An Enhanced Three-Point UHF RFID System Based on AGC and an Automatic Gain Control Method

Abstract

The application discloses an enhanced three-point UHF RFID system based on AGC and an automatic gain control method, and relates to the technical field of RFID systems. The system comprises a host computer, a reader-writer, a plurality of exciters and a plurality of tags. The host computer sends control information to the reader-writer in a wired mode. After receiving the information from the host computer, the reader-writer sends a networking command to activate the exciters, and then sends an inventory command to the exciters. After the exciters and the reader-writer are successfully networked, the inventory command is forwarded to the tags. After receiving the inventory command, the tags return response information to the reader-writer. After receiving and processing the tag return information, the reader-writer sends the information to the host computer. The method considers the nonlinearity of a power detection circuit, calibrates output power, makes feedback input more accurate, considers the timing problem between the reader-writer and the tags, solves the influence caused by modulation power fluctuation, improves the receiving sensitivity of the reader-writer, and solves the problems of low gain and poor dynamic range existing in the prior three-point UHF RFID system.

Description

Technical Field

[0001] This invention belongs to the field of Internet of Things (IoT) technology, specifically relating to an enhanced three-point UHF RFID system based on AGC and an automatic gain control method. Background Technology

[0002] UHF RFID (Ultra-High Frequency Radio Frequency Identification) systems have advantages such as low power consumption, low cost, long identification distance, and high identification efficiency, and are widely used in logistics, warehousing, and positioning. With the rapid development of IoT technology, the application of UHF RFID systems has been further expanded, and it has become one of the key technologies of IoT. Existing UHF RFID systems consist of two parts: a reader and a tag (also known as a classic UHF RFID system). The reader transmits radio frequency signals, and the tag receives the radio frequency signals and returns data.

[0003] In a classic UHF RFID system, the reader transmits a signal via an antenna to the tag, which then returns the information to the reader. Due to factors such as different transmission distances between tags, antenna orientation, and multipath effects, the signal strength received by the reader from the tags fluctuates significantly. Therefore, automatic gain control of the received signal is necessary to ensure the stability and accuracy of the system. Unlike other communication systems that transmit and receive at different frequencies, UHF RFID systems (transmit and receive at the same frequency) require a stable local oscillator signal to eliminate self-interference signals in the tag's return signal. In classic UHF RFID systems, readers often use couplers to couple the transmitted signal as the local oscillator signal. Meanwhile, in three-point UHF RFID systems (reader, exciter, and tag), which evolved from classic UHF RFID systems, the reader uses a coupler to couple the received signal, which is then amplified and used as the local oscillator signal. However, the power of the received signal varies with the distance between the reader and exciter, the reader and the tag, and the exciter and the tag. Furthermore, the reader chip requires a high-power local oscillator signal. Therefore, the three-point UHF RFID system cannot ensure the reliability of the reader's data retrieval by simply amplifying the coupled signal.

[0004] In existing classic UHF RFID systems, the reader communicates directly with the tag, and its local oscillator signal comes from the output signal of the coupled transmitted signal. In contrast, the three-point UHF RFID system introduces an exciter as an auxiliary device to activate the tag. The reader amplifies a portion of the coupled received signal and uses it for demodulating the local oscillator. However, its local oscillator link gain is low, its dynamic range is small, and it is greatly affected by the environment. Classic UHF RFID systems are limited by the tag's receiving sensitivity, restricting the forward link communication distance. The three-point UHF RFID system has poor gain and dynamic range in its RF receiving front-end, and its exciter's envelope detection demodulation method is susceptible to multipath interference at long distances, resulting in poor system reliability and failing to meet practical application requirements.

[0005] Therefore, to improve system reliability, a controllable gain amplifier circuit needs to be designed to increase the dynamic range of the reader's receiving link, thereby enhancing system reliability. The controllable gain amplifier circuit can be implemented manually or automatically. Manual gain adjustment is not only time-consuming and labor-intensive but also difficult to achieve in real-time performance. Existing automatic gain control methods are rarely used in the receiving front-end of UHF RFID readers, and their adaptability to interference and signal attenuation in different environments is poor, resulting in unreliable system performance and failing to meet practical application requirements. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention proposes an enhanced three-point UHF RFID system and an automatic gain control method based on AGC. Through a novel AGC (Automatic Gain Control) circuit, an enhanced three-point UHF RFID system is constructed, solving the problems of low gain and poor dynamic range in existing three-point UHF RFID systems.

[0007] The technical solution adopted in this invention is as follows: an enhanced three-point UHF RFID system based on AGC, comprising: a host computer, a reader / writer, several exciters, and several tags; the host computer sends control information to the reader / writer via a wired connection. After receiving the control information from the host computer, the reader / writer first sends a network formation command to activate the exciter, and then sends an inventory command to the exciter. After the exciter and reader / writer successfully form a network, the exciter forwards the inventory command from the reader / writer to the tag. After receiving the inventory command forwarded by the exciter, the tag returns the response information to the reader / writer. After receiving and processing the information returned by the tag, the reader / writer sends the information to the host computer.

[0008] The host computer connects to the reader via a serial port or network port, and controls the reader to send network or inventory commands. The reader, exciter, and tag communicate wirelessly.

[0009] Furthermore, in the enhanced three-point UHF RFID system, the reader includes an AGC control circuit, the specific structure of which includes: N+1 AGC gain modules, a power divider, a processor, an RX receiving pin, and an LO receiving pin.

[0010] The input signal is connected to the input terminal of the first-stage AGC gain module. One output terminal of the first-stage AGC gain module is connected to the processor, and the other output terminal of the first-stage AGC gain module is connected to the second-stage AGC gain module. The other output terminal of the Xth-stage AGC gain module is connected to the input terminal of the power divider. One output terminal of the power divider is connected to the (X+1)th-stage AGC gain module, and the other output terminal of the power divider is connected to the RX receiving pin. The other output terminal of the Nth-stage AGC gain module is connected to the input terminal of the (N+1)th-stage AGC gain module. One output terminal of the (N+1)th-stage AGC gain module is connected to the processor, and the other output terminal of the (N+1)th-stage AGC gain module is connected to the LO receiving pin.

[0011] The (N+1)th level is the last level, and 1 ≤ X ≤ N. <X+1≤N。

[0012] The AGC gain modules at each level have the same structure, specifically including: BPF (bandpass filter), LNA (low noise amplifier), variable attenuator, coupler, and power detection circuit.

[0013] The input signal is connected to one end of the BPF, the other end of the BPF is connected to one end of the LNA, the other end of the LNA is connected to one end of the variable attenuator, the other end of the variable attenuator is connected to the input end of the coupler, the coupling end of the coupler is connected to one end of the power detection circuit, the other end of the power detection circuit is connected to the processor, the other end of the processor is connected to the control pin of the variable attenuator, and the output signal of the coupler's through end is connected to one end of the BPF in the next stage AGC gain module.

[0014] In this module, the direct-through terminal of the coupler in the Xth stage AGC gain module is connected to the input terminal of the power divider, and one output terminal of the power divider is connected to one end of the BPF in the (X+1)th stage AGC gain module. The direct-through terminal of the coupler in the (N+1)th stage AGC gain module is connected to the LO receiving pin. The RX receiving pin outputs the received signal, which is obtained by coupling from the power divider. The LO outputs the local oscillator signal, which is obtained by the final stage coupler. The power at the RX receiving pin is 10-20 dB lower than the power at the LO receiving pin. The low-noise amplifier (LNA) and the variable attenuator together form the RF gain module.

[0015] Furthermore, in each of the AGC gain modules, the power detection circuits are consistent, and the specific structure includes: inductor L1, capacitor C1, switch LNA, Schottky diode SMS7630, capacitor C2, resistor R1, resistor R2, capacitor C3, operational amplifier, resistor R3, resistor R4, resistor R5, capacitor C4, and analog-to-digital converter ADC.

[0016] The detector input signal is connected to one end of inductor L1. The other end of inductor L1 is connected to capacitor C1 and one end of switch LNA. The other end of capacitor C1 is grounded. The other end of switch LNA is connected to one end of Schottky diode SMS7630. The other end of Schottky diode SMS7630 is connected to one end of capacitor C2, resistor R1, and resistor R2. The other ends of capacitor C2 and resistor R1 are grounded. The other end of resistor R2 is connected to capacitor C3 and the non-inverting input of operational amplifier. The other end of capacitor C3 is grounded. The inverting input of operational amplifier is connected to one end of resistor R3 and resistor R4. The other end of resistor R3 is grounded. The other end of resistor R4 is connected to resistor R5 and the output of operational amplifier. The other end of resistor R5 is connected to capacitor C4 and the input of analog-to-digital converter (ADC). The other end of capacitor C4 is grounded.

[0017] The present invention also provides an automatic gain control method, the specific steps of which are as follows:

[0018] S1. The host computer sends control commands to the processor;

[0019] S2, The reader sends a Query command;

[0020] S3. The ADCs at each stage sample the power multiple times and take the maximum value to obtain the output power of each stage.

[0021] S4. The processor reads the control pin status of each attenuator to obtain the attenuation of each attenuator;

[0022] S5. Based on the RF gain module in the AGC control circuit, obtain the gain difference of each stage and calibrate the output power of each stage.

[0023] S6. Distribute the optimal output power of each stage as the target power to each stage;

[0024] S7. Compare the difference between the target power and the actual power at each stage, provide feedback control for each stage of attenuator, and determine whether the error range is met.

[0025] Furthermore, step S1 is specifically as follows:

[0026] After receiving the AGC control command sent by the host computer, the processor first determines the type of the control command. If it is a control command to adjust the target values ​​of local oscillator power and receiver power, the target values ​​will be adjusted before AGC adjustment is performed; otherwise, the default target values ​​will be used for AGC adjustment.

[0027] The dynamic range of the local oscillator power can be dynamically adjusted according to the local oscillator input power range of the reader chip, and the received power is set to be 10-20dB less than the local oscillator signal power.

[0028] Furthermore, step S2 is specifically as follows:

[0029] The AGC control circuit is in an idle state by default. When it starts working, the reader sends a Query command to the exciter. At the same time, the reader generates a control command to the AGC control circuit, and the AGC control circuit begins to adjust the power.

[0030] Furthermore, step S3 is specifically as follows:

[0031] When performing AGC adjustment, the attenuation levels are first initialized, and then the adjustment is performed in multiple sub-levels. The output power of each level serves as the basis for AGC adjustment. The analog-to-digital converter (ADC) samples the output voltage of the power detection circuit multiple times to obtain the maximum value, and the output power of each level is obtained based on the relationship between output voltage and output power. The output voltage of the power detection circuit is then calibrated against the actual power, taking into account the dynamic range of the power detection circuit itself.

[0032] Furthermore, step S5 is specifically as follows:

[0033] In the AGC control circuit, the low-noise amplifier (LNA) and the variable attenuator together form an RF gain module. Using a multi-stage cascaded approach, the output power of each stage is coupled to the power detection circuit by a coupler and converted into a DC voltage output. The output power is held and sampled multiple times by the ADC inside the processor, and the average value of the samples is compared with the target output power value of each stage.

[0034] Each stage of the RF gain module has an independent power detection circuit after it, and the gain difference between each stage of the gain module is known. Based on the power detection output of the preceding and following stages and the gain difference between them, it is determined whether the output power of a certain stage of the power detection circuit exceeds its dynamic range, and then correction is performed based on the gain difference.

[0035] Furthermore, step S6 is specifically as follows:

[0036] When controlling multi-stage attenuators, the AGC control circuit prioritizes attenuating the final stage attenuator as much as possible while ensuring the linear operation of the LNA, thereby reducing the noise figure of the AGC control circuit. The optimal output power for each stage is obtained from the system noise figure table and allocated to each stage as the target power.

[0037] Furthermore, step S7 is specifically as follows:

[0038] The feedback control algorithm controls the attenuators at each stage. If the target power of each stage is greater than the actual power, the attenuation is increased proportionally; if the target power of each stage is less than the actual power, the attenuation is decreased proportionally.

[0039] Set the error range according to actual needs. If the difference between the actual power of the current test and the actual power of the previous test is less than the set error range, no further adjustment of the attenuation amount will be made, and the system will return to the initial state to complete automatic gain control; otherwise, repeat step S7 until the error range is met.

[0040] The beneficial effects of this invention are as follows: The system of this invention includes: a host computer, a reader / writer, several exciters, and several tags. The host computer sends control information to the reader / writer via a wired connection. After receiving the information from the host computer, the reader / writer first sends a network formation command to activate the exciter, and then sends an inventory command to the exciter. After the exciter and reader / writer successfully form a network, the exciter forwards the inventory command to the tag. After receiving the inventory command, the tag returns response information to the reader / writer. The reader / writer receives and processes the tag's return information and then sends the information to the host computer. This invention's method considers the nonlinearity of the power detection circuit, calibrates the output power to make the feedback input more accurate, considers the timing problem between the reader / writer and the tag, and solves the impact of modulation power fluctuations, enabling the acquisition of optimal power RX and LO signals, improving the reader / writer's receiving sensitivity, and solving the problems of low gain and poor dynamic range in existing three-point UHF RFID systems. The AGC control circuit structure used in this invention is flexible and can be appropriately adjusted for various working scenarios. Attached Figure Description

[0041] Figure 1 This is a structural diagram of an enhanced three-point UHF RFID system based on AGC according to the present invention.

[0042] Figure 2 This is a circuit diagram of the AGC control circuit in the reader / writer in an embodiment of the present invention.

[0043] Figure 3 This is a power detection circuit diagram in the AGC control circuit of this invention.

[0044] Figure 4 This is a flowchart of an automatic gain control method according to the present invention.

[0045] Figure 5 This is a state transition diagram of the AGC control circuit in an embodiment of the present invention.

[0046] Figure 6 This is a flowchart of the AGC adjustment process in an embodiment of the present invention.

[0047] Figure 7 This is a timing diagram of the UHF RFID system in an embodiment of the present invention.

[0048] Figure 8 This is a timing diagram of a three-point UHF system in an embodiment of the present invention.

[0049] Figure 9 This is a schematic diagram of the command 2 baseband signal with PIE encoding in an embodiment of the present invention. Detailed Implementation

[0050] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0051] like Figure 1 As shown, an enhanced three-point UHF RFID system based on AGC according to the present invention includes: a host computer, a reader / writer, several exciters, and several tags; the host computer sends control information to the reader / writer via a wired connection. After receiving the control information from the host computer, the reader / writer first sends a network formation command to activate the exciter, and then sends an inventory command to the exciter. After the exciter and the reader / writer successfully form a network, the exciter forwards the inventory command from the reader / writer to the tag. After receiving the inventory command forwarded by the exciter, the tag returns the response information to the reader / writer. After receiving and processing the information returned by the tag, the reader / writer sends the information to the host computer.

[0052] The host computer connects to the reader via a serial port or network port, and controls the reader to send network or inventory commands. The reader, exciter, and tag communicate wirelessly.

[0053] The exciter is responsible for forwarding commands sent by the reader. Therefore, in the uplink, the reader's RF receiver module receives not only the data returned by the tag but also the carrier wave (CW) signal forwarded by the exciter. Since the tag operates on a backscattering principle, the frequency of the tag's returned signal is the same as the frequency of the CW signal forwarded by the exciter, and the power of the CW signal is greater than the power of the tag's returned signal. Therefore, the CW signal received by the reader from the exciter is filtered and amplified, then used as the local oscillator signal and self-mixed with the tag's returned signal to demodulate the tag's returned baseband signal. As the distance between the reader and the exciter changes, the power of the CW signal received by the reader from the exciter also varies. Since the local oscillator signal generally requires high power and stability, a high-gain and high-dynamic-range AGC control circuit is needed.

[0054] like Figure 2 As shown, in this embodiment, the reader in the enhanced three-point UHF RFID system includes an AGC control circuit, the specific structure of which includes: N+1 AGC gain modules, a power divider, a processor, an RX receiving pin, and an LO receiving pin.

[0055] The input signal is connected to the input terminal of the first-stage AGC gain module. One output terminal of the first-stage AGC gain module is connected to the processor, and the other output terminal of the first-stage AGC gain module is connected to the second-stage AGC gain module. The other output terminal of the Xth-stage AGC gain module is connected to the input terminal of the power divider. One output terminal of the power divider is connected to the (X+1)th-stage AGC gain module, and the other output terminal of the power divider is connected to the RX receiving pin. The other output terminal of the Nth-stage AGC gain module is connected to the input terminal of the (N+1)th-stage AGC gain module. One output terminal of the (N+1)th-stage AGC gain module is connected to the processor, and the other output terminal of the (N+1)th-stage AGC gain module is connected to the LO receiving pin.

[0056] The (N+1)th level is the last level, and 1 ≤ X ≤ N. <X+1≤N。

[0057] The AGC gain modules at each level have the same structure, specifically including: BPF (bandpass filter), LNA (low noise amplifier), variable attenuator, coupler, and power detection circuit.

[0058] The input signal is connected to one end of the BPF, the other end of the BPF is connected to one end of the LNA, the other end of the LNA is connected to one end of the variable attenuator, the other end of the variable attenuator is connected to the input end of the coupler, the coupling end of the coupler is connected to one end of the power detection circuit, the other end of the power detection circuit is connected to the processor, the other end of the processor is connected to the control pin of the variable attenuator, and the output signal of the coupler's through end is connected to one end of the BPF in the next stage AGC gain module.

[0059] In this module, the direct-through terminal of the coupler in the Xth stage AGC gain module is connected to the input terminal of the power divider, and one output terminal of the power divider is connected to one end of the BPF in the (X+1)th stage AGC gain module. The direct-through terminal of the coupler in the (N+1)th stage AGC gain module is connected to the LO receiving pin. The RX receiving pin outputs the received signal, which is obtained by coupling from the power divider. The LO outputs the local oscillator signal, which is obtained by the final stage coupler. The power at the RX receiving pin is 10-20 dB lower than the power at the LO receiving pin. The low-noise amplifier (LNA) and the variable attenuator together form the RF gain module.

[0060] like Figure 3As shown in this embodiment, the power detection circuits in each stage of the AGC gain module are identical, and the specific structure includes: inductor L1, capacitor C1, switch LNA, Schottky diode SMS7630, capacitor C2, resistor R1, resistor R2, capacitor C3, operational amplifier, resistor R3, resistor R4, resistor R5, capacitor C4, and analog-to-digital converter ADC.

[0061] The detector input signal is connected to one end of inductor L1. The other end of inductor L1 is connected to capacitor C1 and one end of switch LNA. The other end of capacitor C1 is grounded. The other end of switch LNA is connected to one end of Schottky diode SMS7630. The other end of Schottky diode SMS7630 is connected to one end of capacitor C2, resistor R1, and resistor R2. The other ends of capacitor C2 and resistor R1 are grounded. The other end of resistor R2 is connected to capacitor C3 and the non-inverting input of operational amplifier. The other end of capacitor C3 is grounded. The inverting input of operational amplifier is connected to one end of resistor R3 and resistor R4. The other end of resistor R3 is grounded. The other end of resistor R4 is connected to resistor R5 and the output of operational amplifier. The other end of resistor R5 is connected to capacitor C4 and the input of analog-to-digital converter (ADC). The other end of capacitor C4 is grounded.

[0062] The switching LNA can switch between pass-through and gain modes based on the power value of the detected input, effectively improving the dynamic range of the power detection circuit. Adjusting the ratio of resistor R3 to resistor R4 ensures that the DC voltage output does not exceed 4 / 5 of the maximum input voltage of the analog-to-digital converter (ADC) when the detected input power is at its maximum and the switching LNA is in gain mode. Adjusting resistor R5 and capacitor C4 can resolve voltage disturbances and improve the sampling accuracy of the ADC.

[0063] like Figure 4 As shown, the present invention also provides an automatic gain control method, the specific steps of which are as follows:

[0064] S1. The host computer sends control commands to the processor;

[0065] S2, The reader sends a Query command;

[0066] S3. The ADCs at each stage sample the power multiple times and take the maximum value to obtain the output power of each stage.

[0067] S4. The processor reads the control pin status of each attenuator to obtain the attenuation of each attenuator;

[0068] S5. Based on the RF gain module in the AGC control circuit, obtain the gain difference of each stage and calibrate the output power of each stage.

[0069] S6. Distribute the optimal output power of each stage as the target power to each stage;

[0070] S7. Compare the difference between the target power and the actual power at each stage, provide feedback control for each stage of attenuator, and determine whether the error range is met.

[0071] In this embodiment, step S1 is specifically as follows:

[0072] To adapt to various application scenarios, the adjustable receive power and local oscillator power of the AGC control circuit in the enhanced three-point UHF RFID system can both be configured by the host computer. The dynamic range of the local oscillator power can be dynamically adjusted according to the power required by the reader chip's local oscillator input (it can be set to 0–10 dBm). Simultaneously, to achieve good reception, the dynamic range of the receive power is set within the power range required by the reader chip, and kept 10-20 dB lower than the local oscillator signal power, which can be set to -30–0 dBm, achieving optimal demodulation results. The dynamic range of both the local oscillator input power and the receive power can be adjusted according to actual application conditions. The dynamic range mentioned in the enhanced three-point UHF RFID system can meet the needs of most scenarios.

[0073] After receiving the AGC control command sent by the host computer, the processor (such as MCU, ARM, DSP, FPGA, CPU, etc.) first determines the type of control command. If it is a control command to adjust the target values ​​of local oscillator power and receiver power, the target values ​​will be adjusted before AGC adjustment; otherwise, the default target values ​​will be used for AGC adjustment.

[0074] In this embodiment, step S2 is specifically as follows:

[0075] The AGC control circuit is in an idle state by default. When it starts working, the reader sends a query command to the actuator. At the same time, the reader generates a control command to the AGC control circuit, and the AGC control circuit begins to adjust the power. The state transition of the AGC control circuit is as follows: Figure 5 As shown.

[0076] like Figure 6 As shown, in this embodiment, step S3 is specifically as follows:

[0077] When performing AGC adjustment, the attenuation of each stage (each stage is represented by a different i value) is first initialized, and then the adjustment is further subdivided into multiple AGC stages. The output power of each stage serves as the basis for AGC adjustment. In order to obtain a more accurate output power for each stage, the analog-to-digital converter (ADC) samples the output voltage of the power detection circuit multiple times and takes the maximum value. The output power of each stage is then obtained based on the relationship between the output voltage and the output power.

[0078] In addition, the output voltage of the power detection circuit needs to be calibrated to match the actual power, and the dynamic range of the power detection circuit itself needs to be considered.

[0079] In this embodiment, step S5 is specifically as follows:

[0080] In the AGC control circuit, the low-noise amplifier (LNA) and the variable attenuator together form an RF gain module. Using a multi-stage cascaded approach, the output power of each stage is coupled to the power detection circuit by a coupler and converted into a DC voltage output. The output power is held and sampled multiple times by the ADC inside the processor, and the average value of the samples is compared with the target output power value of each stage.

[0081] To obtain a relatively accurate output power for each stage, the output voltage of the power detection circuit needs to be calibrated against the actual power. Furthermore, the dynamic range of the power detection circuit itself must be considered. When the signal power is within its dynamic range, the output voltage and actual power of the power detection circuit generally have a linear relationship. However, when the signal power exceeds its dynamic range, this linear relationship no longer exists. Using the multi-stage RF amplification module in the aforementioned AGC control circuit can effectively solve the nonlinearity problem.

[0082] Each stage of the RF gain module has an independent power detection circuit after it, and the gain difference between each stage of the gain module is known. Based on the power detection output of the preceding and following stages and the gain difference between them, it is determined whether the output power of a certain stage of the power detection circuit exceeds its dynamic range, and then correction is performed based on the gain difference.

[0083] In this embodiment, step S6 is specifically as follows:

[0084] Unlike existing AGC control methods, UHF RFID systems require consideration of timing issues between the reader and the tag. Taking a UHF RFID system conforming to ISO / IEC 18000-6C tags as an example, the required link timing between the reader and the tag is as follows: Figure 7 As shown.

[0085] After the reader sends command 2, the tag begins to return a signal after time T1. Therefore, the closed-loop control of the AGC control circuit must be completed before command 2 is sent. In existing three-point UHF RFID systems, the exciter acts as a repeater, sending the same command as the reader, except that the signal has a certain delay, such as... Figure 8 As shown.

[0086] During the transmission of command 2 by the reader, the input to the AGC control circuit comes from the exciter's forwarding signal. This input is not a continuous carrier CW, but an ASK modulated signal with PIE encoding (assuming it meets the 18000-6C protocol). Therefore, the output of the power detection circuit is no longer a stable voltage, but rather similar to the baseband signal of command 2, such as... Figure 9 As shown.

[0087] To address the impact of modulation power fluctuations, based on the characteristics of PIE encoding and the duration limitations of data0 and data1, a complete high level will definitely occur within the data0 period. The processor's ADC is used to sample as many times as possible within the data0 period, and the highest power is selected as the output power of this stage of power detection circuit.

[0088] Furthermore, existing AGC control circuits generally only need to focus on the stability of the final stage output signal. However, in UHF RFID systems, both the reader's RX and LO signals originate from the AGC control circuit output. For different demodulation systems, there exists an optimal demodulation power difference between RX and LO; therefore, the AGC control logic must satisfy this condition as much as possible. To maximize receiver sensitivity, when controlling multi-stage attenuators, the AGC prioritizes attenuating the final stage attenuator while ensuring the LNA's linear operation, thereby reducing the overall noise figure of the AGC control circuit. The optimal output power for each stage can be obtained from the system noise figure table and allocated to each stage as the target power.

[0089] In this embodiment, step S7 is specifically as follows:

[0090] The feedback control algorithm controls the attenuators at each stage, keeping the output power of each stage constant. If the target power of each stage is greater than the actual power, the attenuation is increased proportionally; if the target power of each stage is less than the actual power, the attenuation is decreased proportionally.

[0091] To improve the efficiency of AGC and avoid reducing system efficiency due to excessively frequent AGC adjustments, an error range can be set according to requirements. If the difference between the actual power of the current test and the actual power of the previous test is less than the set error range, the attenuation amount will not be adjusted, and the system will return to the initial state to complete automatic gain control; otherwise, step S7 will be repeated until the error range is met.

[0092] In summary, this invention performs multi-level feedback processing on the signal at the RF front end, enabling rapid response to varying input signal strengths. It maximizes the dynamic range of the received signal while minimizing noise figure degradation. The method considers the nonlinearity of the power detection circuit and calibrates the output power for more accurate feedback input. It also addresses the timing issues between the reader and the tag, mitigating the impact of modulation power fluctuations. This results in optimal RX and LO signals, improving the reader's receiving sensitivity and resolving the low gain and poor dynamic range issues inherent in existing three-point UHF RFID systems. Furthermore, the AGC control circuit used in this invention is flexible and can be adjusted appropriately for various operating scenarios.

[0093] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of this invention.

Claims

1. An enhanced three-point UHF RFID system based on AGC, comprising: One host computer, one reader / writer, several exciters, and several tags; The host computer sends control information to the reader via a wired connection. After receiving the control information from the host computer, the reader first sends a network command to activate the exciter, and then sends an inventory command to the exciter. After the exciter and the reader successfully form a network, the exciter forwards the inventory command from the reader to the tag. After receiving the inventory command forwarded by the exciter, the tag returns the response information to the reader. After receiving and processing the information returned by the tag, the reader sends the information to the host computer. The host computer is connected to the reader via a serial port or network port, and controls the reader to send network or inventory commands. The reader, exciter and tag communicate wirelessly. In the enhanced three-point UHF RFID system, the reader includes an AGC control circuit, the specific structure of which includes: N+1 AGC gain modules, a power divider, a processor, an RX receiving pin, and an LO receiving pin; The input signal is connected to the input terminal of the first-stage AGC gain module. One output terminal of the first-stage AGC gain module is connected to the processor, and the other output terminal of the first-stage AGC gain module is connected to the second-stage AGC gain module. The other output terminal of the Xth-stage AGC gain module is connected to the input terminal of the power divider. One output terminal of the power divider is connected to the (X+1)th-stage AGC gain module, and the other output terminal of the power divider is connected to the RX receiving pin. The other output terminal of the Nth-stage AGC gain module is connected to the input terminal of the (N+1)th-stage AGC gain module. One output terminal of the (N+1)th-stage AGC gain module is connected to the processor, and the other output terminal of the (N+1)th-stage AGC gain module is connected to the LO receiving pin. Among them, the N+1th level is the last level, and , ; The AGC gain modules at each level have the same structure, specifically including: BPF bandpass filter, LNA low noise amplifier, variable attenuator, coupler, and power detection circuit. The input signal is connected to one end of the BPF, the other end of the BPF is connected to one end of the LNA, the other end of the LNA is connected to one end of the variable attenuator, the other end of the variable attenuator is connected to the input end of the coupler, the coupling end of the coupler is connected to one end of the power detection circuit, the other end of the power detection circuit is connected to the processor, the other end of the processor is connected to the control pin of the variable attenuator, and the output signal of the coupler's through end is connected to one end of the BPF in the next stage AGC gain module. In this module, the direct-through terminal of the coupler in the Xth stage AGC gain module is connected to the input terminal of the power divider, and one output terminal of the power divider is connected to one end of the BPF in the (X+1)th stage AGC gain module. The direct-through terminal of the coupler in the (N+1)th stage AGC gain module is connected to the LO receiving pin. The RX receiving pin outputs the received signal, which is obtained by coupling from the power divider. The LO outputs the local oscillator signal, which is obtained by the final stage coupler. The power at the RX receiving pin is 10-20 dB lower than the power at the LO receiving pin. The low-noise amplifier (LNA) and the variable attenuator together form the RF gain module.

2. The enhanced three-point UHF RFID system based on AGC according to claim 1, characterized in that, In each AGC gain module, the power detection circuits are consistent, and the specific structure includes: inductor L1, capacitor C1, switch LNA, Schottky diode SMS7630, capacitor C2, resistor R1, resistor R2, capacitor C3, operational amplifier, resistor R3, resistor R4, resistor R5, capacitor C4, and analog-to-digital converter ADC. The detector input signal is connected to one end of inductor L1. The other end of inductor L1 is connected to capacitor C1 and one end of switch LNA. The other end of capacitor C1 is grounded. The other end of switch LNA is connected to one end of Schottky diode SMS7630. The other end of Schottky diode SMS7630 is connected to one end of capacitor C2, resistor R1, and resistor R2. The other ends of capacitor C2 and resistor R1 are grounded. The other end of resistor R2 is connected to capacitor C3 and the non-inverting input of operational amplifier. The other end of capacitor C3 is grounded. The inverting input of operational amplifier is connected to one end of resistor R3 and resistor R4. The other end of resistor R3 is grounded. The other end of resistor R4 is connected to resistor R5 and the output of operational amplifier. The other end of resistor R5 is connected to capacitor C4 and the input of analog-to-digital converter (ADC). The other end of capacitor C4 is grounded.

3. An automatic gain control method applied to the enhanced three-point UHF RFID system based on AGC according to claim 1, comprising the following specific steps: S1. The host computer sends control commands to the processor; S2, The reader sends a Query command; S3. The ADCs at each stage sample the power multiple times and take the maximum value to obtain the output power of each stage. S4. The processor reads the control pin status of each attenuator to obtain the attenuation of each attenuator; S5. Based on the RF gain module in the AGC control circuit, obtain the gain difference of each stage and calibrate the output power of each stage. S6. Distribute the optimal output power of each stage as the target power to each stage; S7. Compare the difference between the target power and the actual power at each stage, provide feedback control for each stage of attenuator, and determine whether the error range is met.

4. The automatic gain control method according to claim 3, characterized in that, The specific steps of S1 are as follows: After receiving the AGC control command from the host computer, the processor first determines the type of the control command. If it is a control command to adjust the target values ​​of the local oscillator power and the receiver power, the target values ​​will be adjusted before AGC adjustment is performed; otherwise, the default target values ​​will be used for AGC adjustment. The dynamic range of the local oscillator power can be dynamically adjusted according to the local oscillator input power range of the reader chip, and the received power is set to be 10-20dB less than the local oscillator signal power.

5. The automatic gain control method according to claim 3, characterized in that, Step S2 is as follows: The AGC control circuit is in an idle state by default. When it starts working, the reader sends a Query command to the exciter. At the same time, the reader generates a control command to the AGC control circuit, and the AGC control circuit begins to adjust the power.

6. The automatic gain control method according to claim 3, characterized in that, Step S3 is as follows: When performing AGC adjustment, the attenuation of each level is first initialized, and then the adjustment is further subdivided into multiple AGC levels. The output power of each level serves as the basis for AGC adjustment. The analog-to-digital converter (ADC) samples the output voltage of the power detection circuit multiple times to obtain the maximum value, and obtains the output power of each level based on the relationship between the output voltage and the output power. Then, the output voltage of the power detection circuit is calibrated with the actual power, taking into account the dynamic range of the power detection circuit itself.

7. The automatic gain control method according to claim 3, characterized in that, Step S5 is as follows: In the AGC control circuit, the low-noise amplifier (LNA) and the variable attenuator together form an RF gain module. Using a multi-stage cascaded approach, the output power of each stage is coupled to the power detection circuit by a coupler and converted into a DC voltage output. The output power is held and sampled multiple times by the ADC inside the processor, and the average value of the samples is compared with the target output power value of each stage. Each stage of the RF gain module has an independent power detection circuit after it, and the gain difference between each stage of the gain module is known. Based on the power detection output of the preceding and following stages and the gain difference between them, it is determined whether the output power of a certain stage of the power detection circuit exceeds its dynamic range, and then correction is performed based on the gain difference.

8. The automatic gain control method according to claim 3, characterized in that, Step S6 is as follows: When controlling multi-stage attenuators, the AGC control circuit prioritizes attenuating the final stage attenuator to reduce the noise figure of the AGC control circuit while ensuring the linear operation of the LNA. The optimal output power of each stage is obtained from the system noise figure table and allocated to each stage as the target power.

9. The automatic gain control method according to claim 3, characterized in that, Step S7 is as follows: The feedback control algorithm controls the attenuators at each stage. If the target power of each stage is greater than the actual power, the attenuation is increased proportionally; if the target power of each stage is less than the actual power, the attenuation is decreased proportionally. Set the error range according to actual needs. If the difference between the actual power of the current test and the actual power of the previous test is less than the set error range, no further adjustment of the attenuation amount will be made, and the system will return to the initial state to complete automatic gain control; otherwise, repeat step S7 until the error range is met.

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