Resonant circuit tuning system using magnetic field coupled reactive elements

Abstract

A resonant circuit tuning system and a method for tuning are provided. The resonant circuit tuning system may include an LCR circuit and a reactive element magnetically coupled to the LCR circuit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, together with other objects, features and advantages, reference should be made to the following detailed description which should be read in conjunction with the following figures wherein like numerals represent like parts.

FIG. 1 is a block diagram of a resonant circuit tuning system constructed in accordance with an embodiment of the invention having a reactive element.

FIG. 2 is a block diagram of a resonant circuit tuning system constructed in accordance with an embodiment of the invention having a resistive and a capacitive element.

FIG. 3 is a block diagram of a resonant circuit tuning system constructed in accordance with an embodiment of the invention having a resistive and an inductive element.

FIG. 4 is a block diagram of a resonant circuit tuning system constructed in accordance with an embodiment of the invention having a plurality of reactive elements.

FIG. 5 is a block diagram of a resonant circuit tuning system constructed in accordance with an embodiment of the invention having a reactive element and a plurality of taps.

FIG. 6 is a block diagram of a resonant circuit tuning system constructed in accordance with an embodiment of the invention having a plurality of reactive elements and a plurality of magnetically coupled windings.

FIG. 7 is a block diagram of a resonant circuit tuning system constructed in accordance with an embodiment of the invention having a reactive element magnetically coupled to the windings of an LCR circuit.

FIG. 8 is a block diagram of a resonant circuit tuning system constructed in accordance with an embodiment of the invention having a variable inductive element.

FIG. 9 is a block diagram of a resonant circuit tuning system constructed in accordance with an embodiment of the invention having a variable capacitive element.

FIG. 10 is a block diagram of a resonant circuit tuning system constructed in accordance with an embodiment of the invention having a variable capacitive element and a variable resistive element.

FIG. 11 is a block diagram of a resonant circuit tuning system constructed in accordance with an embodiment of the invention having a variable inductive element and a variable resistive element.

DETAILED DESCRIPTION OF THE INVENTION

For simplicity and ease of explanation, the invention will be described herein in connection with various embodiments thereof. Those skilled in the art will recognize, however, that the features and advantages of the various embodiment of the invention may be implemented in a variety of configurations. It is to be understood, therefore, that the embodiments described herein are presented by way of illustration, not of limitation.

Various embodiments of the invention provide a system and method for tuning an LCR circuit using one or more magnetically coupled reactive elements and/or resistive elements. It should be noted that the tuning system and method may be used in connection with any type of electronic system, for example, in electronic systems wherein a coil is used as either a transmitter or receiver. The tuning system and method also may be used in different types of applications, for example, Electronic Article Surveillance (EAS), Radio Frequency Identification (RFID), metal detectors, magnetic imaging systems, remote sensing, communications, etc. However, the various embodiments may be implemented in other applications for use with different electronic devices as desired or needed.

FIG. 1 illustrates a resonant circuit tuning system 30 constructed in accordance with an embodiment of the invention and may include an LCR circuit 32 magnetically coupled to a reactive element 34 with a magnetically coupled winding 36. The LCR circuit 32 may be configured, for example, as a transmitting or receiving antenna, such as, an antenna for an EAS antenna pedestal. Further, the magnetically coupled winding 36 may be any type of magnetically coupled element, for example, any type of magnetic field coupled element. Additionally, the reactive element 34 may be any type of element providing reactance, for example, one or more capacitive elements and/or one or more inductive elements.

The LCR circuit may be a parallel and/or series circuit, and in one embodiment, may include a first capacitive element 38 in series with a parallel combination of a second capacitive element 40 and an inductive element 42. The magnetically coupled winding 36 may include one or more turns that are magnetically coupled to the inductive element 42 of the LCR circuit 32 with the reactive element 34 connected to the magnetically coupled winding 36.

It should be noted that when reference is made herein to a capacitive element, inductive element, resistive element or other element, these elements may be provided, modified or replaced with an equivalent element. For example, when an embodiment is shown having a capacitive element, this may include one or more capacitors or elements providing capacitance. Similarly, and for example, when an embodiment is shown having an inductive element, this may include one or more inductors or elements providing inductance. Also, similarly, and for example, when an embodiment is shown having a resistive element, this may include one or more resistors or elements providing resistance.

The resonant circuit tuning system 30 also may include a controller 44 connected to the reactive element 34 via a switch 46. The controller 44 is configured to control the switch 46, and more particularly, to switch between an on state (connected state) and an off state (disconnected state) to reactively load the LCR circuit 32. The switching of the switch 46 by the controller 44 may be manual, for example, controlled by an operator or user, or may be automatic, for example, controlled by a system controller or program. It should be noted that the switch 46 may be any kind of switching element, for example, switching transistors.

The resonant circuit tuning system 30 also may include and be connected to a communication device 48, for example, a transmitter or receiver. In operation, the switching of the reactive element 34, which may be referred to as a tuning reactance, to reactively load the LCR circuit 32, adjusts the tuning of the LCR circuit 32. The tuning of the communication device 48 connected to the LCR circuit 32 is also thereby adjusted.

FIG. 2 illustrates a resonant circuit tuning system 50 constructed in accordance with another embodiment of the invention and may include an LCR circuit 52 magnetically coupled to a capacitive element 54 (C₂), for example, a loading capacitor via a magnetically coupled winding 56. The LCR circuit 52 may be configured in a series configuration having a capacitive element 58 (C₁), a resistive element 60 (R₁) and an inductive element 62 (L₁). The inductive element 62 is may be referred to as a primary inductance and the capacitive element 58 may be referred to as a resonant capacitance. The magnetically coupled winding 56 may include an inductive element 64 (L₂) and a resistive element 66 (R₂). The inductive element 64 of the magnetically coupled winding 56 is coupled (e.g., magnetically coupled) to the inductive element 62 of the LCR circuit 52 with a coupling coefficient k. The LCR circuit 52 also may be connected to a voltage source 68 (V_s).

The operation and operating characteristics of the resonant circuit tuning system 50 will now be described. This description can be similarly applied to the other various embodiments of resonant circuit tuning systems described herein. In particular, the impedance of the LCR circuit 52 at the voltage source 68 is shown in Equation 1:

$\begin{array}{cc}Z=\frac{V_s}{I_1}& \left(1\right)\end{array}$

Solving for Z from Equation 1, a reduced form of Equation 1 results as follows:

$\begin{array}{cc}Z=\lfloor R_{\mathrm{series}}+R_{\mathrm{coupled}}\rfloor +j·\lfloor X_{\mathrm{series}}+X_{\mathrm{coupled}}\rfloor \mathrm{where}\text{:}& \left(2\right)\\ R_{\mathrm{series}}=R_1& \left(3\right)\\ R_{\mathrm{coupled}}=\frac{{\omega }^2·k^2·L_1·L_2·R_2}{R_2^2+{\left(\omega ·L_2-\frac{1}{\omega ·C_2}\right)}^2}& \left(4\right)\\ X_{\mathrm{series}}=\omega ·L_1-\frac{1}{\omega ·C_1}& \left(5\right)\\ X_{\mathrm{coupled}}=-\frac{{\omega }^2·k^2·L_1·L_2·\left(\omega ·L_2-\frac{1}{\omega ·C_2}\right)}{R_2^2+{\left(\omega ·L_2-\frac{1}{\omega ·C_2}\right)}^2}& \left(6\right)\end{array}$

The resonant frequency of the coupled circuit occurs when the total reactance of Equation 2 is zero:

X _total =X _series +X _coupled=0  (7)

In operation, and for example, the inductance of the inductive element 64, which in one embodiment is a tuning winding, is selected to have a value much lower than the inductance of the inductive element 62. The capacitive element 54 may be selected to have approximately the same magnitude as the capacitive element 58 and adjusted by a controller (not shown) for tuning purposes to be either greater than or less than the capacitive element 52, for example, as needed or desired for tuning purposes.

If the tuning winding, namely inductive element 64, is open circuited, the resonant frequency of the main winding, namely inductive element 62, will occur when reactance of a series winding X_series=0, which occurs at:

$\begin{array}{cc}{\omega }_{\mathrm{res}}=\frac{1}{\sqrt{L1·C1}}& \left(8\right)\end{array}$

As an example, for typical antenna circuits and tuning windings, the capacitive element 54 dominates both the resistance of the resistive element 66 and the inductive reactance of the inductive element 62 as follows:

$\begin{array}{cc}\frac{1}{{\omega }_{\mathrm{res}}·C2}{\omega }_{\mathrm{res}}·L2\mathrm{and}& \left(9\right)\\ \frac{1}{{\omega }_{\mathrm{res}}·C2}R2& \left(10\right)\\ X_{\mathrm{total}}\cong \left(\omega ·L_1-\frac{1}{\omega ·C_1}\right)+{\omega }^3·k^2·L_1·L_2·C_2& \left(11\right)\end{array}$

which reduces to:

$\begin{array}{cc}{X}_{\mathrm{total}}\cong {\omega }^2-\frac{1}{L_1·C_1}+{\omega }^4·k^2·L_2·C_2& \left(12\right)\end{array}$

Thus, the resonance occurs when X_total=0. Finding the roots of the equation yields:

$\begin{array}{cc}{\omega }_{\mathrm{new}}\cong \sqrt{\frac{-1+\sqrt{1+4·k^2\frac{L_2·C_2}{L_1·C_1}}}{2·k^2·L_2·C_2}}\mathrm{for}k,L_2,C_2\ne 0;L_2L_1\mathrm{and}& \left(13\right)\\ {\omega }_{\mathrm{new}}=\sqrt{\frac{1}{L_1·C_1}}\mathrm{for}k=0& \left(14\right)\end{array}$

Additionally, it can be shown that at the new resonant frequency the resonant impedance is:

Z _total(ω_new)=R₁+(ω_new⁴·k²·L₁·L₂·C₂²)·R₂  (15)

or expressed in terms of the reactances of the mutual inductance and the tuning capacitance:

$\begin{array}{cc}{Z}_{\mathrm{total}}\left({\omega }_{\mathrm{new}}\right)=R_1+{\left(\frac{X_{M_{12}}\left({\omega }_{\mathrm{new}}\right)}{X_{C_2}\left({\omega }_{\mathrm{new}}\right)}\right)}^2·R_2& \left(16\right)\\ \mathrm{where}\text{:}X_{M_{12}}\left(\omega \right)=\omega ·k·\sqrt{L_1·L_2}& \left(17\right)\\ \mathrm{and}X_{C_2}\left(\omega \right)=\frac{1}{\omega ·C_2}& \left(18\right)\end{array}$

From these equations, it can be seen that the increase of real impedance to the circuit from the resistive element 66 is very small when X_C2>>X_M12.

In another embodiment as shown in FIG. 3, a resonant circuit tuning system 70 is provided that is similar to the resonant circuit tuning system 50 (shown in FIG. 2), and accordingly, like reference numerals identify like components. Unlike the resonant circuit tuning system 50, the capacitive element 54 may be replaced with an inductive element 72 (L₃). Using a similar analytical technique as described above with respect to the resonant circuit tuning system 50, the impedance at the voltage source 68 is:

$\begin{array}{cc}{Z}_{\mathrm{total}}=R_{\mathrm{series}}+R_{\mathrm{coupled}}+j·\left[X_{\mathrm{series}}+X_{\mathrm{reactive}}\right]\mathrm{where}\text{:}& \left(19\right)\\ R_{\mathrm{series}}=R_1& \left(20\right)\\ R_{\mathrm{coupled}}=\frac{{\omega }^2·k^2·L_1·L_2·R_2}{R_2^2+{\omega }^2·{\left(L_1+L_2\right)}^2}& \left(21\right)\\ X_{\mathrm{series}}=\omega ·L_1-\frac{1}{\omega ·C_1}& \left(22\right)\\ X_{\mathrm{coupled}}=-\frac{{\omega }^2·k^2·L_1·L_2·\left[\left(\omega ·L_1+L_2\right)\right]}{R_2^2+{\omega }^2·{\left(L_1+L_2\right)}^2}& \left(23\right)\end{array}$

Again, for many applications, the following assumptions are made:

L₃>>L₂  (24)

and

ω·L₃>>R₂  (25)

solving for the resonant frequency, as described above, results in the following:

$\begin{array}{cc}{\omega }_{\mathrm{new}}\cong \sqrt{\frac{1}{\left(1-k^2·\frac{L_2}{L_3}\right)·L_1·C_1}}& \left(26\right)\end{array}$

and the impedance at the resonant frequency is approximately:

$\begin{array}{cc}{Z}_{\mathrm{res}}\cong R_1+\frac{k^2·L_1·L_2·R_2}{L_3^2}\mathrm{when}& \left(27\right)\\ L_3L_2& \left(28\right)\end{array}$

It should be noted that the solution for the resonant frequency of a parallel LCR circuit can be estimated using, for example, circuit simulation software such as SPICE (Simulation Program with Integrated Circuit Emphasis), a product commercially available from many sources, or graphically solving for the impedance.

In another embodiment as shown in FIG. 4, a resonant circuit tuning system 80 is provided that is similar to the resonant circuit tuning system 30 (shown in FIG. 1), and accordingly, like reference numerals identify like components. Unlike the resonant circuit tuning system 30, the reactive element 34 may be replaced with a plurality of reactive elements 84. The controller 44 is configured to control a plurality of switches 82, one corresponding to each of the reactive elements 82, and more particularly, to switch between an on state (connected state) and an off state (disconnected state) to reactively load the LCR circuit 32. The switching of the switches 82 by the controller 44 may be manual, for example, controlled by an operator or user, or may be automatic, for example, controlled by a system program.

In another embodiment as shown in FIG. 5, a resonant circuit tuning system 90 is provided that is similar to the resonant circuit tuning system 30 (shown in FIG. 1), and accordingly, like reference numerals identify like components. Unlike the resonant circuit tuning system 30, the reactive element 34 may be connected to a plurality of taps. More particularly, the reactive element 34 may be connected to a plurality of taps 82 that provides tapping of the reactive element 34 to the magnetically coupled winding 36. The tapping allows, for example, for selection of a different number of turns or windings of the magnetically coupled winding 36 to be included in an active portion of the magnetically coupled winding 36. It should be noted that more than one tap 82 with a corresponding switching element may be provided to a single winding.

In operation, the controller 44 connects the reactive element 34 to one or more taps 82 of the magnetically coupled winding 36. Each of the taps 82 provides a different coupling of the reactive element 34 to the LCR circuit 32. The controller 44 may adjust the tuning of the LCR circuit 32 by connecting the reactive element 34 to different taps 82 in the magnetically coupled winding 36.

In another embodiment as shown in FIG. 6, a resonant circuit tuning system 100 is provided that is similar to the resonant circuit tuning system 30 (shown in FIG. 1), and accordingly, like reference numerals identify like components. Unlike the resonant circuit tuning system 30, another reactive element 102 is provided, with the LCR circuit 32 magnetically coupled to the reactive element 102 with a magnetically coupled winding 104. The controller 44 is connected to the reactive element 104 via a switch 106. In this embodiment, the controller 44 is configured to control the switch 46 and switch 106 to adjust tuning of the LCR circuit 32. More particularly, the reactive elements 34 and 102 are magnetically coupled to the LCR circuit 32 with the magnetically coupled winding 36 and the magnetically coupled winding 104, respectively. It should be noted that additional reactive elements may be added to the resonant circuit tuning system 100 in a similar manner.

In another embodiment as shown in FIG. 7, a resonant circuit tuning system 110 is provided that is similar to the resonant circuit tuning system 30 (shown in FIG. 1) and accordingly, like reference numerals identify like components. Unlike the resonant circuit tuning system 30, the resonant circuit tuning system 110 includes a plurality of taps 112 on the windings 114 of the inductive element 42 of the LCR circuit 32 and does not include the magnetically coupled winding 36. The reactive element 34 may be connected to the plurality of taps 112 that provide tapping of the reactive element 34 to the windings 114 of the inductive element 42. The tapping allows, for example, for selection of a different number of turns or windings 114 of the inductive element 42 to be included in an active portion of the resonant circuit tuning system 110.

In operation, the controller 44 connects the reactive element 34 to one or more taps 112 of the inductive element 42. Each of the taps 112 provides a different coupling of the reactive element 34 to the LCR circuit 32. The controller 44 may adjust the tuning of the LCR circuit 32 by connecting the reactive element 34 to different taps 112 in the inductive element 42. As an example, the windings of, for example, an antenna may be used in this embodiment to magnetically couple the reactive element 34.

In another embodiment as shown in FIG. 8, a resonant circuit tuning system 120 is provided that is similar to the resonant circuit tuning system 30 (shown in FIG. 1) and accordingly, like reference numerals identify like components. In this embodiment, the reactive element is a variable inductive element, such as a variable inductor 122 and the controller 44 may be configured to control the operation of the switch 46 and to vary the inductance of the variable inductor 122. For example, separate control lines providing separate control signals may be included. In operation, the controller 44 may be configured to switch between an on state (connected state) and an off state (disconnected state) of the variable inductor 122, as well as adjust the inductive value of the variable inductor 122 to provide variable adjustment to the tuning of the LCR circuit 32.

In another embodiment as shown in FIG. 9, a resonant circuit tuning system 130 is provided that is similar to the resonant circuit tuning system 30 (shown in FIG. 1) and accordingly, like reference numerals identify like components. In this embodiment, the reactive element is a variable capacitive element, such as a variable capacitor 132 also referred to as a varactor. In this embodiment, the controller 44 may be configured to control the operation of the switch 46 and to vary the capacitance of the variable capacitor 132. For example, separate control lines providing separate control signals may be included. In operation, the controller 44 may be configured to switch between an on state (connected state) and an off state (disconnected state) of the variable capacitor, as well as adjust the capacitive value of the variable capacitor 132 to provide variable adjustment to the tuning of the LCR circuit 32.

In another embodiment as shown in FIG. 10, a resonant circuit tuning system 140 is provided that is similar to the resonant circuit tuning system 130 (shown in FIG. 9) and accordingly, like reference numerals identify like components. In this embodiment, a variable resistive element, such as a variable resistor 142 is also provided. In this embodiment the controller 44 may be configured to control the operation of the switch 46 and to vary the capacitance of the variable capacitor 132 and the resistance of the variable resistor 142. For example, separate control lines providing separate control signals may be included. In operation, the controller 44 may be configured to switch between an on state (connected state) and an off state (disconnected state) of the variable capacitor 132 and variable resistor 142, which may be provided in a parallel connection. The controller 44 also may be configured to adjust the capacitive value of the variable capacitor 132 and the resistive value of the variable resistor 142 to provide variable adjustment to the tuning of the LCR circuit 32. Specifically, the Q, the resonant frequency, or both of the LCR circuit 32 may be adjusted.

In another embodiment as shown in FIG. 11, a resonant circuit tuning system 150 is provided that is similar to the resonant circuit tuning system 120 (shown in FIG. 8) and accordingly, like reference numerals identify like components. In this embodiment, a variable resistive element, such as a variable resistor 152 is also provided. In this embodiment, the controller 44 may be configured to control the operation of the switch 46 and to vary the inductance of the variable inductor 122 and the resistance of the variable resistive element 152. For example, separate control lines providing separate control signals may be included. In operation, the controller 44 may be configured to switch between an on state (connected state) and an off state (disconnected state) of the variable inductor 122 and variable resistor 152, which may be provided in a parallel connection. The controller 44 also may be configured to adjust the inductive value of the variable inductor 122 and the resistive value of the variable resistor 152 to provide variable adjustment to the tuning of the LCR circuit 32. Specifically, the Q, the resonant frequency, or both of the LCR circuit 32 may be adjusted.

Thus, various embodiments of the invention provide a resonant circuit tuning system wherein one or more of a reactive element, inductive element and resistive element are magnetically coupled to an LCR circuit to provided tuning thereof. The coupled elements may be variable to provide variable adjustment of the Q, resonant frequency, or both of the LCR circuit.

It is to be understood that variations and modifications of the present invention can be made without departing from the scope of the invention. It is also to be understood that the scope of the invention is not to be interpreted as limited to the specific embodiments disclosed herein, but only in accordance with the appended claims when read in light of the forgoing disclosure.

Claims

1. A resonant circuit tuning system comprising: an LCR circuit; anda reactive element magnetically coupled to the LCR circuit.

2. A resonant circuit tuning system in accordance with claim 1 wherein the reactive element comprises at least one of an inductive element and a capacitive element.

3. A resonant circuit tuning system in accordance with claim 1 wherein the reactive element comprises at least one of a variable inductive element and a variable capacitive element.

4. A resonant circuit tuning system in accordance with claim 1 further comprising a resistive element magnetically coupled to the LCR circuit.

5. A resonant circuit tuning system in accordance with claim 4 wherein the resistive element comprises a variable resistive element.

6. A resonant circuit tuning system in accordance with claim 1 wherein the LCR circuit is configured in at least one of a series and a parallel arrangement.

7. A resonant circuit tuning system in accordance with claim 1 further comprising at least one magnetically coupled winding coupling the reactive element to the LCR circuit.

8. A resonant circuit tuning system in accordance with claim 1 further comprising at least one of a transmitter and receiver connected to the LCR circuit.

9. A resonant circuit tuning system in accordance with claim 1 wherein the LCR circuit comprises an antenna configured to provide at least one of transmission and reception.

10. A resonant circuit tuning system in accordance with claim 1 further comprising a controller connected to the reactive element and configured to control the operation of the reactive element.

11. A resonant circuit tuning system in accordance with claim 8 further comprising a switch connected to the controller to control switching of the reactive element.

12. A resonant circuit tuning system in accordance with claim 10 further comprising a plurality of reactive elements.

13. A resonant circuit tuning system in accordance with claim 10 further comprising a plurality of resistive elements

14. A resonant circuit tuning system in accordance with claim 1 further comprising a plurality of taps connecting the reactive element to at least one coil of the LCR circuit.

15. A resonant circuit tuning system in accordance with claim 1 further comprising a plurality of taps connecting the reactive element to at least one coil of the LCR circuit, the plurality of taps connected to an inductor winding of the LCR circuit.

16. A resonant circuit tuning system in accordance with claim 1 wherein the reactive element is magnetically coupled to an inductive winding of the LCR circuit.

17. A resonant circuit tuning system in accordance with claim 1 further comprising a resistive element magnetically coupled to the LCR circuit and a controller configured to control at least one of a resonant frequency and a Q value of the LCR circuit using the reactive element and the resistive element.

18. A resonant circuit tuning system in accordance with claim 1 wherein the LCR circuit is configured to operate in connection with an Electronic Article Surveillance (EAS) system.

19. An electronic article surveillance (EAS) system comprising: at least one of a transmitter and a receiver;at least one antenna connected to the at least one transmitter and receiver; anda tuning circuit configured to tune the at least one antenna, the tuning circuit comprising at least one reactive element magnetically coupled to the antenna.

20. An EAS system in accordance with claim 19 further comprising a controller configured to control at least one of (i) switching the reactive element and (ii) varying a level of the reactive element.

21. An EAS system in accordance with claim 19 wherein the tuning circuit further comprises at least one resistive element magnetically coupled to the antenna.

22. An EAS system in accordance with claim 21 further comprising a controller configured to control at least one of (i) switching the resistive element and (ii) varying a level of the resistive element.

23. A method for tuning an LCR circuit, the method comprising: magnetically coupling a reactive element to an inductor of the LCR circuit; andcontrolling a resonant frequency of the LCR circuit using the reactive element.

24. A method in accordance with claim 23 wherein the magnetically coupling comprises tapping the reactive element to coils of the inductor of the LCR circuit.

25. A method in accordance with claim 23 wherein the LCR circuit further comprises an antenna.

26. A method in accordance with claim 23 further comprising magnetically coupling a resistive element to an inductor of the LCR circuit and controlling a Q value of the LCR circuit using the resistive element.