Pulse density modulation transmitter for radio frequency identification

Abstract

A transmitter (19; 36) configured to drive the transmitter (19; 19 ') with an antenna signal (21) of a substantially sinusoidal current / voltage having a carrier frequency in the RFID frequency range of 100 kHz to 100 MHz; 36), the transmitter (19; 36) comprises: a power amplifier (22) configured to provide an amplified transmitter signal (23); an electromagnetic interference filter (33) configured to filter the amplified transmitter signal (23) and to provide a filtered transmitter signal (34); a matching circuit (35) connected to the electromagnetic interference filter (33) and the RFID antenna (20) and configured to match their impedance and to receive the filtered transmitter signal (34) and to provide the antenna signal (21) to the RFID antenna (20), in which the transmitter (19; 36) comprises a transmitter signal source (25; 37) connected to the power amplifier (22) and configured to provide the pulse density modulated signal as a transmitter signal (26) to the power amplifier (22).

Description

Technical Field

[0001] The present invention relates to a transmitter configured to drive an RFID antenna of the transmitter with an antenna signal having a substantially sinusoidal current / voltage with a carrier frequency within an RFID frequency range of 100 kHz to 100 MHz. The transmitter includes: a power amplifier configured to provide an amplified transmitter signal; an electromagnetic interference filter configured to filter the amplified transmitter signal and provide a filtered transmitter signal; and a matching circuit connected to the electromagnetic interference filter and the RFID antenna and configured to match their impedances, receive the filtered transmitter signal, and provide the antenna signal to the RFID antenna. Background Art

[0002] High-voltage digital power amplifiers, or Class D amplifiers, such as the AMS AS3911, are known for use in radio frequency identification (RFID) devices, such as RFID readers or transmitters, communicating with active or passive transponders. In a typical application, a passive transponder or tag stores the product identity of the product to which it is connected, and a reader is used to retrieve this product information. The reader is powered on and generates a magnetic field that is emitted by its RFID antenna. When the reader and tag are brought into close proximity, the magnetic field generated by the reader is induced into the tag's RFID antenna and used to power the passive tag. The tag also has a transceiver that receives signals from the reader and transmits a response back to the reader.

[0003] Standards such as ISO / IEC 18000-3 or ISO / IEC 14.443 Class A and Class B, ISO 15.693 or ECMA-340 13.56 MHz Near Field Communication (NFC), or company standards such as Sony's Felica define the protocols and modulation types used to transmit information between tags and readers. Some or all of these standards define how a reader transmits digital data as an analog signal to a tag by varying the amount of transmit power. A power amplifier processes the digital data in the low-voltage domain (typically 3.6V or 5V) into an amplified analog signal with a carrier frequency that drives the RFID antenna to resonate, thereby outputting the maximum power of a fundamentally sinusoidal output current. This fundamentally sinusoidal output current must meet regulatory spurious emission levels to avoid noise in other frequency ranges.

[0004] Figure 1The transmit portion of a prior art high-voltage digital power amplifier 1 within a reader or transmitter is shown. The integrated circuit IC1 processes the digital data to be transmitted to a tag or used for wireless charging of a receiver battery. A filter 2, implemented with discrete components, is connected to first and second transmit output pins 3 and 4 to filter harmonics from the differential square-wave digital output signal of IC1 and feed an antenna signal having a carrier frequency resonant with a fundamental sinusoidal current I to an RFID antenna 10. The digital data is implemented by IC1's amplified transmitter signal, providing a 0 volt "0" or "low" bit and a 3.6 to 5 volt "1" or "high" bit between IC1's first and second transmit output pins 3 and 4. An electromagnetic interference filter 5 of filter 2 is connected to transmit output pins 3 and 4 to filter the rectangular digital output signal into a fundamental sinusoidal signal, which is provided at output connections 6 and 7 of the electromagnetic interference filter 5. A matching circuit 8 is connected to output connections 6 and 7 of the electromagnetic interference filter 5. An ohmic resistor 9 is arranged in parallel with the RFID antenna 10 and connected to output connections 11 and 12 of the matching circuit 8. Matching circuit 8 converts the filtered transmitter signal from electromagnetic interference filter 5 into an antenna signal with a substantially sinusoidal current I at the carrier frequency of RFID antenna 10, thereby delivering maximum power to the magnetic field HF for reception by the tag. Ohmic resistor 9 forms a load resistor and generates a substantially sinusoidal voltage U for the antenna signal for RFID antenna 10.

[0005] A disadvantage of this known transmitter is that the EMI filter 5 needs a corner frequency close to the carrier frequency (e.g., 13.56 MHz) to reduce harmonics and thus provide a predominantly sinusoidal antenna signal at the transmitter's RFID antenna 10. The NFC carrier frequency and the corner frequency of the EMI filter 5 are very close together. Therefore, since the RFID antenna 10 is a tuned load connected to the EMI filter 5, the poles of the EMI filter 5 and the RFID antenna 10 interact with each other. The inductance and therefore the resonant frequency of the RFID antenna 10 are affected by the presence of other objects moving in the near field. For applications such as NFC wireless charging, the object to be charged must move in the near field, so this is a normal phenomenon. Due to changes in the impedance of the matching circuit 8, changes in the resonance of the RFID antenna 10 lead to fluctuations in the output power after the EMI filter 5. Such fluctuations in the output power are a significant disadvantage, particularly for applications such as NFC-based wireless charging.

[0006] EP 3 182 585 B1 discloses a transmitter using a power amplifier that generates and supplies a sinusoidal antenna signal using multiple driver blocks. Each driver block includes a capacitor that provides an increment of output current, which together form the sinusoidal antenna signal without the need for an electromagnetic interference filter to block harmonics of the antenna signal's carrier frequency. Figure 2 The transmit portion of this prior art transmitter, a high-voltage digital power amplifier 13, is shown. The integrated circuit IC2 processes the digital data to be transmitted to a reader or tag. A matching circuit 14, comprised of discrete components connected to a first transmission output pin 15 and a second transmission output pin 16, is configured to influence the output signal 17 of the integrated circuit IC2 and feed a substantially sinusoidal output current I at the carrier frequency to the RFID antenna 10. Matching circuit 14 includes a first resonant capacitor C2a connected at its first contact to the first transmission output pin 15 and a second resonant capacitor C2b connected at its first contact to the second transmission output pin 16. Both resonant capacitors C2a and C2b are connected at their second contacts to a ground pin 18 of the integrated circuit IC2. Resonant capacitors C2a and C2b are tuned to form a resonant system with the output impedance of the integrated circuit IC2 at the first and second transmission output pins 15 and 16, and the impedance of the parallel-connected ohmic resistor 9 and the RFID antenna 10, thereby resonating at a carrier frequency of 13.56 MHz.

[0007] A disadvantage of this known transmitter for wireless charging applications is that the power consumption of the power amplifier is higher than that of other known transmitters due to the switching capacitor losses in the driver block. Summary of the Invention

[0008] The object of the present invention is to provide a method and a transmitter which are designed to drive an RFID antenna of the transmitter with an antenna signal having a substantially sinusoidal current and a carrier frequency in the RFID frequency range of 100 kHz to 100 MHz, and to provide a system having such a transmitter which does not comprise the disadvantages of the known transmitters and methods.

[0009] This object is achieved by a transmitter as claimed in claim 1 and a system as claimed in claim 7 as well as a method as claimed in claim 8 .

[0010] Until now, pulse density modulation (PDM) has not been used in the RFID frequency domain because those skilled in the art have overlooked its use to achieve high oversampling rates and high-purity sine waves due to the large number of pulses. Each pulse is a short square wave, which introduces noise, and the goal of RFID technology is to minimize noise to maximize the communication range between transmitter and receiver. The inventors of the present invention have overcome this bias and discovered that using PDM to generate the transmitter signal can achieve the advantage of stable wireless power transmission from transmitter to receiver, which is particularly important for wireless charging applications. If a short distance between transmitter and receiver, such as only 5 or 10 mm, is required, wireless communication between the transmitter and receiver is still possible despite the higher noise caused by PDM. The use of PDM will also be advantageous for other applications where stable wireless power transmission is highly critical and communication distances exceeding 1 or 2 cm are less important.

[0011] In one embodiment of the present invention, a pulse density modulated signal is provided by a transmitter signal source comprising a sampling stage configured to provide a pulse code modulated sinusoidal signal having a sampling rate of at least 32 times the carrier frequency of the antenna signal. Lower sampling rates are possible, but a sampling rate of 32 times or higher of the carrier frequency is advantageous because the higher the sampling rate, the better the waveform accuracy towards a perfect sinusoidal signal can be achieved. A delta-sigma modulator can be used to process the high-resolution pulse code modulated sinusoidal signal from the sampling stage into a low-resolution pulse density modulated signal to implement the transmitter signal. This provides the advantage that the noise caused by the pulse density modulation is shifted to frequencies away from the carrier frequency, for example, 13.56 MHz, which greatly simplifies filtering by the electromagnetic interference filter, since its corner frequency is significantly higher than the carrier frequency.

[0012] In another embodiment of the present invention, a pulse density modulated signal is provided by a transmitter signal source including a memory configured to store a lookup table of at least one period of the pulse density modulated signal. The transmitter signal source also includes a signal loop regenerator configured to continuously provide the stored pulse density modulated signal in a loop to provide the transmitter signal to a power amplifier. This allows for the advantage of reducing or eliminating the effects of lower harmonics of the carrier signal (particularly the third and fifth harmonics) in the pulse density modulated signal stored in the memory. By storing the lookup table in this manner, a delta-sigma modulator can be omitted.

[0013] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.It will be understood by those skilled in the art that the various embodiments may be combined. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 A high voltage digital power amplifier of a transmitter according to the prior art is shown, which uses a square wave as transmitter signal to generate a substantially sinusoidal current for an RFID antenna.

[0015] Figure 2 Another transmitter according to the prior art is shown, which uses a plurality of driver blocks with capacitors to add their current increments to form a basic sinusoidal current for the RFID antenna.

[0016] Figure 3 A transmitter having a ΔΣ modulator according to a first embodiment of the present invention is shown.

[0017] Figures 4 to 7 Shown in accordance with Figure 3 The signal is processed in the transmitter.

[0018] Figure 8 A comparison of the signals is shown.

[0019] Figure 9 A transmitter according to a second embodiment of the present invention is shown, in which a lookup table for one period of a pulse density modulation signal is stored. DETAILED DESCRIPTION

[0020] Figure 3 A transmitter 19 according to a first embodiment of the present invention is shown. Together with a receiver, it is part of a system designed for use in wireless NFC charging applications. The receiver, for example, can be an in-ear headphone with a battery, which is wirelessly charged via a magnetic field generated by transmitter 19 at a carrier frequency of 13.56 MHz. Transmitter 19 includes an RFID antenna 20 and is designed to drive RFID antenna 20 with an antenna signal 21 having a substantially sinusoidal current at 13.56 MHz within the RFID frequency range of 100 kHz to 100 MHz. Other embodiments of the present invention can use carrier frequencies of, for example, 125 kHz or 900 MHz. In principle, the present invention can also be used with higher RFID frequencies, such as 1 GHz, but this would necessitate a sampling frequency of 150 GHz, which is impractical in practice.

[0021] Transmitter 19 includes a power amplifier 22 implemented within an integrated circuit using C-MOS technology and configured to provide an amplified transmitter signal 23 at two output pins 24 of the integrated circuit. Power amplifier 22 is a digital Class C amplifier, which, in simple form, is a powerful CMOS inverter driven by a series of inverters of increasing drive strength, known as a "speaker buffer." Transmitter 19 also includes a transmitter signal source 25 connected to power amplifier 22 and configured to provide a pulse-density modulated signal to implement transmitter signal 26 from power amplifier 22.

[0022] The transmitter signal source 25 comprises a sampling stage 27 which is configured to provide a pulse code modulated sinusoidal signal 28 having a sampling rate of at least 32 times the carrier frequency of the antenna signal 21. To achieve this, the sampling stage 27 provides a stored sinusoidal signal having a carrier frequency of 13.56 MHz which is sampled with a sampling signal 29 at a fixed sampling rate of 48 times the carrier frequency. Figure 4 Such a pulse code modulated sinusoidal signal 28 is shown in FIG. Lower sampling rates are possible, but a sampling rate of 32 times or higher is advantageous for sampling a sinusoidal signal at the carrier frequency, as the higher the sampling rate, the better the waveform accuracy towards a perfect sinusoidal signal can be achieved. To generate the sampled signal 29, the transmitter signal source 25 includes a reference oscillator 30, which is composed of a crystal oscillator that generates a frequency in the range of 10-60 MHz defined by the oscillation mode of a deployed quartz crystal. As known to those skilled in the art, the transmitter PLL 31 of the transmitter signal source 25 is composed of a phase detector, a loop filter, a voltage controlled oscillator (VCO), and a feedback divider. It converts the frequency from the reference oscillator 30 to the appropriate frequency for the sampled signal 29 and the carrier frequency.

[0023] The transmitter signal source 25 further includes a ΔΣ modulator 32 connected to the sampling stage 27 and the power amplifier 22, the ΔΣ modulator 32 being configured to receive the pulse code modulated sinusoidal signal 28 and to provide the transmitter signal 26. The ΔΣ modulator 32 is configured to process the high-resolution pulse code modulated sinusoidal signal 28 from the sampling stage 27 into a low-resolution pulse density modulated signal to realize the transmitter signal 26. Figure 5 Such a transmitter signal 26 is shown provided by the ΔΣ modulator 32, Figure 6 The amplified transmitter signal 23 of the differential power amplifier 22 is shown at a 5 volt supply. This embodiment with the delta-sigma modulator 32 offers the advantage that the noise caused by the pulse density modulation is shifted to frequencies away from the 13.56 MHz carrier frequency, which greatly simplifies filtering of the amplified transmitter signal 23 by the electromagnetic interference filter 33, since its corner frequency is significantly higher than the carrier frequency.

[0024] The electromagnetic interference filter 33 is configured to provide a filtered transmitter signal 34 to a matching circuit 35 connected to the electromagnetic interference filter 33 and the RFID antenna 20. The matching circuit 35 is configured to match the output impedance of the electromagnetic interference filter 33 to the input impedance of the RFID antenna 20 to transfer as much energy as possible from the received filtered transmitter signal 34 to the antenna signal 21 fed into the RFID antenna 20. Depending on the power requirements of the receiver, a resonant system or a slightly detuned system is used using the RFID antenna 20, as known to those skilled in the art. Figure 7The antenna signal 21 is shown as a substantially sinusoidal voltage generated and provided by the transmitter 19 to provide a magnetic field to the receiver to enable wireless charging.

[0025] Figure 3 The transmitter 19 disclosed in

[19] is capable of achieving higher energy transfer efficiency than current methods, which is particularly important for NFC-based wireless charging. Because the higher deployment cutoff frequency means smaller I2R losses in the smaller inductor, losses in the electromagnetic interference filter are lower than with traditional square-wave methods. Furthermore, power consumption is lower compared to switched-capacitor power amplifiers because there are no capacitor charging losses. Furthermore, a smaller die area is achieved compared to direct sinusoidal synthesis, thereby reducing die cost because no internally fabricated switched capacitors are required.

[0026] This approach also allows for a trade-off between EMI filter losses and internal driver losses, allowing for maximum efficiency over a wide range of output powers. For higher power solutions, it is advantageous to use a higher oversampling ratio to reduce EMI filter losses. For lower power solutions, it is advantageous to use a lower oversampling ratio to reduce driver losses.

[0027] Unlike traditional square wave methods, pulse density modulation can vary the output amplitude to achieve the modulation required for NFC communication.

[0028] Traditional square wave methods require an EMI filter with a filter pole close to the antenna's resonant frequency. Therefore, since the RFID antenna is a tuned load connected to the EMI filter, the EMI filter pole and the RFID antenna interact. The inductance of the RFID antenna, and therefore the resonant frequency, is affected by the presence of other objects moving in the near field. NFC wireless charging relies on the object to be charged being in the near field, so this is normal. Due to changes in matching impedance, the resonance of the RFID antenna causes fluctuations in the output power after the EMI filter. Moving the EMI filter pole to a higher frequency (which is possible when using pulse density modulation) can reduce power fluctuations in NFC-based wireless charging applications, which allows the antenna system to better cope with misalignment.

[0029] The use of ΔΣ modulation disperses the noise from the power supply, which results in improved PSRR for both square wave and direct sine wave TX methods.

[0030] Figure 9 FIG. 3 shows a transmitter 36 according to a second embodiment of the present invention having a memory lookup table for one period of a pulse density modulated signal as the transmitter signal 26. Figure 3Stages of the second embodiment that are similar or identical to those of the first embodiment are denoted by the same reference numerals. Transmitter 36 differs from transmitter 19 in that it includes a different transmitter signal source 37 for providing transmitter signal 26. Transmitter signal source 37 includes a lookup table stage 38 having a memory for storing a lookup table for one cycle of a pulse density modulated signal, and a signal loop regenerator for continuously providing the stored pulse density modulated signal in a loop as transmitter signal 26 to power amplifier 22. One advantage of this approach is that the effects of lower harmonics of the carrier frequency, particularly the third and fifth harmonics, may be reduced or eliminated in the stored pulse density modulated signal. This reduces the need for filtering and filtering losses in electromagnetic interference filter 33. Those skilled in the art will know how to store such a pulse density modulated signal as digital data in a memory in an appropriate manner, and will also know how to implement such a signal loop regenerator, which reads the stored data / signal in a loop and provides it as transmitter signal 26.

[0031] Both transmitters 19 and 36 implement a method of driving the RFID antenna 20 of the transmitter 19 or 36 with an antenna signal 21 having a substantially sinusoidal current with a carrier frequency in the RFID frequency range of 100 kHz to 100 MHz, the method comprising the following steps:

[0032] - generating a transmitter signal 26 and amplifying it to provide an amplified transmitter signal 23;

[0033] - filtering the amplified transmitter signal 23 with an electromagnetic interference filter 33 and providing a filtered transmitter signal 34;

[0034] - Matching the impedance of the RFID antenna 20 and the electromagnetic interference filter 33 at their outputs through the matching circuit 35 and driving the RFID antenna 20 to resonate;

[0035] - Using a pulse density modulated signal to generate the transmitter signal 26.

[0036] Wirelessly charging the battery of a receiver using the method described above and using a system having such a transmitter 19 or 36 and a receiver has the advantages described above for the two different embodiments of the transmitters 19 and 36 .

Claims

1. A transmitter (19; 36) configured to drive the transmitter (19; 36) with an antenna signal (21) having a substantially sinusoidal current / voltage with a carrier frequency in the RFID frequency range of 100 kHz to 100 MHz. 36) of an RFID antenna (20), the transmitter (19; 36) comprising: a power amplifier (22) configured to provide an amplified transmitter signal (23); an electromagnetic interference filter (33) configured to filter the amplified transmitter signal (23) and provide a filtered transmitter signal (34); and a matching circuit (35) connected to the electromagnetic interference filter (33) and the RFID antenna (20) and configured to match their impedances and receive the filtered transmitter signal (34) and provide the antenna signal (21) to the RFID antenna (20), It is characterized by: The transmitter (19; 36) comprises a transmitter signal source (25; 37) connected to the power amplifier (22) and configured to provide a pulse density modulated signal as a transmitter signal (26) to the power amplifier (22).

2. The transmitter (19) according to claim 1, characterized in that The transmitter signal source (25) includes a sampling stage (27) configured to provide a pulse code modulated sinusoidal signal (28) having a sampling rate of at least 32 times the carrier frequency of the antenna signal (21).

3. The transmitter (19) according to claim 2, characterized in that The transmitter signal source (25) comprises a ΔΣ modulator (32) connected to the sampling stage (27) and the power amplifier (22) and configured to receive the pulse code modulated sinusoidal signal (28) and provide the transmitter signal (26).

4. The transmitter (19) according to claim 3, characterized in that The ΔΣ modulator (32) is used to convert a high-resolution pulse code modulated sinusoidal signal (28) into a low-resolution transmitter signal (26).

5. The transmitter (19) according to claim 3 or 4, characterized in that The ΔΣ modulator (32) is implemented as a second-order ΔΣ modulator or a higher-than-second-order ΔΣ modulator.

6. The transmitter (36) according to claim 1, characterized in that The transmitter signal source (37) includes a memory configured to store a lookup table of one period of the pulse density modulated signal, and the transmitter signal source (37) includes a signal loop reproducer configured to continuously provide the stored pulse density modulated signal as a transmitter signal (26) to the power amplifier (22) in a loop.

7. A system of a transmitter (19; 36) and a receiver for wirelessly charging a battery of the receiver using power transmitted by an RFID antenna (20) of the transmitter (19, 36) and received by an RFID antenna of the receiver, characterized in that The transmitter (19; 36) is implemented as the transmitter (19; 36) according to any one of claims 1 to 6.

8. A method for driving an RFID antenna (20) of a transmitter (19, 36) with an antenna signal (21) having a substantially sinusoidal current with a carrier frequency in the RFID frequency range of 100 kHz to 5 GHz, the method comprising the steps of: - generating a transmitter signal (26) and amplifying it to provide an amplified transmitter signal (23); - filtering the amplified transmitter signal (23) with an electromagnetic interference filter (33) and providing a filtered transmitter signal (34); - matching the impedance of the RFID antenna (20) and the electromagnetic interference filter (33) at its output through a matching circuit (35), and driving the RFID antenna (20) to resonate; It is characterized by the following steps: - using a pulse density modulated signal to generate the transmitter signal (26).

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