Low-frequency RFID resonant frequency self-calibration method and circuit

Abstract

The invention discloses a low-frequency RFID resonant frequency self-calibration method and circuit, and the circuit comprises a resonant circuit, a resonant capacitor array, a rectification circuit, an energy storage capacitor C2, an oscillation maintenance circuit, a clock recovery circuit, an oscillator, a digital circuit, and a memory. The resonant capacitor array comprises a plurality of resonant capacitors and symmetrical switches. According to the method, calibration is realized through the steps of supplying energy to a base station, starting a test mode to disconnect carrier waves, performing clock recovery and frequency measurement in cooperation with an oscillator, adjusting a resonant capacitor array by a digital circuit, storing parameters, and multiplexing the parameters after power-on next time. According to the scheme, errors are compensated through the capacitor array, the internal circuit autonomously measures frequency, the design is simplified, the cost is reduced, the efficiency is improved through an automatic process, external instruments are not needed, and continuous and reliable calibration is guaranteed.

Description

Technical Field

[0001] This invention relates to the field of low-frequency RFID technology, specifically to a self-calibration method and circuit for the resonant frequency of low-frequency RFID. Background Technology

[0002] In the field of low-frequency RFID, energy harvesting is a common issue. This is typically achieved by an external inductor and an internal capacitor forming an LC resonant circuit. The LC resonant circuit gains energy through mutual inductance with the base station, which is then rectified into DC power to supply the chip. The center frequency of the LC resonant circuit needs to be as close as possible to the base station's transmission frequency for the chip to efficiently harvest energy. However, manufacturing errors in the external resonant inductor and process errors in the internal resonant capacitor can cause a shift in the LC resonant frequency, degrading RFID performance. Therefore, an appropriate resonant capacitor array needs to be added to calibrate this shift error, ensuring the chip harvests energy efficiently and stably.

[0003] Existing low-frequency RFID products suffer from LC resonant frequency shifts due to manufacturing errors in the external resonant inductor and process errors in the internal resonant capacitor, affecting performance and sensitivity. This invention proposes a self-calibration method for the resonant frequency of low-frequency RFID, effectively solving the problems of inaccurate resonant frequency caused by deviations in the resonant inductor and capacitor due to process and manufacturing errors, and the cumbersome calibration process. By automatically adjusting the resonant capacitor array, the accuracy of the resonant frequency is ensured. This method eliminates the need for specialized low-frequency RFID testing equipment. After the chip enters test mode, the internal oscillator detects the resonant frequency, and the number of adjustable resonant capacitors in the array is adjusted, enabling automatic calibration for products with individual differences. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by proposing a self-calibration method and circuit for the resonant frequency of low-frequency RFID.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: a self-calibration method for low-frequency RFID resonant frequency, comprising the following steps:

[0006] S1: The base station transmits a carrier wave, the resonant circuit receives the carrier wave and resonates to generate AC energy, which is converted into DC power by the rectifier circuit to power the system and charge the energy storage capacitor C2.

[0007] S2: The low-frequency front-end circuit receives the test mode command, demodulates it through the demodulation module, and then transmits the data to the digital circuit. The digital circuit control chip enters the test mode, enables the oscillator to generate a high-frequency clock signal, then disconnects the base station carrier, starts the oscillation maintenance circuit to supply power to the resonant circuit, and maintains the self-resonant oscillation of the resonant circuit.

[0008] S3: The clock recovery circuit processes the oscillation signal of the resonant circuit and outputs a clock signal with the same frequency as the resonant frequency to the digital circuit; the digital circuit uses the high-frequency clock signal as a reference, counts the clock signal for a fixed period of time, and calculates the current resonant frequency.

[0009] S4: The digital circuit compares the current resonant frequency with the preset target resonant frequency. If the current resonant frequency exceeds the preset error range, it outputs a control signal to adjust the on / off state of the corresponding switch in the resonant capacitor array. By controlling the connection and disconnection of the resonant capacitors under different combinations, the total connected capacity is changed, thereby adjusting the resonant frequency. Repeat step S3 until the current resonant frequency falls into the error range.

[0010] S5: The digital circuit writes the current control signal of the resonant capacitor array into the memory, then turns off the oscillator and oscillation sustaining circuit, and the system is powered down; the next time the chip is powered on, the digital circuit reads the control signal in the memory and controls the resonant capacitor array to be connected to the resonant circuit according to the calibrated capacitance.

[0011] Preferably, in step S4, the output signal ctrl_ctN-1:0 of ​​the N-bit digital circuit increases sequentially from a state of all zeros according to a preset period; or when the digital circuit determines that the resonant frequency output by the clock recovery circuit is higher or lower than a preset value, it increments or decrements ctrl_ctN-1:0 by 1, and continues to compare until the resonant frequency corresponding to ctrl_ctN-1:0 falls within a preset error range; wherein, each time ctrl_ctN-1:0 changes, the on / off state of one or more sets of switches in the corresponding resonant capacitor array changes once, thereby changing the capacitor combination connected to the resonant circuit, and during this process, the digital circuit uses the faster frequency output by the oscillator to count the resonant frequency output by the clock recovery circuit to determine whether the corresponding resonant frequency meets the preset value.

[0012] Preferably, in step S1, the DC power supply converted by the rectifier circuit charges the energy storage capacitor C2, and the energy storage capacitor C2 provides continuous power supply for the calibration process (steps S2 to S5) to ensure that the resonant circuit can still maintain oscillation after the base station carrier is disconnected.

[0013] Preferably, in step S2, the operation of "disconnecting the base station carrier" is performed after the high-frequency clock signal generated by the oscillator has stabilized, so as to avoid the instability of the high-frequency clock signal affecting the subsequent resonant frequency detection accuracy.

[0014] Preferably, the on / off state of each group of switches in the resonant capacitor array is driven by an independent control signal bit. The high or low level of a single control signal bit directly determines the connection or disconnection of the corresponding capacitor, ensuring the accuracy of capacitor combination adjustment.

[0015] The present invention also provides a self-calibration circuit for the low-frequency RFID resonant frequency that implements the above self-calibration method, including a resonant circuit, a resonant capacitor array, a rectifier circuit, an energy storage capacitor C2, an oscillation sustaining circuit, a clock recovery circuit, an oscillator, a digital circuit, and a memory.

[0016] The resonant circuit is composed of an external inductor L1 and an internal chip capacitor C1 connected in parallel, with the two ends of the parallel circuit being the RFN terminal and the RFP terminal, respectively.

[0017] The resonant capacitor array includes several resonant capacitors and a set of symmetrical switches corresponding to each resonant capacitor. One end of each set of switches is connected to the RFN terminal and the RFP terminal of the resonant circuit, and the other end is connected to the corresponding resonant capacitor. The resonant capacitors are connected between the symmetrical switches. The control terminals of the switches are connected to the digital circuit through a level conversion circuit. The RFN terminal and the RFP terminal of the resonant circuit are connected to the input terminal of the rectifier circuit and the input terminal of the clock recovery circuit, respectively.

[0018] The output terminal of the rectifier circuit is connected to one end of the energy storage capacitor C2, the power supply terminal of the oscillation sustaining circuit, the power supply terminal of the clock recovery circuit, the power supply terminal of the oscillator, the power supply terminal of the digital circuit, and the power supply terminal of the memory, respectively, and the other end of the energy storage capacitor C2 is grounded.

[0019] The signal input terminal of the oscillation sustaining circuit is connected to the output terminal of the clock recovery circuit, and the output terminal of the oscillation sustaining circuit is connected to the RFN terminal of the resonant circuit.

[0020] The output of the clock recovery circuit is also connected to the input of the digital circuit; the enable terminal of the oscillator is connected to the control terminal of the digital circuit, and the clock output terminal of the oscillator is connected to the counting reference terminal of the digital circuit.

[0021] The control signal output terminal of the digital circuit is connected to the switch control terminal of the resonant capacitor array through a level conversion circuit, and the data port of the digital circuit is bidirectionally connected to the memory.

[0022] Preferably, the switching devices in the resonant capacitor array are NMOS transistors or PMOS transistors, and the control signal output by the digital circuit is used for switching control after level conversion.

[0023] Preferably, the control signal output by the digital circuit is an N-bit digital signal that matches the number of resonant capacitors in the resonant capacitor array. Each bit of the digital signal independently controls the on / off state of a switch corresponding to a group of resonant capacitors, thereby adjusting the total capacity of the resonant capacitor array.

[0024] The present invention has the following beneficial effects:

[0025] 1. This invention, through the design of a resonant circuit and a resonant capacitor array, uses a digital circuit to drive a switch to adjust the capacitance of the input capacitor, thereby achieving precise compensation for external inductor manufacturing errors and chip capacitor process errors, ensuring that the resonant frequency matches the base station.

[0026] 2. This invention processes the oscillation signal through a clock recovery circuit, provides a high-frequency counting reference through an oscillator, and enables the digital circuit to autonomously complete frequency detection and comparison, thereby achieving automatic calibration without the need for external professional low-frequency testing instruments and reducing the operational threshold.

[0027] 3. This invention simplifies the rectifier circuit and oscillation maintenance circuit structure, integrates multiple logics with NMOS / PMOS switches and digital circuits in the capacitor array, thereby reducing chip area and controlling device cost, and improving economic efficiency.

[0028] 4. This invention achieves high efficiency in calibration operations by using a fully automated calibration process and storing parameters in a memory for reuse on the next power-on, avoiding repeated calibrations and shortening the R&D testing and mass production delivery cycle. Attached Figure Description

[0029] Figure 1 The present invention provides a schematic diagram of the working principle of a self-calibration circuit for low-frequency RFID resonant frequency;

[0030] Figure 2 The present invention provides a schematic diagram of the working principle of a resonant capacitor array for a self-calibration circuit of low-frequency RFID resonant frequency.

[0031] Figure 3 The present invention provides a schematic diagram of the resonant frequency waveform that varies with an N-bit digital signal. Detailed Implementation

[0032] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0033] Reference Figure 1-3 One embodiment provided by the present invention:

[0034] A self-calibration circuit for the resonant frequency of a low-frequency RFID device includes a resonant circuit, a resonant capacitor array, a rectifier circuit, an energy storage capacitor C2, an oscillation sustaining circuit, a clock recovery circuit, an oscillator, a digital circuit, and a memory; wherein,

[0035] The resonant circuit consists of an external inductor L1 and an internal chip capacitor C1 connected in parallel. The two ends of the parallel circuit are RFN and RFP terminals, respectively. The RFN and RFP terminals are connected to the input terminals of the rectifier circuit and the clock recovery circuit, respectively. At the same time, they are connected to the corresponding resonant capacitors through the switches of the resonant capacitor array. The resonant capacitor array contains several resonant capacitors (such as Ct1, Ct2, and Ct3) and a set of symmetrical switches corresponding to each resonant capacitor (such as Ct1 corresponding to k1n and k1p, Ct2 corresponding to k2n and k2p, and Ct3 corresponding to k3n and k3p). The switch control terminals are connected to the digital circuit through a level conversion circuit.

[0036] The input terminal of the rectifier circuit is connected to the RFN and RFP terminals of the resonant circuit. The output terminal is connected to one end of the energy storage capacitor C2, the power supply terminal of the oscillation sustaining circuit, the power supply terminal of the clock recovery circuit, the power supply terminal of the oscillator, the power supply terminal of the digital circuit, and the power supply terminal of the memory, respectively. The other end of the energy storage capacitor C2 is grounded.

[0037] The power supply terminal of the oscillation sustaining circuit is connected to the output terminal of the rectifier circuit (or the energy storage capacitor C2), the signal input terminal is connected to the output terminal of the clock recovery circuit, and the output terminal is connected to the RFN terminal of the resonant circuit.

[0038] The clock recovery circuit input is connected to the RFN and RFP terminals of the resonant circuit, and the output is connected to the digital circuit input and the oscillation sustaining circuit signal input, respectively.

[0039] The oscillator enable terminal is connected to the digital circuit control terminal, and the clock output terminal is connected to the digital circuit counting reference terminal.

[0040] The digital circuit input terminals receive the clock signal output from the clock recovery circuit and the clock signal output from the oscillator, respectively. The control signal output terminal is connected to the resonant capacitor array switch control terminal via a level conversion circuit. The data port is bidirectionally connected to the memory. The memory is used to store the calibrated resonant capacitor array control signal.

[0041] When the LC resonant frequency shifts due to manufacturing errors in external inductor L1 and process errors in internal capacitor C1, the digital circuit outputs a control signal. After level conversion, this signal drives the switches in the resonant capacitor array to open and close. By controlling the connection and disconnection of resonant capacitors under different combinations, the total capacitance of the resonant circuit is changed, thereby adjusting the resonant frequency. This effectively solves the problem of resonant frequency shift caused by external device factors and chip manufacturing process errors, ensuring that the resonant frequency matches the base station's transmission frequency and guaranteeing that the chip efficiently acquires energy.

[0042] The resonant circuit generates an AC signal by acquiring energy through mutual inductance with the base station. The rectifier circuit converts this AC signal into DC power, part of which powers the various modules, and the other part charges the energy storage capacitor C2. After the base station disconnects the carrier wave, the energy storage capacitor C2 releases its energy to continuously power the system. This eliminates the need for an external power source; internal energy conversion and storage ensure uninterrupted calibration, simplifying the power supply design and improving the practicality of the solution. After the base station disconnects the carrier wave, the oscillation maintenance circuit receives a clock signal from the clock recovery circuit that is at the same frequency as the resonant frequency. This signal powers the resonant circuit, enabling it to maintain its oscillation at its resonant frequency. This ensures stable oscillation even after the base station disconnects the carrier wave, providing a continuous and stable frequency signal source for subsequent resonant frequency detection and calibration, ensuring a smooth calibration process.

[0043] The clock recovery circuit samples and processes the oscillation frequency generated by the resonant circuit, converting the oscillation signal into a standard clock signal with the same frequency as the resonant frequency. This signal is then transmitted to the digital circuit and the oscillation sustaining circuit, respectively. This provides an accurate signal basis for the digital circuit to detect the resonant frequency and provides a frequency reference for the oscillation sustaining circuit, ensuring that the oscillation sustaining circuit can accurately match the frequency of the resonant circuit and guaranteeing calibration accuracy.

[0044] The low-frequency front-end circuit receives the test mode command, demodulates it through the demodulation module, and then transmits the data to the digital circuit. After the chip enters the test mode, the digital circuit outputs an enable signal to start the oscillator. The oscillator generates a clock signal with a frequency much higher than the resonant frequency, which is transmitted to the digital circuit as a counting reference. This provides a high-frequency counting reference for the digital circuit to detect the resonant frequency, ensuring that the digital circuit can accurately calculate the resonant frequency, improving the accuracy of frequency detection, and laying the foundation for accurate calibration.

[0045] The digital circuit uses the high-frequency clock signal output by the oscillator as a reference to count the resonant frequency clock signal output by the clock recovery circuit, calculates the current resonant frequency, compares it with the preset target resonant frequency, and outputs a control signal to adjust the switching state of the resonant capacitor array. When the resonant frequency meets the requirements, the digital circuit writes the control signal into the memory. When the chip is powered on again, the control signal in the memory is read first to control the resonant capacitor array, realizing automatic detection, comparison and automatic adjustment of the resonant frequency and the capacitor array without manual intervention, simplifying the calibration operation. By storing the calibration parameters in the memory, repeated calibration is avoided on the next power-on, improving calibration efficiency and shortening the R&D testing cycle.

[0046] Based on the above-described self-calibration circuit for low-frequency RFID resonant frequency, the self-calibration method for low-frequency RFID resonant frequency of the present invention includes the following steps:

[0047] S1: Energy Acquisition and Power Storage

[0048] The base station transmits a carrier wave, which is received and resonated by a resonant circuit consisting of an external inductor L1 and an internal capacitor C1. This resonant circuit generates AC energy through mutual inductance with the base station. This AC energy is then transferred to a rectifier circuit, which converts it into DC power. The input of the rectifier circuit is connected to the RFN and RFP terminals of the resonant circuit, and the output terminals power the oscillation sustaining circuit, clock recovery circuit, oscillator, digital circuit, and memory, respectively. Simultaneously, it charges the energy storage capacitor C2 (the other end of which is grounded). The energy stored in the energy storage capacitor C2 will provide continuous power for the subsequent calibration processes from S2 to S5, ensuring that the resonant circuit can still oscillate after the base station carrier is disconnected, without relying on an external power source.

[0049] S2: Test Mode Initiation and Carrier Switching

[0050] The low-frequency front-end circuit receives the test mode command, demodulates the data via the demodulation module, and transmits it to the digital circuit. The digital circuit control chip enters the test mode and then outputs an enable signal to enable the oscillator. After the high-frequency clock signal generated by the oscillator stabilizes (to avoid the instability of the high-frequency clock signal affecting the subsequent resonant frequency detection accuracy), the digital circuit controls the base station to disconnect the carrier. Simultaneously, the digital circuit activates the oscillation sustaining circuit. The oscillation sustaining circuit supplies power to the resonant circuit according to the clock signal output by the clock recovery circuit, maintaining the oscillation state of the resonant circuit. At this time, the oscillation of the resonant circuit is determined by its own L1 and C1 characteristics. This step, by disconnecting the base station carrier and maintaining the oscillation by the oscillation sustaining circuit, solves the problem in the background technology where the resonant circuit oscillation is affected by base station carrier interference and cannot truly reflect its own L1 and C1 characteristics, leading to frequency detection distortion. This ensures that the subsequent detection is of the true resonant frequency of the resonant circuit itself. At the same time, the chip enters the test mode via internal commands, eliminating the need for external professional low-frequency RFID testing equipment and simplifying the calibration startup process.

[0051] S3: Resonance Frequency Detection

[0052] The clock recovery circuit samples and processes the signal generated by the resonant circuit (L1 and C1) to sustain oscillation, outputting a clock signal with the same frequency as the resonant frequency. This clock signal is then transmitted to the digital circuit and the oscillation sustaining circuit, respectively. The digital circuit uses the faster, higher-frequency clock signal output by the oscillator as a reference to count the resonant frequency clock signal output by the clock recovery circuit for a fixed period of time. Based on the counting result, it calculates the actual resonant frequency of the current resonant circuit and stores this frequency in an internal register. This step uses a stable, high-frequency clock generated by the internal oscillator as a reference for counting and detection. It eliminates the need for external testing equipment and directly obtains the actual frequency of the resonant circuit, which may deviate due to process and production errors. This solves the problems of low efficiency and equipment-limited accuracy in traditional calibration, which relies on external equipment. It provides accurate frequency data for calibration of products with individual differences.

[0053] S4: Resonant capacitor array adjustment and frequency calibration

[0054] The digital circuit compares the current resonant frequency with the preset target resonant frequency. If the current resonant frequency exceeds the preset error range, it adjusts the on / off state of the corresponding switch in the resonant capacitor array through an N-bit digital signal—the output signal ctrl_ctN-1:0 of ​​the N-bit digital circuit increments from all zero according to a preset period. Figure 3 The resonant frequency changes with the control signal. Alternatively, when the digital circuit determines that the resonant frequency output by the clock recovery circuit is higher or lower than the preset value, it increments or decrements ctrl_ctN-1:0 by 1 and continues the comparison until the resonant frequency corresponding to ctrl_ctN-1:0 falls within the preset error range. (That is, there are two methods: Method 1: ctrl_ctN-1:0 increments sequentially from a state of zero according to a preset period; Method 2: After the digital circuit determines whether the resonant frequency is high or low, it increments or decrements 1 based on the current value of ctrl_ctN-1:0 and then continues the comparison). Starting from zero, it increments sequentially according to the preset period. The process increments sequentially, with each increment corresponding to a change in the on / off state of one or more sets of switches (each set of switches is driven by an independent control signal bit, and the high or low level of a single control signal bit directly determines the connection or disconnection of the corresponding capacitor). By controlling the connection and disconnection of the resonant capacitor under different combinations, the total capacitance of the resonant circuit is changed, thereby adjusting the resonant frequency. During this process, the digital circuit continuously counts the resonant frequency output by the clock recovery circuit using the faster frequency output by the oscillator, determining whether the resonant frequency corresponding to ctrl_ctN-1:0 currently meets the preset value. Step S3 is repeated until the current resonant frequency falls within the preset error range. This step directly compensates for the resonant inductance and capacitance deviations caused by process and production errors by automatically adjusting the switching state of the resonant capacitor array (i.e., adjusting the number of connected capacitors), solving the problems of cumbersome traditional calibration processes that require manual intervention or specialized equipment. Through fine adjustment of the adjustable capacitor array, automatic calibration can be achieved for products with different individual differences, ensuring accurate and stable resonant frequencies.

[0055] S5: Calibration parameter storage, system power-down and power-on recall

[0056] When the resonant frequency corresponding to ctrl_ctN-1:0 is determined to meet the preset value (i.e., the preset target resonant frequency), the digital circuit reads the ctrl_ctN-1:0 signal of the current control resonant capacitor array and writes the calibrated correction value of the signal into the memory through the data port. After confirming that the memory has completed writing the correction value, the digital circuit outputs a shutdown signal, which sequentially shuts down the oscillator and the oscillation sustaining circuit. The resonant circuit stops oscillating, the rectifier circuit stops outputting power, the energy storage capacitor C2 discharges, the system is powered down, and the RFID resonant frequency calibration is completed. The next time the chip is powered on, the digital circuit first reads the ctrl_ctN-1:0 correction value stored in the memory through the data port, transmits it to the switch control terminal of the resonant capacitor array after level conversion, controls the corresponding switch to turn on and off, so that the resonant circuit is connected according to the calibrated capacitor combination (the default value of the resonant capacitor array is taken from the correction value in the memory), enters the precise resonance state, and then the base station sends a carrier wave. The resonant circuit efficiently obtains energy to power the system, and the chip enters the normal RFID working mode. This step ensures that the resonant frequency, after automatic calibration to compensate for process and production errors, remains accurate over a long period by storing calibration parameters and recalling them directly upon the next power-on. This avoids the problem of having to repeat tedious calibration every time the product is powered on due to individual differences, and further improves the practicality of automatic calibration and system stability.

[0057] Finally, it should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A self-calibration method for the resonant frequency of low-frequency RFID, characterized in that, Includes the following steps: S1: The base station transmits a carrier wave, the resonant circuit receives the carrier wave and resonates to generate AC energy, which is converted into DC power by the rectifier circuit to power the system and charge the energy storage capacitor C2. S2: The low-frequency front-end circuit receives the test mode command, demodulates it through the demodulation module, and then transmits the data to the digital circuit. The digital circuit control chip enters the test mode, enables the oscillator to generate a high-frequency clock signal, then disconnects the base station carrier, starts the oscillation maintenance circuit to supply power to the resonant circuit, and maintains the self-resonant oscillation of the resonant circuit. S3: The clock recovery circuit processes the oscillation signal of the resonant circuit and outputs a clock signal with the same frequency as the resonant frequency to the digital circuit; the digital circuit uses the high-frequency clock signal as a reference, counts the clock signal for a fixed period of time, and calculates the current resonant frequency. S4: The digital circuit compares the current resonant frequency with the preset target resonant frequency. If the current resonant frequency exceeds the preset error range, it outputs a control signal to adjust the on / off state of the corresponding switch in the resonant capacitor array. By controlling the connection and disconnection of the resonant capacitors under different combinations, the total connected capacity is changed, thereby adjusting the resonant frequency. Repeat step S3 until the current resonant frequency falls into the error range. S5: The digital circuit writes the current control signal of the resonant capacitor array into the memory, then turns off the oscillator and oscillation sustaining circuit, and the system is powered down; the next time the chip is powered on, the digital circuit reads the control signal in the memory and controls the resonant capacitor array to be connected to the resonant circuit according to the calibrated capacitance.

2. The self-calibration method for low-frequency RFID resonant frequency according to claim 1, characterized in that, In step S4, the output signal ctrl_ctN-1:0 of ​​the N-bit digital circuit increments sequentially from a state of all zeros according to a preset period; or when the digital circuit determines that the resonant frequency output by the clock recovery circuit is higher or lower than a preset value, it increments or decrements ctrl_ctN-1:0 by 1, and continues to compare until the resonant frequency corresponding to ctrl_ctN-1:0 falls within a preset error range; wherein, each time ctrl_ctN-1:0 changes, the on / off state of one or more sets of switches in the corresponding resonant capacitor array changes once, thereby changing the capacitor combination connected to the resonant circuit, and during this process, the digital circuit uses the faster frequency output by the oscillator to count the resonant frequency output by the clock recovery circuit to determine whether the corresponding resonant frequency meets the preset value.

3. The self-calibration method for low-frequency RFID resonant frequency according to claim 1, characterized in that, In step S1, the DC power converted by the rectifier circuit charges the energy storage capacitor C2. The energy storage capacitor C2 provides continuous power for the calibration process from step S2 to step S5, ensuring that the resonant circuit can still oscillate after the base station carrier is disconnected.

4. The self-calibration method for low-frequency RFID resonant frequency according to claim 1, characterized in that, In step S2, the "disconnect base station carrier" operation is performed after the high-frequency clock signal generated by the oscillator has stabilized.

5. The self-calibration method for low-frequency RFID resonant frequency according to claim 1, characterized in that, In the resonant capacitor array, the on / off state of each switch is driven by an independent control signal bit. The high or low level of a single control signal bit directly determines whether the corresponding capacitor is connected or disconnected.

6. A self-calibration circuit for the resonant frequency of a low-frequency RFID, characterized in that, The self-calibration circuit is used to implement the self-calibration method for the resonant frequency of a low-frequency RFID as described in any one of claims 1-5. The self-calibration circuit includes a resonant circuit, a resonant capacitor array, a rectifier circuit, an energy storage capacitor C2, an oscillation sustaining circuit, a clock recovery circuit, an oscillator, a digital circuit, and a memory. The resonant circuit is composed of an external inductor L1 and an internal chip capacitor C1 connected in parallel, with the two ends of the parallel circuit being the RFN terminal and the RFP terminal, respectively. The resonant capacitor array includes several resonant capacitors and a set of symmetrical switches corresponding to each resonant capacitor. One end of each set of switches is connected to the RFN terminal and the RFP terminal of the resonant circuit, and the other end is connected to the corresponding resonant capacitor. The resonant capacitors are connected between the symmetrical switches. The control terminals of the switches are connected to the digital circuit through a level conversion circuit. The RFN terminal and the RFP terminal of the resonant circuit are connected to the input terminal of the rectifier circuit and the input terminal of the clock recovery circuit, respectively. The output terminal of the rectifier circuit is connected to one end of the energy storage capacitor C2, the power supply terminal of the oscillation sustaining circuit, the power supply terminal of the clock recovery circuit, the power supply terminal of the oscillator, the power supply terminal of the digital circuit, and the power supply terminal of the memory, respectively, and the other end of the energy storage capacitor C2 is grounded. The signal input terminal of the oscillation sustaining circuit is connected to the output terminal of the clock recovery circuit, and the output terminal of the oscillation sustaining circuit is connected to the RFN terminal of the resonant circuit. The output of the clock recovery circuit is also connected to the input of a digital circuit. The enable terminal of the oscillator is connected to the control terminal of the digital circuit, and the clock output terminal of the oscillator is connected to the counting reference terminal of the digital circuit. The control signal output terminal of the digital circuit is connected to the switch control terminal of the resonant capacitor array through a level conversion circuit, and the data port of the digital circuit is bidirectionally connected to the memory.

7. The self-calibration circuit for low-frequency RFID resonant frequency according to claim 6, characterized in that, The switching devices in the resonant capacitor array are NMOS transistors or PMOS transistors, and the switching is controlled by the control signal output from the digital circuit after level conversion.

8. The self-calibration circuit for low-frequency RFID resonant frequency according to claim 6, characterized in that, The control signal output by the digital circuit is an N-bit digital signal that matches the number of resonant capacitors in the resonant capacitor array. Each bit of the digital signal independently controls the on / off state of a switch corresponding to a group of resonant capacitors, thereby adjusting the total capacity of the resonant capacitor array.

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