Circuit arrangement and method for phase modulation in a backscattering transponder

This application is related to copending U.S. application Ser. Nos. 10/753,849 and 10/753,859, both filed Jan. 7, 2004, the entire disclosures of which are incorporated herein by reference. This application is further related to the US application entitled “Circuit Arrangement With Simplified Input Circuit For Phase Modulation In A Backscattering Transponder” by U. Friedrich and M. Fischer, being filed Ser. No. 10/855,866 on May 26, 2004, the entire disclosure of which is incorporated herein by reference.

This application is based on and claims the priority under 35 U.S.C. §119 of German Patent Application 103 25 396.3, filed on May 28, 2003, the entire disclosure of which is incorporated herein by reference.

The invention relates to a circuit arrangement as well as a method for producing a phase modulation in a backscattering transponder having a varactor in an input circuit thereof.

There are various known systems in which a wireless or contactless data transmission is carried out between one or more base stations or reader devices and one or more so-called transponders. Such systems are used, for example, as contactless identification systems or so-called RFID (radio frequency identification) systems. In that case, the data to be transmitted relates to an identification protocol, for example. It is further possible to integrate one or more sensors, for example temperature measuring sensors, on the transponder. Such transponders equipped with integrated sensors are then also referred to as remote sensors, whereby the data to be transmitted may comprise the actual values measured by the sensor.

The transponders, or particularly the receiving/backscattering arrangements thereof, typically do not comprise an active transmitter for actively carrying out the data transmission to the base station. Instead, the transponders are non-active systems that can be designated as passive systems if they do not comprise their own power supply, or as semi-passive systems if they do comprise their own power supply. In such non-active systems, the data transmission over a transmission range or distance of substantially more than one meter is generally carried out using a so-called backscattering or backscatter-coupling of UHF waves or microwaves between the receiving/backscattering arrangement (i.e. transponder) and the base station. While such non-active transponders are sometimes also called receiver/transmitters or receiving/transmitting arrangements, this application uses the more accurate term “receiving/backscattering arrangement”, because this arrangement does not actively transmit but rather merely reflects or backscatters the modulated received electromagnetic wave.

In this regard, the base station emits electromagnetic waves, which are received by the transponder, and then modulated by the transponder in accordance with the particular data that are to be transmitted back to the base station. The thusly modulated signal or electromagnetic waves are then reflected or backscattered with the modulation back to the base station. This is generally achieved using an input impedance variation of the receiving/backscattering arrangement, which causes a corresponding variation of the reflection characteristics of an antenna connected thereto. Thereby the reflected or backscattered signal is correspondingly modulated, so that the base station can receive and evaluate the modulated backscattered signal. Various different methods are known for achieving such modulation.

A first known method of carrying out the above described modulation, for example as described in the published European Patent Application EP 1 211 635, the real part of the input impedance is varied by connecting or disconnecting an essentially ohmic (i.e. resistive) load, which causes mainly an amplitude variation or amplitude modulation of the reflected waves. This modulation method is generally referred to as amplitude shift keying (ASK). Disadvantageously, the variable ohmic or resistive load used for the modulation acts as an additional power consumer that loads the voltage supply of the transponder, whereby the maximum operating transmission range between the transponder and the base station is considerably reduced, especially for a passive transponder without its own power supply.

In a second known method of achieving the above discussed modulation, the imaginary part of the input impedance is varied or influenced by varying the capacitance of a capacitor in the input portion or stage of the receiving/transmitting or backscattering arrangement. This causes mainly a phase variation or a phase modulation of the reflected waves, and is generally referred to as phase shift keying (PSK). In comparison to the ASK method, this PSK modulation method practically does not influence the operating voltage, whereby a high operating efficiency of the transponder can be achieved, and the maximum backscattering range between the transponder and the base station is increased.

Such a PSK method as well as a circuit arrangement suitable for carrying out such a method is disclosed, for example in the German Patent Application DE 101 58 442.3 (published Jun. 26, 2003) of the same Assignee as the present application. Also see the counterpart U.S. Published application US 2003/0102961 A1 (published Jun. 5, 2003), the disclosure of which is incorporated herein by reference. According to those publications, a capacitance variation in the input circuit of the device is achieved by means of a varactor, to which a variable control voltage is applied through varactor control means comprising a controllable voltage source. Thereby, in response to the variable voltage applied to the varactor, the capacitance of the varactor is correspondingly varied.

In view of the above, it is an object of the invention to provide a circuit arrangement as well as an associated method which achieve a simple and effective controllability of a varactor in an input circuit of a backscattering transponder device, which is economical in its fabrication, and which makes good efficient use of the total capacitance swing or range of the varactor in order to ensure a reliable and secure data transmission. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.

The above objects have been achieved according to the invention in a circuit arrangement for phase modulation in a backscattering transponder through input impedance variation of an input circuit of the transponder, comprising a varactor in the input circuit of the transponder, and a varactor control arrangement that applies a variable control voltage to the varactor so as to vary the capacitance of the varactor. Particularly according to the invention, the varactor control arrangement comprises at least two voltage sources, a switching arrangement, and a control unit.

The control unit controls the switching arrangement so as to selectively connect the voltage sources with at least one connection terminal of the varactor depending on the respective phase position or phase angle that is to be achieved in the phase modulation. With such an embodiment using a switching arrangement to selectively connect different voltage sources to the varactor, depending on the phase angle to be achieved, the invention can completely avoid the use of a controllable variable voltage source for generating the control voltage. Such a controllable voltage source having a variable voltage output is significantly more complex and more costly than the inventive solution using plural individual voltage sources and a suitable switching arrangement. Thereby, the invention achieves an overall more-economical solution while still providing a high transmission reliability and security.

In a further embodiment of the inventive circuit arrangement, the varactor particularly and preferably has a characteristic response curve of capacitance versus voltage, which is symmetrical, and particularly rotationally symmetrical, about the point of zero voltage. Such varactors are easy and economical to produce, and can provide a large capacitance swing or range with suitable control voltage activation thereof.

In another further embodiment of the circuit arrangement of the invention, the switching arrangement comprises individual switching devices respectively embodied as high impedance CMOS switches (e.g. CMOS transistors). Such switches can be simply and economically implemented in a semiconductor fabrication process, and ensure a relatively small parasitic loading of the input circuit due to parasitic components.

A further preferred feature of the invention is that one of the voltage sources provides the ground voltage while another of the voltage sources provides a supply voltage. Since such voltage sources are already typically present for the voltage supply in a typical transponder, such an embodiment of the invention can be realized with only slight additional effort, cost and complexity compared to previously existing transponders. Alternatively according to the invention, first and second voltage sources can respectively have opposite polarities relative to a ground or zero voltage, whereby the full capacitance swing or range of the varactor is advantageously utilized.

In a particular preferred and advantageous embodiment of the inventive circuit arrangement, the input circuit of the transponder device includes a first capacitor, the varactor, and a second capacitor connected in series between two antenna connection terminals, and each of the two terminals of the varactor is respectively selectively connectable via a respective switching device to either one of the voltage sources. Through such a circuit arrangement, the capacitance swing or range of the varactor can be fully utilized without requiring a negative voltage to be generated and applied, because the voltage applied to the varactor can be selectively switched-over or reversed in polarity by means of suitable closing and opening of the switching devices.

An alternative preferred and advantageous embodiment of the inventive circuit arrangement includes a first capacitor and the varactor connected in series between two antenna connection terminals in the input circuit of the transponder device, wherein one terminal of the varactor is connected with a first voltage source via a first switching device and with a second voltage source via a second switching device. Each voltage source can respectively produce an associated phase position or phase angle of the phase modulation resulting from the consequent capacitance variation of the varactor. Thus, in other words, the two voltage sources can provide, in total, two phase angles of the modulation. It should be understood that additional voltage sources and associated switching devices could be provided in the circuit arrangement in order to respectively produce additional different phase angles.

The above objects have further been achieved according to the invention in a method of producing a phase modulation in a backscattering transponder by varying the input impedance of an input circuit of the transponder. In order to vary the input impedance, the capacitance of a varactor is varied by applying a suitable varied control voltage to a terminal or terminals of the varactor. Particularly according to the invention, the control voltage is generated by selectively connecting a respective one of at least two voltage sources with at least one terminal of the varactor dependent on the phase position or angle that is to be produced in the phase modulation.

In a further preferred embodiment of the inventive method, to generate or produce a first phase position, a first terminal of the varactor is connected with a first one of the at least two voltage sources and a second terminal of the varactor is connected with a second one of the at least two voltage sources, and to generate or produce a second phase position, the first terminal of the varactor is connected with the second voltage source and the second terminal of the varactor is connected with the first voltage source. In this manner, the inventive method achieves a pole reversal of the control voltage being applied to the varactor, whereby the capacitance swing or range of the varactor is well utilized. This is especially true for varactors of a type having a capacitance-voltage characteristic curve that is symmetrical with respect to zero voltage.

It is further advantageously provided according to the invention, that a third phase angle or position can be achieved by connecting both of the terminals of the varactor with the same one of the voltage sources. In this manner, the resulting potential difference across the varactor is 0 V, which leads to an associated third varactor capacitance as well as a corresponding third phase angle or position.

Another embodiment of the inventive method involves producing a first phase angle by connecting a terminal of the varactor with a first one of the voltage sources, and producing a second phase angle by instead connecting that terminal of the varactor with a second one of the voltage sources. In this context, the first and second voltage sources advantageously have a reversed polarity relative to each other, such that the full capacitance swing or range of the varactor is well utilized.

In order that the invention may be clearly understood, it will now be described in connection with example embodiments thereof, with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic block circuit diagram of a first embodiment of an inventive circuit arrangement for phase modulation in a backscattering transponder;

FIG. 2 is a schematic diagram representing the capacitance-voltage characteristic curve of a CMOS varactor used in the circuit according to FIG. 1 ; and

FIG. 3 is a schematic block circuit diagram of a second embodiment of an inventive circuit arrangement for phase modulation in a backscattering transponder.

The circuit arrangement for phase modulation in a backscattering transponder as shown in FIG. 1 is a portion of an input circuit of the transponder, the rest of which is not shown. The inventive circuit arrangement comprises a capacitor C 11 and a CMOS varactor C 12 connected in series between two antenna terminals A 11 and A 12 , a first voltage source U 11 that is selectively connectable via a switching device S 11 to a terminal N 11 of the varactor C 12 (e.g. the terminal N 11 between the varactor C 12 and the capacitor C 11 ), and a second voltage source U 12 that is selectively connectable via a second switching device S 12 to the varactor terminal N 11 . Each of the switching devices S 11 and S 12 is preferably embodied as a high impedance CMOS switch. The circuit arrangement further comprises a control unit SE 1 that is connected to and controls the switching devices S 11 and S 12 .

The switching devices S 11 and S 12 together form a switching arrangement. The voltage sources U 11 and U 12 , the switching devices S 11 and S 12 , as well as the control unit SE 1 , together form a varactor control arrangement for generating and selectively applying a control voltage to the varactor C 12 . More particularly, the control unit SE 1 selectively controls the switching devices S 11 and S 12 in response to and dependent on the phase position or phase angle that is to be achieved in the phase modulation respectively at any given time, so as to correspondingly connect the voltage sources U 11 and U 12 respectively through the switching devices S 11 and S 12 to the varactor terminal N 11 . Namely, depending on the switching position of the switching devices S 11 and S 12 , either the voltage of the first voltage source U 11 or the voltage of the second voltage source U 12 will be applied to the terminal N 11 of the varactor C 12 . It should be understood that only one of the switching devices S 11 and S 12 will be closed or switched conductive at any given time. Preferably, the voltage source U 11 provides a negative voltage while the voltage source U 12 provides a positive voltage.

FIG. 2 shows the characteristic curve K of the capacitance C versus voltage U of the CMOS varactor C 12 . Note that this curve K is substantially point symmetrical or rotationally symmetrical about the intersection point of the curve K with the vertical C axis (i.e. the point of zero voltage). Due to the different (especially opposite) polarities of the voltages of the voltage sources U 11 and U 12 , a large capacitance difference and thus a large capacitance swing or range of the varactor results as negative and positive voltages are applied thereto, as can be seen in FIG. 2 . The resulting phase positions of the arising phase modulation thus also comprise a correspondingly large difference therebetween, so that a reliable and secure data transmission is ensured. Namely, this large capacitance difference between the respective different control states of the varactor ensures correspondingly large differences in the input circuit impedance of the transponder device, which in turn assures correspondingly large differences in the resulting phase shift achieved by the phase modulation in the backscattering.

In the present illustrated example using two distinct voltage sources U 11 and U 12 selectively connected to the varactor C 12 , two different phase positions or phase angles can be achieved in the phase modulation provided by the input circuit of the backscattering transponder device. It should be understood that further phase positions could also be achieved by adding further unique discrete voltage sources and respective associated switching devices (e.g. a third voltage source selectively connectable to the varactor terminal N 11 through a third switching device).

As mentioned above, the control unit SE 1 controls the switching devices S 11 and S 12 in response to and dependent on a control input that depends on or represents the respective phase positions to be produced in the phase modulation, e.g. a data input D representing the data to be phase-modulated in the backscattering. In other words, for example, the control unit SE 1 selectively connects either one or the other of the voltage sources U 11 and U 12 to the varactor C 12 in response to and dependent on a bit sequence of data that is provided to the data input D and that is to be transmitted via the backscattering.

FIG. 3 schematically shows a second embodiment of an inventive circuit arrangement for phase modulation in a backscattering transponder. This circuit arrangement comprises a first capacitor C 21 , a varactor C 22 , and a second capacitor C 23 connected in series, with the varactor between the two capacitors, between two antenna terminals A 21 and A 22 . The circuit arrangement further comprises a first voltage source U 21 and a second voltage source U 22 , which are connected selectively via switching devices S 21 , S 22 , S 23 and S 24 to the connection terminals N 21 and N 22 of the varactor C 22 , and a control unit SE 2 that is connected to the switching devices S 21 , S 22 , S 23 and S 24 for controlling these switching devices. If the varactor C 22 is embodied as a suitably connected field effect transistor, the varactor terminal N 21 may, for example be a gate terminal and the varactor terminal N 22 may be a drain/source terminal of the field effect transistor. The voltage source U 21 provides the ground voltage, while the voltage source U 22 provides a supply voltage, which simultaneously may serve as a voltage supply of the transponder, i.e. of the control unit SE 2 and all other powered components of the transponder (not shown). The control unit may have any conventionally understood construction for achieving the specified control operations or functions. The characteristic curve of the varactor C 22 may be identical to the curve K shown in FIG. 2 for the varactor C 12 of FIG. 1 .

With the switching arrangement in the embodiment of FIG. 3 , either one of the two voltage sources U 21 and U 22 can be selectively connected to either one of the varactor terminals N 21 and N 22 on opposite sides of the varactor C 22 . For generating a first phase position of the phase modulation, the switching devices S 23 and S 22 are closed and the switching devices S 21 and S 24 are opened, such that the voltage source U 21 is connected to the varactor terminal or node N 21 and the voltage source U 22 is connected to the varactor terminal or node N 22 . The resulting voltage difference across the varactor C 22 is given by the difference of the voltage values of the two voltage sources U 21 and U 22 . Since U 21 provides the ground voltage and U 22 provides the supply voltage, it follows that the supply voltage is applied to the varactor C 22 between the terminals N 22 and N 21 . The associated capacitance of the varactor C 22 is given by the characteristic curve K shown in FIG. 2 , for the supply voltage.

Furthermore, it should be recognized that a short circuit of high frequency received signals through the switching devices to ground or to the supply voltage will not occur due to the high impedance, e.g. highly ohmic character, of the switching devices S 21 , S 22 , S 23 and S 24 . Also, a d.c. decoupling or blocking of the voltage sources U 21 and U 22 relative to other circuit components is further provided by the capacitors C 21 and C 23 .

For generating a second phase position or phase angle of the phase modulation, the control unit SE 2 opens the switches S 23 and S 22 and closes the switches S 21 and S 24 . Thereby, the voltage source U 22 is connected with the varactor terminal or node N 21 , while the voltage source U 21 is connected with the varactor terminal or node N 22 . It should be recognized that the voltage difference arising on the varactor C 22 in this switching condition is inverted or reversed relative to the above described opposite switching condition. Namely, the ground and supply voltage have now been switched to opposite sides of the varactor C 22 . In connection with the symmetrical characteristic curve K of the capacitance-voltage characteristic of the varactor C 22 , this achieves an optimum utilization of the capacitance swing or range of the varactor C 22 without requiring the generation of a negative voltage. This is achieved, because the capacitance of the CMOS varactor C 22 is essentially only dependent on the voltage value and the polarity of the voltage applied between the varactor terminals or nodes N 21 and N 22 .

In order to generate a third phase position or angle in the phase modulation using the circuit according to FIG. 3 , both of the varactor terminals N 21 and N 22 are connected with one of the voltage sources U 21 or U 22 . In other words, the switching control unit SE 2 either closes the switching devices S 23 and S 21 and opens the switching devices S 24 and S 22 , or alternatively opens the switching devices S 23 and S 21 and closes the switching devices S 24 and S 22 . Since in either case the same voltage is applied to both sides of the varactor, the resulting voltage difference across the varactor C 22 will be 0 V. Accordingly, this produces a capacitance of the varactor and a corresponding impedance of the input circuit between the values achieved in the above described two opposite switching positions, and thus achieves a third phase position between the above described first and second phase positions in the phase modulation.

As in the embodiment of FIG. 1 , the switching control unit SE 2 controls the switching devices S 21 , S 22 , S 23 and S 24 in response to and dependent on a control input, such as a data input D representing the bit sequence of data that is to be transmitted via the phase modulation of the backscattering.

The illustrated embodiments provide a simple circuit arrangement and method for activating and controlling the varactor C 22 by means of circuit-connected voltage sources for generating a variable control voltage, without using a controllable variable voltage source. In other words, the present invention can make use of simpler, less costly individual voltage sources that each have a respective fixed or nominal voltage value, and avoids the need for a more complex and more costly controllable voltage source having a controllable variable voltage output. Nonetheless, the invention makes optimal utilization of the full available capacitance swing or range of the varactor, so that a reliable and secure data transmission can be achieved. Moreover, it is possible to provide two, three or even more phase positions of the phase modulation, simply by providing the corresponding number of voltage sources along with suitable switching devices.

Although the invention has been described with reference to specific example embodiments, it will be appreciated that it is intended to cover all modifications and equivalents within the scope of the appended claims. It should also be understood that the present disclosure includes all possible combinations of any individual features recited in any of the appended claims.