Wave shaping to prevent overshoot and undershoot

Abstract

A transmitter (1; 21) is disclosed for wirelessly transmitting an amplitude modulated data signal (2) in a radio frequency field (3) to a receiver (4) of a radio frequency identification communication system (5; 22). The transmitter (1; 21) comprises: a wave generator (6) for generating a carrier signal (7) having a specific frequency and waveform; a modulation stage (15) for modulating the carrier signal (7) with respect to the data signal to be transmitted; an antenna (11) connected to the modulation stage (15) or the wave generator (6) via an amplifier (9) and a matching circuit (10) for wirelessly transmitting the amplitude modulated data signal (2) in the radio frequency field (3), characterized in that the transmitter (1; 21) further comprises a shaping stage (16) connected to the wave generator (6) for selecting the waveform of the carrier signal (7) directly or indirectly depending on the data signal.

Description

Technical Field

[0001] The invention relates to a transmitter for wirelessly transmitting an amplitude modulated data signal in a radio frequency field (RF-Field) to a receiver of a radio frequency identification (RFID) communication system, the transmitter comprising: a wave generator for generating a carrier signal with a specific frequency and waveform; a modulation stage for modulating the carrier signal with respect to a data signal to be transmitted; an antenna connected to the modulation stage or the wave generator via an amplifier and a matching circuit for wirelessly transmitting the amplitude modulated data signal in the radio frequency field. Background Art

[0002] Known RFID communication systems use integrated circuits like the ST25R3916 in an RFID reader or transmitter to communicate with an active or passive transponder. In a typical application, a passive transponder or tag stores the product identity of the product to which it is connected and uses a reader to obtain that product information. The reader is powered and generates a magnetic RF field that is emitted by its antenna. When the reader and tag are brought into close proximity to each other, the RF field generated by the reader is induced into the tag's antenna and used to power the passive tag. The tag also has a transceiver that receives signals from the reader and sends a response back to the reader.

[0003] There are many standards, such as ISO / IEC18000-3 or ISO / IEC 14.443 Type A and Type B, or ISO15.693 or ECMA-340 13,56MHz Near Field Communication (NFC) or NFC Forum, or company standards, such as Sony's Felica, that define the protocols and modulation types used to transmit information between tags and readers. Some or all of these standards define the wireless transmission of an amplitude modulated data signal with digital data to a tag in a radio frequency field using amplitude modulation. For example, ISO14.443 Type A further defines the use of modified Miller coding to encode the data signal into a coded data signal for transmission.

[0004] Near Field Communication systems have been implemented in an increasing number of different applications. Many of these systems have unfavorable conditions, such as nearby PCD boards, metal housings, etc., which can have a negative effect on the modulation shape of the AM data signal in the RF field. Figure 1A , 1B , Figure 2A , 2B , Figure 3A , 3B Examples of such negative impacts are shown. Figure 1A An example of an amplitude modulated data signal on the Tx pin of a prior art transmitter is shown, where the Tx pin is connected to an antenna through an amplifier and matching circuit to generate a radio frequency field. Figure 1BThe RF field near the transmitter antenna is shown. The rising edge of the RF field envelope shows an overshoot of the maximum amplitude of the carrier signal, which can lead to non-compliance with standards or even a failure of communication between the transmitter and receiver. Figure 2A Another example of a 10% ASK amplitude modulated data signal at the Tx pin of a prior art transmitter is shown, which results in an undershoot at the end of the falling edge of the RF field envelope and an overshoot at the end of the rising edge of the RF field envelope. Figure 3A Another example of a 100% AM data signal on the Tx pin of a prior art transmitter is shown, which results in an effect called "ringing" at the end of the rising edge of the RF field envelope. Another such negative effect is called "humps", where the fall of the falling edge or the rise of the rising edge of the RF field envelope is not monotonic. This effect is described, for example, in paragraph 5.7.2.2 of the NFC Forum Analog Technical Specifications document, version 2.1.

[0005] To solve these problematic negative effects of the RF field, the transmitter containing the integrated circuit ST25R3916 dynamically changes the transmitter voltage of the carrier signal between different levels. In order to avoid overshoot, the transmitter voltage is dynamically changed at the same time as the modulation edge, within the time frame of the rising edge of the RF field envelope. The disadvantage of this solution is that different supply voltages must be present within the integrated circuit and switching must be performed between these voltages. This requires multiple voltage supplies. Otherwise, the regulator needs to change its output voltage very quickly.

[0006] Another known solution to this problematic overshoot in the RF field, implemented in other generators, is to detune the antenna by changing the Q factor through a controlled resistance variation, thereby dynamically adjusting the antenna characteristics. This solution is described in US 9,654,181 B1 and has the disadvantage that detuning the antenna changes the antenna characteristics and thus the behavior of the entire antenna system. This can result in a reduction in the communication range and a risk that the communication between the transmitter and the tag may be loosened before the relevant information is exchanged. Therefore, any adjustment control must be applied very carefully in order not to have the opposite effect. Summary of the invention

[0007] The object of the present invention is to provide a transmitter with low complexity, wide communication range and good communication stability with a receiver.

[0008] This object is achieved by a transmitter, which also includes: a shaping stage connected to the wave generator for selecting the waveform of the carrier signal directly or indirectly depending on the data signal.

[0009] The shaping stage is constructed to receive the data signal or coded data signal at the same time as the modulation stage receives the data signal or coded data signal, and therefore knows when the rising edge or falling edge of the amplitude modulated data signal occurs. Based on this knowledge, the shaping stage dynamically selects another waveform of the carrier signal generated by the wave generator, having a higher or lower energy content for a limited period of time, to avoid overshoot, undershoot or other problematic negative effects in the RF field that may be caused by interfering objects in the RF field. The energy content of a wave is related to the area contained within or under the wave. For example, if the general waveform of the carrier signal generated by the wave generator is a sine wave, and overshoot at the end of the rising edge of the RF field envelope should be avoided, the shaping stage switches one or more waves of the wave generator from a sine wave to a sawtooth wave or a triangle wave. Since the energy content of a sawtooth wave or a triangle wave having the same frequency and amplitude as a sine wave is lower than that of a sine wave, the one or more waves with lower energy content at the appropriate time help to reduce or avoid overshoot of the RF field envelope.

[0010] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.It will be appreciated by those skilled in the art that the various embodiments may be combined. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1A An example of a 100% AM data signal on the Tx pin of a prior art transmitter is shown, where the Tx pin is connected to an antenna through an amplifier and matching circuit to generate a radio frequency field.

[0012] Figure 1B Shows Figure 1A The RF field of the AM data signal near the transmitter antenna, where the rising edge of the RF field envelope has an overshoot.

[0013] Figure 2A Another example of a 10% ASK amplitude modulated data signal on the Tx pin of a prior art transmitter is shown, where the Tx pin is connected to an antenna through a matching circuit to generate a radio frequency field.

[0014] Figure 2B Shows Figure 2A The RF field near the transmitter antenna of the AM data signal, where the falling edge of the RF field envelope has undershoot and the rising edge has overshoot.

[0015] Figure 3A Another example of a 100% AM data signal on the Tx pin of a prior art transmitter is shown, where the Tx pin is connected to an antenna through a matching circuit to generate a radio frequency field.

[0016] Figure 3B Shows Figure 3AThe RF field near the transmitter antenna of an AM data signal, where the RF field envelope has an overshoot on the rising edge and an effect called "ringing" at the end of the rising edge.

[0017] Figure 4 A first embodiment of a transmitter for transmitting an amplitude modulated data signal according to the present invention is shown.

[0018] Figure 5A Shows Figure 4 The 10% ASK data signal on the Tx pin of the transmitter is shown in Figure 1, in which a square wave and a sawtooth wave are inserted to avoid the Figure 5B Undershoot and overshoot in the RF field envelope are shown.

[0019] Fig. 6A Shows Figure 4 The 100% AM data signal on the Tx pin of the transmitter is shown, with two triangle waves inserted to avoid Figure 6B Overshoot in the RF field envelope is shown.

[0020] Figure 7 A second embodiment of a transmitter for transmitting an amplitude modulated data signal according to the present invention is shown. DETAILED DESCRIPTION

[0021] Figure 4 A transmitter 1 according to a first embodiment of the invention is shown for wirelessly transmitting an amplitude modulated data signal 2 in a radio frequency field 3 to a receiver 4 of a radio frequency identification communication system 5. The transmitter 1 comprises a wave generator 6 for generating a carrier signal 7 having a specific frequency and waveform. The radio frequency identification communication system 5 complies with the near field communication standard ISO 18.092, which defines a resonant frequency of a carrier signal 7 of 13.56 MHz having a sinusoidal waveform. The radio frequency identification communication system 5 also complies with ISO / IEC 14.443 Type A and Type B or ISO 15.693 or ECMA-340 13.56 MHz. The wave generator 6 is implemented in an integrated circuit 8 of the transmitter 1 and provides the amplitude modulated data signal 2 via an amplifier 9 at a Tx pin Tx of the integrated circuit 8. A matching circuit 10 is connected to the Tx pin Tx and matches the impedance to an antenna 11. The matching circuit 10 and the antenna 11 are arranged to resonantly transmit the amplified amplitude modulated data signal 2 to a nearby receiver 4, which may be implemented as a passive or active tag or any other active device (e.g. a mobile phone).

[0022] The data stage 12 of the integrated circuit 8 stores the digital data to be transmitted to the receiver 4. As defined in the near field communication standard, a modified Miller coding must be processed to encode the data signal of the digital data in the encoder stage 13, which provides the encoded data signal 14 to the modulation stage 15. The modulation stage 15 is constructed to control the amplitude of the carrier signal 7 generated by the wave generator 6 to achieve a 10% Amplitude Shift Keying (ASK) modulation of the encoded data signal 14 to provide an amplitude modulated data signal 2 at the Tx pin Tx of the integrated circuit 8. In other embodiments of the present invention, 50% or 100% ASK modulation, or any other modulation or any other coding known to those skilled in the art can be used.

[0023] The transmitter 1 further comprises a shaping stage 16 connected to the wave generator 6 to select the waveform of the carrier signal 7 directly or indirectly in dependence on the data signal. In this embodiment of the invention, when a coded data signal is used, the waveform is selected indirectly from the data signal. To this end, the shaping stage 16 is connected to the encoder stage 13 and receives the coded data signal 14 at the same time as the coded data signal 14 is supplied to the modulation stage 15. The shaping stage 16 thus has information as to when a rising or falling edge of the amplitude modulated data signal 2 will occur. Figure 5A It is shown that the amplitude modulated data signal 2 at the Tx pin Tx includes a falling edge at time t1. Figure 5B The radio frequency field generated by the antenna 11 is shown. Figure 2B If the transmitter is known from the prior art as shown, the radio frequency field will contain an undershoot at time t2. In order to avoid this undershoot, the shaping stage 16 is constructed to select another waveform in a second time frame after time t1 and shortly before time t2 to provide more energy to the radio frequency field 3. This other waveform to be selected by the shaping stage 16 is stored in the shape configuration stage 17, which stores waveforms with higher and lower energy content than the sine waveform. A square waveform contains more energy in the signal than a sine waveform, and the shaping stage 16 selects a square waveform, replacing a single sine wave of the carrier signal 7 with a single square wave, to avoid such an undershoot. Figure 5B An undershoot in the envelope 18 of the amplitude modulated data signal 3 is shown.

[0024] Figure 5A It is further shown that the amplitude modulated data signal 2 at the Tx pin Tx includes a rising edge at time t3. Figure 5B Display, if using Figure 2BAs shown in the transmitter known from the prior art, the radio frequency field at time t4 includes a much higher small overshoot. In order to reduce or even avoid this overshoot, the shaping stage 16 is constructed to select another waveform in a first time frame after time t3 and shortly before time t4 to provide less energy to the radio frequency field 3. This other waveform with less energy to be selected by the shaping stage 16 can be a sawtooth waveform or a triangle waveform, to name just two possible waveforms, which are stored as configuration data in the shape configuration stage 17. In this embodiment, the shaping stage 16 selects a sawtooth waveform with less energy in the signal than a sine waveform, replacing the single sine wave of the carrier signal 7 with a single sawtooth waveform, in order to reduce the overshoot. Figure 5B The overshoot in the envelope 18 of the AM data signal 3 is shown. If two sawtooth waves are used instead of two sine waves, this can be completely avoided. Figure 5B The small overshoot shown in .

[0025] In the same way as explained with regard to overshoots or undershoots in the radio frequency field 3, the so-called "hump" when the decrease of the falling edge or the rise of the rising edge of the envelope of the radio frequency field 3 is not monotonic can be reduced or even avoided completely. By replacing half a wave, or a full wave, or even several waves of the sine wave of the carrier signal 7 at appropriate times with half a wave, or a full wave, or even several waves of a signal with a higher or lower energy content, such as Figure 3B The distortion of the radio frequency field 3 as a "hump" or "ringing" is shown. This has the advantage that even if metal or other objects are close and interfere with the radio frequency field 3, the AM data signal received by the antenna of the receiver 4 fully complies with the standard and can be decoded without errors.

[0026] Such a wave generator 6 capable of generating a sinusoidal waveform wave by wave is known from EP 3 182 585 B1 of the same applicant. This prior art document discloses a digital power amplifier that generates a sinusoidal wave by small increments of M digital waveform forming bits, which add the increments of the output current to generate the sinusoidal wave. As an inventive improvement of this teaching, all kinds of other waveforms can also be generated, and a person skilled in the art will understand how to add the increments of M digital waveform forming bits to generate a triangular waveform instead of a sinusoidal waveform. The same is true for other waveforms such as sawtooth waveforms or even rectangular waveforms. Therefore, the present invention is also characterized by the inventive use of a digital power amplifier known from the above-mentioned prior art. The inventive use of the digital power amplifier is achieved by storing M digital waveform forming bits of different waveforms with higher or lower energy content in the signal wave in the shape configuration stage 17, wherein the selected M digital waveform forming bits are used to control the wave generator 6 through the shaping stage 16. The shape configuration stage 17 stores these M digital waveform forming bits of different waveforms in relation to or in combination with a specific waveform of a data signal or a coded data signal.

[0027] Fig. 6A Shown in Figure 4 The 100% AM data signal on the Tx pin Tx of the transmitter 1 is shown, in which two triangle waves 19 are inserted to avoid Figure 6B 20. In this example, not only has the waveform been changed from a sine wave to a triangle wave to reduce the energy in the carrier signal, but the amplitude of the triangle wave has also been reduced compared to the amplitude of the sine wave, which also helps to reduce the overshoot in the envelope 20 of the RF field. In another embodiment, in addition to changing to a waveform with more energy content, the amplitude of the sine wave can also be increased to avoid undershoot.

[0028] Figure 7 A transmitter 21 according to a second embodiment of the invention is shown for wirelessly transmitting an amplitude modulated data signal 2 in a radio frequency field 3 to a receiver 4 of a radio frequency identification communication system 22. The difference between the transmitter 1 of the first embodiment and the transmitter 21 of the second embodiment is that the wave generator 6 provides a carrier signal 7 which always has the same amplitude and the amplitude modulation of this carrier signal 7 with the coded data signal 14 is processed in the modulation stage 15. With this slightly different arrangement, the same inventive advantages as with the transmitter 1 according to the first embodiment of the invention can be achieved. Furthermore, in this second embodiment of the invention, the amplifier 9 is not part of the integrated circuit 8, but is implemented as an external amplifier 9 connected to the Tx pin Tx.

[0029] In a third embodiment of the invention not shown in the figures, the data signal is not encoded and is therefore used directly to modulate the carrier signal. The shaping stage therefore selects the waveform of the carrier signal directly according to the data signal.

[0030] In another embodiment of the present invention, the wave generator may generate a sawtooth wave or a triangle wave for a commonly used carrier signal. The shaping stage may use a sine waveform with a higher energy content in the second time frame to avoid undershoot in the envelope of the radio frequency field. In another embodiment of the present invention, the wave generator may generate a square wave for a commonly used carrier signal, as well as a sine waveform, a sawtooth waveform or a triangle waveform with a lower energy content to avoid overshoot in the envelope of the radio frequency field. A person skilled in the art will be able to provide various waveform selections depending on which waveform is used as a general waveform for the carrier signal.

[0031] In another embodiment of the present invention, a higher energy content of the carrier signal may be selected for one or more waves when a rising edge appears in the data signal or the coded data signal to reduce the duration of the rising edge in the envelope of the radio frequency field. This may result in overshoot, which is undesirable for some embodiments, but the reduction in the duration of the rising edge helps, for example, to increase the bit rate or to facilitate the detection of bits at the receiver. Similarly, a lower energy content of the carrier signal may be selected for one or more waves when a falling edge appears in the data signal or the coded data signal to reduce the duration of the falling edge in the envelope of the radio frequency field. This may result in undershoot, which is undesirable for some embodiments, but the reduction in the duration of the falling edge helps, for example, to increase the bit rate or to facilitate the detection of bits at the receiver.

Claims

1. A transmitter (1; 21) for wirelessly transmitting an amplitude modulated data signal (2) in a radio frequency field (3) to a receiver (4) of a radio frequency identification communication system (5; 22), the transmitter (1; 21) comprising: A wave generator (6) for generating a carrier signal (7) having a specific frequency and waveform; a modulation stage (15) for modulating the carrier signal (7) according to a data signal to be transmitted; an antenna (11) connected to the modulation stage (15) or the wave generator (6) via an amplifier (9) and a matching circuit (10) for wirelessly transmitting the amplitude modulated data signal (2) in the radio frequency field (3); It is characterized in that The transmitter (1; 21) further comprises: a shaping stage (16), connected to the wave generator (6), for selecting the waveform of the carrier signal (7) directly or indirectly according to the data signal, wherein the shaping stage (16) is constructed to select a waveform of the carrier signal (7) during a rising edge of the envelope (18; 20) of the radio frequency field (3) in a first time frame (t3->t4) after a rising edge of the data signal or the coded data signal (14), the waveform selected having the same frequency but a lower energy content than outside the first time frame (t3->t4), The shaping stage (16) is constructed to select a triangular waveform or a sawtooth waveform of the carrier signal (7) within the first time frame (t3->t4), and to select a sinusoidal waveform outside the first time frame (t3->t4).

2. The transmitter (1; 21) according to claim 1, wherein The transmitter (1; 21) further comprises an encoder stage (13) for encoding the data signal, providing the encoded data signal (14) to the modulation stage (15) for modulating the carrier signal (7), and providing the encoded data signal (14) to the shaping stage (16) for shaping the waveform of the carrier signal (7) within a time frame before and / or after a rising edge and / or a falling edge of the encoded data signal (14).

3. The transmitter (1; 21) according to claim 1 or 2, wherein: The shape configuration stage (17) stores configuration data for different waveforms of the carrier signal (7) for specific waveforms of the data signal and / or the encoded data signal (14).

4. The transmitter (1; 21) according to claim 1 or 2, wherein: The shaping stage (16) is constructed to select a waveform of the carrier signal (7) during a falling edge of the envelope (18) of the radio frequency field (3) in a second time frame (t1->t2) after a falling edge of the data signal or the encoded data signal (14), the waveform selected having the same frequency but a higher energy content than outside the second time frame (t1->t2).

5. The transmitter (1; 21) according to claim 4, wherein The shaping stage (16) is designed to select a square waveform of the carrier signal (7) within the second time frame (t1->t2) and to select a sinusoidal waveform outside the second time frame (t1->t2).

6. The transmitter (1; 21) according to claim 1 or 2, wherein: The wave generator (6) is constructed to generate a carrier signal (7) having a near field communication frequency of 13.56 MHz.

7. A transmitter (1; 21) according to claim 4, wherein the modulation stage (15) or the wave generator (6) also reduces the amplitude of the data signal or the coded data signal (14) in the first time frame (t3->t4) to avoid overshoot in the first time frame (t3->t4), and also increases the amplitude of the data signal or the coded data signal (14) in the second time frame (t1->t2) to avoid undershoot in the second time frame (t1->t2).

8. A transmitter (1; 21) according to claim 5, wherein the modulation stage (15) or the wave generator (6) also reduces the amplitude of the data signal or the coded data signal (14) in the first time frame (t3->t4) to avoid overshoot in the first time frame (t3->t4), and also increases the amplitude of the data signal or the coded data signal (14) in the second time frame (t1->t2) to avoid undershoot in the second time frame (t1->t2).

9. A transmitter (1; 21) according to claim 6, wherein the modulation stage (15) or the wave generator (6) also reduces the amplitude of the data signal or the coded data signal (14) in the first time frame (t3->t4) to avoid overshoot in the first time frame (t3->t4), and also increases the amplitude of the data signal or the coded data signal (14) in the second time frame (t1->t2) to avoid undershoot in the second time frame (t1->t2).

10. Method for using a transmitter (1; 21) according to claim 1 or 2, the transmitter (1; 21) being used for wirelessly transmitting an amplitude modulated data signal (2) in a radio frequency field (3) to a receiver (4) of a radio frequency identification communication system (5; 22), the method comprising the following steps: A carrier signal (7) having a specific frequency and waveform is amplitude modulated using a data signal to be transmitted or a coded data signal (14); Depending on the occurrence of a rising edge or a falling edge in the data signal or the encoded data signal (14), a different waveform having a higher or lower energy content of the carrier signal (7) is selected for one or more waves.

11. The method according to claim 10, wherein: When a rising edge occurs in the data signal or the encoded data signal (14), a lower energy content of the carrier signal (7) is selected for one or more waves to reduce or avoid overshoot in the envelope (18; 20) of the radio frequency field (3), or, when a falling edge occurs in the data signal or the encoded data signal (14), a higher energy content of the carrier signal (7) is selected for one or more waves to reduce or avoid undershoot in the envelope (18; 20) of the radio frequency field (3).

12. The method according to claim 10, wherein: When a rising edge occurs in the data signal or the encoded data signal (14), a higher energy content of the carrier signal (7) is selected for one or more waves to reduce the duration of the rising edge in the envelope (18; 20) of the radio frequency field (3), or, when a falling edge occurs in the data signal or the encoded data signal (14), a lower energy content of the carrier signal (7) is selected for one or more waves to reduce the duration of the falling edge in the envelope (18; 20) of the radio frequency field (3).

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