Method for tuning an oscillating receiver circuit of a transponder built into a rfid system

The invention relates to a method for matching a receiving oscillating circuit Of a transponder in an RFID system to resonance to the frequency Of the interrogation signal generated by a reader, the receiving oscillating circuit Of at least one inductor and of at least one inductor Of a capacity.

In the identification of persons, animals and objects, A system in which a stationary or portable A transponder connected to the object to be identified Is supplied with energy via an alternating field, whereupon the transponder With the transmission of the data stored therein. Owing to the frequency frequency used, radio frequency identification systems are also used, Short RFID.

An RFID transponder generally consists of an antenna coil and An integrated circuit comprising all the necessary electronic circuit blocks, Such as, for example, for voltage supply, clock generation, Sequence control and to store the data necessary for the identification Data. The capacity connected in parallel to the antenna coil Is also often part of the integrated circuit. However, it can Can also be formed by a discrete structural element.

The RFID reader consists of a resonant circuit with a transmitting coil And a capacity derived from a driver stage with a signal having a Generally fixed frequency, eg 125 kHz. Further The reader contains electronic circuit blocks to the ones determined by the absorption modulation Data transmitted by the transponder and In order to transmit data and commands, for example by modulating the field, to the Transponders.

Reader and transponders form at the data or at the A loose coupled transformer. Therefore, the Energy transfer is relatively low.

The achievable range for the contactless transmission of energy and power Data is influenced by the following boundary conditions:<ul><li>Sending energy (limited by statutory provisions)</li><li>Coil dimensions</li><li>Interference level of the environment</li><li>Match the resonance frequencies</li><li>modulation stroke</li><li>Voltage loss across the rectifier</li><li>Transmitting methods used</li></ul>

To ensure that the transponder has a sufficient number for its operation Voltage is available from the antenna coil and the Connected resonant circuit To be operated in resonance with the transmission frequency.

Due to the tolerances of the components during manufacture of the transponder Generally not ensured.

From DE 44 38 287 C1, a system for the contactless energy and energy supply is provided Data transmission in which, for the purpose of equalizing the capacity of the Resonant vibration circuit, a series of parallel to the capacitance Capacitors according to a value stored in a memory To be switched on.

A method is also known from DE 44 38 268 C1, in which, Arrangement according to DE 44 38 287 C1, the optimum adjustment value for the Operation directly on the transponder. For this purpose, a Differential sample hold detector for sampling the voltage signal Provided. This is in connection with the control device and Counter. The adjustment is made with certain changes of the difference Finished. This approach has the disadvantage that, as a function of The number of steps to be performed is the adjustment process Differently long. At time - critical response times of the Transponder system can have different response times at times Can not be tolerated.

The object of the invention is, therefore, to provide a method for automatic Adjustment of a receiving oscillating circuit of a transponder in an RFID System which has constant response tents.

This object is achieved by a method with the features of the claim 1.

The advantageous embodiment of the invention takes place according to the features Of the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG.<dl tsize="13" compact="compact"><dt>FIG</dt><dd>Flowchart of the process</dd><dt>FIG</dt><dd>Sequence of the adjustment according to a second design of the procedure</dd><dt>FIG</dt><dd>10 shows a block diagram of a first circuit arrangement For carrying out the procedure;</dd><dt>FIG</dt><dd>10 shows a block circuit diagram of the circuit arrangement of a second embodiment Circuit arrangement for carrying out the method;</dd><dt>FIG. 5 shows</dt><dd>The change in the winding voltage V_{Vcil1 / 2} At the expiry of the match;</dd><dt>FIG</dt><dd>Shows the change in the output voltage of the filter stage At the end of the adjustment;</dd><dt>FIG</dt><dd>Shows the change in the total capacitance of the receiving oscillating circuit At the end of the adjustment;</dd></dl>

The invention is described below with reference to an exemplary embodiment shown in FIG With the help of the figures.

For example, the application in the form of an immobilizer results By the small terminal coil arranged around the ignition lock Unfavorable transmission conditions. Therefore, the system must be minimized Losses. What is decisive is:<ul><li>Same resonant frequency of base station and transponder</li><li>Time optimized transmission protocols</li><li>Minimal losses in energy transfer</li><li>Maximum modulation stroke when transferring data to the base station (Read)</li><li>Optimized data transmission to transponder (send)</li></ul>

Thus, despite the small electromagnetic fields emitted by the base station Field the energy for the operation of the transponder and for the Transmission of the data, the resonance frequencies should be the same. This is due to the construction tolerances The capacitors, coils, etc. can not be realized.

In the following, a method is described in which the transponder During the initialization phase to the resonance frequency of the base station .

A method has been developed in which the transponder can be switched off during Of the initialization phase to the resonance frequency of the base station. This is done by stepping capacitors to the LC resonant circuit Is switched on and adjusted to the maximum voltage value will. The coil voltage Vcoil is thereby reduced by means of a low-pass filter Rectified and smoothed. This voltage Vtp, 1 is stored. Subsequently, capacitors are connected in parallel in parallel steps The LC resonant circuit, whereby the resonance frequency and Thus also changing the induced voltage or Vtp. This new Vtp, 2 voltage Is compared with the previous Vtp, 1 by means of a comparator. As soon as Vtp, n + 1 is smaller than Vtp, n, this is registered and the Capacitor value of Vtp, n (maximum value). This active Self-tuning of the resonant frequency is performed at each initialization, But can also be addressed by starting from the base station Of a particular sequence is sent to the transponder. FIG. 3 shows an embodiment Block circuit diagram of a circuit arrangement for carrying out the method. FIGS. 5 to 7 show the change in the voltages during the course of the adjustment.

Advantage: better energy transfer -&gt; higher supply voltage + Larger modulation range (in the weak field) -&gt; greater range of functions

FIG. 1 shows a flowchart of the method. The process step "Adjustment" is shown in detail in FIG. In the following The sequence of the method according to FIGS. 1 and 2 is described.

The entire range of the adjustment is initially complete run through. At the end, the capacitors become the receiving oscillating circuit For which the maximum rectified and smoothed Voltage Vtp was stored in the memory.

After the start of the reception of a polling signal and after the integrated Circuit by resetting to a defined output state The coil voltage Vcoil is rectified and By means of a low-pass filter. The thus generated voltage Vtp, 1 becomes Provisionally as a reference value in a first memory SP. In one Second memory, the configuration of the capacitors Cn becomes unique Descriptive reference number. Subsequently, Uniform steps capacitors Cn parallel to the LC resonant circuit Of the receiving oscillating circuit. This changes its resonant frequency And thus the induced coil voltage Vcoil and finally Also the rectified and smoothed voltage Vtp. Switch it on Of a first capacitor C1 changes with the resonance frequency Of the receiving oscillating circuit is also used for the evaluation Voltage Vtp. This new, amended, in a second time Voltage Vtp, 2 is converted by means of a comparator to the previous, Stored in the memory Vtp, 1. Is the second Instantaneous voltage Vtp, 2 is greater than that stored in the memory Value, so the new voltage is Vtp, 2 instead of the previous voltage Vtp, 1 and the new, the configuration of the capacitors Cn uniquely describing Reference number written to the memory, the next capacitor And the comparison with the new pair of values, the New, third voltage Vtp, 3 and the value stored in the memory Second voltage Vtp, 2.

If the voltage Vtp, 2 present in the second instant is smaller than that in the second instant, Memory, then, without changing the memory content, The next capacitor is connected and the comparison with the new capacitor The new, third voltage Vtp, 3 and the voltage stored in the memory Value of the second voltage Vtp, 2.

If the entire range of the adjustment has been completed, The configuration of the capacitors Cn is added to the receiving oscillating circuit Which corresponds to the reference number stored in the memory.

This method of matching is particularly preferable when When the interrogation signal is switched on or when the transponder is immersed In a query field a specific temporal behavior of the transponder it is asked for.

FIG. 3 shows a first exemplary embodiment of a circuit arrangement according to the invention Carry out the procedure in block diagram. This is for comparison Of the receiving oscillating circuit to resonance with the frequency of the interrogation signal The following functional units: The receiving oscillating circuit of the transponder consists of at least one Inductance and at least one capacitance and is connected via the pair Coil1 / Coil2 is connected to the circuit arrangement. This consists From a variety of capacities provided by controllable switches Parallel to the capacitance of the receiving oscillating circuit In FIG. 3 by the circuit block with the designation Adapt. Also A low-pass filter is connected to the pair of terminals Coil1 / Coil2 In FIG. 3 by the circuit block with the designation Front2. The output signal Vtp of the low-pass filter becomes a comparator stage - shown in FIG. 3 by the circuit block with the Description max_detect. In the comparator stage becomes the output signal Vtp of the low-pass filter with the previously recorded Maximum value. Depending on its output signal adapt_out The large number of capacitances are adjusted.

FIG. 4 shows the circuit arrangement once again in an improved embodiment Illustration. Only the functional units of the transponder are shown, Which contribute significantly to the function of the adjustment. The receiving oscillating circuit 1 of the transponder, consisting of at least one inductor And at least one capacitance, with the one not shown Resonant circuit of the reader a loosely coupled transformer. Parallel To the capacitance of the receiving oscillating circuit 1 are a plurality of capacitances 2, which are controlled by controllable switches to the capacity of the receiving oscillating circuit Can be switched. The switches are controlled By a control logic 6. The control logic 6 also receives a control signal from the control logic 6 Is fed to the interrogation signal. On the basis of this clock signal Be a variety of capacitances, controlled by controllable switches Can be switched in parallel with the capacitance of the receiving oscillating circuit Progressively steered. The receiving oscillating circuit 1 is furthermore provided with a Rectifier stage 3 and a clock generator circuit 7. The Rectified by the rectifier stage 3 becomes a filter stage 4 For smoothing. The filter stage is designed as a low-pass filter. The Such rectified and smoothed signal becomes the input of a maximum detector 5. The maximum detector compares this to its Input signal with a stored size and adopted The larger in its memory. This will save the memory Of the maximum detector always has the largest value of the rectified And smoothed signal since the start of the match. The maximum detector 5 is connected to the control logic 6. This sets the switchable capacitors so that Again the maximum value is obtained and points to another output So that the adjustment is completed.

The switchable capacitances advantageously consist of a series circuit A capacitor and a switching transistor, the switching transistor Is controlled by the control logic. One pair each of the same dimension Capacitors are controlled by the control logic First series circuit of capacitor and switching transistor respectively First connection of the receiving oscillating circuit to the ground potential And the second series circuit of capacitor and switching transistor The second terminal of the receiving oscillating circuit with the ground potential .

FIGS. 5 and 6 show the changes in the coil voltage of the receiving oscillating circuit (FIG. 5) and the associated change in the output voltage Of the filter stage (FIG. 6). In FIG. 7 one can see The change in the total capacity of the receiving oscillating circuit, The switching-on of the switchable capacitors C1-Cn is caused becomes. The time axes of FIGS. 5 to 7 are identical.

In an advantageous embodiment, the control logic includes an n-stage Dual counter, the outputs of which control n pairs of capacitors / switches. The values ​​of the individual pairs of the capacitors are selected in such a way, That they are graded in binary-weighted manner and thus have a Wide range of values. For example, a four-level binary Counter at its outputs 4 pairs of capacitors. The Capacities then correspond to the respective assigned counting stages and Are one, two, four and eight times a basic value, for example 10 PF. With such an arrangement, the receiving oscillating circuit can be arranged next to Of its basic setting can be changed in 15 steps.

In a further embodiment of the circuit arrangement all capacitors are Of equal size. They can then be directly controlled Or be grouped together to form the groups described above Binary-weighted values.

By the above-described method and the associated circuit arrangement It is possible to set the receiving oscillating circuit of a transponder To match the frequency to the interrogation signal. This match Can be carried out each time the transponder is put into operation, or else by A predetermined command can be triggered by the reader. Also for One-time factory adjustment to a predetermined frequency around the Component tolerances of the receiving oscillating circuit And circuit arrangement. Then the through A single adjustment, eg the counter reading of the The switches of the capacitors of the capacitors, in a read-only memory Of the transponder permanently and for each initialization Again used for driving the capacitors.