to beconsidered to be merely illustrative of some of the more prominent features and applications. Other beneficial results can be realized by applying the non-limiting embodiments in a different manner or modifying them in ways known to those familiar with the art. The mixing and matching of features, elements and / or functions between various non-limiting embodiment(s) is expressly contemplated herein so that one of ordinary skill in the art would appreciate from this disclosure that features, elements and / or functions of one embodiment may be incorporated into another embodiment as appropriate, unless described otherwise, above. Although the description is made for particular arrangements and methods, the intent and concept thereof may be suitable and applicable to other arrangements and applications.

[0081] In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments.

Claims

CLAIMS:

1. A radio frequency (RF) antenna tuner block (1000) for individually tuning RF antennas (1010), the tuner block comprising: a plurality of selectively switchable tuning elements (1020); wherein the tuner block further comprises: an RF common mode choke (1030) providing an interface between the tuner block (1000) and the RF antennas (1010), and the RF common mode choke (103) is configured to reduce stray currents in the tuning elements of the tuner block.

2. The RF antenna tuner block of claim 1, wherein the RF common mode choke comprises two conductors (1032) wound around a common core (1034), and wherein the RF common mode choke is positioned between the RF antennas (1010) and input terminals (1022) of the tuner block.

3. The RF antenna tuner block of claim 1 or claim 2, wherein the common mode choke of the tuner block is connected to the RF antennas via at least one RF bus (1502).

4. The RF antenna tuner block of any one of the preceding claims, wherein the RF antennas comprise pairs of antennas in figure-8 configurations.

5. The RF antenna tuner block of any one of the preceding claims, wherein the tuner block comprises a series of tuning sub-blocks (1600) each comprising one or more of: an RF block element, a switching element, and a tuning element.

6. The RF antenna tuner block of any one of the preceding claims, wherein the common mode choke of the tuner block is positioned between an active coil (1012) that is conducting an electrical signal, and the tuner block.

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